Electropolymerization on ITO-Coated Glass Slides of a Series of π-Extended BODIPY Dyes with Redox-Active Meso-Substituents

A series of meso-carbazole and meso-pyrene boron dipyrromethene(BDP) dyes have been synthesized using a two-step method. This simplified synthetic method did not require catalysts or oxidizing agents. Solution spectroscopic and electrochemical studies indicate that the HOMO and LUMO energies are dependent on the extent of π-conjugation associated with the pyrroles. Solution electrochemistry of the dyes in chloroform reveal film formation onto glassy carbon electrodes. Electrolysis of chloroform solutions of the dyes using indium tin oxide (ITO) glass slides as the working electrode show, using UV/vis spectroscopy, the formation of films. For two of the dyes, the BODIPY structure stays in tact upon electrolysis, exhibiting sharp absorption peaks on the ITO slides similar to that observed for the same dyes in solution.


Introduction
The fortuitous discovery of boron dipyrromethene (BDP) dyes by Triebs and Kruezer [1] in the late 1960s has, in the past two decades, received considerable attention, leading to tens of thousands of new BDP dyes.Molar absorptivities, often greater than 100,000 M −1 cm −1 , with near-unit quantum yields are two intrinsic properties that have contributed to the ever-growing interest in BDP dyes.Post-functionalization of BDP dyes has engendered bioactivity [2], catalytic properties [3], and has expanded their spectroscopic properties into the near-infrared (NIR) region of the electromagnetic spectrum [4].Many BDP dyes are commercially available, primarily for use as fluorescent dyes for cellular organelles.Given their intense spectroscopic properties, these dyes have also been adapted for use as phototherapeutics [5] and, to a lesser extent, in dye-sensitized solar cells (DSSCs) [6].
DSSCs were first developed in the late 1980s, capturing a great deal of interest due to their relatively easy construction [7].Many of the photosensitizers designed to work in DSSCs contain substituents like carboxylic acid groups to allow for covalent attachment and therefore increase solar conversion [8][9][10][11][12][13][14].In principle TiO 2 powder is coated with the photosensitizer, typically by adsorption, and the photosensitizer-TiO 2 particles are placed on conducting transparent slides followed by heating prior to construction of the cell.Upon light irradiation, electrons from the highest occupied molecular orbital (HOMO) of the photosensitizer are transferred into the lowest energy molecular orbital (LUMO), leading to electron injection into the conduction band of the TiO 2 .Electron transfer from a redox cycler, for example, I 3 − /I − , regenerates the electronic structure of the dye (Scheme 1).One of the first reports of a BDP dye for DSSCs demonstrated the robust photostability of this class of dyes; however, low photocurrent production compared to known solar cell dyes was observed [15].Subsequent studies have taken advantage of the post-functionalization diversity of BDP dyes to generate donor-acceptor photosensitizers for DSSCs [16,17].A number of examples of the electropolymerization of photosensitizers on indium tin oxide (ITO) glass slides have been reported [18][19][20][21].This method has been shown to generate a more stable ITO film compared to spin-coating with the hope of generating efficient solar cells.To our knowledge, this technique has not been applied to BDP dyes.This report describes the straightforward synthesis of three meso-phenyl carbazole BDP dyes with varying degrees of pyrrole conjugation, Figure 1.Their solution phase spectroscopic and electrochemical properties are described.The ability of the dyes to electropolymerize onto ITO-coated glass slides is investigated by UV/vis spectroscopy.A fourth isoquinol-pyrene BDP dye is also investigated for its ability to electropolymerize onto ITO glass slides.

Synthesis and Spectroscopy
For the synthesis of BDP1, a mechanochemical technique [22] was used in which the commercially available N-(4-formylphenyl) carbazole and 2,4-dimethyl pyrrole were combined and mixed using a mortar and pestle with the addition of a few drops of trifluoroacetic acid followed by the addition of the oxidizing agent of p-chloranil.After 1 min of mixing, triethylamine followed by boron trifuoride etherate were added with A number of examples of the electropolymerization of photosensitizers on indium tin oxide (ITO) glass slides have been reported [18][19][20][21].This method has been shown to generate a more stable ITO film compared to spin-coating with the hope of generating efficient solar cells.To our knowledge, this technique has not been applied to BDP dyes.This report describes the straightforward synthesis of three meso-phenyl carbazole BDP dyes with varying degrees of pyrrole conjugation, Figure 1.Their solution phase spectroscopic and electrochemical properties are described.The ability of the dyes to electropolymerize onto ITO-coated glass slides is investigated by UV/vis spectroscopy.A fourth isoquinol-pyrene BDP dye is also investigated for its ability to electropolymerize onto ITO glass slides.A number of examples of the electropolymerization of photosensitizers on indium tin oxide (ITO) glass slides have been reported [18][19][20][21].This method has been shown to generate a more stable ITO film compared to spin-coating with the hope of generating efficient solar cells.To our knowledge, this technique has not been applied to BDP dyes.This report describes the straightforward synthesis of three meso-phenyl carbazole BDP dyes with varying degrees of pyrrole conjugation, Figure 1.Their solution phase spectroscopic and electrochemical properties are described.The ability of the dyes to electropolymerize onto ITO-coated glass slides is investigated by UV/vis spectroscopy.A fourth isoquinol-pyrene BDP dye is also investigated for its ability to electropolymerize onto ITO glass slides.

Synthesis and Spectroscopy
For the synthesis of BDP1, a mechanochemical technique [22] was used in which the commercially available N-(4-formylphenyl) carbazole and 2,4-dimethyl pyrrole were combined and mixed using a mortar and pestle with the addition of a few drops of trifluoroacetic acid followed by the addition of the oxidizing agent of p-chloranil.After 1 min of mixing, triethylamine followed by boron trifuoride etherate were added with

Synthesis and Spectroscopy
For the synthesis of BDP1, a mechanochemical technique [22] was used in which the commercially available N-(4-formylphenyl) carbazole and 2,4-dimethyl pyrrole were combined and mixed using a mortar and pestle with the addition of a few drops of trifluoroacetic acid followed by the addition of the oxidizing agent of p-chloranil.After 1 min of mixing, triethylamine followed by boron trifuoride etherate were added with additional grinding for 1 min.Chromatography of the reaction mixture yielded BDP1 with a 28% yield (Scheme 2).This reaction methodology requires about 10 min to complete with yields similar to that achieved by longer, more traditional methods.
additional grinding for 1 min.Chromatography of the reaction mixture yielded BDP1 with a 28% yield (Scheme 2).This reaction methodology requires about 10 min to complete with yields similar to that achieved by longer, more traditional methods.
This laboratory has described a two-step synthetic route for BDP dyes containing highly conjugated pyrroles [23].This solvent-free reaction does not require an oxidizing agent to produce the dipyrrin and can be completed in approximately 30 min.The appropriate pyrrole (naphthyl pyrrole or fluoranthene pyrrole) with N-(4-formylphenyl) carbazole was placed in a round bottom flask and dissolved in a minimum of chloroform to make a homogeneous solution.The solvent was subsequently removed under reduced pressure with heating for 1 min, generating a deep purple/red paste.The flask containing the reaction mixture was placed in an oil bath set at 80 °C with a nitrogen purge while dry toluene was added followed by triethylamine and boron trifluoride etherate.BDP2 and BDP3 were isolated by column chromatography with yields of 16.5% and 10.0%, respectively, Scheme 3.This laboratory has described a two-step synthetic route for BDP dyes containing highly conjugated pyrroles [23].This solvent-free reaction does not require an oxidizing agent to produce the dipyrrin and can be completed in approximately 30 min.The appropriate pyrrole (naphthyl pyrrole or fluoranthene pyrrole) with N-(4-formylphenyl) carbazole was placed in a round bottom flask and dissolved in a minimum of chloroform to make a homogeneous solution.The solvent was subsequently removed under reduced pressure with heating for 1 min, generating a deep purple/red paste.The flask containing the reaction mixture was placed in an oil bath set at 80 • C with a nitrogen purge while dry toluene was added followed by triethylamine and boron trifluoride etherate.BDP2 and BDP3 were isolated by column chromatography with yields of 16.5% and 10.0%, respectively, Scheme 3.
with yields similar to that achieved by longer, more traditional methods.This laboratory has described a two-step synthetic route for BDP dyes containing highly conjugated pyrroles [23].This solvent-free reaction does not require an oxidizing agent to produce the dipyrrin and can be completed in approximately 30 min.The appropriate pyrrole (naphthyl pyrrole or fluoranthene pyrrole) with N-(4-formylphenyl) carbazole was placed in a round bottom flask and dissolved in a minimum of chloroform to make a homogeneous solution.The solvent was subsequently removed under reduced pressure with heating for 1 min, generating a deep purple/red paste.The flask containing the reaction mixture was placed in an oil bath set at 80 °C with a nitrogen purge while dry toluene was added followed by triethylamine and boron trifluoride etherate.BDP2 and BDP3 were isolated by column chromatography with yields of 16.5% and 10.0%, respectively, Scheme 3.  UV/vis spectroscopy of BDP dyes shows strong absorption in the visible region of the electromagnetic spectrum resulting from HOMO (π) to LUMO (π*) transitions localized on the dipyrrin core with BF 2 serving to align the aromatic pyrrole units.The meso-substituent typically plays a limited role in the energy of the absorption since these substituents are orthogonal to the dipyrrin core.Therefore, shifting the absorption to lower energy often requires post-functionalization of the BDP dye, adding synthetic steps and decreasing the yield [24].Our methodology allows for predictable bathochromic shifts in the spectrum as a function of the degree of conjugation of the pyrroles used.The electronic spectra of the carbazole-substituted BDP dyes show intense absorption peaks at 504 nm (BDP1, red), 604 nm (BDP2, blue), and 644 nm (BDP3, violet), consistent with the extent of their dipyrrin conjugation, Figure 2A.Molar absorptivities of the dyes range from 66,760 M −1 cm −1 , 55,480 M −1 cm −1 , and 43,340 M −1 cm −1 for BDP1, BDP2, and BDP3, respectively.Excitation at their absorption maxima show sharp intense emission (Figure 2B) with peaks at 514 nm (BDP1, red), 611 nm (BDP2, blue), and 649 nm (BDP3, violet) with quantum yields of 0.48, 0.47, and 0.37, respectively.

R PEER REVIEW 4 of 15
UV/vis spectroscopy of BDP dyes shows strong absorption in the visible region of the electromagnetic spectrum resulting from HOMO (π) to LUMO (π*) transitions localized on the dipyrrin core with BF2 serving to align the aromatic pyrrole units.The meso-substituent typically plays a limited role in the energy of the absorption since these substituents are orthogonal to the dipyrrin core.Therefore, shifting the absorption to lower energy often requires post-functionalization of the BDP dye, adding synthetic steps and decreasing the yield [24].Our methodology allows for predictable bathochromic shifts in the spectrum as a function of the degree of conjugation of the pyrroles used.The electronic spectra of the carbazole-substituted BDP dyes show intense absorption peaks at 504 nm (BDP1, red), 604 nm (BDP2, blue), and 644 nm (BDP3, violet), consistent with the extent of their dipyrrin conjugation, Figure 2A.Molar absorptivities of the dyes range from 66,760 M −1 cm −1 , 55,480 M −1 cm −1 , and 43,340 M −1 cm −1 for BDP1, BDP2, and BDP3, respectively.Excitation at their absorption maxima show sharp intense emission (Figure 2B) with peaks at 514 nm (BDP1, red), 611 nm (BDP2, blue), and 649 nm (BDP3, violet) with quantum yields of 0.48, 0.47, and 0.37, respectively.

Solution Cyclic Voltammetry
Solution cyclic voltammetry was performed on chloroform solutions of the BDP dyes using a three-electrode one-compartment cell with a glassy carbon working electrode, platinum wire auxiliary electrode, and a non-aqueous Ag/Ag + reference electrode.Scanning in the anodic direction, all three dyes display an intense oxidation wave with an Epa of 1.35 V, 1.31 V, and 1.31 V vs. Ag/Ag + for BDP1, BDP2, and BDP3, respectively.These oxidation waves are coupled to a weaker reduction wave at 0.99 V, 1.18 V, and 1.20 V vs. Ag/Ag + , Figure 3A-C.BDP2 and BDP3 show an additional reduction wave at 0.88 V and 0.92 V, respectively.In the cathodic direction, reversible redox waves with E1/2 values −1.24 V (ΔE 70 mV), −0.92 V (ΔE 60 mV), and −0.76 V (ΔE 70 mV) for BDP1, BDP2, and BDP3, respectively, which are highlighted in Figure 3D where BDP1 (blue), BDP2 (red), and BDP3 (green).The one-electron reversible cathodic redox couple for BDP1, BDP2 and BDP3 shows a shift to less negative values, respectively, consistent with the increasing conjugation of the dyes; therefore, these redox couples are assigned as the reduction and oxidation of the LUMO localized on the dipyrrin core, in agreement with the spectroscopic data.The similarity of the irreversible oxidation waves for the three dyes is likely due to the oxidation of the meso-substituent carbazole.

Solution Cyclic Voltammetry
Solution cyclic voltammetry was performed on chloroform solutions of the BDP dyes using a three-electrode one-compartment cell with a glassy carbon working electrode, platinum wire auxiliary electrode, and a non-aqueous Ag/Ag + reference electrode.Scanning in the anodic direction, all three dyes display an intense oxidation wave with an E pa of 1.35 V, 1.31 V, and 1.31 V vs. Ag/Ag + for BDP1, BDP2, and BDP3, respectively.These oxidation waves are coupled to a weaker reduction wave at 0.99 V, 1.18 V, and 1.20 V vs. Ag/Ag + , Figure 3A-C.BDP2 and BDP3 show an additional reduction wave at 0.88 V and 0.92 V, respectively.In the cathodic direction, reversible redox waves with E 1/2 values −1.24 V (∆E 70 mV), −0.92 V (∆E 60 mV), and −0.76 V (∆E 70 mV) for BDP1, BDP2, and BDP3, respectively, which are highlighted in Figure 3D where BDP1 (blue), BDP2 (red), and BDP3 (green).The one-electron reversible cathodic redox couple for BDP1, BDP2 and BDP3 shows a shift to less negative values, respectively, consistent with the increasing conjugation of the dyes; therefore, these redox couples are assigned as the reduction and oxidation of the LUMO localized on the dipyrrin core, in agreement with the spectroscopic data.The similarity of the irreversible oxidation waves for the three dyes is likely due to the oxidation of the meso-substituent carbazole.Scanning repeatedly in the anodic direction led to the formation of a film on the glassy carbon electrode resulting from electro-polymerization of the carbazole moieties [25].Once the electro-oxidative film of BDP1, BDP2, and BDP3 dyes was formed on the glassy carbon electrodes, they were removed and dipped in chloroform.The electrodes were then placed into chloroform solutions containing only supporting electrolyte (0.1 M TBAPF6) and cycled between 1.2 V and −1.5 V, Figure 3.The redox couples observed in the anodic region are indicative of adsorbed reversible electron transfer [26].All three adsorbed dyes show anodic redox couples (Figure 4A-C) at 0.98 V vs. Ag/Ag + with reduction waves at −1.26 V (BDP1), −1.06 V (BDP2), and −0.88 V (BDP3), consistent with increasing π-conjugation, respectively.Scanning repeatedly in the anodic direction led to the formation of a film on the glassy carbon electrode resulting from electro-polymerization of the carbazole moieties [25].Once the electro-oxidative film of BDP1, BDP2, and BDP3 dyes was formed on the glassy carbon electrodes, they were removed and dipped in chloroform.The electrodes were then placed into chloroform solutions containing only supporting electrolyte (0.1 M TBAPF 6 ) and cycled between 1.2 V and −1.5 V, Figure 3.The redox couples observed in the anodic region are indicative of adsorbed reversible electron transfer [26].All three adsorbed dyes show anodic redox couples (Figure 4A-C) at 0.98 V vs. Ag/Ag + with reduction waves at −1.26 V (BDP1), −1.06 V (BDP2), and −0.88 V (BDP3), consistent with increasing π-conjugation, respectively.
The CVs illustrated in Figure 4 show an overlay of the ferrocene redox couple run under the same conditions.Using the Fc/Fc + redox couple, the HOMO/LUMO energies associated with each of the dyes can be calculated [27], resulting in a qualitative Jablonski diagram, Figure 5.The HOMO orbitals are linked to the anodic redox couple resulting from the electrooxidative polymerization of the carbazole moieties common to all three dyes and therefore occurring at the same energies.The LUMO orbitals are localized on the dipyrrin core of the BDP dyes and their energies are determined from the cathodic reduction waves, Figure 4A-C  The CVs illustrated in Figure 4 show an overlay of the ferrocene redox coup under the same conditions.Using the Fc/Fc + redox couple, the HOMO/LUMO en associated with each of the dyes can be calculated [27], resulting in a qualitative Jab  Generation of polycarbazoles has been studied for several decades.Chemical polymerizations of carbazoles have been performed by using various oxidizing agents [28].The electrochemical oxidation of carbazoles has been extensively studied due to their ability to form conductive films on platinum, glassy carbon, and ITO surfaces [29][30][31][32].A vast array of applications for electrochemically generated conductive polycarbazole films has been noted [33][34][35][36][37][38][39][40][41][42][43][44].While there exist a number of mechanisms for the electrochemical formation of polycarbazoles, it is generally agreed that an initial one-electron oxidation generates a radical with the 3,6-positions of the carbazole, representing the most reactive positions, inset-red Scheme 4, resulting in a chain reaction culminating in the formation of polycarbazole films [38].Employing suggested polycarbazole film structures, Scheme 4 illustrates a proposed polycarbazole-BDP film formed via the oxidation of BDP1 with a similar structure resulting from the oxidative electropolymerization of BDP2 and BDP3.Scheme 4. Proposed structure of electropolymerized BDP1 on ITO glass slides.

ITO Film Formation of BDP1-BDP3
Square ITO-coated glass slides were placed in millimolar solutions of BDP1, BDP2, and BDP3 containing TBAPF6 as a supporting electrolyte with a platinum flag auxiliary Generation of polycarbazoles has been studied for several decades.Chemical polymerizations of carbazoles have been performed by using various oxidizing agents [28].The electrochemical oxidation of carbazoles has been extensively studied due to their ability to form conductive films on platinum, glassy carbon, and ITO surfaces [29][30][31][32].A vast array of applications for electrochemically generated conductive polycarbazole films has been noted [33][34][35][36][37][38][39][40][41][42][43][44].While there exist a number of mechanisms for the electrochemical formation of polycarbazoles, it is generally agreed that an initial one-electron oxidation generates a radical with the 3,6-positions of the carbazole, representing the most reactive positions, inset-red Scheme 4, resulting in a chain reaction culminating in the formation of polycarbazole films [38].Employing suggested polycarbazole film structures, Scheme 4 illustrates a proposed polycarbazole-BDP film formed via the oxidation of BDP1 with a similar structure resulting from the oxidative electropolymerization of BDP2 and BDP3.
reduction waves, Figure 4A-C.The associated energies of the LUMOs vary as a function of the increasing π-conjugation going from BDP1 to BDP3.Generation of polycarbazoles has been studied for several decades.Chemical polymerizations of carbazoles have been performed by using various oxidizing agents [28].The electrochemical oxidation of carbazoles has been extensively studied due to their ability to form conductive films on platinum, glassy carbon, and ITO surfaces [29][30][31][32].A vast array of applications for electrochemically generated conductive polycarbazole films has been noted [33][34][35][36][37][38][39][40][41][42][43][44].While there exist a number of mechanisms for the electrochemical formation of polycarbazoles, it is generally agreed that an initial one-electron oxidation generates a radical with the 3,6-positions of the carbazole, representing the most reactive positions, inset-red Scheme 4, resulting in a chain reaction culminating in the formation of polycarbazole films [38].Employing suggested polycarbazole film structures, Scheme 4 illustrates a proposed polycarbazole-BDP film formed via the oxidation of BDP1 with a similar structure resulting from the oxidative electropolymerization of BDP2 and BDP3.Scheme 4. Proposed structure of electropolymerized BDP1 on ITO glass slides.

ITO Film Formation of BDP1-BDP3
Square ITO-coated glass slides were placed in millimolar solutions of BDP1, BDP2, and BDP3 containing TBAPF6 as a supporting electrolyte with a platinum flag auxiliary

ITO Film Formation of BDP1-BDP3
Square ITO-coated glass slides were placed in millimolar solutions of BDP1, BDP2, and BDP3 containing TBAPF 6 as a supporting electrolyte with a platinum flag auxiliary electrode and a nonaqueous Ag/Ag + reference electrode.Figure 6, left, illustrates cyclic voltammograms of these solutions from 0 V to 1.5 V for 10 cycles.The CV's in Figure 6 with square ITO glass slides as the working electrode show the buildup of the BDP films.
To obtain a detectable film on the ITO slides by UV/vis spectroscopy, bulk electrolysis of the mM solutions was performed at 1.5 V vs. Ag/Ag + reference electrode for 100 s, at which point, the slides were rinsed with chloroform and air-dried.UV/vis spectra of the ITO-films for the respective dyes are illustrated in Figure 6 (right).The UV/vis spectrum of the ITO-BDP1-coated glass slide shows two broad absorption bands with peaks at 440 nm and 530 nm.The absorption spectra of ITO slides electrocoated with BDP2 and BDP3 show similar broad bands with peaks at 440 nm but do not show any additional absorption bands.
electrode and a nonaqueous Ag/Ag + reference electrode.Figure 6, left, illustrates cyclic voltammograms of these solutions from 0 V to 1.5 V for 10 cycles.The CV's in Figure 6 with square ITO glass slides as the working electrode show the buildup of the BDP films.
To obtain a detectable film on the ITO slides by UV/vis spectroscopy, bulk electrolysis of the mM solutions was performed at 1.5 V vs. Ag/Ag + reference electrode for 100 s, at which point, the slides were rinsed with chloroform and air-dried.UV/vis spectra of the ITOfilms for the respective dyes are illustrated in Figure 6 (right).The UV/vis spectrum of the ITO-BDP1-coated glass slide shows two broad absorption bands with peaks at 440 nm and 530 nm.The absorption spectra of ITO slides electrocoated with BDP2 and BDP3 show similar broad bands with peaks at 440 nm but do not show any additional absorption bands.To better understand the discrepancies of the UV/vis spectra of the ITO-coated slides, chloroform solutions of N-(4-formylphenyl) carbazole were bulk-electrolyzed at 2.0 V vs. a nonaqueous Ag/Ag + reference electrode utilizing an ITO glass slide as the working electrode.After 100 s of electrolysis, the ITO glass slide was removed from the solution, thoroughly rinsed with chloroform and air-dried.To better understand the discrepancies of the UV/vis spectra of the ITO-coated slides, chloroform solutions of N-(4-formylphenyl) carbazole were bulk-electrolyzed at 2.0 V vs. a nonaqueous Ag/Ag + reference electrode utilizing an ITO glass slide as the working electrode.After 100 s of electrolysis, the ITO glass slide was removed from the solution, thoroughly rinsed with chloroform and air-dried.
The UV/vis spectrum, Figure 7, shows an absorption peak at 440 nm, identical in shape and peak absorption to the peaks observed after electrolysis of BDP1, BDP2, and BDP3 from Figure 6.It is logical to conclude that the absorption bands at 440 nm observed for BDP1-BDP3 ITO-coated slides result from polycarbazole.In the case of the ITO-BDP1 slide, the absorption band at 530 nm is the result of surface-confined BDP1 dye, consistent with the solution absorption spectrum of BDP1.In contrast, the ITO-BDP2 and ITO-BDP3 electrocoated slides do not show any evidence for BDP2 and BDP3 which should show absorption bands near 600 and 640 nm, respectively.One possibility is that the more highly conjugated BDP dyes (e.g., naphthyl and fluoranthene pyrrole units) may undergo electrooxidation at the potential applied to oxidize the carbazole moieties, resulting in the decomposition of the BDP structure.
Molecules 2023, 28, x FOR PEER REVIEW 9 of 15 The UV/vis spectrum, Figure 7, shows an absorption peak at 440 nm, identical in shape and peak absorption to the peaks observed after electrolysis of BDP1, BDP2, and BDP3 from Figure 6.It is logical to conclude that the absorption bands at 440 nm observed for BDP1-BDP3 ITO-coated slides result from polycarbazole.In the case of the ITO-BDP1 slide, the absorption band at 530 nm is the result of surface-confined BDP1 dye, consistent with the solution absorption spectrum of BDP1.In contrast, the ITO-BDP2 and ITO-BDP3 electrocoated slides do not show any evidence for BDP2 and BDP3 which should show absorption bands near 600 and 640 nm, respectively.One possibility is that the more highly conjugated BDP dyes (e.g., naphthyl and fluoranthene pyrrole units) may undergo electrooxidation at the potential applied to oxidize the carbazole moieties, resulting in the decomposition of the BDP structure.

Investigation of a Pyrene-Substituted Isoquinol BDP Dye
While BDP1 showed the desired ability to polymerize onto ITO slides maintaining the BDP photo-properties, this was not the case with BDP2 and BDP3 due to a proposed breakdown of the BDP-core upon electrooxidation.In a previous study, this laboratory has reported a series of isoquinol-based BDP dyes [45].In particular, one of these dyes showed evidence of film formation on glassy carbon electrodes upon anodic scanning in chloroform solutions.The structure of this BDP dye (BDP4), illustrated in Figure 8, incorporates highly conjugated pyrene moieties.Polypyrene resulting from anodic electrochemistry has been well documented [46][47][48].The cyclic voltammetry of BDP4 in chloroform, Figure 8A, shows that, when scanned in the anodic direction, the current begins to go offscale.Upon scanning between 0.0 V and 1.5 V vs. the Ag/Ag + reference electrode for several cycles, the glassy carbon electrode was removed from the BDP4/TBAP6 chloroform, rinsed with chloroform, placed in a 0.1 M TBAPF6 chloroform solution and cycled between 2.0 V and −1.5 V vs. the Ag/Ag + reference electrode, Figure 8B.The coated glassy carbon electrode displays an irreversible oxidation at 1.70 V followed by two irreversible reduction waves with the most pronounced at −0.40 V vs. the Ag/Ag + reference electrode.

Investigation of a Pyrene-Substituted Isoquinol BDP Dye
While BDP1 showed the desired ability to polymerize onto ITO slides maintaining the BDP photo-properties, this was not the case with BDP2 and BDP3 due to a proposed breakdown of the BDP-core upon electrooxidation.In a previous study, this laboratory has reported a series of isoquinol-based BDP dyes [45].In particular, one of these dyes showed evidence of film formation on glassy carbon electrodes upon anodic scanning in chloroform solutions.The structure of this BDP dye (BDP4), illustrated in Figure 8, incorporates highly conjugated pyrene moieties.Polypyrene resulting from anodic electrochemistry has been well documented [46][47][48].The cyclic voltammetry of BDP4 in chloroform, Figure 8A, shows that, when scanned in the anodic direction, the current begins to go off-scale.Upon scanning between 0.0 V and 1.5 V vs. the Ag/Ag + reference electrode for several cycles, the glassy carbon electrode was removed from the BDP4/TBAP 6 chloroform, rinsed with chloroform, placed in a 0.1 M TBAPF 6 chloroform solution and cycled between 2.0 V and −1.5 V vs. the Ag/Ag + reference electrode, Figure 8B.The coated glassy carbon electrode displays an irreversible oxidation at 1.70 V followed by two irreversible reduction waves with the most pronounced at −0.40 V vs. the Ag/Ag + reference electrode.
From these observations, mM solutions of BDP4 in 0.1 M TBAPF 6 chloroform containing a square ITO-coated glass slide as working electrode, platinum flag as auxiliary, and a non-aqueous Ag/Ag + reference electrode were electrolyzed at 1.5 V for 100 s.The ITO slide was removed, rinsed with chloroform, and air-dried.The UV/vis spectrum of the BDP4 coated ITO slide shows a broad absorption band at 440 nm and a sharper lower energy absorption band at approximately 600 nm, Figure 9 blue line.The absorption spectrum of a chloroform solution of BDP4 is overlayed, Figure 9 red line, suggesting that the anodic film formation on ITO slides maintain absorption properties and, by extension, the structure of the dye.From these observations, mM solutions of BDP4 in 0.1 M TBAPF6 chloroform containing a square ITO-coated glass slide as working electrode, platinum flag as auxiliary, and a non-aqueous Ag/Ag + reference electrode were electrolyzed at 1.5 V for 100 s.The ITO slide was removed, rinsed with chloroform, and air-dried.The UV/vis spectrum of the BDP4 coated ITO slide shows a broad absorption band at 440 nm and a sharper lower energy absorption band at approximately 600 nm, Figure 9 blue line.The absorption spectrum of a chloroform solution of BDP4 is overlayed, Figure 9 red line, suggesting that the anodic film formation on ITO slides maintain absorption properties and, by extension, the structure of the dye.For comparison, an overlay of the spectrum of electorpolymerized BDP1 on an ITO slide (Figure 10 blue) and the electropolymerized BDP4 on an ITO slide (Figure 10 red) illustrates the region of the electromagnetic spectrum which can be covered by these films.For comparison, an overlay of the spectrum of electorpolymerized BDP1 on an ITO slide (Figure 10 blue) and the electropolymerized BDP4 on an ITO slide (Figure 10 red) illustrates the region of the electromagnetic spectrum which can be covered by these films.The objective of this study was to explore whether BDP dyes affixed with redox-active substituents could be electro-coated onto ITO slides without destroying the photophysical structure of the dyes.Solution cyclic voltammetry of the dyes revealed film formation on glassy carbon electrodes.However, when electropolymerized onto ITO slides, BDP2 and BDP3 clearly form films but lose their associated BDP absorption peaks.A potential explanation may be due to the ability of the extended aromatic pyrroles of BDP2 and BDP3 (i.e., naphthyl or fluoranthene) to undergo an oxidative process within the same potential window of the oxidation of the carbazole moiety.This leads to film formation but may also result in decomposition of the BDP-core.This may explain the differences observed for the solution cyclic voltammograms of BDP1, BDP2, and BDP3 (Figure 3).For BDP1, the reduction of the dipyrrin core in the cathodic region, a one-electron process, shows approximately 1/3 the current intensity of the oxidation of the carbazole, a threeelectron process, in the anodic region.On the other hand, the current intensity of the oxidative process observed for BDP2 and BDP3 is considerably greater than the cathodic redox process, suggesting something other than the carbazole is being oxidized.Our contention is that the additional oxidative process is due to the naphtyl (BDP2) and fluoranthene (BDP3) dipyrrin core ultimately leading to decomposition of the BDP dyes.The objective of this study was to explore whether BDP dyes affixed with redox-active substituents could be electro-coated onto ITO slides without destroying the photophysical structure of the dyes.Solution cyclic voltammetry of the dyes revealed film formation on glassy carbon electrodes.However, when electropolymerized onto ITO slides, BDP2 and BDP3 clearly form films but lose their associated BDP absorption peaks.A potential explanation may be due to the ability of the extended aromatic pyrroles of BDP2 and BDP3 (i.e., naphthyl or fluoranthene) to undergo an oxidative process within the same potential window of the oxidation of the carbazole moiety.This leads to film formation but may also result in decomposition of the BDP-core.This may explain the differences observed for the solution cyclic voltammograms of BDP1, BDP2, and BDP3 (Figure 3).For BDP1, the reduction of the dipyrrin core in the cathodic region, a one-electron process, shows approximately 1/3 the current intensity of the oxidation of the carbazole, a three-electron process, in the anodic region.On the other hand, the current intensity of the oxidative process observed for BDP2 and BDP3 is considerably greater than the cathodic redox process, suggesting something other than the carbazole is being oxidized.Our contention is that the additional oxidative process is due to the naphtyl (BDP2) and fluoranthene (BDP3) dipyrrin core ultimately leading to decomposition of the BDP dyes.
In contrast, ITO glass slides anodically coated with BDP1 show a distinct BDP-like absorption band similar in shape and energy to that observed for BDP1 in solution, Figure 2 red and Figure 10 blue, respectively.A similar process is observed for BDP4 coated ITO glass slides (Figure 10 red) resulting from anodic oxidation of the pyrene moieties.The ITO-coated slides are stable, exhibiting similar spectroscopic properties for several days.It is apparent that while this method shows promise for affixing BDP dyes to ITO slides, judicious choice of the redox-active meso-substituents is needed to ensure that the BDP-core is unaffected by the oxidative process.

General Procedures
All chemicals were reagent grade and used without further purification.Naphtha[1,2c]pyrrole, fluorantho[2,3-c]pyrrole [49] and BDP4 [45] were synthesized as previously described.Chromatography was performed on a Teledyne Combiflash Rf + equipped with UV detection.High-resolution mass spectral analysis was performed at the Mass Spectrometry and Proteomics facility at the Ohio State University. 1 H NMR spectra were recorded on a Bruker 400 MHz NMR spectrophotometer at 298 K (Bruker, Billerica, MA, USA).

Spectroscopic Measurements
Electronic absorption spectra were recorded at room temperature using an HP8453 photodiode array spectrophotometer with 2 nm resolution.All spectra were recorded at 298 K. Room temperature luminescence spectra in a 1 cm quartz spectrophotometer fluorescence cell (Starna Cells, Atascadero, CA, USA) in CHCl 3 were run on a Cary Eclipse fluorescence spectrophotometer.Luminescence quantum yields were determined at room temperature in HPLC-grade CHCl 3 relative to Rhodamine 6G as the reference (φ fl = 0.95, in ethanol) [50].The quantum yields were obtained using the following equation: where "s" and "r" indicate the sample and reference, respectively, A is the absorbance at the excitation wavelength, η is the average refractive index of the solution, and D is the integrated area under the emission spectrum.

Electrochemical Measurements
Cyclic voltammograms were recorded under a nitrogen atmosphere using a onecompartment, three-electrode cell, CH-Instruments, equipped with a platinum wire auxiliary electrode.The working electrode was a 2.0 mm diameter glassy carbon disk from CH-Instruments, which was polished first using 0.30 mm followed by 0.05 mm alumina polish (Buehler, Lake Bluff, IL, USA) and then sonicated for 10 s prior to use.Potentials were referenced to a Ag/Ag + non-aqueous electrode, CH-Instruments.The supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) and the measurements were made in dry CHCl 3 .Bulk electrolysis of mM CHCl 3 solutions of the BDP dyes was performed on 2.54 cm × 2.54 cm indium tin oxide-coated square glass slides with 70-100-ohm surface resistivity (Sigma-Aldrich, St. Louis, MO, USA).The auxiliary electrode was a square platinum flag and the reference was a nonaqueous Ag/Ag + electrode (CH-Instruments, Austin, TX, USA).

Synthesis
BDP1 was synthesized via a variation on the previous reports using a mechanochemical technique [22].2,4-dimethyl pyrrole (0.25 mL, 2.4 mmol and N-(4-formylphenyl)carbazole (325 mg, 1.20 mmol) were placed in a mortar with approximately 3 mL of DCM.Approximately 10 drops of trifluoroacetic acid were added and the mixture was ground for 1 min.At this point, 454 mg (1.90 mmol) of p-chloranil was added and the mixture was ground for an additional minute.Using a syringe, 0.50 mL of triethylamine followed by 0.50 mL of boron trifluoro etherate was added while the mixture was continuously ground for another 2 min.The dark mixture was dissolved in DCM and chromatographed on silica gel using DCM with increasing amounts of methanol.Orange powder (164 mg, 28%), 1  Under reduced pressure at 70-75 • C, the solvent was removed, giving a dark purple or greenish paste, respectively.The round-bottom flask was placed in an oil bath set at 80 • C under a nitrogen purge and approximately 5 mL of dry toluene was added followed by 250 µL of triethylamine and 300 mL of boron trifluoroetherate.The reaction mixture was stirred at 80 • C for 20 min, at which point the solution was chromatographed on silica gel with hexanes and increasing amounts of chloroform.The fluorescent products were dried, resulting in 31.5 mg (0.050 mmol, 16.5% yield) of BDP2 and 23.2 mg (0.030 mmol, 10.0% yield) of BDP3.

Conclusions
A series of four BDP dyes with varying degrees of aromaticity affixed with redox-active meso-substituents have been synthesized.Solution phase spectroscopic and electrochemical properties reveal the expected HOMO-LUMO gaps associated with increasing aromaticity of the BDP-pyrroles.The carbazole substituents of BDP1, BDP2, and BDP3 undergo oxidative redox processes, resulting in film formation on glassy carbon electrodes.Of these three dyes, only BDP1 shows the ability to maintain the BDP-core spectroscopic properties upon oxidative film formation on ITO glass slides.Utilizing a pyrene-substituted isoquinol BDP dye (BDP4) shows the ability of anodic film formation with π-extended BDP dyes.Further studies are underway to determine the best meso-substituents (e.g., 4-hydroxyphenyl, 3,4-dihydroxyphenyl, and aniline) needed to allow for oxidative film formation without destruction of the BDP-core.In addition, future work will include a study of layering ITO slides with BDP-dyes with varying HOMO-LUMO gaps; ultimately, these slides will be tested as anodes in DSSCs.

Scheme 1 .
Scheme 1. Electron collection at the anode of a DSSC.

Scheme 1 .
Scheme 1. Electron collection at the anode of a DSSC.
. The associated energies of the LUMOs vary as a function of the increasing π-conjugation going from BDP1 to BDP3.

Figure 4 .
Figure 4. Cyclic voltammetry of glassy carbon electrodes with adsorbed BDP1 (A), BDP2 ( BDP3 (C) in 0.1 M TBAPF6 chloroform solutions, with a platinum wire auxiliary electrode nonaqueous Ag/Ag + reference electrode.Fc/Fc + redox couple measured in under the same tions is overlayed.Scan rates of 100 mV/s.

Figure 4 .
Figure 4. Cyclic voltammetry of glassy carbon electrodes with adsorbed BDP1 (A), BDP2 (B), and BDP3 (C) in 0.1 M TBAPF 6 chloroform solutions, with a platinum wire auxiliary electrode and a nonaqueous Ag/Ag + reference electrode.Fc/Fc + redox couple measured in under the same conditions is overlayed.Scan rates of 100 mV/s.

Figure 5 .
Figure 5. Qualitative Jablonski diagram of the HOMO/LUMO energies determined from cyclic voltammograms of the adsorbed BDP dyes from Figure 3.

Figure 5 .
Figure 5. Qualitative Jablonski diagram of the HOMO/LUMO energies determined from cyclic voltammograms of the adsorbed BDP dyes from Figure 3.

Figure 5 .
Figure 5. Qualitative Jablonski diagram of the HOMO/LUMO energies determined from cyclic voltammograms of the adsorbed BDP dyes from Figure 3.

Figure 6 .
Figure 6.(top) Cyclic voltammetry of mM chloroform solutions of BDP dyes with 0.1 M TBAPF6 as supporting electrolyte, a square ITO glass slide as working electrode, platinum flag auxiliary electrode, and non-aqueous Ag/Ag + reference electrode.(bottom) UV/vis spectra of ITO slides after electrolysis of the BDP dye solutions for 100 s at 1.5 V.

Figure 6 .
Figure 6.(top) Cyclic voltammetry of mM chloroform solutions of BDP dyes with 0.1 M TBAPF 6 as supporting electrolyte, a square ITO glass slide as working electrode, platinum flag auxiliary electrode, and non-aqueous Ag/Ag + reference electrode.(bottom) UV/vis spectra of ITO slides after electrolysis of the BDP dye solutions for 100 s at 1.5 V.

Molecules 2023 , 15 Figure 8 .
Figure 8. Structure of BDP4.(A) Cyclic voltammogram of a mM solution of BDP4 in 0.1 M TBAPF6 in chloroform with a glassy carbon working electrode and a nonaqueous Ag/Ag + reference electrode.(B) Cyclic voltammogram of coated glassy carbon electrode in 0.1 M TBAPF6 with a nonaqueous Ag/Ag + reference electrode overlayed with Fc/Fc + redox couple.

Figure 8 . 15 Figure 9 .Figure 9 .
Figure 8. Structure of BDP4.(A) Cyclic voltammogram of a mM solution of BDP4 in 0.1 M TBAPF 6 in chloroform with a glassy carbon working electrode and a nonaqueous Ag/Ag + reference electrode.(B) Cyclic voltammogram of coated glassy carbon electrode in 0.1 M TBAPF 6 with a nonaqueous Ag/Ag + reference electrode overlayed with Fc/Fc + redox Molecules 2023, 28, x FOR PEER REVIEW 11 of 15

Figure 9 .
Figure 9. Electronic spectrum of an ITO glass slide electro-coated with BDP4, blue line and the spectrum of BDP4 in chloroform solution, red line.