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; [email protected] * Correspondence: [email protected] (I.P.); [email protected] (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 1H 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 on the arrangement of aromatic rings and the number of nitrogen atoms in the modified phenazine Oxygen Sensitizers Based on the 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, Publisher’s Note: MDPI stays neutral contributes to the aging of organisms and the development of cancer. Changes in the with regard to jurisdictional claims in properties of oxygen and its ability to initiate a number of processes are conditioned by published maps and institutional affil- 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. 3 form ( 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 configuration of the oxygen molecule changes from a triplet to a singlet 1O . Thus, in the conditions of the Creative Commons 2 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, 3 i.e., irradiation of the 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 3 its excited singlet state *S(S1) to the ground-state oxygen molecule 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.

3 1 1 *S(S1) + O2 → *S(T1) + O2 ( ∆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.

3 3 *S(S1) + O2 → *S(T1) + O2 (3) Materials 2021, 14, 3085 3 of 19

3 3 *S(S1) + O2 → S(S0) + 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 Table1) 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,, 14,, xx FORFOR PEERPEER REVIEWREVIEW 4 of 20 Materials 2021, 14, x FOR PEER REVIEW 4 of 20 Materials Materials 20212021,,, 1414,,, xxx FORFORFOR PEERPEERPEER REVIEWREVIEWREVIEW 44 ofof 2020

lene were purchased from Alfa Aesar Co., Ward Hill, MA, United States; 9,10-phenan- Materials 2021, 14, 3085 lenelene werewere purchasedpurchased fromfrom AlfaAlfa AesarAesar Co.,Co., WardWard Hill,Hill, MA,MA, UnitedUnited States;States; 9,10-phenan-9,10-phenan-4 of 19 threnequinone, 1,2-naphthoquinone, 9,10-diaminophenanthrene, 2,3-diaminopyridine, 4, threnequinone,threnequinone, 1,2-naphthoqui1,2-naphthoquinone,none, 9,10-diaminophenanthrene9,10-diaminophenanthrene,, 2,3-diaminopyridine,2,3-diaminopyridine, 4,4, 5-diamino-pyrimidine, 4-bromo-1,2-diaminobenzene, and 2,3-diamino-5-bromopyridine 5-diamino-pyrimidine,5-diamino-pyrimidine, 4-bromo-1,2-diaminobenzene,4-bromo-1,2-diaminobenzene, andand 2,3-diamino-5-bromopyridine2,3-diamino-5-bromopyridine were purchased from Sigma-Aldrich Co, Saint Louis, MO, United States; 1,2-diammono- werewere purchasedpurchased fromfrom Sigma-AldrichSigma-Aldrich Co,Co, SaintSaint Louis,Louis, MO,MO, UnitedUnited States;States; 1,2-diammono-1,2-diammono- 5,6-dionecyclohexane were purchased and 1,10-phenanthr from Acros Organicsoline-5,6-dione Co, Pittsburgh, were purchased PA, United from States; Acros and Organics 5-bromo- Co, cyclohexanecyclohexane andand 1,10-phenanthr1,10-phenanthroline-5,6-dioneoline-5,6-dione werewere purchasedpurchased fromfrom AcrosAcros OrganicsOrganics Co,Co, 2,3-diaminopyrazinePittsburgh, PA, wasUnited purchased States; fromand Fluorochem5-bromo-2,3-diaminopyrazine Co, Hadfield, UK. was purchased from Pittsburgh,Pittsburgh, PA,PA, UnitedUnited States;States; andand 5-brom5-bromo-2,3-diaminopyrazineo-2,3-diaminopyrazine waswas purchasedpurchased fromfrom Fluorochem Co, Hadfield, UK. TableFluorochemFluorochem 1. Structures, Co,Co, basic Hadfield,Hadfield, spectroscopic UK.UK. properties, and quantum yields of fluorescence for the dyes Table 1. Structures, basic spectroscopic properties, and quantum yields of fluorescence tested.TableTable 1.1. Structures,Structures, basicbasic spectroscospectroscopicpic properties,properties, andand quantumquantum yieldsyields ofof fluorescencefluorescence

for the dyes tested. forfor thethe dyesdyes tested.tested. Ab Ab −1 − Fl Dye Structure 𝑨𝒃λ , nm 𝑨𝒃 e 𝟏, M cm𝟏 1 𝑭𝒍λ , nm Φ Dye Structure 𝝀𝑨𝒃𝒎𝒂𝒙max,𝒏𝒎,𝒏𝒎 𝝐𝑨𝒃𝒎𝒂𝒙max,𝑴,𝑴𝟏𝒄𝒎𝟏 𝝀𝑭𝒍𝒎𝒂𝒙max,𝒏𝒎,𝒏𝒎 𝚽𝑭𝒍Fl Dye Structure 𝝀𝑨𝒃𝒎𝒂𝒙𝑨𝒃 ,𝒏𝒎 𝝐𝑨𝒃𝒎𝒂𝒙𝑨𝒃 ,𝑴𝟏𝟏𝒄𝒎𝟏𝟏 𝝀𝑭𝒍𝒎𝒂𝒙𝑭𝒍 ,𝒏𝒎 𝚽𝑭𝒍 DyeDye StructureStructure 𝝀𝝀𝒎𝒂𝒙𝒎𝒂𝒙,𝒏𝒎,𝒏𝒎,𝒏𝒎 𝝐𝝐𝒎𝒂𝒙𝒎𝒂𝒙,𝑴,𝑴,𝑴 𝒄𝒎𝒄𝒎 𝝀𝝀𝒎𝒂𝒙𝒎𝒂𝒙,𝒏𝒎,𝒏𝒎,𝒏𝒎 𝚽𝚽𝑭𝒍𝑭𝒍 N N 345 8060 NN 345345 80608060 FN1FN1 345345 80608060 383.0383.0 0.110 0.110 FN1 N 362362 10,40010,400 383.0 0.110 FN1 N 362 10,400 383.0 0.110 NN 362362 10,40010,400

N N 375 9700 NN 375375 97009700 FN2FN2 375 9700 413.6413.6 0.044 0.044 FN2 375 9700 413.6 0.044 FN2 N 395395 11,40011,400 413.6 0.044 FN2 N 395 11,400 413.6 0.044 NN 395395 11,40011,400

N N 402 10,060 NN 402402 10,06010,060 FN3FN3 402402 10,06010,060 532.8532.8 0.045 0.045 FN3 N 425425 13,50013,500 532.8 0.045 FN3 N 425 13,500 532.8 0.045 NN 425425 13,50013,500

N N 392 9900 415.2 FN4 NN 392392 99009900 415.2415.2 0.027 FN4FN4 392392 99009900 415.2415.2 0.0270.027 FN4FN4 N 414414 15,90015,900 438.4438.4 0.0270.027 N 414 15,900 438.4 NN 414414 15,90015,900 438.4438.4

N N 392 9700 414.2 FN5 NN 392392 97009700 414.2414.2 0.049 FN5FN5 392392 97009700 414.2414.2 0.0490.049 FN5 N 414 16,300 438.0 0.0490.049 FN5 N 414414 16,30016,300 438.0438.0 NN 414414 16,30016,300 438.0438.0

386 9500 N 386 9500 FN6 N 386386 95009500 437.8 0.025 FN6 NN 386386 95009500 437.8 0.025 FN6FN6FN6 407 10,900 437.8437.8437.8 0.0250.025 0.025 N 407407 10,90010,900 N 407 10,900 N N 409 14,000 N 409 14,000 FN7 N 409 14,000 543.8 0.025 FN7 NN 409409409 14,00014,00014,000 543.8 0.025 FN7FN7FN7 432 10,800 543.8543.8543.8 0.0250.025 0.025 N 432432 10,80010,800 N 432 10,800 N N N N 383 9600 NN 383 9600 460 0.045 FN8 383383383 960096009600 460 0.045 FN8FN8 N N 393 9620 460460460 0.0450.045 0.045 FN8 N N 393 9620 NN NN 393393393 962096209620

Br N Br N 382 20,500 BrBr NN 382 20,500 424.2 0.033 FN9 382382382 20,50020,50020,500 424.2 0.033 FN9FN9 N 403 25,600 424.2424.2424.2 0.0330.033 0.033 FN9 N 403 25,600 NN 403403403 25,60025,60025,600

Br N Br N 390 15,900 BrBr NN 390 15,900 428 0.028 FN10 390390 15,90015,900 428 0.028 FN10FN10 N N 407 16,300 428428 0.0280.028 N N 407 16,300 NN NN 407407 16,30016,300

Materials 2021, 14, x FOR PEER REVIEW 4 of 20

lene were purchased from Alfa Aesar Co., Ward Hill, MA, United States; 9,10-phenan- threnequinone, 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-diammono- cyclohexane and 1,10-phenanthroline-5,6-dione were purchased from Acros Organics Co, Pittsburgh, PA, United States; and 5-bromo-2,3-diaminopyrazine was purchased from Fluorochem Co, Hadfield, UK. Table 1. Structures, basic spectroscopic properties, and quantum yields of fluorescence for the dyes tested. 𝑨𝒃 𝑨𝒃 𝟏 𝟏 𝑭𝒍 Dye Structure 𝝀𝒎𝒂𝒙,𝒏𝒎 𝝐𝒎𝒂𝒙,𝑴 𝒄𝒎 𝝀𝒎𝒂𝒙,𝒏𝒎 𝚽𝑭𝒍

N 345 8060 FN1 383.0 0.110 N 362 10,400

N 375 9700 FN2 413.6 0.044 N 395 11,400

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

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

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

386 9500 FN6 N 437.8 0.025 407 10,900 N 409 14,000 FN7 N 543.8 0.025 432 10,800 N

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

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

Br N 390 15,900 MaterialsMaterials 20212021,, 1414,, xx FORFOR PEERPEER REVIEWREVIEW 390 15,900 428 0.02855 ofof 2020 Materials 2021, 14, x FOR PEER REVIEWFN10FN10 428 0.028 5 of 20 N N 407407 16,30016,300

BrBr NN NN Br N N 395395395 14,30014,30014,300 FN11FN11 395 14,300 445.6445.6445.6 0.0420.042 0.042 FN11FN11 411 15,800 445.6 0.042 NN NN 411411 15,80015,800 N N 411 15,800

NN NN N N 360360 14,50014,500 FN12 360360 14,50014,500 412.4 0.047 FN12FN12FN12 412.4412.4412.4 0.0470.047 0.047 NN NN 378378378 13,70013,70013,700 N N 378 13,700

NN NN N FN13 N 437 11,490 477 0.100 FN13FN13FN13 437437437 11,49011,490 11,490 477477 477 0.1000.100 0.100 NN NN N N

OO CQ O 472 40 - - CQCQCQ 472472472 4040 40------OO O

AllAll solventssolvents forfor synthesis,synthesis, photochemicalphotochemical ststudies,udies, andand NMRNMR analysis,analysis, includingincluding aceticacetic All solventsAll solvents for for synthesis, synthesis, photochemical photochemical studies, studies, andand NMR analysis, analysis, including including acetic acid,acid, ethylethyl acetate,acetate, deutdeuteratederated chloroformchloroform (CDCl(CDCl33),), chloroform,chloroform, andand 1-methyl-2-pyrroli-1-methyl-2-pyrroli- aceticacid, acid, ethyl ethyl acetate, acetate, deut deuteratederated chloroform chloroform (CDCl (CDCl3), chloroform,), chloroform, and and 1-methyl-2-pyrroli- 1-methyl-2- dinonedinone (MP),(MP), werewere purchasedpurchased fromfrom Sigma-AldrichSigma-Aldrich Co,Co,3 SaintSaint Louis,Louis, MO,MO, UnitedUnited States.States. pyrrolidinonedinone (MP), (MP), were were purchased purchased from from Sigma-Aldrich Sigma-Aldrich Co, Co, Saint Saint Louis, Louis, MO, MO, United United States. TheThe DC-PlastikfolienDC-Plastikfolien SilicaSilica gelgel 6060 F254F254 thinthin layerlayer chromatographychromatography platesplates (0.2(0.2 mm)mm) werewere States.The The DC-Plastikfolien DC-Plastikfolien Silica Silica gel gel 60 60F254 F254 thin thin layer layer chromatography chromatography plates plates (0.2 (0.2 mm) mm) were fromfrom MerckMerck Co,Co, Kenilworth,Kenilworth, NJ,NJ, USA.USA. werefrom from Merck Merck Co, Co, Kenilworth, Kenilworth, NJ, NJ, USA. USA. CamphorquinoneCamphorquinone (CQ,(CQ, initiator),initiator), [(3,[(3,4-dimethoxyphenyl)sulfanyl]acetic4-dimethoxyphenyl)sulfanyl]acetic acidacid CamphorquinoneCamphorquinone (CQ, initiator),(CQ, initiator), [(3,4-dimethoxyphenyl)sulfanyl]acetic [(3,4-dimethoxyphenyl)sulfanyl]acetic acid (MKTFO, acid (MKTFO,(MKTFO, co-initiator),co-initiator), andand trimethylolpropanetrimethylolpropane triacrylatetriacrylate (TMPTA,(TMPTA, monomer)monomer) werewere ob-ob- co-initiator),(MKTFO, and co-initiator), trimethylolpropane and trimethylolpropane triacrylate (TMPTA, triacrylate monomer) (TMPTA, were monomer) obtained fromwere ob- tainedtained fromfrom Sigma-AldrichSigma-Aldrich Co,Co, SaintSaint Louis,Louis, MO,MO, USA.USA. Sigma-Aldrichtained from Co, Sigma-Aldrich Saint Louis, MO,Co, Saint USA. Louis, MO, USA. 2.2. Synthesis 2.2.2.2. Synthesis2.2. SynthesisSynthesis 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN110,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN110,11,12,13-Tetrahydrodibenzo[a,c]phenazine-FN1 1,2-diaminocyclohexane (0.548 g, 4.8 mmol) was refluxed with 9,10-phenrenequi- 1,2-diaminocyclohexane1,2-diaminocyclohexane1,2-diaminocyclohexane (0.548 (0.548(0.548 g, 4.8 g,g, mmol) 4.84.8 mmol)mmol) was refluxed waswas refluxedrefluxed with 9,10-phenrenequinone withwith 9,10-phenrenequi-9,10-phenrenequi- none (1 g, 4.8 mmol) in glacial acetic acid (40 mL) for 2 h. The compound was obtained as (1 g,nonenone 4.8 mmol) (1(1 g,g, 4.84.8 in glacialmmol)mmol) acetic inin glacialglacial acid aceticacetic (40 mL) acidacid for (40(40 2 h. mL)mL) The forfor compound 22 h.h. TheThe compoundcompound was obtained waswas as obtainedobtained yellow asas yellowyellow crystals:crystals: CC2020HH1818NN22,, 286.38286.38 g/mol,g/mol, 1.121.12 g,g, yieldyield 82%,82%, m.p.m.p.◦ 174–176174–176 °C,°C,◦ lit.lit. 173173 °C°C [51].[51]. crystals:yellow C20 crystals:H18N2, 286.38C20H18N g/mol,2, 286.38 1.12 g/mol, g, yield 1.12 82%, g, yield m.p. 82%, 174–176 m.p. 174–176C, lit. 173 °C,C[ lit.51 173]. °C [51]. 1 11H1H NMRNMR (400(400 MHz,MHz, CDClCDCl33,, δδ):): 9.24–9.219.24–9.21 (d,(d, 1H),1H), 8.62–8.598.62–8.59 (d,(d, 2H),2H), 7.78–7.707.78–7.70 (m,(m, 4H),4H), 3.28–3.28– 1HH NMR NMR (400 (400 MHz, MHz, CDCl CDCl3,3 δ, ):δ): 9.24–9.21 9.24–9.21 (d, (d, 1H), 1H), 8.62–8.59 8.62–8.59 (d, (d, 2H), 2H), 7.78–7.70 7.78–7.70 (m, (m, 4H), 4H), 3.28– 3.25 (m, 5H), 2.02–2.08 (m, 5H) 3.28–3.253.253.25 (m,(m, (m, 5H),5H), 5H), 2.02–2.082.02–2.08 2.02–2.08 (m,(m, (m, 5H)5H) 5H) 131313 13CC NMRNMR (100(100 MHz,MHz, CDClCDCl33,, δδ):):δ 152.1,152.1, 138.2,138.2, 135.8,135.8, 131.0,131.0, 129.2,129.2, 129.1,129.1, 127.5,127.5, 125.23,125.23, 122.6,122.6, 13CC NMR NMR (100 (100 MHz, MHz, CDCl CDCl3,3 δ, ): ):152.1, 152.1, 138.2, 138.2, 135.8, 135.8, 131.0, 131.0, 129.2, 129.2, 129.1, 129.1, 127.5, 127.5, 125.23, 125.23, 122.6, 32.4, 22.75 122.6,32.4,32.4, 32.4, 22.7522.75 22.75 Dibenzo[a,c]phenazine-FN2 Dibenzo[a,c]phenazine-FN2Dibenzo[a,c]phenazine-FN2Dibenzo[a,c]phenazine-FN2 1,2-diaminobenzene (0.519 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone (1 1,2-diaminobenzene1,2-diaminobenzene1,2-diaminobenzene (0.519 (0.519(0.519 g, 4.8 g,g, mmol) 4.84.8 mmol)mmol) was refluxed waswas refluxedrefluxed with 9,10-phenrenequinone withwith 9,10-phenrenequinone9,10-phenrenequinone (1 g, (1(1 4.8 mmol)g,g, 4.84.8 mmol) inmmol) glacial inin acetic glacialglacial acid aceticacetic (80 acid mL)acid for(80(80 2 mL)mL) h. The forfor compound 22 h.h. TheThe compoundcompound was obtained waswas as obtainedobtained pale, yellow asas pale,pale, g, 4.8 mmol) in glacial acetic acid (80 mL) for 2 h. The compound◦ was obtained◦ as pale, crystals:yellowyellow C crystals:crystals:H N , 280.33CC2020HH1212NN g/mol,22,, 280.33280.33 1.34 g/mol,g/mol, g, yield 1.341.34 80%, g,g, yieldyield m.p. 80%,80%, 224–225 m.p.m.p. 224–225C,224–225 lit. 224–226 °C,°C, lit.lit. 224–226224–226C[56]. °C°C yellow20 crystals:12 2 C20H12N2, 280.33 g/mol, 1.34 g, yield 80%, m.p. 224–225 °C, lit. 224–226 °C [56].1 δ [56].[56].H NMR (400 MHz, CDCl3, ): 9.34 (d, 2H), 9,32 (d, 2H), 8.50–8.48 (d, 2H), 8.27–8.24 1 (m, 2H),11HH NMRNMR 7.870–7.65 (400(400 MHz,MHz, (m, 6H) CDClCDCl33,, δδ):): 9.349.34 (d,(d, 2H),2H), 9,329,32 (d,(d, 2H),2H), 8.50–8.488.50–8.48 (d,(d, 2H),2H), 8.27–8.248.27–8.24 (m,(m, 1H NMR (400 MHz, CDCl3, δ): 9.34 (d, 2H), 9,32 (d, 2H), 8.50–8.48 (d, 2H), 8.27–8.24 (m, 2H), 7.870–7.65 (m, 6H) 2H),2H), 7.870–7.657.870–7.65 (m,(m, 6H)6H) 13 1313CC NMRNMR (100(100 MHz,MHz, CDClCDCl33,, δδ):): 141.8,141.8, 141.3,141.3, 132.0,132.0, 130.7,130.7, 130130.2,.2, 129.3,129.3, 128.9,128.9, 128.0,128.0, 126.5,126.5, 13C NMR (100 MHz, CDCl3, δ): 141.8, 141.3, 132.0, 130.7, 130.2, 129.3, 128.9, 128.0, 126.5, 122,9 122,9122,9 Tribenzo[a,c,i]phenazine-FN3 Tribenzo[a,c,i]phenazine-FN3Tribenzo[a,c,i]phenazine-FN3 2,3-diaminonaphthalene (0.76 g, 4.8 mmol) was refluxed with 9,10-phenrenequinone 2,3-diaminonaphthalene2,3-diaminonaphthalene (0.76(0.76 g,g, 4.84.8 mmol)mmol) waswas refluxedrefluxed withwith 9,10-phenrenequinone9,10-phenrenequinone (1 g, 4.8 mmol) in glacial acetic acid (100 mL) for 2 h. The compound was obtained as (1(1 g,g, 4.84.8 mmol)mmol) inin glacialglacial aceticacetic acidacid (100(100 mL)mL) forfor 22 h.h. TheThe compoundcompound waswas obtainedobtained asas yellowyellow crystals:crystals: CC2424HH1414NN22,, 330.39330.39 g/mol,g/mol, 1.361.36 g,g, yieldyield 86%,86%, m.p.m.p. 292–293292–293 °C,°C, lit.lit. 291–293291–293 °C°C yellow crystals: C24H14N2, 330.39 g/mol, 1.36 g, yield 86%, m.p. 292–293 °C, lit. 291–293 °C [57]. [57].[57].

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: C24H14N2, 330.39 g/mol, 1.36 g, yield 86%, m.p. 292–293 C, lit. 291–293 C[57]. 1 H NMR (400 MHz, CDCl3, δ): 9.38–9.36 (d, 2H), 8.88 (s, 2H), 8,47–8.45 (d, 2H), 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: C24H14N2, 330.39 g/mol, 1.0 g, yield 63%, m.p. 257–258 C, lit. 278–279 C[58]. 1 H NMR (400 MHz, CDCl3, δ): 9.78 (d, 1H), 9.62 (d, 2H), 8.93 (d, 1H), 8.69 (m, 2H), 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: C28H16N2, 380.45 g/mol, 1.1 g, yield 60%, m.p., lit. 462 C[59]. 1 HNMR (400 MHz, DMSO-d6, δ): 8.33–8.31 (d, 4H), 8.05 (d, 2H), 8.03 (d, 2H), 7.82–7.77 (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: C16H10N2, 230.27 g/mol, 0.94 g, yield 65%, m.p. 230–232 C, lit. 230–233 C[52]. 1 HNMR (400 MHz, CDCl3, δ): 9.65 (1H, d), 8.50 (1H, d), 8.37 (1H, d), 8.23 (1H, d), 7.99–7.60 (m, 6H) 13 C NMR (100 MHz, CDCl3, δ): 142.9, 142.5, 132.9, 131.1, 131.0, 130.6, 130.3, 129.9, 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: C20H12N2, 280.33 g/mol, 0.94 g, yield 87%, m.p. 247–248 C, lit. 247 C[60]. 1 HNMR (400 MHz, CDCl3, δ): 9.32 (d, 1H), 9.03 (s, 1H), 8.94 (s, 1H), 8.15 (m, 3H), 8.03 (d, 1H), 7.86–7.77 (m, 3H), 7.61–7.59 (m, 2H) 13 C NMR (100 MHz, CDCl3, δ): 143.9, 138.7, 136.8, 134.7, 134.2, 132.8, 130.8, 130.5, 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: C19H11N3, 281.32 g/mol, 1.1 g, yield 81%, m.p. 218–219 C, lit. 217–219 C[48,49]. 1 HNMR (400 MHz, CDCl3, δ): 9.52-9.49 (d, 1H), 9.31-9.27 (m, 2H), 8.66-8.64 (d, 1H), 8.52-8.50 (d, 2H), 7.84-7.68 (m, 5H) 13 C NMR (100 MHz, CDCl3, δ): 154.4, 149.8, 145.0, 143.6, 138.3, 137.3, 132.5, 132.3, 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: C20H11BrN2, 359.23 g/mol, 1.24 g, yield 72%, m.p. 274–275 C, lit. 274–276 ◦C[46]. 1 HNMR (400 MHz, CDCl3, δ): 9.38–9.35 (m, 2H), 8,59–8.57 (d, 2H), 8.53 (d, 1H), 8.21–8.19 (d, 1H), 7.94–7.91 (m, 1H), 7.85–7.74 (m, 4H) 13 C NMR (100 MHz, CDCl3, δ): 133.7, 132.3, 132.26, 131.2, 131.0, 130.9, 130.2, 128.2, 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: C19H10BrN3, 360.21 g/mol, 1.42 g, yield 77%, m.p. 235–238 C, lit. 235–238 ◦C[49,57]. 1 HNMR (400 MHz, CDCl3, δ): 9.43–9.41 (d, 1H), 9.25 (s, 1H), 9.20–9.18 (d, 1H), 8.77 (s, 1H), 8.52–8.50 (d, 2H), 7.85–7.68 (m, 4H) 13 C NMR (100 MHz, CDCl3, δ): 155.3, 148.0, 145.1, 144.1, 139.5, 137.4, 132.5, 132.5, 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: C18H9BrN4, 361.21 g/mol, 1.19 g, yield 69%, m.p. 279–281 C. 1 HNMR (400 MHz, CDCl3, δ): 9.56 (d, 1H), 8.22–8.19 (d, 2H), 8.04–8.02 (d, 2H), 7.76–7.72 (m, 2H), 7.51–7.47 (m, 2H) 13 C NMR (100 MHz, CDCl3, δ): 180.3, 135.9, 135.8, 131.0, 130.5, 129.9, 129.5, 129.5, 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: C18H10N4, 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, CDCl3, δ): 9.71 (d, 2H), 9.33–9.32 (d, 2H), 8.40–8.38 (m, 2H), 7.97–7.95 (m, 2H), 7.86–7.83 (m, 2H) 13 C NMR (100 MHz, CDCl3, δ): 175.7, 152.4, 148.2, 142.5, 141.1, 133.9, 130.7, 129.5, 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: C18H10N4, 282.30 g/mol, 0.96 g, yield 71%, m.p. 320–321 C. 1 HNMR (400 MHz, DMSO-d6, δ): 9.55–9.52 (d, 2H), 9.22–9.21 (d, 2H), 8.41–8.37 (d, 2H), 8.08–8.04 (d, 2H), 7.96–7.93 (d, 2H) 13 C NMR (100 MHz, DMSO-d6, δ): 172.4, 152.8, 148.3, 142.2, 141.3, 133.5, 131.7, 129.6, 127.4, 125.0

2.3. Methods The 1H 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

Materials 2021, 14, x FOR PEER REVIEWused for the excitation. Transient absorbances at preselected wavelengths were monitored8 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 digitalphotomultiplier storage oscilloscope was processed and an by Acorn-compatible a Hewlett-Packard/Agilent computer. an Agilent Infiniium 54810AThe digital quantum storage yield os ofcilloscope the triplet and state an formationAcorn-compatible of the synthesized computer. dyes was deter- minedThe by quantum two methods. yield Theof the first triplet one wasstate described formation by of Lament the synthesized et al. [64] dyes and waswas baseddeter- onmined the analysisby two methods. of transient The absorption first one was spectra, described whereas by Lament the second et al. one[64] wasand describedwas based byon Silvermanthe analysis and of transient Foote [32 ].absorption The latter spec methodtra, whereas was based the second on the analysisone was described of 1H NMR by 1 spectraSilverman recorded and Foote during [32]. the The singlet latter oxygen method oxidation was based of 2,3-diphenyl- on the analysisp-dioxene of H NMR (Scheme spectra1). Therecorded photooxidation during the reaction singlet wasoxygen carried oxidation out in aof solution 2,3-diphenyl- containingp-dioxene 0.6 mL (Scheme of deuterated 1). The chloroformphotooxidation in which reaction both was 10 mgcarried of 2,3-diphenyl- out in a solutionp-dioxene containing and 5 0.6 mg mL of theof deuterated dye were dissolved.chloroform The in which solution both was 10 mg irradiated of 2,3-diphenyl- with LEDp-dioxene F (dental-lamp-emitted) and 5 mg of the dye radiation were dis- in thesolved. range: The 390–480 solution nm. was The irradiated radiation with intensity LED was F (dental-lamp-emitted) 18 mW/cm2. During irradiation,radiation in thethe solutionrange: 390–480 was stirred nm. withThe radiation a stream of intensity oxygen. was The 18 formation mW/cm of2. During the photooxidation irradiation, the product solu- wastionmonitored was stirred by with recording a stream the of1H oxygen. NMR spectra. The formation of the photooxidation product was monitored by recording the 1H NMR spectra.

O O O O C O OC 1 O temp. + O2 CH CH O 2 2 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. TheThe quantumquantum yieldyield ofof singletsinglet oxygenoxygen productionproduction waswas calculatedcalculated basedbased on on the the rate rate of of thethe ethyleneethylene glycol glycol dibenzoate dibenzoate formation formation by by the the testedtested sensitizerssensitizers compared compared to to the the rate rate of of thisthis product’s product’s formation formation in in the the presence presence of of camphorquinone, camphorquinone, which which was was used used asa as standard. a stand- Theard. quantum The quantum yield yield of singlet of sing oxygenlet oxygen generation generation by camphorquinone by camphorquinone was 1.0 was [65 1.0,66 ].[65,66]. With aWith large a excesslarge excess of oxygen, of oxygen, the quantum the quantum yield of yield singlet of singlet oxygen oxygen formation formation was equal was to equal the quantumto the quantum yield of yield the triplet of the statetriplet formation state formation [24]. [24].

Φ Φ1 =Φ= Φ T (5)(5) O2 TheThe effectivenesseffectiveness ofof thethe synthesizedsynthesized dyes dyes as as radical radical polymerization polymerization photosensitizers photosensitizers waswas testedtested byby the the microcalorimetric microcalorimetric method method [ 67[67,68].,68]. The The measurements measurements were were performed performed inin aa homemade homemade microcalorimetermicrocalorimeter usingusing aa semiconductingsemiconducting diodediode immersedimmersed inin aa 2–3 2–3 mm mm thickthick layerlayer (0.25 (0.25 mL) mL) of of a cureda cured sample sample as aas temperature a temperature sensor. sensor. The initialThe initial rate of rate poly- of merizationpolymerization was determinedwas determined by the by measurements the measurements of the rateof the of therate heat of the evolution heat evolution during polymerizationduring polymerization for the initialfor the time.initial Atime. diode-pumped A diode-pumped solid-state solid-state laser laser (408 (408 nm) nm) with with an intensityan intensity of 18 of mW/cm 18 mW/cm2 (Mean2 (Mean Well Well model model NES-15-12) NES-15-12) was was used used as the as light the light source. source. The lightThe light intensity intensity was measuredwas measured by a Coherentby a Cohere modelnt model Fieldmaster Fieldmaster power power meter. meter. The distance The dis- oftance the sampleof the sample from the from light the source light wassource the wa sames the for same all measurements. for all measurements. Each measurement Each meas- wasurement repeated was atrepeated least three at least times. three times. TheThe photopolymerization photopolymerization compositions compositions were were composed composed of 0.9of g0.9 of g trimethylolpropane of trimethylolpro- triacrylatepane triacrylate (TMPTA) (TMPTA) and 0.1 and mL of0.1 1-methyl-2-pyrrolidinone mL of 1-methyl-2-pyrrolidinone (MP) containing (MP) containing appropriate ap- amountspropriate of amounts the photosensitizer of the photosensitizer (synthesized (synthesized phenazine phenazine based dyes) based and dyes) the co-initiator and the co- [(3,4-dimethoxyphenyl)sulfanyl]aceticinitiator [(3,4-dimethoxyphenyl)sulfanyl]acetic acid (MKTFO). acid (MKTFO). The concentration The concentration of the dyes of was the −3 −4 2dyes× 10 was–5.63 2×10×−3–5.63×1010 M− (depending4 M (depending on the on molarthe molar absorption absorption coefficient coefficient at an at excitation an excita- wavelength),tion wavelength), whereas whereas the concentration the concentration of the co-initiatorof the co-initiator was 0.1 was M. As 0.1 a M. reference As a reference sample, asample, polymerizing a polymerizing mixture withoutmixture awithout co-initiator a co-initiator was used. was used. InIn orderorder to to compare compare the the quantum quantum efficiency efficiency of tripletof triplet state state formation formation and theand photoini- the pho- tiatingtoinitiating efficiency efficiency of the of TMPTA, the TMPTA, radical radical polymerization polymerization by the synthesizedby the synthesized compounds com- pounds with a commercial compound, the camphorquinone (λmax = 472 nm, ε = 40 M−1cm‒ 1 in ethyl acetate), was used. Its concentration in the polymerization experiments was 0.675 M.

Materials 2021, 14, x FOR PEER REVIEW 9 of 20

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 −1 -1 towith 530 nm. a commercial Thus, the compound,compounds theafforded camphorquinone deeply colored (λmax solutions= 472 nm,withε molar= 40M extinctioncm in coefficientsethyl acetate), ranging was used.from 0.96 Its concentration × 104 M−1cm−1 into 2.56 the polymerization× 104 M−1cm−1 for experiments the most intense was 0.675 band, M. which is typical for π→π* transitions. The basic spectral data are collected in Table 1. 3. ResultsThe position of the absorption bands was clearly influenced by the dye structure, i.e., the numberThe analysis of conjugated of the double electronic bonds, absorption the arrangement spectra of of aromatic the synthesized rings, and compounds the num- berindicated and location that the of spectranitrogen were atoms. typical The forincr heterocyclicease in the aromaticnumber of compounds conjugated with double con- bondsdensed in ringsthe molecule and that (FN1, they wereFN2, characterizedand FN3; FN6 by and several FN7) absorptionand the angular maxima arrangement in the long- ofwavelength the rings (FN4 region versus (Figure FN3)1 ).caused They a displayed bathochromic main effect. absorption The red bands shift inof the absorption range from maximum350 nm to was 530 also nm. observed Thus, the with compounds an increase afforded in the deeply number colored of nitrogen solutions atoms with in molar the moleculeextinction (FN9, coefficients FN10, FN11) ranging as well from as 0.96 the ×presence104 M− of1cm a −heavy1 to 2.56 atom× (FN2104 M and−1cm FN9)−1 for (Fig- the uremost S1 in intense ESI). band, which is typical for π→π* transitions. The basic spectral data are collected in Table1.

1.00 FN10 FN2 FN4 0.75 FN7

0.50 Absorbance

0.25

0.00 350 400 450 500 Wavelength, nm

FigureFigure 1. 1. FragmentsFragments of of the the electronic electronic absorption absorption spectr spectraa of of the the selected selected dyes dyes in in ethyl ethyl acetate. 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. Figure2 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 Table1 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 (Table2).

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1.00 FN10 FN2 FN4 0.75 FN7

0.50 Fluorescence 0.25

0.00 400 500 600 700 Wavelength, nm

FigureFigure 2. 2. FluorescenceFluorescence spectra spectra of of the the selected selected dyes dyes in in ethyl ethyl acetate. acetate.

Table 2. Quantum yield of the singlet oxygen formation (Φ1 ), quantum yield of the triplet state As for absorption spectra, the position of the fluorescenceO2 maximum depended on (Φ ), and initial rate of photoinitiated polymerization (R ) for the dyes tested. theT dye structure. The number of both conjugated pdouble bonds and nitrogen atoms in −1 the molecule,Dye the angular arrangementR , µmol s of the rings, Φand1 the presence of a Φheavy atom p O2 T affected the fluorescence band position and its intensity (Figure S2 in ESI). The data in FN1 44.1 0.06 - Table 1 showFN2 that the fluorescence 112.0 quantum yield varied 0.32 from 0.025 (FN6 and 0.30 FN7) for compoundsFN3 with a high singlet-oxygen-gener 107.7ating ability 0.36 to 0.11 (FN1) for the 0.37 compound showing aFN4 very low singlet oxygen 180.2 quantum yield (Table 0.77 2). 0.75 FN5 101.7 0.32 - FN6 196.3Φ 0.92 0.90 Table 2. Quantum yield of the singlet oxygen formation ( ), quantum yield of the triplet state FN7 183.9 0.80 0.79 (ΦT), and initial rate of photoinitiated polymerization (Rp) for the dyes tested. FN8 107.6 0.32 0.30 FN9𝟏 147.9𝚽 𝟏 0.67 - Dye 𝑹𝒑,𝝁𝒎𝒐𝒍 𝒔 𝐎𝟐 𝚽𝑻 FN10 136.7 0.51 - FN1FN11 44.1 115.9 0.06 - 0.31 - FN2FN12 112.0 103.30.32 0.30 0.30 - FN3FN13 107.7 54.4 0.36 0.37 0.09 - FN4CQ 180.2 234.2 0.77 0.75 1.00 - FN5 101.7 0.32 - TheFN6 quantum yield196.3 of the triplet 0.92 state formation0.90 was determined by the comparative methodFN7 using camphorquinone183.9 as a standard. 0.80 The0.79 method was based on the oxidation of 2,3-diphenyl-FN8 p-dioxene107.6 by singlet oxygen 0.32 produced0.30 in a photochemical process involving the excitationFN9 of triplet147.9 oxygen with UV–vis 0.67 light in - the presence of a sensitizing dye. The 1 formationFN10 of the photooxidation136.7 product 0.51 was monitored - by the analysis of the H NMR spectra recorded for the initial solution and after different times of irradiation (Figures3 and4). 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).

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

dioxene in CDCl3 beforeFigure irradiation. 3. 1H 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 NMR spectradioxene of a mixture in CDCl of dibenzo[a.c]phenazine3; irradiation time: A-10 (sensitizer) min., B-20 and min., 2,3-diphenyl- and C-30 min.p-dioxene in CDCl ; Figure 4. 1H NMR spectra of a mixture of dibenzo[a.c]phenazine (sensitizer) and 2,3-diphenyl-p- 3 irradiation time: A-10 min, B-20 min, and C-30 min. dioxene in CDCl3; irradiationThe time: 1H NMR A-10 spectra min., B-20 of the min., tested and compounds C-30 min. in the presence of 2,3-diphenyl-p-di- 1 oxeneThe in HCDCl NMR3, recorded spectra of after the different tested compounds irradiation in times, the presence showed of the 2,3-diphenyl- formation ofp- pho- The 1H NMR dioxenespectratooxidation inof CDCl the products 3tested, recorded according compounds after to different Equation in the irradiation (5) presence (Figure times, 3 of and 2,3-diphenyl- showed Figure theS3 in formation ESI).p-di- Their of anal- photooxidation products according to Equation (5) (Figure3 and Figure S3 in ESI). Their oxene in CDCl3, recordedysis allowed after usdifferent to distinguish irradiation the substrate times, showedfrom the theoxidation formation product, of pho- 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 in Table 3. tooxidation productsrelevant according values ofto characteristicEquation (5) chemical (Figure proton 3 and shifts Figure are presented S3 in ESI). in TableTheir3. anal- 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 characteristicpeaks after the specifiedchemical irradiation proton time shifts assigned are presented from 1H NMR in spectra. Table 3.

Area Table 3. Characteristic values of Compoundchemical shifts for individualσ, ppmcompounds and areas under the 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 Area Compoundethylene glycol dibenzoateσ, ppm 4.5916 (s, 4H) 1.2199 2.2026 3.4956 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) 1.2199 2.2026 3.4956

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

Area Compound σ, ppm Materials 2021, 14, x FOR PEER REVIEW 10 min 20 min 30 min12 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- 1 phenyl-Thep-dioxene analysis ofin CDClH NMR3 indicated spectra a ofwell-separated the tested compounds singlet for intwo a mixturemethylene with groups 2,3- diphenyl-from the p2,3-diphenyl--dioxene in CDClp-dioxene.3 indicated After a well-separatedgradual irradiation, singlet the for proton two methylene spectrum groups of the fromsame thesolution 2,3-diphenyl- showed pconsiderable-dioxene. After changes gradual in chemical irradiation, shifts. the It protonbecame spectrum evident that of the the samesignal solution intensity showed at 4.3612 considerable ppm decreased changes gradually in chemical accompanied shifts. Itby became the appearance evident that of a thenew signal signal intensity at 4.5916 at ppm. 4.3612 The ppm intensity decreased of the gradually new singlet accompanied increased proportionally by the appearance with ofthe a newirradiation signal attime. 4.5916 The ppm. location The of intensity the signal of the was new in singletperfect increasedagreement proportionally with the two withmethylene the irradiation groups in time. ethylene The location glycol dibe of thenzoate, signal which was in was perfect formed agreement during with photooxida- the two methylenetion of the groups 2,3-diphenyl- in ethylenep-dioxene glycol dibenzoate,sensitized by which the wastested formed compounds. during photooxidation The quantum ofyield the of 2,3-diphenyl- singlet oxygenp-dioxene production sensitized in the by presen the testedce of compounds.the tested sensitizers The quantum is collected yield ofin singletTable 3. oxygen These productionvalues can inbe theconsidered presence equa of thel to tested the quantum sensitizers yield is collected of the triplet in Table state3. Theseformation values when can determined be considered under equal the to conditions the quantum of excess yield ofoxygen. the triplet state formation whenThe determined quantum under yield theof the conditions triplet state of excess formation oxygen. for selected sensitizers was also de- terminedThe quantum from the analysis yield of of the transient triplet stateabsorption formation spectra for obtained selected using sensitizers the nanosecond was also determinedlaser flash photolysis from the analysistechnique. of transientAn exempl absorptionary spectrum spectra of FN2 obtained in deoxygenated using the nanosec- acetoni- ondtrile lasersolution flash is photolysisshown in Figure technique. 5. A readil An exemplaryy detectable spectrum transient of absorption FN2 in deoxygenated spectra peak- acetonitrileing at 390 nm, solution 470 nm, is shown and 620 in Figure nm decay5. A readily in a few detectable milliseconds transient according absorption to first-order spectra peakingkinetics. at They 390 nm,were 470 not nm, observed and 620 in nm andecay oxygen-saturated in a few milliseconds 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 1.5μs 2.5μs

0.05 Absorbance

0.00

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

FigureFigure 5.5. LaserLaser flashflash photolysisphotolysis spectra spectra of of the the dye dye FN2 FN2 in in acetonitrile acetonitrile recorded recorded after after irradiation irradiation with laserwith pulseslaser pulses of 355 of nm. 355 Delay nm. Delay times times shown shown in legends. in legends.

TheThe quantumquantum yieldsyields ofof thethe triplettriplet statestate formationformation ofof thethe testedtested sensitizerssensitizers werewere deter-deter- minedmined fromfrom the the triplet triplet formation formation curves curves using using methodology methodology given given by Lamentby Lament et al. et [64 al.] and[64] byand us by [36 us]. [36]. The methodThe method is based is based on quantitative on quantitative analysis analysis of the of ground the ground state state recovery recovery and itsand deconvolution, its deconvolution, which which concerns concerns an instrumental an instrumental response response function. function. A typical A typical example ex- ofample such of approach such approach is shown is shown in Figure in Figure5, and 5, obtained and obtained data data are summarizedare summarized in Table in Table2. According to Equation (6), the yield of triplet formation (Φ ) is proportional to the ratio 2. According to Equation (6), the yield of triplet formation (ΦT T) is proportional to the ratio of the initial increment in absorbance immediately after the flash (∆At) and the value at of the initial increment in absorbance immediately after the flash (ΔAt) and the value at the maximum ground state depletion (the former is compared to the long delay, “plateau” region) (cf. Figure 6):

Δ𝐴 (6) Φ = ΔA

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Materials 2021, 14, x FOR PEER REVIEWthe maximum ground state depletion (the former is compared to the long delay, “plateau”13 of 20 Materials 2021, 14, x FOR PEER REVIEW 13 of 20 region) (cf. Figure6): ∆At ΦT = (6) ∆Amax

0.00 0.00 ΔA Δ t At -0.01 -0.01

-0.02 -0.02 Absorbance Absorbance -0.03 ΔA Δ max -0.03 Amax 370 nm -0.04 370 nm -0.04 0.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 0.0 2.0x10-6 4.0x10-6 6.0x10-6 8.0x10-6 Time, s Time, s

Figure 6. Analysis of ΔA(t) curve recorded for FN2 in acetonitrile at 370 nm. FigureFigure 6. 6. AnalysisAnalysis of of Δ∆A(t)A(t) curve curve recorded recorded for FN2 in acetonitrile at 370 nm. The comparison of the data collected in Table 2 and illustrated in Figure 7 shows TheThe comparison comparison of of the the data data collected in in Table Table 22 andand illustratedillustrated inin FigureFigure7 7shows shows unequivocally the equivalence of the methods for assessing the quantum yield of the tri- unequivocallyunequivocally the equivalenceequivalence ofof the the methods methods for for assessing assessing the the quantum quantum yield yield of theof the triplet tri- plet state formation and the validity of Equation (5). pletstate state formation formation and and the validitythe validity of Equation of Equation (5). (5).

Equation y = a + b*x FN6 0.9 PlotEquation y = Ca + b*x FN6 0.9 WeightPlot No WeightingC InterceptWeight No-0.00635 Weighting FN7 0.8 SlopeIntercept 0.96877-0.00635 FN7 0.8 ResidualSlope Sum of Sq 6.81334E-40.96877 FN4 Pearson'sResidual Sumr of Sq 6.81334E-40.99909 FN4 0.7 R-Square(COD)Pearson's r 0.998190.99909 0.7 Adj.R-Square(COD) R-Square 0.997730.99819 Adj. R-Square 0.99773 T 0.6 T Φ Φ 1O 0.6 0.8 2 Φ Φ1 Φ O2 2 0.8 T

O Φ 1 0.5 2 0.6 T O Φ 1

0.5 , 0.6 Φ T

, 0.4 Φ FN3 T 0.4

0.4 Φ 0.4 FN3 0.2 FN2 0.2 0.3 FN2 FN8 0.0 0.3 0.0 FN2FN3FN4FN6FN7FN8 FN8 FN2FN3FN4FN6FN7FN8 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.3 0.4 0.51 0.6 0.7 0.8 0.9 1.0 1Φ ΦO2 O 2 FigureFigure 7. 7. ComparisonComparison of of the the quantum quantum yield yield of ofsinglet singlet oxygen oxygen generation generation to tothe the quantum quantum yield yield of of tripletFigure state 7. Comparison formation forof theselected quantum sensitizers. yield of singlet oxygen generation to the quantum yield of triplettriplet state state formation formation for for selected selected sensitizers. sensitizers. FromFrom the the viewpoint viewpoint of of the the photochemical photochemical pr propertiesoperties of of the the tested tested compounds, compounds, the the From the viewpoint of the photochemical properties of the tested compounds, the datadata presented presented in Table 2 2 indicate indicate that that the the structure structure of of the the sensitizer sensitizer tested tested had had a significant a signifi- data presented in Table 2 indicate that the structure of the sensitizer tested had a signifi- cantinfluence influence on the on abilitythe ability to generate to generate singlet single oxygent oxygen and, and, thus, thus, to form to form a triplet a triplet state. state. cant influence on the ability to generate singlet oxygen and, thus, to form a triplet state. IntroductionIntroduction of aa bromidebromide atom atom to to the the phenazine phenazine scaffold scaffold (dye (dye FN2 FN2 vs. FN9) vs. increasedFN9) in- Introduction of a bromide atom to the phenazine scaffold (dye FN2 vs. FN9) in- creasedthe ability the ability to generate to generate singlet singlet oxygen oxygen in the in photochemical the photochemical process. process. Furthermore, Furthermore, the creased the ability to generate singlet oxygen in the photochemical process. Furthermore, thequantum quantum yields yields of tripletof triplet state state formation formation decreased decreased from from about about 0.67 0.67 to 0.51to 0.51 and and to 0.31to 0.31 for the quantum yields of triplet state formation decreased from about 0.67 to 0.51 and to 0.31 fordyes dyes containing containing two two (FN9), (FN9), three three (FN10), (FN10),and and fourfour (FN11) nitrogen nitrogen atoms, atoms, respectively. respectively. for dyes containing two (FN9), three (FN10), and four (FN11) nitrogen atoms, respectively. ThisThis proves proves that that the the modification modification of of the the phen phenazineazine structure structure by by introducing introducing more more nitrogen nitrogen This proves that the modification of the phenazine structure by introducing more nitrogen atoms into the molecule leads to a reduction in the ability of the compounds to sensitize atoms into the molecule leads to a reduction in the ability of the compounds to sensitize singlet oxygen. The ability to generate singlet oxygen by the phenazine derivatives was singlet oxygen. The ability to generate singlet oxygen by the phenazine derivatives was also influenced by the number of conjugated double bonds and the arrangement of aro- also influenced by the number of conjugated double bonds and the arrangement of aro- matic rings (linear and/or angular system). The highest quantum yield of singlet oxygen matic rings (linear and/or angular system). The highest quantum yield of singlet oxygen

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atoms into the molecule leads to a reduction in the ability of the compounds to sensitize Materials 2021, 14, x FOR PEER REVIEW 14 of 20 singlet oxygen. The ability to generate singlet oxygen by the phenazine derivatives was also influenced by the number of conjugated double bonds and the arrangement of aro- matic rings (linear and/or angular system). The highest quantum yield of singlet oxygen productionproduction was was characterized characterized by by phenazine phenazine derivatives derivatives with with angular angular arrangements arrangements of of the the aromaticaromatic rings rings (FN4 (FN4 compared compared to to FN3 FN3 and and FN6 FN6 compared compared to to FN FN 7). 7). SinceSince the the quantum quantum yield yield of of singlet singlet oxygen oxygen generation generation at at a ahigh high excess excess of of oxygen oxygen translatestranslates into into the the quantum quantum yield yield of of the the triplet triplet state formationformation (which(which isis a a desired desired feature feature of ofphotoinitiators), photoinitiators), the the obtained obtained sensitizers sensitizers were were used used to to initiate initiate the the TMPTA TMPTA polymerization. polymeriza- tion.According According to Equationto Equation (7) (7) [67 ,[67,68],68], the the rate rate of of photoinitiated photoinitiated radical radical polymerization polymerization is isdirectly directly proportional proportional to to the the square square root root of of the the quantum quantum yield yield of of triplet triplet state state formation: formation: s (7) 𝑑𝑀d[M] 𝐼𝛷 IaΦT 𝑅 =−Rp = − =𝑘=𝑀kp[M] (7) dt dt 𝑘 kt

where:where: M—theM—the molar molar concentration concentration of the of the monomer; monomer; kp—the polymerization rate constant; kp—the polymerization rate constant; Ia—the absorbed radiation intensity; Ia—the absorbed radiation intensity; φT—the quantum yield of triplet state formation; ϕT—the quantum yield of triplet state formation; t kt—thek —the macroradical macroradical termination termination rate rate constant; constant; The efficiency of the polymerization initiated by phenazine dye-[(3,4-dimethoxy- The efficiency of the polymerization initiated by phenazine dye-[(3,4-dimethoxyphenyl)- phenyl)sulfanyl]acetic acid (MKTFO) systems was assessed from the heat flow during the sulfanyl]acetic acid (MKTFO) systems was assessed from the heat flow during the visible visible light irradiation. Typical kinetic curves obtained for the photoinitiated polymeri- light irradiation. Typical kinetic curves obtained for the photoinitiated polymerization zationof the of TMPTA-MP the TMPTA-MP (9:1) mixture(9:1) mixture recorded recorded for selected for selected dyes dyes are shown are shown in Figures in Figures8 and 98 andfor 9 illustration. for illustration. The ratesThe rate of photoinitiateds of photoinitiated polymerization polymerization determined determined for all for the all tested the testedphotoredox photoredox pairs arepairs collected are collected in Table in2 Table. 2.

0.8

0.6

0.4

laser on Heat evoluted, V 0.2

0.0 0 1020304050 Time, s

FigureFigure 8. 8.FamilyFamily of kinetic of kinetic curves curves reco recordedrded during during the measurements the measurements of the of heat the flow heat emitted flow emitted duringduring the the photoinitiated photoinitiated polymerization polymerization of of the the TMPTA-MP TMPTA-MP mixture mixture initiated initiated by by tested tested dyes— dyes—[(3,4- [(3,4-dimethoxyphenyl)sulfanyl]aceticdimethoxyphenyl)sulfanyl]acetic acid acid couples. couples. The The co-initiator co-initiator concentration concentration was was 0.1 0.1 M, M, and and the thelight light intensity intensity of of the the diode diode laser laser (408 (408 nm) nm) was was 18 18 mW/cm mW/cm2.. The The applied applied dyes dyes possessed possessed various various ⁎ ▲ ■ chromophores:chromophores: —FN4,*—FN4, N—FN2,—FN2, ●•—FN3,—FN3, —FN1.—FN1.

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0.80.8

0.60.6

0.4

0.4

Heat evoluted, V 0.2 laser on Heat evoluted, V 0.2 laser on

0.00.0 00 1020304050 1020304050 Time,Time, ss

FigureFigureFigure 9.9. 9. FamilyFamilyFamily ofof kinetickinetic of kinetic curvescurves curves recoreco recordedrdedrded duringduring during thethe measurementsmeasurements the measurements ofof thethe of heatheat the flowflow heat emittedemitted flow emitted duringduringduring thethe the photoinitiatedphotoinitiated photoinitiated polymerizationpolymerization polymerization ofof of thethe the TMPTA-MPTMPTA-MP TMPTA-MP mixturemixture mixture initiatedinitiated initiated byby testedtested dyes—dyes— dyes—[(3,4- [(3,4-dimethoxyphenyl)sulfanyl]acetic[(3,4-dimethoxyphenyl)sulfanyl]aceticdimethoxyphenyl)sulfanyl]acetic acid acidacid couples. couples.couples. The TheThe co-initiator co-initiatorco-initiator concentration concentrationconcentration was waswas 0.1 0.10.1 M, M,M, and andand the 2 thethelight lightlight intensity intensityintensity of ofof the thethe diode diodediode laser laserlaser (408 (408(408 nm) nm)nm) was waswas 18 1818 mW/cm mW/cmmW/cm22.. TheThe The appliedapplied applied dyesdyes dyes possessedpossessed possessed variousvarious various chromophores:chromophores: ■■—FN2,—FN2, ⁎⁎—FN12,—FN12, ●●—FN8,—FN8, ▲▲—FN13.—FN13. chromophores: —FN2, *—FN12, •—FN8, N—FN13.

The analysis of the presented curves and the Rp values collected in Table 2 indicated TheThe analysis analysis of ofthe the presented presented curves curves and and the theRp valuesRp values collected collected in Table inTable 2 indicated2 indi- thatthatcated thethe that highesthighest the highest efficiencyefficiency efficiency ofof photoinitiatingphotoinitiating of photoinitiating polymerizationpolymerization polymerization waswas characterizedcharacterized was characterized byby thosethose by phenazinephenazinethose phenazine derivativesderivatives derivatives forfor whichwhich for the whichthe quantumquantum the quantum yieldsyields yieldsofof singletsinglet of singlet oxygenoxygen oxygen generationgeneration generation (for-(for- mationmation(formation ofof aa triplettriplet of a triplet state)state) state) werewere were thethe highest.highest. the highest. TheTheThe effectivenesseffectiveness effectiveness ofof of thethe synthesized synthesized dyesdyesdyes in ininthe thethe radical radicalradical polymerization polymepolymerizationrization photoinitiation, photoinitia-photoinitia- p tion,tion,characterized characterizedcharacterized by the byby initial thethe initialinitial rate Rrateratep, was RRp,, was comparedwas comparedcompared to the toto camphorquinone—a thethe camphorquinone—acamphorquinone—a commercial com-com- mercialmercialphotoinitiator photoinitiatorphotoinitiator used in usedused dentistry inin dentistrydentistry (Figure (Figure(Figure 10). 10).10).

1.01.0

0.80.8

0.60.6

0.40.4 laserlaser onon Heat evoluted, V Heat evoluted, V 0.20.2

0.00.0 00 1020304050 1020304050 Time,Time, ss

Figure 10. Family of kinetic curves recorded during polymerization of TMPTA-MP photoinitiated FigureFigure 10.10. FamilyFamily ofof kinetickinetic curvescurves recordedrecorded duringduring polymerizationpolymerization ofof TMPTA-MPTMPTA-MP photoinitiatedphotoinitiated withwith ●•—CQ,—CQ, ■—FN7,—FN7, ▲N—FN9—FN9 in in the the presence presence of of the the MKTFO MKTFO co co-initiator-initiator (0.1 (0.1 M); M); the the light light inten- intensity with ●—CQ, ■—FN7, ▲—FN9 in the presence2 of the MKTFO co-initiator (0.1 M); the light inten- of diode laser (408 nm) was 18 mW/cm . 2. sitysity ofof diodediode laserlaser (408(408 nm)nm) waswas 1818 mW/cmmW/cm2. The results clearly indicated that the initial rate of radical polymerization photoini- TheThe resultsresults clearlyclearly indicatedindicated thatthat thethe initialinitial raterate ofof radicalradical polymerizationpolymerization photoiniti-photoiniti- tiated by phenazine derivatives was comparable to the polymerization rate in which atedated byby phenazinephenazine derivativesderivatives waswas comparablecomparable toto thethe polymerizationpolymerization raterate inin whichwhich com-com- mercialcommercial CQ was CQ used was used as a asphotoinitiator a photoinitiator (Figure (Figure 10). 10 However,). However, the the concentration concentration of of the the mercialsynthesized CQ was dyes used in theas a polymerizing photoinitiator composition (Figure 10). was However, about 2000 the concentration times lower than of the the

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synthesized dyes in the polymerizing composition was about 2000 times lower than the CQ. The CQ molar absorption coefficient at the excitation wavelength (λmax = 408 nm) was λ onlyCQ. ε The = 8 CQM−1 molarcm−1, whereas absorption forcoefficient the synthesized at the excitationdyes, e.g., wavelength FN9 (λmax = (408max nm),= 408 it nm)was wasε = −1 −1 15,400only ε M=− 81cm M−1. cmThis made, whereas it possible for the to synthesized obtain thick dyes, polymer e.g., FN9layers (λ max(4 mm)= 408 that nm), did it not was −1 −1 containε = 15,400 significant M cm amounts. This made (about it possible 0.675 M) to of obtain unreacted thick polymer CQ and layersco-products (4 mm) formed that did duringnot contain the photochemical significant amounts reaction (about (the 0.675radical M) resulting of unreacted from CQ the and dye co-products usually does formed not initiateduring a thechain photochemical reaction but participates reaction (the in radical side reactions). resulting from the dye usually does not initiateAccording a chain to reaction Equation but participates(7), the radicals’ in side formation reactions). takes place in the triplet excited state ofAccording the dye. If to this Equation assumption (7), the is correct radicals’ for formation the tested takes photoinitiator place in the systems, triplet the excited ini- state of the dye. If this assumption is correct for the tested photoinitiator systems, the tial rate (Rp) of TMPTA photoinitiated polymerization should depend linearly on the initial rate (R ) of TMPTA photoinitiated polymerization should depend linearly on the square root of pthe quantum yield of the triplet state. On the basis of the correlation shown square root of the quantum yield of the triplet state. On the basis of the correlation shown in Figure 11, it can be concluded that the electron transfer process from the electron donor in Figure 11, it can be concluded that the electron transfer process from the electron donor (MKTFO) to the photosensitizers (FN1–FN13) occured in the triplet excited state. (MKTFO) to the photosensitizers (FN1–FN13) occured in the triplet excited state.

CQ 200 FN7 FN4 FN6

150 FN9 FN2 FN10 FN8 mol/s FN12 FN11 μ 100 , FN3

p FN5 R

50 FN13 FN1

0 0.0 0.2 0.4 0.6 0.8 1.0 Φ 0,5 T FigureFigure 11. 11. CorrelationCorrelation between between the the rate rate of of photoinitiated photoinitiated polymerization polymerization of of TMPTA TMPTA and and the the square ○ squareroot of root the of quantum the quantum yield ofyield the tripletof the triplet state formation state formation for FN1–FN13 for FN1–FN13 and –CQand in–CQ the in presence the of presence of MKTFO co-initiator (0.1 M); the light intensity of diode laser (408 nm) was 18 mW/cm2. MKTFO co-initiator (0.1 M); the light intensity of diode laser (408 nm) was# 18 mW/cm2.

4.4. Conclusions Conclusions TheThe obtained obtained results results show show that that phenazine phenazine derivatives derivatives are are interesting interesting and and efficient efficient singletsinglet oxygen oxygen sensitizers. sensitizers. In In some some cases, cases, the the quantum quantum yield yield of of generating generating singlet singlet oxygen oxygen waswas close close to to 1.0. 1.0. Increasing Increasing the thenumber number of ni oftrogen nitrogen atoms atoms in the inmodified the modified phenazine phenazine struc- turesstructures clearly clearly decreased decreased the quantum the quantum yield yieldof the oftriplet the triplet state formation. state formation. This proves This proves that thethat modified the modified phenazine phenazine structures structures containing containing large number large number of nitrogen of nitrogen atoms atomsare weak are singletweak singletoxygen oxygen sensitizers. sensitizers. In addition, In addition, the ability the abilityto generate to generate singlet singletoxygen oxygen by the bymod- the ifiedmodified phenazine phenazine structures structures was also was influenced also influenced by the bynumber the number of conjugated of conjugated double doublebonds andbonds the and arrangement the arrangement of aromatic of aromatic rings. rings. Angu Angularlar arrangement arrangement of aromatic of aromatic rings rings in in the the structurestructure of of phenazine phenazine derivatives derivatives guarantees guarantees obtaining obtaining good good singlet singlet oxygen oxygen sensitizers. sensitizers. InIn conditions conditions of of high high oxygen oxygen excess, excess, the the quantum quantum yield yield of of singlet singlet oxygen oxygen generation generation translatestranslates into into the the quantum quantum yield yield of of the the triplet triplet state state formation, formation, which which allows allows the the obtained obtained photosensitizersphotosensitizers to to be be used used in in the the TMPTA TMPTA radical radical polymerization polymerization process. process. The The initiation initiation efficiencyefficiency of of TMPTA TMPTA polymerization polymerization increases increases with with the the increase increase in in the the quantum quantum yield yield of of thethe singlet singlet oxygen oxygen production production and, and, thus, thus, the the qu quantumantum yield yield of of the the triplet triplet state state formation. formation. TheThe greatest greatest application application potential potential reveals reveals compounds compounds having having a he aavy heavy atom atom or an or an- an gularangular arrangement arrangement of the of the rings. rings. The The analysis analysis of ofthe the data data described described in in this this study study and and in in our our previousprevious articles articles [69–71] [69–71] leads toto thethe conclusion conclusion that that the the appropriate appropriate modification modification of theof the dye structure allowed the obtainment of compounds that are not only effective photosensitizers in systems initiating radical polymerization but also sensitizers of singlet oxygen.

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Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/ma14113085/s1, Figure S1. Normalized electronic absorption spectra of phenazine derivatives in ethyl acetate. Figure S2. Normalized fluorescence spectra of phenazine derivatives in ethyl acetate illustrating the influence of dye structure on the bands position; Ex = 366 nm or 404 nm. Figure S3. 1H NMR spectra of a mixture of dibenzo[a.c]phenazine FN2 (sensitizer) and 2,3-diphenyl-p-dioxene in CDCl3 after different irradiation time; dental lamp, 18 mW. Author Contributions: Conceptualization I.P. and Z.K.; synthesized the compounds, performed all experiments, analyzed the obtained data, prepared the Polish version of the article, created the figures, and collected data in tables, I.P.; wrote the English version of the article, corrected some figures and collected data, discussed the results and conclusions, B.J. All authors have read and agreed to the published version of the manuscript. Funding: The financial support of the Ministry of Science and Higher Education Republic of Poland (BN 7/2019) is gratefully acknowledged. Data Availability Statement: The data is provided by Ilona Pyszka (UTP University of Science and Technology), please contact [email protected]. The data is not publicly available apart from the data contained in the article or Supplementary Materials. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Hayyan, M.; Hashim, M.A.; Alnashef, I.M. Superoxide Ion: Generation and Chemical Implications. Chem. Rev. 2016, 116, 3029–3085. [CrossRef][PubMed] 2. Wilkinson, F.; Helman, W.P.; Ross, A.B. Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. J. Phys. Chem. Ref. Data 1993, 22, 113–262. [CrossRef] 3. Olea, A.; Wilkinson, F. Singlet Oxygen Production from Excited Singlet and Triplet States of Anthracene Derivatives in Acetonitrile. J. Phys. Chem. 1995, 99, 4518–4524. [CrossRef] 4. Wilkinson, F.; Helman, W.P.; Ross, A.B. Rate Constants for the Decay and Reactions of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and Revised Compilation. J. Phys. Chem. Ref. Data 1995, 24, 663–677. [CrossRef] 5. Khan, A.U.; Kasha, M. Red Chemiluminescence of Molecular Oxygen in Aqueous Solution. J. Chem. Phys. 1963, 39, 2105. [CrossRef] 6. Seliger, H. A photoelectric method for the measurement of spectra of light sources of rapidly varying intensities. Anal. Biochem. 1960, 1, 60–65. [CrossRef] 7. Foote, C.S.; Wexler, S.; Ando, W.; Higgins, R. Chemistry of singlet oxygen. IV. Oxygenations with hypochlorite-hydrogen peroxide. J. Am. Chem. Soc. 1968, 90, 975–981. [CrossRef] 8. Wasserman, H.H.; Scheffer, J.R. Singlet oxygen reactions from photoperoxides. J. Am. Chem. Soc. 1967, 89, 3073–3075. [CrossRef] 9. Turro, N.J.; Chow, M.-F.; Blaustein, M.A. Generation, diffusivity, and reactivity of singlet oxygen in polymer matrices. A convenient and sensitive chemiluminescent technique. J. Am. Chem. Soc. 1978, 100, 7110–7112. [CrossRef] 10. Steer, R.; Darnall, K.; Pitts, J. The base-induced decomposition of peroxyacetylnitrate. Tetrahedron Lett. 1969, 10, 3765–3767. [CrossRef] 11. Murray, R.W.; Kaplan, M.L. Singlet oxygen sources in ozone chemistry. Chemical oxygenations using the adducts between phosphite esters and ozone. J. Am. Chem. Soc. 1969, 91, 5358–5364. [CrossRef] 12. Wasserman, E.; Murray, R.W.; Kaplan, M.L.; Yager, W.A. Electron paramagnetic resonance of 1.DELTA. oxygen from a phosphite- ozone complex. J. Am. Chem. Soc. 1968, 90, 4160–4161. [CrossRef] 13. Corey, E.J.; Taylor, W.C. A Study of the Peroxidation of Organic Compounds by Externally Generated Singlet Oxygen Molecules. J. Am. Chem. Soc. 1964, 86, 3881–3882. [CrossRef] 14. Foner, S.N.; Hudson, R.L. Metastable Oxygen Molecules Produced by Electrical Discharges. J. Chem. Phys. 1956, 25, 601. [CrossRef] 15. Krieger-Liszkay, A. Singlet oxygen production in photosynthesis. J. Exp. Bot. 2004, 56, 337–346. [CrossRef][PubMed] 16. Harrison, R.M. Pollution: Causes, Effects and Control; Royal Society of Chemistry: Cambridge, UK, 2001. 17. Wentworth, P.; McDunn, J.E.; Wentworth, A.D.; Takeuchi, C.; Nieva, J.; Jones, T.; Bautista, C.; Ruedi, J.M.; Gutierrez, A.; Janda, K.D.; et al. Evidence for Antibody-Catalyzed Ozone Formation in Bacterial Killing and Inflammation. Science 2002, 298, 2195–2199. [CrossRef] 18. Gollnick, K. Type II Photooxygenation Reactions in Solution. In Advances in Photochemistry; Wiley: Hoboken, NJ, USA, 2007; pp. 1–122. 19. Nowakowska, M. Solvent effect on the quantum yield of the self-sensitized photoperoxidation of 1,3-diphenylisobenzofuran. J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases 1984, 80, 2119–2126. [CrossRef] Materials 2021, 14, 3085 18 of 19

20. Kawaoka, K.; Khan, A.U.; Kearns, D.R. Role of Singlet Excited States of Molecular Oxygen in the Quenching of Organic Triplet States. J. Chem. Phys. 1967, 46, 1842–1853. [CrossRef] 21. Gollnick, K.; Schenck, G.O. Mechanism and stereoselectivity of photosensitized oxygen transfer reactions. Pure Appl. Chem. 1964, 9, 507–526. [CrossRef] 22. DeRosa, M.C. Photosensitized singlet oxygen and its applications. Coord. Chem. Rev. 2002, 233–234, 351–371. [CrossRef] 23. Atmaca, G.Y. Investigation of The Differences Between Sono-Photochemical and Photochemical Studies for Singlet Oxygen Generation of Indium Phthalocyanine. Inorg. Chim. Acta 2021, 515, 120052. [CrossRef] 24. P ˛aczkowski,J. Monomeryczne i Polimeryczne Pochodne Rózu˙ Bengalskiego; ATR: Bydgoszcz, Poland, 1988. 25. Schweitzer, C.; Schmidt, R. Physical Mechanisms of Generation and Deactivation of Singlet Oxygen. Chem. Rev. 2003, 103, 1685–1758. [CrossRef] 26. Redmond, R.W.; Gamlin, J.N. A Compilation of Singlet Oxygen Yields from Biologically Relevant Molecules. Photochem. Photobiol. 1999, 70, 391–475. [CrossRef] 27. Juzeniene, A.; Peng, Q.; Moan, J. Milestones in the development of photodynamic therapy and fluorescence diagnosis. Photochem. Photobiol. Sci. 2007, 6, 1234–1245. [CrossRef][PubMed] 28. He, Y.; Liu, S.H.; Yin, J.; Yoon, J. Sonodynamic and chemodynamic therapy based on organic/organometallic sensitizers. Coord. Chem. Rev. 2021, 429, 213610. [CrossRef] 29. Kumar, P.P.P.; Rahman, A.; Goswami, T.; Ghosh, H.N.; Neelakandan, P.P. Fine-Tuning Plasmon-Molecule Interactions in Gold- BODIPY Nanocomposites: The Role of Chemical Structure and Noncovalent Interactions. ChemPlusChem 2021, 86, 87–94. [CrossRef] 30. Atmaca, G.Y. Investigation of singlet oxygen efficiency of di-axially substituted silicon phthalocyanine with sono-photochemical and photochemical studies. Polyhedron 2021, 193, 114894. [CrossRef] 31. Bartusik, D.; Aebisher, D.; Lyons, A.M.; Greer, A. Bacterial Inactivation by a Singlet Oxygen Bubbler: Identifying Factors Controlling the Toxicity of 1O2 Bubbles. Environ. Sci. Technol. 2012, 46, 12098–12104. [CrossRef] 32. Silverman, S.K.; Foote, C.S. Singlet oxygen and electron-transfer mechanisms in the dicyanoanthracene-sensitized photooxidation of 2,3-diphenyl-1,4-dioxene. J. Am. Chem. Soc. 1991, 113, 7672–7675. [CrossRef] 33. Fouassier, J.P.; Allonas, X. Dyes and Chromophores in Polymer Science; John Wiley & Sons: Hoboken, NJ, USA, 2015. 34. Czech, Z.; Kabatc, J.; Bartkowiak, M.; Licbarski, A.; Mozelewska, K.; Kwiatkowska, D. Novel Photoreactive Pressure-Sensitive Adhesives (PSA) Based on Acrylics Containing Additionable Photoinitiators. Materials 2020, 13, 5151. [CrossRef] 35. Balcerak, A.; Kwiatkowska, D.; Iwi´nska,K.; Kabatc, J. Highly efficient UV-Vis light activated three-component photoinitiators composed of tris(trimethylsilyl)silane for polymerization of acrylates. Polym. Chem. 2020, 11, 5500–5511. [CrossRef] 36. Scigalski,´ F.; J˛edrzejewska,B. Structural effect of oxazolone derivatives on the initiating abilities of dye-borate photoredox systems in radical polymerization under visible light. RSC Adv. 2020, 10, 21487–21494. [CrossRef] 37. J˛edrzejewska,B.; Wejnerowska, G. Highly Effective Sensitizers Based on Merocyanine Dyes for Visible Light Initiated Radical Polymerization. Polymers 2020, 12, 1242. [CrossRef] 38. Topa, M.; Hola, E.; Galek, M.; Petko, F.; Pilch, M.; Popielarz, R.; Morlet-Savary, F.; Graff, B.; Lalevée, J.; Ortyl, J. One-component cationic photoinitiators based on coumarin scaffold iodonium salts as highly sensitive photoacid generators for 3D printing IPN photopolymers under visible LED sources. Polym. Chem. 2020, 11, 5261–5278. [CrossRef] 39. Topa, M.; Ortyl, J. Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts. Materials 2020, 13, 4093. [CrossRef][PubMed] 40. Tomal, W.; Chachaj-Brekiesz, A.; Popielarz, R.; Ortyl, J. Multifunctional biphenyl derivatives as photosensitisers in various types of photopolymerization processes, including IPN formation, 3D printing of photocurable multiwalled carbon nanotubes (MWCNTs) fluorescent composites. RSC Adv. 2020, 10, 32162–32182. [CrossRef] 41. Tomal, W.; Pilch, M.; Chachaj-Brekiesz, A.; Galek, M.; Morlet-Savary, F.; Graff, B.; Dietlin, C.; Lalevée, J.; Ortyl, J. Photoinitiator- catalyst systems based on meta-terphenyl derivatives as photosensitisers of iodonium and thianthrenium salts for visible photopolymerization in 3D printing processes. Polym. Chem. 2020, 11, 4604–4621. [CrossRef] 42. Orzeł, A.; Podsiadły, R.; Podemska, K.; Strzelczyk, R.; Koli´nska,J.; Sokołowska, J. Dyes based on the 6,7-dichloro-5,8- quinolinedione skeleton as new type II photoinitiators for radical polymerisation. Color. Technol. 2014, 130, 185–190. [CrossRef] 43. Podemska, K.; Podsiadly, R.; Orzeł, A.; Kowalska, A.; Maruszewska, A.; Marcinek, A.; Sokolowska, J. 6-Pyridinium benzo[a]phenazine-5-oxide derivatives as visible photosensitisers for polymerisation. Color. Technol. 2014, 130, 250–259. [CrossRef] 44. Xiao, P.; Zhang, J.; Dumur, F.; Tehfe, M.A.; Morlet-Savary, F.; Graff, B.; Gigmes, D.; Fouassier, J.P.; Lalevée, J. Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions. Prog. Polym. Sci. 2015, 41, 32–66. [CrossRef] 45. Hinsberg, O. Ueber Chinoxalinbasen. Eur. J. Org. Chem. 1887, 237, 327–372. [CrossRef] 46. Go, A.; Lee, G.; Kim, J.; Bae, S.; Lee, B.M.; Kim, B.H. One-pot synthesis of quinoxalines from reductive coupling of 2-nitroanilines and 1,2-diketones using indium. Tetrahedron 2015, 71, 1215–1226. [CrossRef] 47. Zhang, K.; Dai, Y.; Zhang, X.; Xiao, Y. Synthesis and photophysical properties of three ladder-type chromophores with large and rigid conjugation structures. Dye. Pigment. 2014, 102, 1–5. [CrossRef] Materials 2021, 14, 3085 19 of 19

48. Carlier, L.; Baron, M.; Chamayou, A.; Couarraze, G. Use of co-grinding as a solvent-free solid state method to synthesize dibenzophenazines. Tetrahedron Lett. 2011, 52, 4686–4689. [CrossRef] 49. Kumbhar, A.; Kamble, S.; Barge, M.; Rashinkar, G.; Salunkhe, R. Brönsted acid hydrotrope combined catalyst for environmentally benign synthesis of quinoxalines and pyrido[2,3-b]pyrazines in aqueous medium. Tetrahedron Lett. 2012, 53, 2756–2760. [CrossRef] 50. Zhang, L.; Zuo, Q. A series of blue-green-yellow-red emitting Cu(I) complexes: Molecular structure and photophysical perfor- mance. Spectrochim. Acta Part A 2019, 223, 117280. [CrossRef] 51. Einat, M.; Nagler, A.; Lishner, M.; Amiel, A.; Yarkoni, S.; Rudi, A.; Gellerman, G.; Kashman, Y.; Fabian, I. Potent antileukemic activity of the novel agents norsegoline and dibezine. Clin. Cancer Res. 1995, 1, 823–829. [PubMed] 52. Hussain, H.; Specht, S.; Sarite, S.R.; Saeftel, M.; Hoerauf, A.; Schulz, B.; Krohn, K. A New Class of Phenazines with Activity against a Chloroquine ResistantPlasmodium falciparumStrain and Antimicrobial Activity. J. Med. Chem. 2011, 54, 4913–4917. [CrossRef] 53. Chen, J.; Chen, Y.; Wu, Y.; Wang, X.; Yu, Z.; Xiao, L.; Liu, Y.; Tian, H.; Yao, J.; Fu, H. Modulated emission from dark triplet excitons in aza-acene compounds: Fluorescence versus phosphorescence. New J. Chem. 2017, 41, 1864–1871. [CrossRef] 54. Mamada, M.; Perez-Bolivar, C.; Kumaki, D.; Esipenko, N.A.; Tokito, S.; Anzenbacher, P. Benzimidazole Derivatives: Synthesis, Physical Properties, and n-Type Semiconducting Properties. Chem. A Eur. J. 2014, 20, 11835–11846. [CrossRef] 55. Zhou, C.; Zhang, X.; Pan, G.; Tian, X.; Xiao, S.; Liu, H.; Zhang, S.; Yang, B. Investigation on excited-state properties and electroluminescence performance of Donor−Acceptor materials based on quinoxaline derivatives. Org. Electron. 2019, 75, 105414. [CrossRef] 56. Patil, M.U.; Shinde, S.K.; Patil, S.P.; Patil, S.S. [BBSA-DBN][HSO4]: A novel –SO3H functionalized Bronsted acidic ionic liquid for easy access of quinoxalines. Res. Chem. Intermed. 2020, 46, 4923–4938. [CrossRef] 57. Sahoo, P.K.; Giri, C.; Haldar, T.S.; Puttreddy, R.; Rissanen, K.; Mal, P. Mechanochemical Synthesis, Photophysical Properties, and X-ray Structures of N-Heteroacenes. Eur. J. Org. Chem. 2016, 2016, 1283–1291. [CrossRef] 58. Turner, H.S. 482. The formation of isomeric azo-compounds in the coupling of diazonium salts with 1-naphthylamine. J. Chem. Soc. 1949, 10, 2282–2289. [CrossRef] 59. Otani,¯ S.; Watanabe, S.; Ogino, H.; Iijima, K.; Koitabashi, T. High Modulus Carbon Fibers from Pitch Materials. Bull. Chem. Soc. Jpn. 1972, 45, 3710–3714. [CrossRef] 60. VanAllan, J.A.; Adel, R.E.; Reynolds, G.A. Polynuclear Heterocycles. II. Addition Reactions of Benzophenazines. J. Org. Chem. 1962, 27, 2873–2878. [CrossRef] 61. Roy, N.; Sen, U.; Chaudhuri, S.R.; Muthukumar, V.; Moharana, P.; Paira, P.; Bose, B.; Gauthaman, A.; Moorthy, A. Mitochondria specific highly cytoselective iridium(iii)–Cp* dipyridophenazine (dppz) complexes as cancer cell imaging agents. Dalton Trans. 2021, 50, 2268–2283. [CrossRef] 62. Brouwer, A.M. Standards for photoluminescence quantum yield measurements in solution (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 2213–2228. [CrossRef] 63. Olmsted, J. Calorimetric determinations of absolute fluorescence quantum yields. J. Phys. Chem. 1979, 83, 2581–2584. [CrossRef] 64. Lament, B.; Karpiuk, J.; Waluk, J. Determination of triplet formation efficiency from kinetic profiles of the ground state recovery. Photochem. Photobiol. Sci. 2003, 2, 267–272. [CrossRef][PubMed] 65. Monroe, B.M.; Weiner, S.A. Mechanisms of photochemical reactions in solution. LVIII. Photoreduction of camphorquinone. J. Am. Chem. Soc. 1969, 91, 450–456. [CrossRef] 66. Pyszka, I.; Kucybała, Z.; P ˛aczkowski,J. Reinvestigation of the Mechanism of the Free Radical Polymerization Photoinitiation Process by Camphorquinone-Coinitiator Systems: New Results. Macromol. Chem. Phys. 2004, 205, 2371–2375. [CrossRef] 67. P ˛aczkowski,J.; Pietrzak, M.; Kucybała, Z. Generalization of the Kinetic Scheme for Photoinduced Polymerization via an Intermolecular Electron Transfer Process. 2. Application of the Marcus Theory. Macromolecules 1996, 29, 5057–5064. [CrossRef] 68. Paczkowski, J.; Kucybala, Z. Generalization of the Kinetic Scheme for a Dye-Photosensitized Free-Radical Polymerization Initiating System via an Intermolecular Electron-Transfer Process. Application of Marcus Theory. Macromolecules 1995, 28, 269–273. [CrossRef] 69. Pyszka, I.; J˛edrzejewska,B.; Kucybala, Z. Modified dibenzophenazine structures as photoinitiators for radical polymerization of trimethylolpropane triacrylate: The effect of the number of nitrogen atoms in the structure. Przemysł Chem. 2020, 99, 1605–1609. [CrossRef] 70. Pyszka, I.; J˛edrzejewska,B.; Kucybala, Z. Phenazine derivatives as dye photoinitiators of radical polymerization of trimethylol- propane triacrylate: Impact of quantum yield of triplet state formation on photoinitiation efficiency. Przemysł Chem. 2020, 99, 691–695. [CrossRef] 71. Pyszka, I.; Kucybala, Z. Benzo- and dibenzoderivatives of phenazine as dye photoinitiators of radical polymerization of trimethylolpropane triacrylate: The substituent effect in the photoredox pair. Przemysł Chem. 2019, 98, 1290–1294. [CrossRef]