Effect of Triarylamine Structure on the Photoinduced Electron Transfer

ARTICLE pubs.acs.org/JPCC

Effect of Triarylamine Structure on the Photoinduced Electron Transfer to Boron Subphthalocyanine Brett A. Kamino,† Graham E. Morse,† and Timothy P. Bender*,†,‡ †

Department of Chemical Engineering and Applied Chemistry, The University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5 ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6

bS Supporting Information ABSTRACT: The photoinduced electron transfer (PET) reaction between a phenoxy-boronsubphthalocyanine derivative and a series of triarylamine electron donors was investigated. A series of triarylamines ranging in oxidation potentials and number of redox centers were prepared to study the effect of triarylamine structure on the photoinduced electron transfer (PET) reaction. In the case of multiple nitrogen centers, the triarylamines were dendritic in nature and were synthesized by a convergent strategy relying on successive C
N coupling and thermolytic deprotection steps. The efficiency of the PET reaction was found to be exponentially dependent on the oxidation potential of the triarylamine beyond a certain threshold. The free-energy change of the PET reaction was estimated using the Rehm
Weller equation, and this framework could be used to adequately explain the observed behavior of the system. We have concluded that the specific structure of the triarylamine is not important in the PET reaction and that efficiency of electron transfer is almost solely dictated by the oxidation potential of the triarylamine donor.

’ INTRODUCTION Single and multi-nitrogen centered triarylamines are an important class of functional materials in the area of organic electronics. Owing to their well-behaved chemical and electrochemical oxidation, this class of organic semiconductors represents one of the most frequently studied electron-donating materials (aka hole-transporting materials) in the field.1 Beyond their basic ability to function as stable and reversible electron donors, the adoption and study of these materials is aided by the demonstrated ability to modify their electronic and physical properties over a wide range. Fine control over their oxidation potentials2 and access to relatively stable polycations can be achieved by the appropriate use of electron-withdrawing/-donating groups and by the construction of large molecules containing multiple conjugated triarylamine (we will use the term triarylamines to refer to all triarylamine structures regardless of number of nitrogen centers) centers.3 Their physical properties can range from crystalline solids,4 to morphologically stable glasses,5 and even to molecular liquids.6 Because of their highly tunable properties, triarylamines have become standard materials in some organic electronic devices such as xerographic photoreceptors, light-emitting diodes, field effect transistors, bulk heterojunction solar cells,7 and solid-state dye-sensitized solar cells.8 As well, triarylamine moieties have been incorporated into the molecular structure of light-absorbing oligomers, polymers,9 and photosensitizers.10 However, there has been comparatively little done to understand the effects of specific triarylamine molecular structures and their associated substituents on r 2011 American Chemical Society

electron transfer processes with complementary materials. One very important factor affecting the ultimate performance of these devices is how well the triarylamines are able to donate an electron into a complementary acceptor material upon photoexcitation of the acceptor. Such an interaction can be an important factor in optimizing the charge separation and charge extraction processes in an organic solar cell (for example) and thus improving device efficiencies11 for a selected group or pairing of materials.12 In this paper, we study the effect of triarylamine chemical structure on photoinduced electron transfer efficiencies to a light-absorbing electron acceptor. This was done by studying the fluorescence quenching in solution of a model fluorescent electron acceptor with various triarylamines acting as electron donors. For the fluorophore and electron acceptor, a soluble boron subphthalocyanine (BsubPc) derivative was chosen: 3,4-dimethylphenoxyboronsubphthalocyanine (3,4-DMPhO-BsubPc, Figure 1).13 Beyond its pleasing magenta color, this acceptor was chosen because BsubPc derivatives are currently of interest for application in both organic photovoltaics14 and organic lightemitting diode devices.15 As well, the established position of its HOMO allows a wide range of triarylamine donors to be used as fluorescent quenchers. While chloroboronsubphthalocyanine Received: July 2, 2011 Revised: August 24, 2011 Published: September 08, 2011 20716

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Figure 1. Structures of 3,4-DMPhO-BsubPc and the triarylamines used in this study (containing either one (1a
i) or two nitrogen centers (2a
c)).

(Cl-BsubPc) is typically used as an electron-donating material, recent studies have shown the potential of phenoxy-substituted BsubPc derivatives to act as electron acceptors/n-type charge transporting materials.16 A series of triarylamines that spanned both a range of oxidation potentials and a variety of conjugated molecular structures were paired with 3,4-DMPhO-BsubPc. Obtaining triarylamines that include one or two nitrogen centers was facilitated by our previous work in the area2a and by the straightforward synthesis of triarylamines with two nitrogen centers (such molecules are commonly known in the literature). However, to access triarylamines with a higher number of nitrogen centers and as a consequence very low oxidation potentials, we purposefully synthesized dendritic triarylamines for this study. Such dendrimers possess a high degree of conjugation and associated charge stabilization while maintaining reasonable solubility due to their nonplanar structures. These unique attributes result in materials that contain very small energy gradients17 and stable electrochemistry.18 They have been studied as models for charge transfer19 as well as the generation of high-spin polycations.20 By synthesizing and adding these dendritic structures to a series of more conventional structures, we hoped to better understand the range of structural and electronic properties that may affect the photoinduced electron transfer reaction between a triarylamine and 3,4DMPhO-BsubPc.

’ EXPERIMENTAL SECTION Materials. All reagents and starting materials were used as received. All solvents were purchased from Caledon Laboratories

(Ontario, Canada) and used as received except toluene which was purified through a commercial solvent purification system prior to use. Deuterated NMR solvents were purchased from Cambridge Isotopes. External standards were prepared by sealing the standard in a melting point tube with a flame and placing it freely within the NMR tube. NMR spectra were acquired on a Varian 400 NMR system with a field strength of 400 MHz. Size exclusion chromatography was performed using Waters Styragel HR0.5 and a Waters Styragel HR1 placed in series, each having a column size of 4.6  300 mm. GPC was operated with THF as the mobile phase at a rate of 0.75 mL/min. Detection was achieved by a UV
vis photodiode array. Fluorescence spectroscopy for the quenching measurements was performed in a dimly lit room with a Perkin-Elmer L55 spectrometer. Mass spectroscopy for the dendritic triarylamines was acquired on an AccuTOF mass spectrometer (JEOL USA Inc. Peabody, MA) with a DART-SVP ion source (Ionsense Inc., Saugus, MA) using He Gas at 300
500 °C. Samples were dissolved in CH2Cl2 and introduced into the sampling region using glass melting-point capillaries. Compounds 6a and 6b required volatilization using a butane torch directly on the sample. Mass spectroscopy for samples 2a, 2b, and 2c was achieved using an AB/Sciex QStar mass spectrometer with an ESI source (50:50 methanol and water). Cyclic voltammetry was performed with a Bioanalytical Systems C3 electrochemical cell setup. The working electrode was a 1 mm platinum disk with a platinum wire used as a counter electrode. The reference electrode was a Ag/AgCl saturated salt solution. All electrochemistry was done in “Spectro” grade dichloromethane from Caledon Laboratories. Decamethylferrocene was added to the solutions as an internal reference, and all electrochemical half-wave potentials are corrected to its 20717

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0.012 V (vs Ag/AgCl). The synthesis of 3,4-DMPhO-BsubPc16 and the syntheses of compounds 1a
1i1 have been previously reported. Synthesis of 2a. 1,4-Phenylene diamine (1.000 g, 9.25 mmol), sodium tert-butoxide (5.332 g, 55.5 mmol), and bis(dibenzylideneacetone)palladium (106 mg, 0.18 mmol) were added to a round-bottom flask. This flask was sealed under an argon atmosphere. Anhydrous toluene (50 mL), 4-bromoanisole (7.611 g, 40.69 mmol), and tri-tert-butylphosphine (29.9 mg, 0.15 mmol, added as a stock solution in toluene) were added. This mixture was refluxed under an inert atmosphere for 2 h. Upon cooling, acidic clay (10 g, montmorillonite K10) and acidic alumina (1 g, standard basic) were added to the mixture. This slurry was filtered, washing with toluene to yield a clear, pale yellow solution. This solution was precipitated into methanol to yield 4.237 g of a fine pale white powder (86.3% yield). 1H NMR (400 MHz, C6D6): δ7.14 (d, J = 9.08 Hz, integration obscured by solvent peak), 7.08 (s, 4H), 6.73 (d, J = 9.08 Hz, 8H), 3.30 (s, 12H). 13C NMR (100 MHz, C6D6): 156.34, 143.84, 142.55, 126.35, 124.17, 115.45, 53.39. HRMS [M+] calculated 532.2356, found 532.2372. Synthesis of 2b. 1,3-Phenylene diamine (1.000 g, 9.25 mmol), sodium tert-butoxide (5.332 g, 55.5 mmol), and bis(dibenzylideneacetone)palladium (106 mg, 0.18 mmol) were added to a round-bottom flask. This flask was sealed under an argon atmosphere. Anhydrous toluene (50 mL), 4-bromoanisole (7.611 g, 40.69 mmol), and tri-tert-butylphosphine (29.9 mg, 0.15 mmol, added as a stock solution in toluene) were added. This mixture was refluxed under an inert atmosphere for 2 h. Upon cooling, acidic clay (10 g, montmorillonite K10) and acidic alumina (1 g, standard basic) were added to the mixture. This slurry was filtered washing with toluene to yield a clear, pale yellow solution. This solution was concentrated under vacuum and precipitated into methanol. A light yellow powder was collected and dried under vacuum to obtain 3.728 g of dry material (75.7% yield). 1H NMR (400 MHz, C6D6): δ 7.11 (d, J = 9.08 Hz, 8H), 6.73 (dd, J1 = 7.91 Hz, J2 = 2.34 Hz, 2H), 7.05 (t, J = 2.34 Hz, 1H), 7.02 (d, J = 7.92 Hz, 1H), 6.67 (d, J = 9.08 Hz, 8H), 3.28(s, 12H). 13C NMR (100 MHz, C6D6): 156.56, 150.48, 141.92, 130.30, 127.04, 115.36, 115.30, 114.71, 53.31. HRMS [M+H] calculated 533.2434, found 533.2429. Synthesis of 2c. Bis(4-methoxyphenyl)amine (3.824 g, 16.8 mmol), 4,40 -dibromobiphenyl (2.500 g, 8.0 mmol), sodium tertbutoxide (1.922 g, 20 mmol), palladium(II) acetate (72 mg, 0.3 mmol), tri-tert-butylphosphine (52 mg, 0.3 mmol, added from stock solution in anhydrous toluene), and anhydrous toluene (20 mL) were reacted for 3 h at reflux under an inert atmosphere. After cooling, acidic clay (2 g, montmorillonite K10) and acidic alumina (1 g, standard basic) were added, and the mixture was filtered. The compound was precipitated into methanol and recrystallized from EtOAc. Slightly crystalline yellow flakes were collected (4.880 g, 70%). 1H NMR (400 MHz, C6D6): δ 7.46 (d, J = 8.61 Hz, 4H), 7.17 (m, obscured by residual solvent), 7.12 (d, J = 9.00 Hz, 8H), 6.74 (d, J = 9.00 H, 8H), 3.31 (s, 12H). 13C NMR (100 MHz, C6D6): δ 156.43, 148.16, 141.72, 133.98, 127.56, 126.82, 122.04, 55.06. Synthesis of N-tBOC-bis(4-bromophenyl)amine (4). Bis(4bromophenyl)amine (25 g, 76.43 mmol), di-tert-butyl dicarbonate (18.32 g, 84.07 mmol), 4-dimethylaminopyridine (1.872 g, 15.29 mmol), and tetrahydrofuran (125 mL) were added to an oven-dried flask. The flask was refluxed for 24 h and allowed to cool. Approximately 75 mL of solvent was removed by rotary

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distillation, and the flask was cooled to 5 °C overnight. Large white crystals were isolated from the mother liquor and washed lightly with cold methanol. The final pure product was dried overnight under vacuum (26.14 g, 80% yield). 1H NMR (400 MHz, CDCl3): δ 7.43 (d, J = 8.86 Hz, 4H), 7.06 (d, J = 8.86 Hz, 4H), 1.44 (s, 12H). 13C NMR (100 MHz, CDCl3): δ 153.35, 141.93, 132.15, 128.68, 119.52, 82.16, 28.35. Synthesis of Generation 1 Dendron (5a). Palladium acetate (146 mg, 0.65 mmol) and tri-tert-butylphosphine (105.25 mg, 0.52 mmol) were added to an oven-dried flask under a flow of argon and allowed to stir for 30 min. 4 (27.77 g, 65.01 mmol), bis(3,4-dimethylphenyl)amine (3) (30.766 g, 136.54 mmol), sodium tert-butoxide (16.66 g, 173.36 mmol), and 90 mL of toluene were then added to the flask. The solution was refluxed for 2 h and allowed to cool. When the solution was at room temperature, an additional 100 mL of toluene was added, and the solution was treated with 5 g of basic alumina and 5 g of montmorillonite K10 clay. The solution was filtered, and the yellow mother liquor was collected and concentrated under vacuum. The now concentrated and viscous solution was precipitated into 50 mL of methanol and allowed to stir. The collected solid was dried and placed into a vessel with 10 mL of tetralin (1,2,3,4-tetrahydronaphthalene) and heated at 200 °C under an argon atmosphere overnight. The still yellow solution was allowed to cool and precipitated into 50 mL of methanol to yield a white solid (40.721 g, 78% yield). Mass Spectroscopy [M+H]: 616.3690. Expected [M+H]: 616.3691. Synthesis of Generation 1 Dendrimer (5b). Bis(dibenzylideneacetone)palladium (14 mg, 0.024 mmol) and tri-tertbutylphosphine (4 mg, 0.02 mmol, added as a stock solution in toluene) were added to an oven-dried flask under a flow of argon and allowed to stir for 30 min. Compound 5a (300 mg, 0.49 mmol), 4-bromotoluene (13 mg, 0.73 mmol), sodium tertbutoxide (94 mg, 1.0 mmol), and 2 mL of anhydrous toluene were added, and the solution was refluxed overnight. Upon completion, the solution was allowed to cool and then treated with basic alumina (0.25 g) and montmorillonite K10 clay (0.25 g). The solids were filtered, washing with additional toluene, and reduced to a viscous liquor. This liquor was precipitated by dropwise addition into 50 mL of stirring methanol. A white, fluffy powder was collected (305 mg, 89% yield). The compound was characterized by NMR, mass spectroscopy, and gel permeation chromatography. 1H NMR (400 MHz, CDCl3): δ 7.15
7.09 (m, 14H), 7.03 (dd, J1 = 8.19 Hz, J2 = 2.14 Hz, 4H), 6.92 (d, J = 7.99 Hz, 4H), 6.87 (d, J = 8.19 Hz, 2H), 2.07 (s, 3H), 2.00 (s, 12H), 1.91 (s, 12H). Mass Spectroscopy [M+H]: 706.4143. Expected [M+H]: 706.4161. Synthesis of Generation 2 Dendron (6a). Palladium acetate (217 mg, 0.97 mmol) and tri-tert-butylphosphine (156 mg, 0.77 mmol) were added to an oven-dried flask under a flow of argon and allowed to stir for 30 min. 5a (20 g, 32.47 mmol), 4 (6.866 g, 16.08 mmol), sodium tert-butoxide (4.057 g, 42.22 mmol), and 100 mL of toluene were then added to the flask. The solution was refluxed for 18 h and allowed to cool. When the solution was at room temperature, an additional 150 mL of toluene was added, and the solution was treated with 5 g of basic alumina and 5 g of montmorillonite K10 clay. The solution was filtered, and the yellow mother liquor was collected and concentrated under vacuum. The now concentrated and viscous solution was precipitated into 50 mL of methanol and allowed to stir. The collected solid was dried and placed into a vessel with 10 mL of tetralin (1,2,3,4-tetrahydronaphthalene) and heated at 200 °C 20718

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Scheme 1. Synthetic Pathway Towards Triarylamine Dendrimers (5b and 6b)a

a

Conditions: (i) sodium tert-butoxide, bis(dibenzylideneacetone)palladium or palladium acetate (see Experimental Section), tri-tert-butylphosphine, toluene, reflux; (ii) 1,2,3,4-tetrahydronaphthalene, 200 °C, overnight.

under an argon atmosphere overnight. The still yellow solution was allowed to cool and precipitated into 50 mL of methanol to yield a white solid (22.455 g, 91% yield). Mass Spectroscopy [M+H]: 1396.8. Expected [M+H]: 1396.8. Synthesis of Generation 2 Dendrimer (6b). Bis(dibenzylideneacetone)palladium (6 mg, 0.01 mmol) and tri-tert-butylphosphine (1.74 mg, 0.009 mmol, added as a stock solution in toluene) were added to an oven-dried flask under a flow of argon and allowed to stir for 30 min. Compound 6a (300 mg, 0.215 mmol), 4-bromotoluene (55 mg, 0.322 mmol), sodium tert-butoxide (41 mg, 0.430 mmol), and 2 mL of anhydrous toluene were added, and the solution was refluxed overnight. Upon completion, the solution was allowed to cool and then treated with basic alumina (0.25 g) and montmorillonite K10 clay (0.25 g). The solids were filtered, washing with additional toluene, and reduced to a viscous liquor. This liquor was precipitated by adding dropwise into 50 mL of stirring methanol. A yellow, fluffy powder was collected (295 mg, 92% yield). Compound was characterized by NMR, mass spectroscopy, and gel permeation chromatography. 1H NMR (400 MHz, CDCl3): δ 7.14
6.86 (m, 52), 2.09 (s, 3H), 2.00 (s, 24H), 1.90 (s, 24H). 13 C NMR (100 MHz, C6D6): δ 147.11, 144.23, 143.57, 143.55, 143.14, 137.94, 132.30, 131.19, 131.16, 131.00, 130.58, 128.27, 126.47, 125.45, 125.39, 125.01, 124.85, 122.80, 21.12, 20.14, 19.42. Mass Spectroscopy [M+H]: 1486.8. Expected [M+H]: 1486.8.

’ RESULTS AND DISCUSSION A series of triarylamine donors were used or purposefully prepared to study their ability to act as electron donors to 3,4-DMPhO-BsubPc

upon photoexcitation. This series incorporates single triarylamines (1a
i, Figure 1) bearing various combinations of electron-donating groups as well as two nitrogen-centered triarylamines which are constructed with different molecular fragments separating the two nitrogen centers (2a
c, Figure 1). The chemistry utilized to synthesize single or two-nitrogen centered triarylamines relies on either Buchwald
Hartwig21 or Ullman amination to construct the necessary C
N linkages. The detailed syntheses of compounds 1a
i are described elsewhere (Figure 1).2a Triarylamines with two nitrogen centers (2a
c) were synthesized in a single step using conventional methods. Their synthesis is illustrated in the Supporting Information accompanying this article (Figure S1). To gain access to triarylamines containing multiple nitrogen centers as well as materials with very low oxidation potentials, dendritic triarylamines were synthesized. The dendrimers were prepared in a convergent approach utilizing an alternating C
N coupling and deprotection sequence to systematically increase generation size (Scheme 1). Catalytic hydrogenolysis of benzyl18 or diphenyl methyl17 protecting groups or the acidpromoted cleavage of a t-butylcarbamate (tBOC) group by trifluoroacetic acid have been used elsewhere to deprotect diphenylamine moieties.19,20 We, however, developed a simplified deprotection strategy which avoided hydrogenolysis and the use of strong organic acids. This was necessitated by our observation that the use of trifluoroacetic acid, which while considered to be a very weakly oxidizing acid compared to mineral acids such as H2SO4 or HNO3 resulted in the oxidation of the triarylamine dendrimers as evidenced by a bright green color present in the reaction medium characteristic of the radical cation. We found that removal of the tBOC group was 20719

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Table 1. Electrochemical Oxidation Potentials, Fluorescence Quenching Efficiency, and Free Energy Change upon Photoinduced Electron Transfer Reaction with 3,4-DMPhOBsubPc for Triarylamines (1a
i, 2a
c, 5b, and 6b) compound

Figure 2. Size exclusion chromatograms of the triarylamine dendrimers (5a and 6a) and a single triarylamine analogue (1a) as detected by UV
vis absorbance.

facilitated by simple heating (thermolysis) at 200 °C.22 This reaction proceeds cleanly resulting in no observable oxidized product and is performed best when a small amount of a high boiling solvent (in this case tetralin) is added to help melt the substrate and lower the viscosity of the melt. Using this synthetic strategy, dendrons up to the second generation were prepared in good yields. Attempts to produce dendrons of a higher generation by this strategy resulted in partial substitution, a limitation that is likely a result of steric factors. For each generation dendron, the free amine was finally capped with a p-tolyl group resulting in dendrimers 5b and 6b. Both dendrimers were isolated as fine white powders, and their purity and composition were established by 1H NMR, mass spectroscopy, and size exclusion chromatography (SEC). 1H NMR analysis of both the free amine and p-tolyl-capped dendrimers (5a
b and 6a
b, respectively) resulted in many overlapping resonances in the aromatic region of the spectra. We were unable to acquire satisfactory 13C NMR spectra for all but compound 6b. Solubilities in benzene-d6 were too low to achieve a good signal-to-noise ratio. Attempts to obtain spectra in chlorinated solvents (chloroform-d and dichloromethane-d2) quickly resulted in oxidation of the dendrimers as evidenced by the evolution of a characteristic bright green color which is attributable to the presence of the radical cation. The presence of the paramagnetic radical cations prevented NMR analysis in these solvents. The dendrimers were run through a low molecular weight size exclusion column, and the chromatograms show well-resolved peaks for each molecule and illustrate the progression of molecular size between each generation as well as the purity of each (Figure 2). With this broad series of triarylamines in hand, each previously unreported compound was characterized by solution cyclic voltammetry to determine their relative oxidation potentials (Table 1). The electrochemistry was performed in a dichloromethane solution with tetrabutylammonium perchlorate as the supporting electrolyte at a scan rate of 50 mV/s. A platinum disk working electrode, platinum wire counter electrode, and saturated Ag/AgCl pseudo reference electrode were also used. A small amount of decamethylferrocene was added to each sample as an internal standard. All oxidation potentials are corrected to the accepted half wave oxidation potential of decamethylferrocene (
0.012 V vs Ag/AgCl).23 Several triarylamines used in this study (1a
i) have been previously characterized under identical conditions, and the literature values were incorporated into our data set. All of the studied triarylamines displayed at least a single well-defined and reversible oxidation. This allowed for an accurate determination of the half-wave oxidation potentials of the entire series. With respect to the range of half-wave oxidation

a

number of redox centers

E1ox (V vs Ag/AgCl)

K (mol
1)

ΔGPET (eV)

1a

1

820

10.0 ( 1.82


0.098

1b

1

735

14.4 ( 0.48


0.187

1c

1

690

18.7 ( 0.80


0.228

1d

1

654

23.4 ( 1.16


0.260

1e

1

814

5.2 ( 0.21


0.127

1f 1g

1 1

844 912

4.9 ( 0.15 2.8 ( 0.23


0.132
0.064

1h

1

981

0.7 ( 0.07

0.005

1i

1

1138

2a

2

417

0a 71.4 ( 4.28

0.045
0.569

2b

2

693

18.4 ( 0.52


0.284

2c

2

643

26.4 ( 0.17


0.333

5b

3

471

44.6 ( 5.56


0.561

6b

7

273

187.7 ( 6.89


0.843

Value obtained is less than experimental error, considered to be 0.

potentials, triarylamines with single nitrogen centers ranged from 1138 to 654 mV vs Ag/AgCl, while the two nitrogen-centered amines ranged from 693 mV for the weakly conjugated 2b to 417 mV vs Ag/AgCl for the more conjugated 2a. The triarylamine dendrimers extended this range of half-wave oxidation potentials down to 273 mV for the second-generation dendrimer (6b), whereas the first-generation dendrimer (5b) was found to have an oxidation potential of 471 mV vs Ag/AgCl. The firstgeneration dendrimer (5b) was found to undergo two fully reversible one-electron oxidations which is consistent with similar structures in the literature (Figure 3).19 Increasing the potential further resulted in a third irreversible oxidation. The second-generation dendrimer (6b) undergoes three distinct and reversible oxidation events (Figure 3) likely attributable to the large number of conjugated nitrogen centers present in this molecule. The ability of the complete series of triarylamines to undergo an electron transfer event with a photoexcited BsubPc was investigated using a standard steady state fluorescence quenching experiment in solution. The BsubPc (3,4-DMPhO-BsubPc) was prepared as a 1  10
5 M solution in anhydrous toluene having varying amounts of triarylamine present. The solution was photoexcited at 550 nm, and the intensity of the emission peak at 578 nm was recorded. The relative efficiency of the photoinduced electron transfer reaction was determined by the Stern
Volmer (equation 1) F0 ¼ 1 þ K 3 ½Q  F

ð1Þ

where F0 is the fluorescence intensity without any quencher added; F is the fluorescence intensity at quencher concentration [Q]; and K is the quenching coefficient. For each triarylamine, a plot of F0/F vs [Q] (a Stern
Volmer plot) was found to be linear and gave a good correlation when fitted with a linear regression. The fluorescence quenching efficiencies (K) are summarized in Table 1. The full Stern
Volmer plots are included in the Supporting Information. The quenching coefficients range from 20720

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Figure 3. Solution electrochemistry of triarylamine dendrimers (5b, 6b) and a representative single triarylamine (1a). Voltammagrams are corrected to the internal standard decamethylferrocene (E1/2,red =
0.012 V).

those where no significant quenching was observed (1i) to relatively large quenching coefficients (6b). To determine the

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Figure 5. Experimentally determined Stern
Volmer constant (K) plotted against the free energy change estimated by a modified Rehm
Weller equation (eq 2). Error bars are not included as they are small relative to the size of the data marker point.

This correlation was developed for measurements in polar solvents. Because our measurements are carried out in a nonpolar solvent (toluene, ε = 2.38), a modified version of the Rehm
Weller equation is used to take into account the measured redox potential differences in solvents used which are of different dielectric constants as follows25 ΔGPET ¼ ðEox ðDÞ
Ered ðAÞÞDCM
Eex     e2 1 1 1 e2 1 1 þ þ

þ

3 þ þ 2R RDA 2R 4πε0 εtoluene 2R 4πε0 εDCM 2R

ð2Þ

Figure 4. Experimentally determined Stern
Volmer constants (K) plotted against half-wave oxidation potentials (E1/2,ox) of the triarylamine donor. Error bars are not included as they are small relative to the size of the data marker point.

effect of triarylamine structure and oxidation potential on the photoinduced electron transfer (PET) process, the quenching coefficients were plotted against the electrochemical half-wave oxidation potentials (Figure 4) in a linear-log plot. From this plot, it is apparent that there is a correlation between oxidation potential and quenching efficiencies. Triarylamines with stronger electron-donating substituents or lower oxidation potentials had higher quenching efficiencies than triarylamines with high potentials. Perhaps more surprisingly, the majority of the triarylamines showed an exponential dependence of quenching coefficient with oxidation potential. This relationship deviates only for the triarylamines with the highest oxidation potentials. Fitting a line through the nine best quenchers results in a particularly good fit with R2 = 0.98. As the oxidation potential increases past 825 mV (vs Ag/AgCl), the quenching constant quickly drops off. For example, triarylamine 1i has the highest oxidation potential and did not quench 3,4-DMPhO-BsubPc significantly. To better understand these observations and to confirm whether our system is behaving as a normal photoinduced electron transfer system, the free energy change of the electron transfer process was estimated using the Rehm
Weller equation.24

where Eox(D) is this oxidation potential of the electron donor (triarylamine); Ered(A) is the reduction potential of the acceptor (3,4-DMPhO-BsubPc);13 Eex is the energy of the excitation; e is the elementary charge; ε0 is the vacuum permittivity; εtoluene and εDCM are the dielectric constants of toluene and dichloromethane, respectively; R+ and R
are the average ion radii of the donor and acceptor, respectively; and RDA is the average donor
acceptor distance (estimated as 6 Å).25a The average ion radii of the donor and acceptor molecules were roughly estimated using calculated values for molecular volume assuming that the molecules are spheres. These molecular volumes were obtained through molecular modeling calculations on each of the molecules studied and are included in the Supporting Information.26 The average donor
acceptor distance was estimated from molecular dynamics simulations using molecular mechanics force fields (MM+). Center to center separation distances typically varied between 5.5 and 6.5 Å; a median estimate of 6 Å was used for these calculations. The resulting ΔGPET values (Table 1) were plotted against the measured Stern
Volmer constants (Figure 5). The plot shows an expected trend; as the free energy change becomes more favorable, the fluorescence quenching efficiency increases. As well, a decrease in quenching efficiency occurs near ΔGPET = 0 eV, thus confirming that our system is well behaved and fits well into this theoretical framework. This observation is expected given the lack of thermodynamic driving force for the PET to the photoexcited 3,4-DMPhO-BsubPc for ΔGPET > 0 eV and indicates that our system is well behaved and can be described by the Rehm
Weller model over the range studied. Looking at the quenching efficiencies of the materials with the most favorable energy changes, it is interesting to note that no plateau in 20721

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The Journal of Physical Chemistry C quenching efficiency is observed. A diffusion limited plateau in quenching efficiency at high values of ΔGPET is predicted by Rehm
Weller theory and is observed in most systems.27 In our case, it may simply be that no materials have a sufficiently high enough ΔGPET value to result in diffusion limited electron transfer. When looking at this analysis, it should be emphasized that several assumptions were made in the calculation of the Coulombic attraction energy term, particularly in regards to the ion radii (R+ and R
) and the donor
acceptor distance (RDA). Regardless of the assumptions associated with this calculation, several important observations can be drawn from the data. Looking first at the effect of triarylamine structure on quenching efficiencies, there is no relation between structure and the fluorescence quenching efficiency. Triarylamines with single or two nitrogen centers of similar oxidation potentials have equivalent quenching efficiencies. Therefore, a conclusion can be drawn that there is no need to synthesize dendritic triarylamines to act as electron donors to BsubPcs. It can also be seen that once the PET reaction is energetically favorable, the quenching efficiency very closely follows an exponential relationship with solution oxidation potential. The one deviation from this behavior is for compound 1e which has a similar oxidation potential to 1a but a much lower quenching efficiency. This may be explained by the much bulkier substitution pattern (t-butyl vs methyl) intuitively suggesting that bulky nonconjugated substituents inhibit the electron transfer process via a steric effect.

’ CONCLUSIONS In summary, the effect of triarylamine structure and oxidation potential on the photoinduced electron transfer (PET) reaction to 3,4-DMPhO-BsubPc was investigated. This was performed using a series conventional triarylamines and novel triarylamine dendrimers which spanned a large range of oxidation potentials, number of conjugated redox centers, and structures. Relying on steady-state fluorescence quenching, it was found that two regimes of PET reactions could be found. Under a certain oxidation potential, the quenching efficiency was found to scale exponentially with oxidation potential of the triarylamine regardless of it chemical structure. This implies that oxidation potential is the primary factor in determining the efficiency of electron transfer in this system and that triarylamine structure is not particularly important for this process. By placing our data into the Rehm
Weller theoretical framework we have determined that this system is well behaved including the expected deviation from the exponential dependence of quenching on the oxidation potential of the donor coinciding with a loss of thermodynamic driving at values for ΔGPET > 0 eV. The results of this study will directly aid in the design of triarylamines as effective electrondonating materials to BsubPc derivatives. It also suggests a general methodology by which other donor/acceptor materials can be selected and optimized for PET.

’ ASSOCIATED CONTENT

bS

Supporting Information. NMR spectra, electrochemistry, or new compounds and Stern
Volmer plots of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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