Effective Singlet Oxygen Sensitizers Based on the Phenazine Skeleton as Efficient Light Absorbers in Dye Photoinitiating Systems for Radical ...

materials
Article
Effective Singlet Oxygen Sensitizers Based on the Phenazine
Skeleton as Efficient Light Absorbers in Dye Photoinitiating
Systems for Radical Polymerization of Acrylates
Ilona Pyszka * , Zdzisław Kucybała and Beata J˛edrzejewska *

 Faculty of Chemical Technology and Engineering, UTP University of Science and Technology,
 85-326 Bydgoszcz, Poland; kucybala@utp.edu.pl
 * Correspondence: Ilona.Pyszka@utp.edu.pl (I.P.); beata@utp.edu.pl (B.J.); Tel.: +48-52-374-9039 (I.P.);
 +48-52-374-9046 (B.J.)

 Abstract: A series of dyes based on the phenazine skeleton were synthesized. They differed in the
 number of conjugated double bonds, the arrangement of aromatic rings (linear and/or angular
 system), as well as the number and position of nitrogen atoms in the molecule. These compounds
 were investigated as potential singlet oxygen sensitizers and visible light absorbers in dye photoiniti-
 ating systems for radical polymerization. The quantum yield of the singlet oxygen formation was
 determined by the comparative method based on the 1 H NMR spectra recorded for the tested dyes
 in the presence of 2,3-diphenyl-p-dioxene before and after irradiation. The quantum yield of the
 triplet state formation was estimated based on the transient absorption spectra recorded using the
 nanosecond flash photolysis technique. The effectiveness of the dye photoinitiating system was
 



 
 characterized by the initial rate of trimethylolpropane triacrylate (TMPTA) polymerization. In the
 investigated photoinitiating systems, the sensitizer was an electron acceptor, whereas the co-initiator
Citation: Pyszka, I.; Kucybała, Z.;
 was an electron donor. The effectiveness of TMPTA photoinitiated polymerization clearly depended
J˛edrzejewska, B. Effective Singlet
Oxygen Sensitizers Based on the
 on the arrangement of aromatic rings and the number of nitrogen atoms in the modified phenazine
Phenazine Skeleton as Efficient Light structure as well as the quantum yield of the triplet state formation of the photosensitizer in the
Absorbers in Dye Photoinitiating visible light region.
Systems for Radical Polymerization
of Acrylates. Materials 2021, 14, 3085. Keywords: phenazine derivatives; sensitizers; singlet oxygen; dye photoinitiating systems; radi-
https://doi.org/10.3390/ma14113085 cal polymerization

Academic Editor: Dario Pasini

Received: 26 April 2021 1. Introduction
Accepted: 31 May 2021
 Oxygen is the most abundant element on Earth and is involved in many chemical
Published: 4 June 2021
 and physical processes. Alchemists called it the “food of life” since it is necessary for
 life. Despite its huge positive role, oxygen can also destroy cell structures, which, in turn,
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 contributes to the aging of organisms and the development of cancer. Changes in the
with regard to jurisdictional claims in
published maps and institutional affil-
 properties of oxygen and its ability to initiate a number of processes are conditioned by
iations.
 the presence of light. Because of the interaction of electromagnetic radiation with oxygen,
 reactive oxygen species (ROS), widely described in literature, may be formed [1]. They are
 highly energetic, and the excess energy is responsible for their high reactivity. One such
 form is singlet oxygen.
 In the ground state, the oxygen molecule has two unpaired electrons having their
Copyright: © 2021 by the authors.
 spins parallel and occupying perpendicular anti-bonding orbitals π, so it is in the triplet
Licensee MDPI, Basel, Switzerland.
 form (3 O2 ). Thus, the O2 molecule is paramagnetic—the spin quantum number is 1. When
This article is an open access article
 the molecule gains energy upon excitation, the electron spins are paired, and the electron
distributed under the terms and
conditions of the Creative Commons
 configuration of the oxygen molecule changes from a triplet to a singlet 1 O2 . Thus, in the
Attribution (CC BY) license (https://
 excited state, the spin quantum number is 0. Such a system of energy levels distinguishes
creativecommons.org/licenses/by/ the oxygen molecule from the vast majority of organic compounds for which the electronic
4.0/).

Materials 2021, 14, 3085. https://doi.org/10.3390/ma14113085 https://www.mdpi.com/journal/materials

Materials 2021, 14, 3085 2 of 19

 ground state is the singlet state S0 , whereas the lowest excited energy state is the triplet
 state T1 [2–4].
 This chemically useful singlet oxygen form can be prepared by two main methods,
 i.e., irradiation of the 3 O2 form and chemical synthesis. The latter method produces the
 singlet oxygen as one of the products of a chemical reaction. The following examples
 can be given: (i) the reaction of hydrogen peroxide with sodium chlorate (I) [5–7], (ii)
 the decomposition of some peroxides [8–10], and (iii) the decomposition of adducts of
 organophosphorus compounds and ozone [11,12]. The other way is to produce singlet
 oxygen by an atmospheric discharge [13,14]. In nature, singlet oxygen is commonly formed
 from water during photosynthesis, using the energy of sunlight [15]. It is also produced in
 the troposphere by the photolysis of ozone by light of short wavelength [16] and by the
 immune system as a source of active oxygen [17].
 However, a more interesting way to generate singlet oxygen is a photochemical
 process based on an excitation of triplet oxygen with ultraviolet light in the presence of a
 sensitizing dye. In that case, oxygen molecule takes energy from the sensitizer being in the
 triplet excited state. A singlet oxygen sensitizer can be any compound that forms a triplet
 state and fulfils the following conditions:
 1. It does not react with oxygen, i.e., it does not undergo auto-oxidation [18,19];
 2. It has a high molar absorption coefficient at the excitation wavelength and the ap-
 propriate spectral characteristics enabling selective excitation of the sensitizer, while
 limiting the excitation of other molecules present in the solution;
 3. It has a low triplet state energy [20].
 The above conditions are met by a number of organic compounds that are effective
 singlet oxygen sensitizers, e.g., erythrosine, methylene blue, chlorophyll, hematoporphyrin,
 rose bengal, perinaftenone, acridine, and others [21–23].
 Singlet oxygen sensitizers usually operate in the same phase in which the photooxida-
 tion is performed. In that case, the sensitization reaction is carried out in solution. However,
 a number of heterogeneous sensitizers have now been developed. Their use allows for
 photooxidation without side effects. There are several ways to prepare heterogeneous
 sensitizers. One of them is using a polymeric carrier (e.g., chloromethylated polystyrene
 beads or modified silica gel) to which the sensitizer is chemically attached [24,25].
 Upon excitation the sensitizer molecule undergoes a change from the singlet ground
 state S(S0 ) to the singlet excited state *S(S1 ).

 S(S0 ) + hν → *S(S1 ) (1)

 As a result of the energy transfer (or electron transfer) from the sensitizer molecule in
 its excited singlet state *S(S1 ) to the ground-state oxygen molecule 3 O2 through collisions,
 the sensitizer molecule goes to the triplet state *S(T1 ). The singlet oxygen formed in the
 excited state can oxidize other compounds.

 *S(S1 ) + 3 O2 → *S(T1 ) + 1 O2 (1 ∆g) (2)

 Only those sensitizers with a lifetime of the S1 excited state greater than ns will be
 effective in interacting with oxygen molecules. Furthermore, when the energy gap for
 the sensitizer is lower than the triplet-singlet energy difference for the oxygen molecule,
 the process described by Equation (2) is not allowed energetically. In such a case, the
 oxygen molecule may be a medium participating in the formation of the triplet state of
 the sensitizer molecule (Equation (3)) or may influence the process of deactivating the
 excited singlet state of the sensitizer *S(S1 ) to the ground state S(S0 ) (Equation (4)). Both
 of these processes can occur through the formation of an exciplex. Polycyclic aromatic
 hydrocarbons mostly undergo the processes described by Equations (3) and (4) because of
 the insufficient energy difference between the states S1 and T1 .

 *S(S1 ) + 3 O2 → *S(T1 ) + 3 O2 (3)

Materials 2021, 14, 3085 3 of 19

 *S(S1 ) + 3 O2 → S(S0 ) + 3 O2 (4)
 The most effective method of generating singlet oxygen under laboratory conditions
 is the triplet (T1 ) quenching of the sensitizer molecule. This method plays an important
 role in many applications of oxygen.
 The study of reactive oxygen species is crucial for several groups of scientists, includ-
 ing photochemists, biologists, and physiologists [2,26–30]. The search for new, efficient
 singlet oxygen sensitizers seems to be important because of the positive and negative role of
 singlet oxygen for humans. Singlet oxygen is used in medicine because it can be involved,
 for example, in the destruction of cancer cells [23]. In addition, photo-oxidation occurring
 under the influence of singlet oxygen is used to sterilize blood and surgical instruments
 as well as to purify water [22,31]. The negative effects of singlet oxygen include polymer
 degradation and photo-oxidation of food products [32].
 In the present work, our intention was to apply dyes based on the phenazine skeleton,
 which effectively generated singlet oxygen, as visible light absorbers in the photoinitiating
 systems for radical polymerization. Such dye photoinitiators operate through a long-
 lived triplet state, which is beneficial in terms of generating radicals through the electron
 transfer process. Therefore, we performed a comparative study on the quantum yield of
 the triplet state formation of the phenazine derivatives. We investigated the dye-sensitized
 photooxidation of 2,3-diphenyl-1,4-dioxene (DPD) to determine the quantum yield of
 singlet oxygen as it is equal to the quantum yield of triplet state formation when large
 excess oxygen is used. Additionally, we used a nanosecond flash photolysis technique
 since the analysis of transient absorption spectra allowed us to determine the quantum
 yield of the triplet state formation as well.
 We have attempted to test phenazine derivatives as photosensitizers in radical poly-
 merization, as new dye structures are still being sought because of the high demands
 placed on photoinitiators in modern applications such as dental filling materials, reprog-
 raphy (photoresists, printing plates, integrated circuits), laser-induced 3D curing, holo-
 graphic recordings, and nanoscale micromechanics. Modifications of the photosensitizer
 structure were carried out by introducing heavy atoms and electron-donating or electron-
 withdrawing substituents to various heterocyclic rings [33–43] or by the preparation of
 new chemical skeletons. Our strategy for synthesis followed the modification of phenazine
 structure by increasing either nitrogen atoms to 3 or 4 in the dye molecule or aromatic rings
 in the angular and/or linear positions. Such phenazine-functionalized dyes fulfilled the
 requirements for photoinitiators of radical polymerization, i.e., they exhibited high molar
 absorption coefficients in visible light, efficiently interacted with additives (e.g., electron
 donors), and revealed a long-lived triplet state. Moreover, their activity in the visible light
 region allowed them to meet economic and environmental constraints because visible light
 is safer, cheaper, and more easily available than ultraviolet light; therefore, inexpensive irra-
 diation setups like low-power light-emitting diodes (LEDs) and diode-pumped solid-state
 (DPSS) lasers can be used to initiate the polymerization reaction [44].
 The synthesized dyes were used as a primary light absorber, which formed the
 photoinitiating system with a suitable co-initiator. In our study, we used commercially
 available [(3,4-dimethoxyphenyl)sulfanyl]acetic acid as the co-initiator.

 2. Materials and Methods
 2.1. Reagents
 Dye sensitizers based on the phenazine skeleton (FN1–FN13) (see Table 1) were
 synthesized by the condensation of appropriate diamines with quinones using the method
 described in the literature [45–55]. The crude product was purified by recrystallization from
 ethyl acetate. Reagents for synthesis: 1,2-diaminobenzene and 2,3-diaminonaphthalene were
 purchased from Alfa Aesar Co., Ward Hill, MA, United States; 9,10-phenanthrenequinone, 1,2-
 naphthoquinone, 9,10-diaminophenanthrene, 2,3-diaminopyridine, 4, 5-diamino-pyrimidine,
 4-bromo-1,2-diaminobenzene, and 2,3-diamino-5-bromopyridine were purchased from Sigma-
 Aldrich Co, Saint Louis, MO, United States; 1,2-diammonocyclohexane and 1,10-phenanthroline-

Materials 2021,
 Materials 2021, 14,
 14, xx FOR
 FOR PEER
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 REVIEW 444 of
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Materials 2021, 14, 3085 lene were
 lene were purchased
 purchased fromfrom Alfa
 Alfa Aesar
 Aesar Co.,
 Co., Ward
 Ward Hill,
 Hill, MA,MA, United
 United States;
 States; 9,10-phenan-
 9,10-phenan-
 4 of 19
 lene were
 lene were purchased
 purchased fromfrom Alfa
 Alfa Aesar
 Aesar Co.,
 Co., Ward
 Ward Hill,
 Hill, MA,MA, United
 United States;
 States; 9,10-phenan-
 9,10-phenan-
 threnequinone,
 threnequinone, 1,2-naphthoquinone,
 1,2-naphthoquinone, 9,10-diaminophenanthrene,
 9,10-diaminophenanthrene, 2,3-diaminopyridine,
 2,3-diaminopyridine, 4,
 4,
 threnequinone, 1,2-naphthoquinone,
 threnequinone, 1,2-naphthoquinone, 9,10-diaminophenanthrene,
 9,10-diaminophenanthrene, 2,3-diaminopyridine,
 2,3-diaminopyridine, 4, 4,
 5-diamino-pyrimidine,
 5-diamino-pyrimidine, 4-bromo-1,2-diaminobenzene,
 4-bromo-1,2-diaminobenzene, and
 and 2,3-diamino-5-bromopyridine
 2,3-diamino-5-bromopyridine
 5-diamino-pyrimidine, 4-bromo-1,2-diaminobenzene,
 5-diamino-pyrimidine, 4-bromo-1,2-diaminobenzene, and and 2,3-diamino-5-bromopyridine
 2,3-diamino-5-bromopyridine
 were
 were purchased
 purchased from
 from Sigma-Aldrich
 Sigma-Aldrich Co,
 Co, Saint
 Saint Louis,
 Louis, MO,
 MO, United
 United States;
 States; 1,2-diammono-
 1,2-diammono-
 werewere
 were
 5,6-dione purchased
 purchased
 cyclohexane from
 from
 purchased
 and Sigma-Aldrich
 Sigma-Aldrich
 from Acros Organics Co,Co,
 Co,
 1,10-phenanthroline-5,6-dione Saint
 Saint Louis,
 Louis,
 Pittsburgh,
 were MO,
 MO, United
 United
 PA, United
 purchased States;
 States;
 States;
 from 1,2-diammono-
 1,2-diammono-
 and
 Acros 5-bromo- Co,
 Organics
 cyclohexane
 cyclohexane and
 and 1,10-phenanthroline-5,6-dione
 1,10-phenanthroline-5,6-dione were
 were purchased
 purchased from
 from Acros
 Acros Organics
 OrganicsfromCo,
 Co,
 2,3-diaminopyrazine
 Pittsburgh,
 Pittsburgh, PA,
 PA, was purchased
 United
 United States;
 States; from
 and
 and Fluorochem Co, Hadfield,
 5-bromo-2,3-diaminopyrazine
 5-bromo-2,3-diaminopyrazine UK. was
 was purchased
 purchased from
 Pittsburgh, PA, United States; and 5-bromo-2,3-diaminopyrazine
 Pittsburgh, PA, United States; and 5-bromo-2,3-diaminopyrazine was purchased from was purchased from
 Fluorochem
 Fluorochem Co,
 Co, Hadfield,
 Hadfield, UK.
 UK.
 TableFluorochem
 Fluorochem Co,
 1. Structures,Co, Hadfield,
 Hadfield,
 basic UK. properties, and quantum yields of fluorescence for the dyes
 UK.
 spectroscopic
 Table
 Table 1.
 1. Structures,
 Structures, basic
 basic spectroscopic
 spectroscopic properties, and
 properties, and quantum
 quantum yields
 yields ofof fluorescence
 fluorescence
 Table 1. Structures, basic spectroscopic properties, and
 Table 1. Structures, basic spectroscopic properties,
 tested. and quantum
 quantum yields
 yields ofof fluorescence
 fluorescence
 for the
 for the dyes
 dyes tested.
 tested.
 for the
 for the dyes
 dyes tested.
 tested. −1 −1
 Dye
 Dye Structure
 Structure λAb
 
 , 
 , nm Ab , M
 emax
 , 
 cm λFl
 
 , 
 , nm ΦFl
 Dye
 Dye Structure
 Structure 
 ,, 
 
 max
 
 ,, 
 
 ,, 
 
 max
 
 Dye Structure , , 
 
 , 
 
 N
 N
 N
 N
 N
 N
 345
 345
 345 8060
 8060
 8060
 FN1
 FN1 N 345
 345 8060
 8060 383.0
 383.0
 383.0 0.110
 0.110
 0.110
 FN1
 FN1
 FN1 N
 N
 362
 362 10,400
 10,400 383.0
 383.0 0.110
 0.110
 N
 N
 N 362
 362
 362 10,400
 10,400
 10,400
 N
 N

 N
 N
 N
 N
 N
 N
 N 375
 375
 375 9700
 9700
 9700
 FN2
 FN2
 FN2 375
 375 9700
 9700 413.6
 413.6
 413.6 0.044
 0.044
 0.044
 FN2
 FN2 N
 N
 N
 395
 395
 395 11,400
 11,400
 11,400 413.6
 413.6 0.044
 0.044
 N
 N
 N
 N
 395
 395 11,400
 11,400

 N
 N
 N
 N
 N
 N
 N 402
 402
 402 10,060
 10,060
 10,060 532.8 0.045
 FN3
 FN3
 FN3 402
 402 10,060
 10,060 532.8
 532.8 0.045
 0.045
 FN3
 FN3 N
 N 425
 425 13,500
 13,500 532.8
 532.8 0.045
 0.045
 N
 N
 N
 N
 425
 425
 425 13,500
 13,500
 13,500
 N

 N
 N
 N
 FN4
 N
 N
 N
 N 392
 392
 392 9900
 9900
 9900 415.2
 415.2
 415.2 0.027
 FN4
 FN4
 FN4
 392
 392 9900
 9900 415.2
 415.2 0.027
 0.027
 0.027
 FN4
 FN4 N
 N 414
 414
 414 15,900
 15,900
 15,900 438.4
 438.4
 438.4 0.027
 N
 N
 N
 N
 N 414
 414 15,900
 15,900 438.4
 438.4

 N
 N
 N
 FN5
 N
 N
 N
 N 392
 392
 392 9700
 9700
 9700 414.2
 414.2
 414.2 0.049
 FN5
 FN5
 FN5
 392
 392 9700
 9700 414.2
 414.2 0.049
 0.049
 0.049
 FN5 N
 N
 N
 414
 414
 414 16,300
 16,300
 16,300 438.0
 438.0
 438.0 0.049
 N
 N
 N
 N 414
 414 16,300
 16,300 438.0
 438.0

 N
 N
 N
 386
 386 9500
 9500
 FN6
 FN6 N
 N
 N
 N
 386
 386 9500
 9500 437.8
 437.8 0.025
 0.025
 FN6
 FN6
 FN6 407
 407
 407 10,900
 10,900
 10,900 437.8
 437.8
 437.8 0.025
 0.025
 0.025
 N
 N
 N
 N
 407
 407 10,900
 10,900
 N
 N
 N

 N
 N
 N 409
 409 14,000
 14,000 543.8 0.025
 FN7
 FN7
 FN7 N
 N
 N
 N 409
 409
 409 14,000
 14,000
 14,000 543.8 0.025
 FN7
 FN7
 FN7 432
 432 10,800
 10,800 543.8
 543.8
 543.8 0.025
 0.025
 0.025
 N
 N 432
 432
 432 10,800
 10,800
 10,800
 N
 N
 N
 N
 N

 N
 N
 N
 N 383
 383 9600
 9600
 FN8
 FN8
 N 383
 383
 383 9600
 9600
 9600 460
 460 0.045
 0.045
 FN8
 FN8
 FN8 N
 N N
 N 393
 393 9620
 9620 460
 460
 460 0.045
 0.045
 0.045
 N
 N N
 N 393
 393
 393 9620
 9620
 9620
 N N

 Br
 Br N
 N
 Br
 Br
 Br
 Br
 N
 N
 N
 N
 382
 382 20,500
 20,500
 FN9
 FN9 382
 382
 382 20,500
 20,500
 20,500 424.2
 424.2 0.033
 0.033
 FN9
 FN9
 FN9 N
 N
 403
 403 25,600
 25,600 424.2
 424.2
 424.2 0.033
 0.033
 0.033
 N
 N
 N
 N
 403
 403
 403 25,600
 25,600
 25,600

 Br
 Br N
 N
 Br
 Br
 Br N
 N 390 15,900
 Br N
 N 390 15,900
 FN10
 FN10 390
 390 15,900
 15,900 428
 428 0.028
 0.028
 FN10
 FN10 N
 N N
 N
 407
 407 16,300
 16,300 428
 428 0.028
 0.028
 N
 N
 N N
 N
 N 407 16,300
 N N

432 10,800
 N

 N
 383 9600
 FN8 460 0.045
Materials 2021, 14, 3085 N N 393 9620 5 of 19

 Br N
 382 1. Cont. 20,500
 Table
 FN9 424.2 0.033
 N 403 25,600
 Dye Structure λAb Ab , M−1 cm−1 λFl ΦFl
 max , nm emax max , nm

 Br N
 Materials
 Materials 2021,
 Materials 2021, 14,
 14, xxx
 2021, 14, FOR
 x FOR PEER
 FOR PEER REVIEW
 PEER REVIEW
 390
 390 15,900
 15,900 5 of 20
 Materials 2021, 14, FOR PEER REVIEW
 FN10
 FN10
 REVIEW 428
 428 0.028555 of
 0.028 of 20
 of 20
 20
 N N
 407
 407 16,300
 16,300

 Br
 Br
 Br N
 N N
 N
 Br N N 395 14,300
 N N 395
 395
 395
 395 14,300
 14,300
 14,300
 14,300 445.6 0.042
 FN11
 FN11
 FN11
 FN11
 FN11 411 15,800 445.6
 445.6
 445.6
 445.6 0.042
 0.042
 0.042
 0.042
 N
 N N
 N
 411
 411
 411 15,800
 15,800
 15,800
 N
 N N
 N 411 15,800

 N
 N
 N N
 N
 N
 N N 360
 360
 360
 360 14,500
 14,500
 14,500
 14,500
 FN12
 FN12
 FN12
 FN12 360 14,500 412.4
 412.4
 412.4
 412.4 0.047
 0.047
 0.047
 0.047
 FN12 N N 378
 378
 378
 378 13,700
 13,700
 13,700
 13,700 412.4 0.047
 N
 N N
 N 378 13,700
 N N

 N
 N
 N
 N
 N
 N N
 FN13
 FN13
 FN13 N 437
 437
 437 11,490
 11,490
 11,490 477
 477
 477 0.100
 0.100
 0.100
 FN13
 FN13 N N
 437
 437 11,490
 11,490 477
 477 0.100
 0.100
 N
 N N
 N
 N N

 O
 O
 O
 O
 CQ
 CQ
 CQ
 CQ
 CQ
 472
 472
 472
 472
 472
 40
 40
 40 40
 40 ---- - -----
 O
 O
 O
 O

 All
 All solvents
 All solvents for
 solvents for synthesis,
 for synthesis, photochemical
 synthesis, photochemical studies,
 photochemical studies,
 studies, and and NMR
 and NMR analysis,
 NMR analysis, including
 analysis, including acetic
 including acetic
 acetic
 All solvents
 All solvents for for synthesis,
 synthesis, photochemical
 photochemical studies,
 studies, and
 and NMR
 NMR analysis,
 analysis,including
 includingacetic
 acid,
 acid, ethyl
 ethyl
 acid, ethyl acetate,
 acetate,
 ethyl acetate, deuterated
 deuterated
 acetate, deuterated chloroform
 chloroform
 deuterated chloroform
 chloroform (CDCl(CDCl
 (CDCl
 (CDCl33),3
 3 ),
 ), chloroform,
 chloroform,
 ), chloroform,
 chloroform, and and
 and 1-methyl-2-pyrroli-
 1-methyl-2-pyrroli-
 and 1-methyl-2-pyrroli-
 1-methyl-2-pyrroli-
 acid,
 acetic acid, (MP),
 ethyl acetate, deuterated chloroform (CDClCo, 3 ), chloroform, and 1-methyl-2-
 dinone
 dinone
 dinone (MP),
 (MP), were
 were
 were purchased
 purchased
 purchased from
 from
 from Sigma-Aldrich
 Sigma-Aldrich
 Sigma-Aldrich Co,
 Co, Saint
 Saint
 Saint Louis,
 Louis,
 Louis, MO,
 MO,
 MO, United
 United States.
 United States.
 States.
 dinone (MP),
 pyrrolidinone were
 (MP), were purchased
 purchased fromfromSigma-Aldrich
 Sigma-Aldrich Co,Co, Saint Louis,
 Saint MO,
 Louis, MO, United States.
 United
 The
 The
 The DC-Plastikfolien
 DC-Plastikfolien
 DC-Plastikfolien Silica
 Silica
 Silica gel
 gel
 gel 60
 60
 60 F254
 F254
 F254 thin
 thin
 thin layer
 layer
 layer chromatography
 chromatography
 chromatography plates
 plates
 plates (0.2
 (0.2
 (0.2 mm)
 mm)
 mm) were
 were
 were
 TheThe
 States. DC-Plastikfolien
 DC-Plastikfolien Silica gelgel
 Silica 6060F254
 F254thin
 thinlayer
 layer chromatography
 chromatography plates
 plates(0.2 mm)
 (0.2 mm) were
 from
 from
 from Merck
 Merck
 Merck Co,
 Co,
 Co, Kenilworth,
 Kenilworth,
 Kenilworth, NJ,
 NJ,
 NJ, USA.
 USA.
 USA.
 werefrom
 from Merck
 MerckCo, Co,Kenilworth,
 Camphorquinone Kenilworth,
 (CQ,
 NJ, USA.
 NJ,initiator),
 USA. [(3,4-dimethoxyphenyl)sulfanyl]acetic acid
 Camphorquinone
 Camphorquinone
 Camphorquinone (CQ,
 (CQ,
 (CQ, initiator),
 initiator),
 initiator), [(3,4-dimethoxyphenyl)sulfanyl]acetic
 [(3,4-dimethoxyphenyl)sulfanyl]acetic
 [(3,4-dimethoxyphenyl)sulfanyl]acetic acid
 acid
 Camphorquinone (CQ, initiator), [(3,4-dimethoxyphenyl)sulfanyl]acetic acid (MKTFO,acid
 (MKTFO,
 (MKTFO,
 (MKTFO,and co-initiator),
 co-initiator),
 co-initiator), and
 and trimethylolpropane
 trimethylolpropane
 and trimethylolpropane
 trimethylolpropane triacrylate
 triacrylate
 triacrylate (TMPTA,
 (TMPTA,
 (TMPTA, monomer)
 monomer)
 monomer) were
 were ob-
 were ob-
 ob-
 (MKTFO,
 co-initiator), co-initiator), and
 trimethylolpropane triacrylate (TMPTA, triacrylate (TMPTA,
 monomer) weremonomer) were
 obtained from ob-
 tained
 tained
 tained from
 from
 from Sigma-Aldrich
 Sigma-Aldrich
 Sigma-Aldrich Co,
 Co,
 Co, Saint
 Saint
 Saint Louis,
 Louis,
 Louis, MO,
 MO,
 MO, USA.
 USA.
 USA.
 tained fromCo,
 Sigma-Aldrich Sigma-Aldrich
 Saint Louis, MO,Co, Saint
 USA.Louis, MO, USA.
 2.2.
 2.2. 2.2. Synthesis
 2.2. Synthesis
 Synthesis
 Synthesis
 2.2. Synthesis
 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1
 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1
 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1
 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1
 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1
 1,2-diaminocyclohexane
 1,2-diaminocyclohexane
 1,2-diaminocyclohexane
 1,2-diaminocyclohexane (0.548(0.548 (0.548
 g, 4.8 g,
 (0.548 g,
 g, 4.8
 4.8
 mmol) mmol)
 4.8 mmol)
 mmol) was
 was refluxed
 was refluxed
 was refluxed refluxed with
 with 9,10-phenrenequi-
 with 9,10-phenrenequi-
 9,10-phenrenequi-
 with 9,10-phenrenequinone
 1,2-diaminocyclohexane (0.548 g, 4.8 mmol) was refluxed with 9,10-phenrenequi-
 none
 none
 none
 (1 g, none (1
 (1
 (1
 4.8 mmol) g,
 g,
 g, 4.8
 4.8
 4.8 mmol)
 mmol)
 mmol)
 in glacial in
 in
 in glacial
 glacial
 glacial acetic
 acetic
 acetic acid
 acid
 acid (40
 (40
 (40 mL)
 mL)
 mL) for
 for
 for 22
 2 h.
 h.
 h. The
 The
 The compound
 compound
 compound was
 was
 was obtained
 asobtained
 yellow as
 obtained as
 as
 (1 g, 4.8 mmol)aceticin glacial acid acetic (40 mL) for(40
 acid 2 h.mL)
 Theforcompound was obtained
 2 h. The compound was obtained as
 yellow
 yellow
 yellow crystals:
 crystals:
 crystals: CC 20
 C20 H
 H
 H1818 N
 N
 N 2 ,, 286.38
 286.38 g/mol,
 g/mol, 1.12
 1.12 g,
 g, yield
 yield 82%,
 82%, m.p.
 m.p. ◦174–176
 174–176 °C,
 °C,◦ lit.
 lit. 173
 173 °C
 °C [51].
 [51].
 crystals: C20crystals:
 H18 N2 , 286.38 22,, 286.38
 g/mol, 286.38 1.12 g/mol, 1.1282%,
 g, yield g, yield
 yield 82%,
 m.p.82%, m.p. 174–176
 174–176 174–176 °C,Clit.
 C, lit. 173°C, lit. 173 °C
 [51]. °C [51].
 [51].
 20 18 2
 yellow C 20H 18N g/mol, 1.12 g, m.p. 173
 1H
 111H
 1H
 NMR
 HHNMR
 NMR
 NMR (400
 (400
 (400
 (400 MHz,
 MHz,
 MHz,
 MHz, CDCl
 CDCl
 CDCl
 CDCl 333,,, δ):δ):
 δ):
 , δ):9.24–9.21
 9.24–9.21
 9.24–9.21
 9.24–9.21 (d,
 (d,
 (d,
 (d,1H),
 1H),
 1H),
 1H), 8.62–8.59
 8.62–8.59
 8.62–8.59
 8.62–8.59 (d,
 (d,
 (d, 2H),
 2H),
 2H),
 (d, 2H), 7.78–7.70
 7.78–7.70
 7.78–7.70
 7.78–7.70 (m,
 (m,
 (m,
 (m,4H),
 4H),
 4H),
 4H), 3.28–
 3.28–
 3.28–
 NMR (400 MHz, CDCl3, δ): 9.24–9.21 (d, 1H), 8.62–8.59 (d, 2H), 7.78–7.70 (m, 4H), 3.28–
 3
 3.25
 3.25
 3.28–3.25 (m,
 3.25 (m,
 (m, 5H),
 (m,5H), 2.02–2.08
 5H),2.02–2.08
 5H), 2.02–2.08(m,
 2.02–2.08 (m,
 (m,5H)
 (m, 5H)
 5H)
 5H)
 3.25 (m, 5H), 2.02–2.08 (m, 5H)
 13
 13 13
 13CC
 CCNMR
 NMR
 NMR
 NMR (100
 (100
 (100
 (100 MHz,
 MHz,
 MHz,
 MHz, CDCl
 CDCl
 CDCl
 CDCl 333,,,3δ):
 δ):
 δ): 152.1,
 , δ):152.1,
 152.1,
 152.1, 138.2,
 138.2,
 138.2,
 138.2, 135.8,
 135.8,
 135.8,
 135.8,131.0,
 131.0,
 131.0,
 131.0, 129.2,
 129.2,
 129.2,
 129.2,129.1,
 129.1,
 129.1,
 129.1,127.5,
 127.5,
 127.5, 125.23,
 125.23,
 127.5,125.23, 122.6,
 122.6,
 125.23,122.6,
 13 C NMR (100 MHz, CDCl3, δ): 152.1, 138.2, 135.8, 131.0, 129.2, 129.1, 127.5, 125.23, 122.6,
 122.6,32.4,
 32.4,32.4,
 32.4, 22.75
 22.75
 22.75
 22.75
 32.4, 22.75
 Dibenzo[a,c]phenazine-FN2
 Dibenzo[a,c]phenazine-FN2
 Dibenzo[a,c]phenazine-FN2
 Dibenzo[a,c]phenazine-FN2
 Dibenzo[a,c]phenazine-FN2
 1,2-diaminobenzene
 1,2-diaminobenzene
 1,2-diaminobenzene
 1,2-diaminobenzene (0.519 (0.519
 (0.519
 (0.519 g, 4.8g, g, 4.8
 4.8
 g,mmol) mmol)
 4.8 mmol) was
 was refluxed
 was refluxed
 was refluxed
 mmol) with
 with 9,10-phenrenequinone
 with 9,10-phenrenequinone
 with 9,10-phenrenequinone
 refluxed 9,10-phenrenequinone (1 g, (1 (1
 (1
 1,2-diaminobenzene (0.519 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone (1
 g,
 g,
 4.8 mmol)
 g, 4.8
 4.8
 4.8 mmol)
 mmol)
 in glacial
 mmol) in
 in
 in glacial
 glacial
 acetic
 glacial acetic
 acetic
 acid
 acetic (80 acid
 acid
 mL)
 acid (80
 (80
 for
 (80 2mL)
 mL)
 h.
 mL) Thefor
 for
 for 22 h.
 h.
 compound
 2 h. The
 The
 The compound
 compound
 was
 compoundobtainedwas
 was
 was asobtained
 obtained
 pale,
 obtained as
 as
 yellow
 as pale,
 pale,
 pale,
 g, 4.8 mmol) in glacial acetic acid (80 mL) for 2 h. The compound was obtained as pale,
 yellow
 yellow crystals:
 C20crystals: C
 C H N 222,,, 280.33
 280.33 g/mol, 1.34 g, yield 80%, m.p. ◦ C,
 224–225 °C, lit.◦224–226 °C
 crystals:
 yellow
 yellow crystals: C 20H
 H12 N2 , 280.33
 crystals: C2020
 20 H 12N
 H1212
 12
 g/mol,
 N
 N 280.33
 2, 280.33
 1.34g/mol,
 g/mol,
 g/mol, 1.34
 g, yield
 1.34
 1.34 g,
 g, yield
 80%,
 g, m.p.80%,
 yield
 yield 80%,
 80%, m.p.
 m.p. 224–225
 224–225
 m.p. 224–225
 224–225 °C,
 °C, lit.
 lit. 224–226
 °C, lit. 224–226
 lit. C [56]. °C
 224–226
 224–226 °C
 °C
 1
 [56].
 H NMR (400 MHz, CDCl , 9.34 (d, 2H), 9,32 (d, 2H), 8.50–8.48 (d, 2H), 8.27–8.24
 [56].
 [56].
 [56]. 3 δ):
 111H
 (m, 2H),
 1H
 H NMR (400
 7.870–7.65
 H NMR
 NMR (400 MHz,
 (m, 6H)
 (400 MHz,
 MHz, CDCl
 CDCl3333,,,, δ):
 CDCl δ): 9.34
 δ): 9.34
 9.34 (d,(d, 2H),
 (d, 2H), 9,32
 2H), 9,32 (d,
 9,32 (d, 2H),
 (d, 2H), 8.50–8.48
 2H), 8.50–8.48 (d,
 8.50–8.48 (d, 2H),
 (d, 2H), 8.27–8.24
 2H), 8.27–8.24
 8.27–8.24 (m,(m,
 (m,
 NMR (400 MHz, CDCl δ): 9.34 (d, 2H), 9,32 (d, 2H), 8.50–8.48 (d, 2H), 8.27–8.24 (m,
 2H),
 2H),
 2H), 7.870–7.65
 7.870–7.65
 7.870–7.65 (m,
 (m,
 (m, 6H)
 6H)
 6H)
 2H), 7.870–7.65 (m, 6H)
 13
 1313CC NMR
 C NMR
 NMR (100 (100 MHz,
 (100 MHz,
 MHz, CDCl CDCl
 CDCl3333,,,, δ): δ): 141.8,
 δ): 141.8, 141.3,
 141.8, 141.3, 132.0,
 141.3, 132.0, 130.7,
 132.0, 130.7, 130.2,
 130.7, 130.2, 129.3,
 130.2, 129.3, 128.9,
 129.3, 128.9, 128.0,
 128.9, 128.0, 126.5,
 128.0, 126.5,
 126.5,
 13 C NMR (100 MHz, CDCl δ): 141.8, 141.3, 132.0, 130.7, 130.2, 129.3, 128.9, 128.0, 126.5,
 122,9
 122,9
 122,9
 122,9
 Tribenzo[a,c,i]phenazine-FN3
 Tribenzo[a,c,i]phenazine-FN3
 Tribenzo[a,c,i]phenazine-FN3
 Tribenzo[a,c,i]phenazine-FN3
 2,3-diaminonaphthalene
 2,3-diaminonaphthalene
 2,3-diaminonaphthalene (0.76 (0.76
 (0.76 g, g, 4.8
 g, 4.8 mmol)
 4.8 mmol)
 mmol) was was refluxed
 was refluxed with
 refluxed with 9,10-phenrenequinone
 with 9,10-phenrenequinone
 9,10-phenrenequinone
 2,3-diaminonaphthalene (0.76 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone

Materials 2021, 14, 3085 6 of 19

 13 C NMR (100 MHz, CDCl3 , δ): 141.8, 141.3, 132.0, 130.7, 130.2, 129.3, 128.9, 128.0,
 126.5, 122,9
 Tribenzo[a,c,i]phenazine-FN3
 2,3-diaminonaphthalene (0.76 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone (1
 g, 4.8 mmol) in glacial acetic acid (100 mL) for 2 h. The compound was obtained as yellow
 crystals: C24 H14 N2 , 330.39 g/mol, 1.36 g, yield 86%, m.p. 292–293 ◦ C, lit. 291–293 ◦ C [57].
 1 H NMR (400 MHz, CDCl , δ): 9.38–9.36 (d, 2H), 8.88 (s, 2H), 8,47–8.45 (d, 2H),
 3
 8.14–8.11 (m, 4H), 7.76–7.51 (m, 4H)
 Tribenzo[a,c,h]phenazine-FN4
 1,2-naphthoquinone (0.76 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone (1 g,
 4.8 mmol) in glacial acetic acid (150 mL) for 1 h. The compound was obtained as yellow
 crystals: C24 H14 N2 , 330.39 g/mol, 1.0 g, yield 63%, m.p. 257–258 ◦ C, lit. 278–279 ◦ C [58].
 1 H NMR (400 MHz, CDCl , δ): 9.78 (d, 1H), 9.62 (d, 2H), 8.93 (d, 1H), 8.69 (m, 2H),
 3
 8.47-8.44 (m, 2H), 8.23 (d, 2H), 7.93-7.86 (m, 4H)
 Tetrabenzo[a,c,h,i]phenazine-FN5
 9,10-diaminophenanthrene (1.0 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone
 (1 g, 4.8 mmol) in glacial acetic acid (150 mL) for 2 h. The compound was obtained as
 yellow crystals: C28 H16 N2 , 380.45 g/mol, 1.1 g, yield 60%, m.p., lit. 462 ◦ C [59].
 1 HNMR (400 MHz, DMSO-d , δ): 8.33–8.31 (d, 4H), 8.05 (d, 2H), 8.03 (d, 2H), 7.82–7.77
 6
 (d, 5H), 7.56–7.53 (m, 3H)
 Benzo[a]phenazine-FN6
 1,2-diaminobenzene (0.68 g, 6.3 mmol) was refluxed with 1,2-naphthoquinone (1 g,
 6.3 mmol) in glacial acetic acid (100 mL) for 2 h. The compound was obtained as yellow
 crystals: C16 H10 N2 , 230.27 g/mol, 0.94 g, yield 65%, m.p. 230–232◦ C, lit. 230–233 ◦ C [52].
 1 HNMR (400 MHz, CDCl , δ): 9.65 (1H, d), 8.50 (1H, d), 8.37 (1H, d), 8.23 (1H, d),
 3
 7.99–7.60 (m, 6H)
 13 C NMR (100 MHz, CDCl , δ): 142.9, 142.5, 132.9, 131.1, 131.0, 130.6, 130.3, 129.9,
 3
 128.6, 128.2, 127.4, 125.80
 Dibenzo[a,i]phenazine-FN7
 2,3-diaminonaphthalene (1.0 g, 6.3 mmol) was refluxed with 1,2-naphthoquinone (1 g,
 6.3 mmol) in glacial acetic acid (100 mL) for 2 h. The compound was obtained as yellow
 crystals: C20 H12 N2 , 280.33 g/mol, 0.94 g, yield 87%, m.p. 247–248 ◦ C, lit. 247 ◦ C [60].
 1 HNMR (400 MHz, CDCl , δ): 9.32 (d, 1H), 9.03 (s, 1H), 8.94 (s, 1H), 8.15 (m, 3H), 8.03
 3
 (d, 1H), 7.86–7.77 (m, 3H), 7.61–7.59 (m, 2H)
 13 C NMR (100 MHz, CDCl , δ): 143.9, 138.7, 136.8, 134.7, 134.2, 132.8, 130.8, 130.5,
 3
 129.0, 128.8, 128.5, 128.4, 128.1, 127.7, 127.2, 125.6, 124.6, 124.4
 Dibenzo[f, h]pyrido[2, 3-b]quinoxaline-FN8
 1,2-diaminopyridine (0.53 g, 4.8 mmol) was refluxed with 9,10-phenanthrenequinone
 (1 g, 4.8 mmol) in glacial acetic acid (80 mL) for 2 h. The compound was obtained as yellow
 crystals: C19 H11 N3 , 281.32 g/mol, 1.1 g, yield 81%, m.p. 218–219 ◦ C, lit. 217–219 ◦ C [48,49].
 1 HNMR (400 MHz, CDCl , δ): 9.52-9.49 (d, 1H), 9.31-9.27 (m, 2H), 8.66-8.64 (d, 1H),
 3
 8.52-8.50 (d, 2H), 7.84-7.68 (m, 5H)
 13 C NMR (100 MHz, CDCl , δ): 154.4, 149.8, 145.0, 143.6, 138.3, 137.3, 132.5, 132.3,
 3
 131.3, 130.8, 129.7, 129.5, 128.1, 128.0, 127.3, 126.4, 124.8, 123.0, 122.8
 11-Bromodibenzo[a, c]phenazine-FN9
 4-bromo-1,2-diaminobenzene (0.898 g, 4.8 mmol) was refluxed with 9,10-phenanthren-
 equinone (1 g, 4.8 mmol) in glacial acetic acid (80 mL) for 30 min. The compound was
 obtained as yellow crystals: C20 H11 BrN2 , 359.23 g/mol, 1.24 g, yield 72%, m.p. 274–275 ◦ C,
 lit. 274–276 ◦ C [46].
 1 HNMR (400 MHz, CDCl , δ): 9.38–9.35 (m, 2H), 8,59–8.57 (d, 2H), 8.53 (d, 1H),
 3
 8.21–8.19 (d, 1H), 7.94–7.91 (m, 1H), 7.85–7.74 (m, 4H)
 13 C NMR (100 MHz, CDCl , δ): 133.7, 132.3, 132.26, 131.2, 131.0, 130.9, 130.2, 128.2,
 3
 128.1, 126.6, 126.6, 123.0, 123.0
 12-Bromo-dibenzo[f, h]pyrido[2, 3-b]quinoxaline-FN10

Materials 2021, 14, 3085 7 of 19

 5-bromo-1,2-diaminobenzene (0.898 g, 4.8 mmol) was refluxed with 9,10-phenanthrene-
 quinone (1 g, 4.8 mmol) in glacial acetic acid (80 mL) for 2 h. The compound was obtained
 as yellow crystals: C19 H10 BrN3 , 360.21 g/mol, 1.42 g, yield 77%, m.p. 235–238 ◦ C, lit.
 235–238 ◦ C [49,57].
 1 HNMR (400 MHz, CDCl , δ): 9.43–9.41 (d, 1H), 9.25 (s, 1H), 9.20–9.18 (d, 1H), 8.77
 3
 (s, 1H), 8.52–8.50 (d, 2H), 7.85–7.68 (m, 4H)
 13 C NMR (100 MHz, CDCl , δ): 155.3, 148.0, 145.1, 144.1, 139.5, 137.4, 132.5, 132.5,
 3
 131.4, 131.2, 129.4, 129.1, 128.3, 128.1, 127.4, 126.6, 123.0, 122.9, 120.5
 Dibenzo[f, h]pyrazino[2,3-b]quinoxaline-FN11
 5-bromo-2,3-diaminopyrazine (0.9 g, 4.8 mmol) was refluxed with 9,10-phenanthrene-
 quinone (1 g, 4.8 mmol) in glacial acetic acid (80 mL) for 2 h. The compound was obtained
 as yellow crystals: C18 H9 BrN4 , 361.21 g/mol, 1.19 g, yield 69%, m.p. 279–281 ◦ C.
 1 HNMR (400 MHz, CDCl , δ): 9.56 (d, 1H), 8.22–8.19 (d, 2H), 8.04–8.02 (d, 2H),
 3
 7.76–7.72 (m, 2H), 7.51–7.47 (m, 2H)
 13 C NMR (100 MHz, CDCl , δ): 180.3, 135.9, 135.8, 131.0, 130.5, 129.9, 129.5, 129.5,
 3
 123.9, 123.6
 Dipirydo[3,2-a:20 ,30 -c]phenazine-FN12
 2,3-diaminobenzene (1.0 g, 9.2 mmol) was refluxed with 1,10-phenanthroline-5,6-dione
 (2 g, 9.2 mmol) in glacial acetic acid (150 mL) for 2 h. The compound was obtained as yellow
 crystals: C18 H10 N4 , 282.30 g/mol, 1.74 g, yield 65%, m.p. 248–250 ◦ C, lit. 254–255 ◦ C [61],
 248–250 ◦ C [57].
 1 HNMR (400 MHz, CDCl , δ): 9.71 (d, 2H), 9.33–9.32 (d, 2H), 8.40–8.38 (m, 2H),
 3
 7.97–7.95 (m, 2H), 7.86–7.83 (m, 2H)
 13 C NMR (100 MHz, CDCl , δ): 175.7, 152.4, 148.2, 142.5, 141.1, 133.9, 130.7, 129.5,
 3
 127.6, 124.2
 Phenanthro[9, 10-g]pteridine-FN13
 4,5-diaminopyrimidine (1.0 g, 9.1 mmol) was refluxed with 9,10-phenanthrenequinone
 (1.9 g, 9.1 mmol) in glacial acetic acid (150 mL) for 2 h. The compound was obtained as
 yellow crystals: C18 H10 N4 , 282.30 g/mol, 0.96 g, yield 71%, m.p. 320–321 ◦ C.
 1 HNMR (400 MHz, DMSO-d , δ): 9.55–9.52 (d, 2H), 9.22–9.21 (d, 2H), 8.41–8.37 (d,
 6
 2H), 8.08–8.04 (d, 2H), 7.96–7.93 (d, 2H)
 13 C NMR (100 MHz, DMSO-d , δ): 172.4, 152.8, 148.3, 142.2, 141.3, 133.5, 131.7, 129.6,
 6
 127.4, 125.0

 2.3. Methods
 The 1 H NMR (400 MHz) spectra were recorded on Bruker AscendTM 400 NMR spectrom-
 eters, Billerica, MA, United States. Deuterated chloroform (CDCl3 ) and dimethylsulfoxide
 (DMSO-d6) was used as the solvent. Chemical shifts are reported in ppm relative to internal
 tetramethylsilane (TMS) reference, and coupling constants are reported in Hertz. Splitting
 patterns are described as singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m).
 The melting points (uncorrected) were determined with the use of a Boëthius appara-
 tus, Vernon Hills, IL, United States.
 For thin layer chromatography, Merck Co, Kenilworth, NJ, United States., silica gel
 60 F254 sheets (0.2 mm thickness) and chloroform as an eluent were used.
 The electronic absorption (c ≈ 10−4 –10−3 M) and fluorescence spectra in ethyl acetate
 (EtOAc) were recorded at room temperature using Shimadzu UV-1280, Kioto, Japan and
 Hitachi F-7100 spectrophotometers, Tokio, Japan, respectively. The fluorescence quantum
 yield (FQY) was determined by a comparative method [62] using Coumarin 1 in ethanol
 (ϕref = 0.64 [63]) as the reference. The absorbances of both the dye and reference solution at
 an excitation wavelength (366 nm or 404 nm) was ca. 0.1.
 The nanosecond laser flash photolysis experiments were performed using LKS.60
 Laser Flash Photolysis apparatus (Applied Photophisics, Leatherhead, United Kingdom).
 Laser irradiation at 355 nm from the third harmonic of the Q-switched Nd:YAG laser from
 a Lambda Phisik/model LPY 150 operating at 65 mJ/pulse (pulse width about 4–5 ns) was

Materials 2021, 14, 3085 8 of 19

 used
 Materials 2021, 14, x FOR PEER REVIEW for the excitation. Transient absorbances at preselected wavelengths were monitored 8 of 20
 by a detection system consisting of a monochromator, a photomultiplier tube (Hamamatsu
 R955), and a pulsed xenon lamp (150 W) as a monitoring source. The signal from the
 photomultiplier was processed by a Hewlett-Packard/Agilent an Agilent Infiniium 54810A
 photomultiplier
 digital was processed
 storage oscilloscope and an by a Hewlett-Packard/Agilent
 Acorn-compatible computer. an Agilent Infiniium
 54810A digital storage oscilloscope and an Acorn-compatible computer.
 The quantum yield of the triplet state formation of the synthesized dyes was deter-
 minedThe by quantum yieldThe
 two methods. of the
 firsttriplet statedescribed
 one was formationbyofLament
 the synthesized
 et al. [64] dyes wasbased
 and was deter-
 mined by two methods. The first one was described by Lament et al.
 on the analysis of transient absorption spectra, whereas the second one was described [64] and was based
 on the analysis of transient absorption spectra, whereas the second
 by Silverman and Foote [32]. The latter method was based on the analysis of H NMR one was described
 1 by
 Silverman and Foote [32]. The latter method was based on the analysis of
 spectra recorded during the singlet oxygen oxidation of 2,3-diphenyl-p-dioxene (Scheme 1).
 1H NMR spectra

 recorded
 The during thereaction
 photooxidation singlet was
 oxygen oxidation
 carried out in aofsolution
 2,3-diphenyl-p-dioxene
 containing 0.6 mL (Scheme 1). The
 of deuterated
 photooxidation
 chloroform reaction
 in which bothwas carried
 10 mg out in a solution containing
 of 2,3-diphenyl-p-dioxene and 5 0.6
 mg mL of deuterated
 of the dye were
 chloroform in which both 10 mg of 2,3-diphenyl-p-dioxene and 5
 dissolved. The solution was irradiated with LED F (dental-lamp-emitted) radiationmg of the dye were dis-
 in
 solved. The solution was irradiated with LED F (dental-lamp-emitted)
 2
 the range: 390–480 nm. The radiation intensity was 18 mW/cm . During irradiation, radiation inthe
 the
 range: 390–480
 solution nm.with
 was stirred The radiation
 a stream of intensity
 oxygen.was The 18 mW/cmof
 formation 2. During irradiation, the solu-
 the photooxidation product
 tion was stirred with a stream of
 1
 was monitored by recording the H NMR spectra.oxygen. The formation of the photooxidation product
 was monitored by recording the 1H NMR spectra.

 O O
 O O C O O C
 1 O temp.
 + O2 CH2 CH2
 O
 O O

 Scheme 1. Oxidation of 2,3-diphenyl-p-dioxene with singlet oxygen.
 Scheme 1. Oxidation of 2,3-diphenyl-p-dioxene with singlet oxygen.
 The
 Thequantum
 quantumyield
 yieldof
 ofsinglet
 singletoxygen
 oxygenproduction
 productionwas wascalculated
 calculatedbased
 basedon onthe
 therate
 rateofof
 the
 the ethylene glycol dibenzoate formation by the tested sensitizers compared to the rateofof
 ethylene glycol dibenzoate formation by the tested sensitizers compared to the rate
 this
 thisproduct’s
 product’sformation
 formationininthe
 thepresence
 presence of of
 camphorquinone,
 camphorquinone, which
 whichwaswas
 used as aas
 used standard.
 a stand-
 The quantum yield of singlet oxygen generation by camphorquinone was 1.0
 ard. The quantum yield of singlet oxygen generation by camphorquinone was 1.0 [65,66].[65,66]. With
 aWith
 largeaexcess of oxygen, the quantum yield of singlet oxygen formation was
 large excess of oxygen, the quantum yield of singlet oxygen formation was equal equal to the
 quantum yield ofyield
 to the quantum the triplet
 of the state formation
 triplet [24]. [24].
 state formation
 Φ Φ1 O= =
 Φ ΦT (5)
 (5)
 2

 Theeffectiveness
 The effectivenessof ofthe
 thesynthesized
 synthesizeddyes dyesas asradical
 radicalpolymerization
 polymerizationphotosensitizers
 photosensitizers
 wastested
 was testedby bythe
 themicrocalorimetric
 microcalorimetricmethod method[67,68].
 [67,68].TheThemeasurements
 measurementswere wereperformed
 performed
 inaahomemade
 in homemademicrocalorimeter
 microcalorimeterusing usingaasemiconducting
 semiconductingdiode diodeimmersed
 immersedin inaa2–3
 2–3mm mm
 thick layer (0.25 mL) of a cured sample as a temperature sensor.
 thick layer (0.25 mL) of a cured sample as a temperature sensor. The initial rate of poly- The initial rate of
 polymerization
 merization was determined
 was determined by theby the measurements
 measurements of the
 of the rate rateheat
 of the of the heat evolution
 evolution during
 during polymerization
 polymerization for thetime.
 for the initial initialAtime. A diode-pumped
 diode-pumped solid-state
 solid-state laser laser
 (408 (408
 nm) nm)
 withwithan
 an intensity
 intensity of 18ofmW/cm
 18 mW/cm 2 (Mean
 2 (MeanWellWell
 model model NES-15-12)
 NES-15-12) was was
 usedused as theaslight
 the light source.
 source. The
 light intensity
 The light was measured
 intensity was measured by a Coherent model
 by a Coherent Fieldmaster
 model power
 Fieldmaster meter.meter.
 power The distance
 The dis-
 of the sample
 tance from the
 of the sample light
 from thesource
 light was thewas
 source same forsame
 the all measurements.
 for all measurements.Each measurement
 Each meas-
 was repeated
 urement wasat least three
 repeated times.
 at least three times.
 The
 Thephotopolymerization
 photopolymerizationcompositions compositions were
 were composed
 composed of 0.9of g0.9
 of gtrimethylolpropane
 of trimethylolpro-
 triacrylate (TMPTA)
 pane triacrylate and 0.1and
 (TMPTA) mL of0.11-methyl-2-pyrrolidinone
 mL of 1-methyl-2-pyrrolidinone (MP) containing appropriate
 (MP) containing ap-
 amounts
 propriateofamounts
 the photosensitizer (synthesized
 of the photosensitizer phenazinephenazine
 (synthesized based dyes) anddyes)
 based the co-initiator
 and the co-
 [(3,4-dimethoxyphenyl)sulfanyl]acetic
 initiator [(3,4-dimethoxyphenyl)sulfanyl]acetic acid (MKTFO). The concentration
 acid (MKTFO). of the dyesofwas
 The concentration the
 2dyes −3 –5.63 ×
 × 10was 10 −4 M −4 (depending on the molar absorption coefficient at an excitation
 2×10 –5.63×10 M (depending on the molar absorption coefficient at an excita-
 −3

 wavelength),
 tion wavelength), whereas the concentration
 whereas of the co-initiator
 the concentration was 0.1 was
 of the co-initiator M. As 0.1a M.
 reference sample,
 As a reference
 asample,
 polymerizing mixture without a co-initiator was
 a polymerizing mixture without a co-initiator was used. used.
 In
 Inorder
 ordertotocompare
 compare thethequantum
 quantum efficiency of triplet
 efficiency statestate
 of triplet formation
 formationand the
 andphotoini-
 the pho-
 tiating efficiency
 toinitiating of theof
 efficiency TMPTA,
 the TMPTA, radicalradical
 polymerization by the synthesized
 polymerization compounds
 by the synthesized com-
 pounds with a commercial compound, the camphorquinone (λmax = 472 nm, ε = 40 M cm‒ −1

 1 in ethyl acetate), was used. Its concentration in the polymerization experiments was 0.675

 M.

3. Results
 The analysis of the electronic absorption spectra of the synthesized compounds indi-
Materials 2021, 14, 3085 9 of 19
 cated that the spectra were typical for heterocyclic aromatic compounds with condensed
 rings and that they were characterized by several absorption maxima in the long-wave-
 length region (Figure 1). They displayed main absorption bands in the range from 350 nm
 towith
 530 nm. Thus, the compound,
 a commercial compounds the afforded deeply colored
 camphorquinone solutions
 (λmax withεmolar
 = 472 nm, = 40 M −1 cm-1 in
 extinction
 coefficients ranging
 ethyl acetate), from 0.96
 was used. × 10 M cm in
 4 −1
 Its concentration −1 to 2.56 × 10 M cm forexperiments
 4 −1
 the polymerization −1 the most intense band,
 was 0.675 M.
 which is typical for π→π* transitions. The basic spectral data are collected in Table 1.
 3. Results
 The position of the absorption bands was clearly influenced by the dye structure, i.e.,
 the number of conjugated
 The analysis of thedouble bonds,
 electronic the arrangement
 absorption spectra of of aromatic rings, and
 the synthesized the num-
 compounds
 ber and location
 indicated of spectra
 that the nitrogenwereatoms. The for
 typical increase in the aromatic
 heterocyclic number of conjugatedwith
 compounds double
 con-
 densed
 bonds in rings and that(FN1,
 the molecule they were
 FN2, characterized
 and FN3; FN6by several
 and FN7)absorption maxima
 and the angular in the long-
 arrangement
 ofwavelength
 the rings (FN4region (Figure
 versus FN3)1). Theyadisplayed
 caused bathochromic maineffect.
 absorption
 The redbands in the
 shift of theabsorption
 range from
 350 nm towas
 maximum 530 also
 nm. observed
 Thus, the with
 compounds
 an increaseafforded
 in thedeeply
 number colored solutions
 of nitrogen withinmolar
 atoms the
 extinction
 molecule coefficients
 (FN9, ranging
 FN10, FN11) the ×
 fromas0.96
 as well 104 M−of
 presence 1 cm−1 to 2.56 × 104 M−1 cm−1 for the
 a heavy atom (FN2 and FN9) (Fig-
 most
 ure ESI). band, which is typical for π→π* transitions. The basic spectral data are
 intense
 S1 in
 collected in Table 1.

 1.00 FN10
 FN2
 FN4
 0.75 FN7
 Absorbance

 0.50

 0.25

 0.00
 350 400 450 500
 Wavelength, nm
 Fragmentsofofthe
 Figure1.1.Fragments
 Figure theelectronic
 electronicabsorption
 absorptionspectra
 spectraofofthe
 theselected
 selecteddyes
 dyesininethyl
 ethylacetate.
 acetate.

 The position of the absorption bands was clearly influenced by the dye structure,
 In general, the compounds had high molar absorption coefficients and appropriate
 i.e., the number of conjugated double bonds, the arrangement of aromatic rings, and the
 spectral characteristics enabling selective excitation of the sensitizer. It follows that the
 number and location of nitrogen atoms. The increase in the number of conjugated double
 dyes met the requirements for singlet oxygen sensitizers.
 bonds in the molecule (FN1, FN2, and FN3; FN6 and FN7) and the angular arrangement of
 Figure 2 shows the fluorescence spectra for selected dyes.
 the rings (FN4 versus FN3) caused a bathochromic effect. The red shift of the absorption
 maximum was also observed with an increase in the number of nitrogen atoms in the
 molecule (FN9, FN10, FN11) as well as the presence of a heavy atom (FN2 and FN9)
 (Figure S1 in ESI).
 In general, the compounds had high molar absorption coefficients and appropriate
 spectral characteristics enabling selective excitation of the sensitizer. It follows that the
 dyes met the requirements for singlet oxygen sensitizers.
 Figure 2 shows the fluorescence spectra for selected dyes.
 As for absorption spectra, the position of the fluorescence maximum depended on the
 dye structure. The number of both conjugated double bonds and nitrogen atoms in the
 molecule, the angular arrangement of the rings, and the presence of a heavy atom affected
 the fluorescence band position and its intensity (Figure S2 in ESI). The data in Table 1 show
 that the fluorescence quantum yield varied from 0.025 (FN6 and FN7) for compounds with
 a high singlet-oxygen-generating ability to 0.11 (FN1) for the compound showing a very
 low singlet oxygen quantum yield (Table 2).

Materials 2021, 14, x FOR PEER REVIEW 10 of 20
Materials 2021, 14, 3085 10 of 19

 1.00 FN10
 FN2
 FN4
 0.75 FN7

 Fluorescence 0.50

 0.25

 0.00
 400 500 600 700
 Wavelength, nm

 Fluorescencespectra
 Figure2.2.Fluorescence
 Figure spectraof
 ofthe
 theselected
 selecteddyes
 dyesin
 inethyl
 ethylacetate.
 acetate.

 Table 2. Quantum yield of the singlet oxygen formation (Φ1 O2 ), quantum yield of the triplet state
 As for absorption spectra, the position of the fluorescence maximum depended on
 (ΦT ), and initial rate of photoinitiated polymerization (Rp ) for the dyes tested.
 the dye structure. The number of both conjugated double bonds and nitrogen atoms in
 the molecule,
 Dye the angular arrangement
 Rp , µmol s−1 of the rings, Φ
 and
 1 O the presence of a Φ
 heavy
 T
 atom
 2
 affected the fluorescence band position and its intensity (Figure S2 in ESI). The data in
 FN1 44.1 0.06 -
 Table 1 show
 FN2 that the fluorescence
 112.0quantum yield varied
 0.32 from 0.025 (FN6 and0.30 FN7) for
 compounds FN3with a high singlet-oxygen-generating
 107.7 ability
 0.36to 0.11 (FN1) for the compound
 0.37
 showing aFN4
 very low singlet oxygen180.2quantum yield (Table
 0.772). 0.75
 FN5 101.7 0.32 -
 FN6
 Table 2. Quantum 196.3
 yield of the singlet oxygen formation (Φ 0.92 ), quantum yield of the0.90
 triplet state
 FN7 rate of photoinitiated
 (ΦT), and initial 183.9
 polymerization (Rp) for0.80
 the dyes tested. 0.79
 FN8 107.6 0.32 0.30
 Dye FN9 , 147.9 
 0.67 -
 FN10 136.7 0.51 -
 FN1FN11 44.1 115.9 0.06 - 0.31 -
 FN2FN12 112.0 103.3 0.32 0.30 0.30 -
 FN3FN13 107.7 54.4 0.36 0.37 0.09 -
 FN4 CQ 180.2 234.2 0.77 0.75 1.00 -
 FN5 101.7 0.32 -
 The
 FN6quantum yield 196.3of the triplet0.92
 state formation
 0.90was determined by the comparative
 methodFN7using camphorquinone
 183.9 as a standard.
 0.80 The0.79
 method was based on the oxidation of
 2,3-diphenyl-p-dioxene
 FN8 by
 107.6 singlet oxygen
 0.32 produced in a photochemical process involving
 0.30
 the excitation
 FN9 of triplet oxygen with UV–vis
 147.9 0.67 light in -the presence of a sensitizing dye. The
 formation of the photooxidation product was monitored 1
 FN10 136.7 0.51 - by the analysis of the H NMR spectra
 recorded for the initial solution and after different times of irradiation (Figures 3 and 4).
 FN11 115.9 0.31 -
 FN12 103.3 0.30 -
 FN13 54.4 0.09 -
 CQ 234.2 1.00 -

 The quantum yield of the triplet state formation was determined by the comparative
 method using camphorquinone as a standard. The method was based on the oxidation of
 2,3-diphenyl-p-dioxene by singlet oxygen produced in a photochemical process involving
 the excitation of triplet oxygen with UV–vis light in the presence of a sensitizing dye. The
 formation of the photooxidation product was monitored by the analysis of the 1H NMR
 spectra recorded for the initial solution and after different times of irradiation (Figures 3
 and 4).

FOR PEER REVIEW 11 of 20

 Materials 2021, 14, 3085 11 of 19
 Materials 2021, 14, x FOR PEER REVIEW 11 of 20

 Figure 3. 1H NMR spectrum of a mixture of dibenzo[a.c]phenazine sensitizer and 2,3-diphenyl-p-
 dioxene in CDCl3 before irradiation.
 Figure 3. 1 H NMR spectrum of a mixture of dibenzo[a.c]phenazine sensitizer and 2,3-diphenyl-p-
 Figure 3. 1H NMR spectrum of a mixture of dibenzo[a.c]phenazine sensitizer and 2,3-diphenyl-p-
 dioxene in CDCl3 before irradiation.
 dioxene in CDCl3 before irradiation.

 Figure 4. 1H NMR spectra of a mixture of dibenzo[a.c]phenazine (sensitizer) and 2,3-diphenyl-p-
 Figure 4. 1H dioxene in CDCl
 NMR spectra of a mixture 3; irradiation time: A-10
 of dibenzo[a.c]phenazine min., B-20and
 (sensitizer) min., and C-30 min.
 2,3-diphenyl-p-dioxene in CDCl3 ;
 Figure 4. H NMR spectra of a mixture of
 1 dibenzo[a.c]phenazine (sensitizer) and 2,3-diphenyl-p-
 irradiation time: A-10 min, B-20 min, and C-30 min.
 dioxene in CDCl3; irradiation
 Thetime: A-10spectra
 1H NMR min., B-20 min.,
 of the andcompounds
 tested C-30 min. in the presence of 2,3-diphenyl-p-di-
 Thein H 1
 NMR spectra ofafter
 the different
 tested compounds
 oxene CDCl 3, recorded irradiationintimes,
 the presence
 showedofthe2,3-diphenyl-p-
 formation of pho-
 The 1H NMR dioxene
 spectra in
 of CDCl
 the , recorded
 tested
 3 after
 compounds different
 in irradiation
 the presence times,
 of showed the formation
 2,3-diphenyl-p-di-
 tooxidation products according to Equation (5) (Figure 3 and Figure S3 in of anal-
 ESI). Their
 photooxidation products according to Equation (5) (Figure 3 and Figure S3 in ESI). Their
 oxene in CDCl3, recorded after us
 ysis allowed different irradiation
 to distinguish times, showed
 the substrate from thethe formation
 oxidation of pho-
 product, for which the
 analysis allowed us to distinguish the substrate from the oxidation product, for which the
 relevant values of characteristic chemical proton shifts are presented
 tooxidation products according to Equation (5) (Figure 3 and Figure S3 in ESI). Their anal- in Table 3.
 relevant values of characteristic chemical proton shifts are presented in Table 3.
 ysis allowed us to distinguish the substrate from the oxidation product, for which the
 Table 3. Characteristic values of chemical shifts for individual compounds and areas under the
 relevant values of characteristic chemical
 peaks after the specified protontime
 irradiation shifts are presented
 assigned from 1H NMR inspectra.
 Table 3.

 Table 3. Characteristic values of Compound
 chemical shifts for individual Areathe
 compounds and areas under
 σ, ppm
 peaks after the specified irradiation time assigned from 1H NMR spectra. 10 min 20 min 30 min
 2,3-diphenyl-p-dioxene 4.3612 (s, 4H) 16.1453 14.8573 12.6815
 ethylene glycol dibenzoate 4.5916 (s, 4H) Area
 1.2199 2.2026 3.4956
 Compound σ, ppm
 10 min 20 min 30 min
 2,3-diphenyl-p-dioxene 4.3612 (s, 4H) 16.1453 14.8573 12.6815

Materials 2021, 14, 3085 12 of 19

 Table 3. Characteristic values of chemical shifts for individual compounds and areas under the peaks
 after the specified irradiation time assigned from 1 H NMR spectra.

 Area
 Compound σ, ppm
 Materials 2021, 14, x FOR PEER REVIEW 10 min 20 min 30 min
 12 of 20
 2,3-diphenyl-p-dioxene 4.3612 (s, 4H) 16.1453 14.8573 12.6815
 ethylene glycol dibenzoate 4.5916 (s, 4H) 1.2199 2.2026 3.4956
 The analysis of 1H NMR spectra of the tested compounds in a mixture with 2,3-di-
 phenyl-p-dioxene
 The analysis of in 1CDCl
 H NMR3 indicated
 spectraaof well-separated singlet for in
 the tested compounds two methylene
 a mixture withgroups
 2,3-
 from the 2,3-diphenyl-p-dioxene.
 diphenyl-p-dioxene in CDCl3 indicated Aftera well-separated
 gradual irradiation,singletthe
 forproton spectrumgroups
 two methylene of the
 samethe
 from solution showed considerable
 2,3-diphenyl-p-dioxene. changes
 After gradualin chemical shifts.
 irradiation, theItproton
 becamespectrum
 evident that the
 of the
 signalsolution
 same intensityshowed
 at 4.3612 ppm decreased
 considerable gradually
 changes accompanied
 in chemical shifts. Itbybecame
 the appearance of a
 evident that
 the
 newsignal
 signalintensity
 at 4.5916atppm.
 4.3612
 Theppm decreased
 intensity of thegradually
 new singlet accompanied by the appearance
 increased proportionally with
 of
 thea new signal at
 irradiation 4.5916
 time. Theppm. The of
 location intensity
 the signalof the
 wasnewin singlet
 perfectincreased
 agreement proportionally
 with the two
 with the irradiation
 methylene groups in time. The location
 ethylene of the signalwhich
 glycol dibenzoate, was inwasperfect agreement
 formed duringwith the two
 photooxida-
 methylene groups in ethylene glycol dibenzoate, which was formed
 tion of the 2,3-diphenyl-p-dioxene sensitized by the tested compounds. The quantum during photooxidation
 of the of
 yield 2,3-diphenyl-p-dioxene
 singlet oxygen production sensitized
 in thebypresence
 the testedof compounds. The quantum
 the tested sensitizers yield of
 is collected in
 singlet
 Table 3. oxygen
 These production
 values can in bethe presenceequal
 considered of thetotested sensitizers
 the quantum is collected
 yield in Table
 of the triplet 3.
 state
 These values
 formation whencan determined
 be considered equal
 under thetoconditions
 the quantum yield of
 of excess the triplet state formation
 oxygen.
 whenThe determined
 quantumunder yield the conditions
 of the of excess
 triplet state oxygen.
 formation for selected sensitizers was also de-
 The quantum
 termined yield of of
 from the analysis the triplet state
 transient formation
 absorption forobtained
 spectra selected using
 sensitizers was also
 the nanosecond
 determined
 laser flash photolysis technique. An exemplary spectrum of FN2 in deoxygenatednanosec-
 from the analysis of transient absorption spectra obtained using the acetoni-
 ond
 trile laser flash
 solution is photolysis technique.
 shown in Figure An exemplary
 5. A readily detectable spectrum
 transientof FN2 in deoxygenated
 absorption spectra peak-
 acetonitrile
 ing at 390 nm, solution is shown
 470 nm, in Figure
 and 620 nm decay 5. A readily
 in a fewdetectable transient
 milliseconds absorption
 according spectra
 to first-order
 peaking
 kinetics.atThey
 390 nm,
 were470notnm, and 620innm
 observed andecay in a few milliseconds
 oxygen-saturated solution.according to first-order
 kinetics. They were not observed in an oxygen-saturated solution.

 0.3μs
 0.15
 0.5μs
 0.7μs
 1.0μs
 0.10
 Absorbance

 1.5μs
 2.5μs

 0.05

 0.00

 -0.05
 300 350 400 450 500 550 600 650
 Wavelength, nm

 Figure5.5.Laser
 Figure Laserflash
 flashphotolysis
 photolysisspectra
 spectraofofthe
 thedye
 dyeFN2
 FN2ininacetonitrile
 acetonitrilerecorded
 recordedafter
 afterirradiation with
 irradiation
 laser
 withpulses of 355of
 laser pulses nm.
 355Delay times times
 nm. Delay shownshown
 in legends.
 in legends.

 The
 Thequantum
 quantumyields
 yieldsof ofthe
 the triplet
 triplet state
 state formation
 formation of
 of the
 the tested
 tested sensitizers
 sensitizers were
 were deter-
 deter-
 mined from the triplet formation curves using methodology given
 mined from the triplet formation curves using methodology given by Lament et by Lament et al. [64]
 al.and
 [64]
 by us [36]. The method is based on quantitative analysis of the ground state
 and by us [36]. The method is based on quantitative analysis of the ground state recovery recovery and
 its
 anddeconvolution, whichwhich
 its deconvolution, concerns an instrumental
 concerns response
 an instrumental function.
 response A typical
 function. example
 A typical ex-
 of
 ample of such approach is shown in Figure 5, and obtained data are summarized Table
 such approach is shown in Figure 5, and obtained data are summarized in 2.
 in Table
 According to Equation (6), the yield of triplet formation (Φ T ) is proportional
 2. According to Equation (6), the yield of triplet formation (ΦT) is proportional to the ratio to the ratio
 of
 ofthe
 theinitial
 initialincrement
 incrementin inabsorbance
 absorbanceimmediately
 immediatelyafter
 afterthe
 theflash
 flash(∆A
 (ΔAt )t)and
 andthe
 thevalue
 valueatat
 the maximum ground state depletion (the former is compared to the long delay, “plateau”
 region) (cf. Figure 6):
 Δ (6)
 Φ =
 ΔA