Influence of Surface-Attachment Functionality on the Aggregation

Apr 5, 2012 - (80, 81) TiO2 films were 4.1 ± 0.9 μm thick and consisted of anatase TiO2 particles with average diameters of 36 ± 6 nm. ..... For al...
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Influence of Surface-Attachment Functionality on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO2 Kacie R. Mulhern,† Alexandra Orchard,† David F. Watson,* and Michael R. Detty* Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States S Supporting Information *

ABSTRACT: Chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids were compared as sensitizers of nanocrystalline TiO2 in dye-sensitized solar cells (DSSCs). The dyes were constructed around a 3,6-bis(dimethylamino)chalcogenoxanthylium core and varied in the 9 substituent: 5-carboxythien-2-yl in dyes 1-E (E = O, Se), 4-carboxyphenyl in dyes 2-E (E = O, S), 5-phosphonothien-2-yl in dyes 3-E (E = O, Se), and 4-phosphonophenyl in dyes 4-E (E = O, Se). All dyes adsorbed to TiO2 as mixtures of H aggregates and monomers, which exhibited broadened absorption spectra relative to those of purely amorphous monolayers. Surface coverages of dyes and the extent of H aggregation varied minimally with the surface-attachment functionality, the structure of the 9-aryl group, and the identity of the chalcogen heteroatom. Carboxylic acid-functionalized dyes 1-E and 2-E desorbed rapidly and completely from TiO2 into acidified CH3CN, but phosphonic acid-functionalized dyes 3-E and 4-E persisted on TiO2 for days. Short-circuit photocurrent action spectra of DSSCs corresponded closely to the absorptance spectra of dye-functionalized films; thus, H aggregation did not decrease the electroninjection yield or charge-collection efficiency. Maximum monochromatic incident photon-to-current efficiencies (IPCEs) of DSSCs ranged from 53 to 95% and were slightly higher for carboxylic acid-functionalized dyes 1-E and 2-E. Power-conversion efficiencies of DSSCs under white-light illumination were low (250 °C. 1H NMR (400 MHz, CD3OD): δ 7.68 (d, 2 H, J = 9.6 Hz), 7.65 (m, 1 H), 7.52 (d, 2 H, J = 2.8 Hz), 7.22 (s, 1 H), 7.06 (dd, 2 H, J = 2.4 Hz, J = 10 Hz), 3.27 (s, 12 H). 31P NMR (121.5 MHz, (CD3)2NC(O)D): δ 106.0 (septet, J = 717 Hz), 5.4. λmax (0.1 M HPF6 in 25% aqueous CH3CN) 403 (ε = 1.1 × 104 M−1cm−1), 590 (sh), 629 nm (ε = 5.6 × 104 M−1cm−1). HRMS (ESI) m/z 493.0244. Calcd for C21H22O3N2PS80Se+, 493.0248. Preparation of 9-(5-(Diethoxyphosphoryl)thien-2-yl)-3,6bis(dimethylamino)-xanthylium Hexafluorophosphate (7-O). n-Butyllithium (0.90 M in hexanes, 3.54 mL, 3.19 mmol), N,N,N′,N′tetramethylethylenediamine (0.481 mL, 3.19 mmol), and thiophene (0.255 mL, 3.19 mmol) in 50 mL of anhydrous THF at −78 °C and diethyl phosphorochloridate (0.461 mL, 3.19 mmol) dissolved in 5 mL of anhydrous THF were treated as described for the preparation of 7-Se. To the resulting solution, LDA prepared from diisopropylamine

(0.495 mL, 3.51 mmol) and n-butyllithium (0.90 M in hexanes, 3.54 mL, 3.19 mmol) in 5 mL of anhydrous THF were added, and the resulting solution and 3,6-bis(dimethylamino)xanthen-9-one (5-O, 300 mg, 1.06 mmol) were treated as described for the preparation of 7-Se. The product was purified via column chromatography (SiO2, 1:9 diethyl ether/CH2Cl2 followed by 10% CH3OH/CH2Cl2) to give 365 mg (55%) of 7-O as green crystals, mp 216−218 °C. 1H NMR (500 MHz, CD2Cl2): δ 7.81 (dd, 1 H, J = 3.5 Hz, J = 8.5 Hz), 7.64 (d, 2 H, J = 9.5 Hz), 7.43 (t, 1 H, J = 3.5 Hz), 7.01 (dd, 2 H, J = 2.5 Hz, J = 9.5 Hz), 6.82 (d, 2 H, J = 2.5 Hz), 4.26 (q, 4 H, J = 7.0 Hz), 3.32 (s, 12 H), 1.41 (t, 6 H, J = 7.5 Hz). 13C NMR (75.5 MHz, CD2Cl2): δ 157.9, 157.7, 149.3, 138.6 (d, J = 6.8 Hz), 136.6 (d, J = 11 Hz), 134.1 (d, J = 205 Hz), 133.0 (d, J = 17 Hz), 132.7, 131.6, 120.4, 114.9, 114.2, 97.0, 63.6 (d, J = 5.7 Hz), 41.2, 16.5 (d, J = 7 Hz). λmax (CH2Cl2) 593 nm (ε = 1.08 × 105 M−1cm−1). HRMS (EI) m/z 485.1651. Calcd for C25H29O4N2PS+, 485.1658. Preparation of 3,6-bis(Dimethylamino)-9-(5-phosphonothien-2-yl)xanthylium Hexafluorophosphate (3-O). Chlorotrimethylsilane (0.302 mL, 2.38 mmol), NaI (357 mg, 2.38 mmol), and dye 7-O (75 mg, 0.12 mmol) in 8 mL of anhydrous CH3CN were treated as described for the preparation of 3-Se. The crude product was then stirred in a solution of aqueous KPF6 (5% w/w, 100 mL) for 24 h at 40 °C. The crude product was collected via filtration and washed with water (5 mL) and diethyl ether (10 mL). The crude product was recrystallized from CH3OH/CH2Cl2 to give 40 mg (59%) of 3-O as pink crystals, mp >250 °C. 1H NMR (500 MHz, CD3OD, CD2Cl2): δ 7.78 (dd, 1 H, J = 3.5 Hz, J = 8.0 Hz), 7.66 (d, 2 H, J = 9.0 Hz), 7.44 (t, 1 H, J = 1.5 Hz), 7.12 (dd, 2 H, J = 2.5 Hz, J = 9.0 Hz), 6.93 (d, 2 H, J = 2.5 Hz), 3.23 (s, 12 H). 31P NMR (121.5 MHz, CD3OD/(CD3)2NC(O)D): δ 106.0 (septet, J = 717 Hz), 11.9. λmax (0.01 M HCl in CH3OH) 400 (ε = 1.1 × 104 M−1 cm−1), 560 (sh), 594 nm (ε = 5.6 × 104 M−1cm−1). HRMS (ESI) m/z 429.1047. Calcd for C21H22O4N2PS+, 429.1032. Preparation of 3,6-bis(Dimethylamino)-9-(4-bromophenyl)selenoxanthylium Hexafluorophosphate (8-Se). sec-Butyllithium (1.05 M in hexanes, 2.48 mL) was added to 1,4-dibromobenzene (615 mg, 2.61 mmol) in THF (50 mL) at −78 °C. The resulting mixture was stirred for 3 min and was then transferred via cannula into a solution of 5-Se (300 mg, 0.869 mmol) in THF (10 mL) at ambient temperature. The resulting mixture was stirred for 10 min at 40 °C before it was cooled again to ambient temperature. Acetic acid (1 mL) was added, and the mixture was then poured into a 10% v/v solution of stirring, cold, aqueous HPF6 (100 mL). The precipitate was collected via filtration and washed with water (5 mL) and diethyl ether (10 mL). The product was purified via column chromatography (SiO2, 10% diethyl ether/CH2Cl2), followed by recrystallization from diethyl ether/CH2Cl2 to give 455 mg (83%) of 8-Se as green crystals, mp >250 °C. 1H NMR (400 MHz, CD3CN): δ 7.80 (dd, 2 H, J = 2.0 Hz, J = 8.8 Hz), 7.43 (t, 2 H, J = 2.6 Hz), 7.36 (dd, 2 H, J = 1.6 Hz, J = 8.0 Hz), 7.25 (dd, 2 H, J = 1.6 Hz, J = 8.0 Hz), 6.92 (dd, 2 H, J = 9.6 Hz, J = 2.4 Hz), 3.20 (s, 12 H). 13C NMR (75 MHz, CD2Cl2): δ 153.4, 146.2, 138.6, 136.3, 132.3, 131.2, 123.9, 120.0, 115.3, 109.2, 40.9. λmax (CH2Cl2) 603 nm (ε = 1.25 × 105 M−1cm−1). HRMS m/z 485.0119. Calcd for C23H2279BrN280Se+, 485.0126. Preparation of 3,6-bis(Dimethylamino)-9-(4-bromophenyl)xanthylium Hexafluorophosphate (8-O). 1,4-Dibromobenzene (2.51 g, 10.6 mmol), sec-butyllithium (1.4 M in hexanes, 7.59 mL), and xanthone 5-O (1.00 g, 3.54 mmol) in THF (200 mL) were treated as described for the preparation of 8-Se. The final product was recrystallized from diethyl ether/CH2Cl2 to give 752 mg (38%) of 8-O as green crystals, mp 227−231 °C. 1H NMR (400 MHz, CD3CN): δ 7.84 (d, 2 H, J = 8.0 Hz), 7.36 (d, 2 H, J = 8.8 Hz), 7.34 (d, 2 H, J = 9.6 Hz), 7.01 (dd, 2 H, J = 2.4 Hz, J = 9.6 Hz), 6.84 (d, 2 H, J = 2.4 Hz), 3.26 (s, 12 H). 13C NMR (125.7 MHz, CD3CN): δ 158.6, 158.3, 157.5, 132.8, 132.2, 132.1, 124.9, 115.2, 114.1, 97.1, 41.2. λmax (CH2Cl2) 573 nm (ε = 1.09 × 105 M−1cm−1). HRMS m/z 421.0923. Calcd for C23H2279BrN2O+, 421.0910. Preparation of 9-(4-(Diethoxyphosphoryl)phenyl)-3,6-bis(dimethylamino)selenoxanthylium Hexafluorophosphate (9-Se). Triethylamine (0.201 mL, 1.44 mmol) was added to a solution of 7073

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described previously.80,81 TiO2 films were 4.1 ± 0.9 μm thick and consisted of anatase TiO2 particles with average diameters of 36 ± 6 nm.80,81 For the FTO-coated glass electrodes, a dense TiO2 blocking layer was added by spray pyrolysis before spreading the TiO2.82 TiO2 films were functionalized by immersion in concentrated (0.3−2 mM) solutions of the dyes in CH2Cl2 (for carboxylic acid-functionalized dyes 1-E and 2-E) or CH3OH (for phosphonic acid-functionalized dyes 3-E and 4-E) for at least 12 h. Surface Coverage of Dyes on TiO2. The amounts of the adsorbed dyes per projected surface area, herein referred to as surface coverages, were determined by two methods. Surface coverages of 1-E and 2-E were determined by completely desorbing the dyes from TiO2 into 0.1 M HCl in CH3CN. Concentrations of dyes in the resulting solutions were determined from UV/vis absorption spectra using molar absorption coefficients at the absorption maxima (εmax) of 1-E and 2-E in acidified CH3CN (Table S1). Because 3-E and 4-E did not desorb significantly into acidified CH3CN, their surface coverages on TiO2 were determined by quantifying the concentration of dye in adsorption solutions before and after exposure to TiO2 films. Values of εmax of the dyes in CH3OH (Table 1) were used to quantify the

diphenylphosphinoferrocene (133 mg, 0.241 mmol), Pd(OAc)2 (27.0 mg, 0.120 mmol), and KOAc (23.6 mg, 0.241 mmol) in THF (15 mL) heated to reflux. After 15 min, diethylphosphite (0.154 mL, 1.20 mmol) and selenoxanthylium dye 8-Se (830 mg, 1.32 mmol) were added. The resulting mixture was stirred at reflux for 48 h and cooled to ambient temperature, and water (5 mL) was added. The resulting mixture was poured into a 10% v/v solution of cold, aqueous HPF6 (100 mL). After 1 h, the precipitate was collected via filtration and washed with water (5 mL) and diethyl ether (5 mL). The crude product was purified via column chromatography (SiO2, 1:9 diethyl ether/CH2Cl2 followed by 10% CH3OH/CH2Cl2) to give 26 mg (4.4%) of 9-Se as green crystals, mp 240−242 °C. 1H NMR (400 MHz, CD3CN): δ 8.00 (d, 1 H, J = 7.6 Hz), 7.97 (d, 1 H, J = 7.2 Hz), 7.48 (m, 4 H), 7.29 (d, 2 H, J = 10.0 Hz), 6.91 (dd, 2 H, J = 9.6 Hz, J = 2.0 Hz), 4.19 (m, 4 H), 3.20 (s, 12 H), 1.36 (t, 6 H, J = 7.0 Hz). 13C NMR (125.7 MHz, CD3CN): δ 160.6, 153.9, 146.2, 142.2, 138.7, 132.5 (d, J = 9 Hz), 130.8 (d, J = 190 Hz), 130.2 (d, J = 15 Hz), 119.9, 115.9, 109.9, 63.2 (d, J = 5.8 Hz), 40.9, 16.6 (d, J = 5.7 Hz). λmax (CH2Cl2) 602 nm (ε = 1.32 × 105 M−1cm−1). HRMS m/z 543.1310. Calcd for C27H32O3N2P80Se+, 543.1310. Preparation of 3,6-bis(Dimethylamino)-9-(4phosphonophenyl)selenoxanthylium Hexafluorophosphate (4-Se). Diethyl phosphonate selenoxanthylium dye 9-Se (500 mg, 0.727 mmol) and hydrochloric acid (10 mL, 6 M) were stirred at 40 °C for 12 h and then cooled to ambient temperature. Potassium hexafluorophosphate (500 mg, 2.72 mmol) was added, and the resulting mixture was stirred for 12 h. The precipitate was collected via filtration and washed with water (5 mL) and diethyl ether (10 mL). The product was purified via recrystallization from CH3OH to give 156 mg (56%) of 4-Se as a purple solid, mp >250 °C. 1H NMR (400 MHz, CD3OD): δ 8.06 (dd, 2 H, J = 7.6 Hz, J = 19.6 Hz), 7.54 (d, 2 H, J = 2.4 Hz), 7.41 (m, 4 H), 7.00 (dd, 2 H, J = 2.4 Hz, J = 9.6 Hz), 3.26 (s, 12 H). 31P NMR (121.5 MHz, (CD3)2NC(O)D): δ 106.0 (septet, J = 715 Hz), 10.7. λmax (DMF) 393 (ε = 6.6 × 103 M−1cm−1), 570 (sh), 605 nm (ε = 3.33 × 104 M−1cm−1). HRMS m/z 487.0681. Calcd for C23H24O3N2P80Se+, 487.0684. Preparation of 9-(4-(Diethoxyphosphoryl)phenyl)-3,6-bis(dimethylamino) xanthylium Hexafluorophosphate (9-O). Triethylamine (0.134 mL, 0.963 mmol), diphenylphosphinoferrocene (89.0 mg, 0.161 mmol), Pd(OAc)2 (18.0 mg, 0.080 mmol), and KOAc (15.8 mg, 0.161 mmol) in THF (10 mL) were treated as described for the preparation of 9-Se. Diethylphosphite (0.103 mL, 0.803 mmol) and xanthylium dye 8-O (500 mg, 0.883 mmol) were added and treated as described for the preparation of 9-Se. The product was purified via column chromatography (SiO2, 1:9 diethyl ether/CH2Cl2 followed by 10% CH3OH/CH2Cl2) to give 70 mg (11%) of 9-O as green crystals, mp 231−233 °C. 1H NMR (400 MHz, CD3CN): δ 8.00 (d, 1 H, J = 7.6 Hz), 7.97 (d, 1 H, J = 7.2 Hz), 7.48 (m, 4 H), 7.29 (d, 2 H, J = 10.0 Hz), 6.91 (dd, 2 H, J = 9.6 Hz, J = 2.0 Hz), 4.19 (m, 4 H), 3.20 (s, 12 H), 1.36 (t, 6 H, J = 7.0 Hz). 13C NMR (75.5 MHz, CD2Cl2): δ 158.2, 157.8, 157.4, 134.9 (d, J = 200 Hz), 132.4 (d, J = 9 Hz), 131.9, 129.9 (d, J = 15 Hz), 114.7, 113.7, 97.0, 62.9 (d, J = 5.8 Hz), 41.2, 16.6 (d, J = 5.7 Hz). λmax (CH2Cl2) 574 nm (ε = 4.65 × 104 M−1cm−1). HRMS m/z 543.1310. Calcd for C27H32O4N2+, 543.1310. Preparation of 3,6-bis(Dimethylamino)-9-(4-phosphonophenyl) xanthylium Hexafluorophosphate (4-O). Chlorotrimethylsilane (0.285 mL, 2.24 mmol), NaI (336 g, 2.24 mmol), and xanthylium dye 9-O (70.0 mg, 0.112 mmol) in 5 mL of anhydrous CH3CN were treated as described for the preparation of 4-Se. The crude product was purified via recrystallization from CH2Cl2 to give 45 mg (71%) of 4-O as green crystals, mp >250 °C. 1H NMR (400 MHz, CD3OD): δ 7.68 (d, 2 H, J = 9.6 Hz), 7.65 (m, 1 H), 7.52 (d, 2 H, J = 2.8 Hz), 7.22 (s, 1 H), 7.06 (dd, 2 H, J = 2.4 Hz, J = 10 Hz), 3.27 (s, 12 H). 31P NMR (121.5 MHz, (CD3)2NC(O)D): δ 106.0 (septet, J = 715 Hz), 11.2. λmax (DMF) 385 (ε = 1.5 × 104 M−1cm−1), 545 (sh), 573 nm (ε = 7.4 × 104 M−1cm−1). HRMS (ESI) m/z 493.0244. Calcd for C23H24O4N2P+, 493.0248. Preparation of TiO2 Thin Films and Sensitized TiO2 Electrodes. Nanocrystalline TiO2 films were deposited onto glass or fluorinedoped tin oxide (FTO)-coated glass (Pilkington; 12−14 Ω/square), as

Table 1. Photophysical and Adsorption Data for Dyes 1-E through 4-E dye

λmax (nm)a

1-O 1-Se 2-O 2-S 3-O 3-Se 4-O 4-Se

565 600 553 572 567 600 550 580

εmax (M−1cm−1)a 9.6 8.4 1.1 7.0 1.1 3.9 8.8 7.2

× × × × × × × ×

104 104 105 104 104 104 104 104

Γ0 (mol cm−2)b (2.3 (1.8 (2.2 (1.7 (1.1 (1.3 (1.2 (1.5

± ± ± ± ± ± ± ±

0.1) 0.1) 0.1) 0.1) 0.1) 0.1) 0.2) 0.1)

× × × × × × × ×

10−7c,e 10−7c,e 10−7c 10−7c 10−7d 10−7d 10−7d 10−7d

Rb 2.4 1.2 1.1 0.9 1.3 0.7 1.0 0.6

± ± ± ± ± ± ± ±

0.2 0.2 0.1 0.1 0.2 0.1 0.1 0.1

a In CH3OH. bStandard deviations are relative to averages of at least four dye-functionalized TiO2 films; cFrom the desorption of dye into 0.1 M HCl in CH3CN. dFrom decreased absorbance of the adsorption solution. eFrom ref 44.

concentrations of dyes. When the adsorptive method was used for 1-O and 2-O, average surface coverages were (1.8 ± 1.3)-fold greater than the average surface coverages from the desorptive method. Thus, the two methods yielded similar, but not identical, estimates of surface coverage. Desorption of Dyes from TiO2. TiO2 films functionalized with dyes at saturation surface coverages were used for desorption experiments. Dye-functionalized films were immersed into 6 mL of a 0.1 M solution of HCl in CH3CN. The films were removed at various times after immersion, and UV/vis absorption spectra were obtained for both the film and the surrounding solution. Films were then placed into a fresh 6 mL aliquot of the HCl/CH3CN solution. Experiments were run for at least 46 h. The rates at which absorbances at λmax of dye-functionalized films decreased served as a qualitative basis for comparing the kinetics of desorption. Photoelectrochemistry. Short-circuit photocurrent action spectra and photocurrent−photovoltage (J−V) data were obtained as described previously.32,83 A custom-made 500 μL Teflon cell housed the FTO/TiO2/dye working electrode and a Pt mesh counter electrode. The electrolyte was 0.05 M I2, 0.5 M LiI, 0.1 M 4tBP, and 0.15 M HCl in CH3CN. Higher photocurrents were measured when protons were added to the electrolyte, consistent with previous reports.84−86 TiO2 films functionalized with N3 dye were characterized as controls. The electrolyte for the FTO/TiO2/N3 working electrodes consisted of 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium thiocyanate, 0.5 M 4tBP, and 0.6 M PMII in CH3CN.87,88 Experiments involving timedependent photoelectrochemistry were typically run by acquiring three photocurrent action spectra followed by two J−V measurements and then repeating. UV/vis absorption spectra of FTO/TiO2/N3 working 7074

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Scheme 1. Syntheses of Dyes 3-E

Scheme 2. Syntheses of Dyes 4-E

electrodes were acquired before and after the acquisition of photoelectrochemical data.

via lithium−bromine exchange with 1 equiv of s-BuLi. The resulting 4-bromolithiobenzene was added to chalcogenoxanthones 5-O or 5-Se to give 9-(4-bromophenyl)chalcogenorosamines 8-O and 8-Se in 42 and 83% isolated yields, respectively, following workup with aqueous HPF6. The coupling of HPO(OEt)2 with 8-O and 8-Se using Pd(OAc)2 gave diethyl phosphonate derivatives 9-O and 9-Se in 11 and 44% isolated yields, respectively.94 The phosphonate esters were hydrolyzed with 6 N HCl and then stirred with an aqueous solution of KPF6 to give phosphonic acid derivatives 4-O and 4-Se in 71 and 56% isolated yields, respectively. Compounds 4-O and 4-Se were also sparingly soluble in most organic solvents. The 1H and 31P NMR spectra of phosphonic acid-functionalized dyes 3-E and 4-E (Appendix S1 in the Supporting Information) were consistent with the proposed structures. The 1H NMR spectra were consistent with structures 3-E (mirror-plane symmetry) and 4-E (C2 symmetry) displaying only one set of signals for each of two equivalent positions in the chalcogenoxanthylium core. The 31P NMR spectra of these compounds displayed the septet expected for the hexafluorophosphate salt (1JP−F ≈ 700−720 Hz) as well as a second signal as a singlet for the 5-phosphono-2-thienyl group (δ 5.4 and 11.9 for 3-Se and 3-O, respectively) or the 4-phosphonophenyl group (δ 10.7 and 11.2 for 3-Se and 3-O, respectively), which is consistent with the introduction of the phosphonic acid functionality. The 13C NMR spectra of diethyl phosphonate precursors 7-E and 9-E (Appendix S1 in the Supporting Information) to phosphonic acidfunctionalized dyes 3-E and 4-E, respectively, displayed the doublet patterns expected from the splitting of the 13C signals associated with the 9-thienyl groups of dyes 7-E and the 9-aryl groups of dyes 9-E by 31P in the phosphono substituents. One would expect the same patterns in 3-E and 4-E, which were not

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RESULTS AND DISCUSSION Syntheses of Carboxylic Acid-Functionalized Dyes 1-E and 2-E. Chalcogenoxanthones 5-E89,90 (Chart 1) are the key intermediates for the syntheses of both carboxylic acidfunctionalized dyes 1-E and 2-E and phosphonic acidfunctionalized dyes 3-E and 4-E. Dyes 1-E were prepared as previously described via the addition of lithium 5-lithiothiophene-2-carboxylate to 5-E, followed by a workup with aqueous HPF6.78 Dyes 2-E were prepared as previously described via the conversion of chalcogenoxanthones 5-E to the corresponding triflates, followed by Pd-catalyzed coupling of the triflates to 4-carboxyphenylboroxin.79 Syntheses of Phosphonic Acid-Functionalized Dyes 3-E and 4-E. Dyes 3-O and 3-Se were synthesized by an approach similar to the preparation of dyes 1-E. As shown in Scheme 1, thiophene was deprotonated with n-BuLi/TMEDA to give 2-lithiothiophene and quenched with diethylchlorophosphidate to give thiophene derivative 6. Thiophene 6 was deprotonated with LDA in THF at −78 °C to give 6-Li, which was transferred to a THF solution of chalcogenoxanthones 5-O or 5-Se. Workup with aqueous HPF6 gave phosphonate esters 7-O and 7-Se in 55 and 57% isolated yields, respectively. Phosphonate esters 7-O and 7-Se were deprotected using trimethylsilyl iodide,91−93 followed by workup with a solution of aqueous KPF6 to give 3-O and 3-Se in isolated yields of 59 and 84%, respectively. Compounds 3-O and 3-Se were sparingly soluble in most organic solvents. Dyes 4-O and 4-Se were prepared by a different approach as shown in Scheme 2. 1,4-Dibromobenzene was monolithiated 7075

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Figure 1. Absorbance spectra of (a) 1-O and (b) 3-O dissolved in CH3OH and adsorbed to nanocrystalline TiO2 films at various surface coverages. The spectra of solvated dyes are normalized to an absorbance of 1.0 at λmax, and the baseline absorbance is offset such that the dashed line corresponds to 0. Arrows indicate the increase in absorbance with surface coverage.

12−20 nm (550−630 cm−1) relative to those of corresponding thiophene-substituted dyes 1-E and 3-E. For a given structure and chalcogen heteroatom, replacing a carboxylic acid with a phosphonic acid caused only a minimal shift in λmax (≤3 nm) (Table 1). Adsorption and Aggregation of Dyes. Saturation surface coverages (Γ0) of the dyes on nanocrystalline TiO2 films, which were typically generated by immersing films in 1 to 2 mM adsorption solutions and allowing the mixtures to equilibrate, were on the order of 10−7 mol cm−2 (Table 1), consistent with reported values for alkanoic acids, alkylphosphonic acids, and various dyes.66,81,86,96,97 For a given structure and chalcogen heteroatom, values of Γ0 for carboxylic acidfunctionalized dyes were approximately 1.4- to 2.1-fold greater than for the phosphonic acid-functionalized analogues. When each of the dyes was adsorbed to TiO2, the absorbance of the higher-energy band increased significantly relative to that of the lower-energy band (Figure 1 and Figure S2 in the Supporting Information), indicating that the dyes formed H aggregates.14,41−43 For a given dye, the relative intensities of the bands differed slightly from film to film; thus, mixtures of H aggregates and nonaggregated dyes were present on TiO2. The broadened absorption spectra of such mixtures increased the light-harvesting efficiency of dye-functionalized films. The ratio (R) of the absorbance of the higher-energy band (arising from H aggregates) and the lower-energy band (arising from monomers) provides an approximate indication of the extent of aggregation of dyes.39,40 Average values of R for our dyes at subsaturation surface coverages (such that absorbances were on scale) are reported in Table 1; the data reveal several trends. First, values of R for O-containing dyes were approximately 1.5to 2-fold greater than those of the corresponding Se-containing dyes. Dye 1-O underwent significantly more H aggregation than any other dye. Second, thiophene-substituted dyes 1-O and 3-O had approximately 1.3- to 2.2-fold larger values of R than corresponding phenyl-substituted dyes 2-O and 4-O. (Values of R were similar for 3-Se and 4-Se.) This trend may arise from a lower barrier to coplanarity of the 9-thienyl group and the xanthylium core in comparison to that of the 9-phenyl group and the xanthylium core.32,95,98 Finally, for thiophenesubstituted dyes, the replacement of the carboxylic acid with a phosphonic acid resulted in an approximately 1.7- to 1.9-fold decrease in R. Despite these subtle differences, modifying the

sufficiently soluble to acquire 13C NMR spectra readily. The two alkyl carbons of the ethyl esters of 7-E and 9-E were also split by 31P. Finally, the high-resolution mass spectra of dyes 3-E and 4-E were consistent with the expected molecular formulas for the cationic xanthylium core for each structure. The absorption spectra of dyes 3-E posed several interesting problems. In polar aprotic solvents such as DMF, evidence of H aggregation was observed. Dye 3-O, in particular, displayed two distinct bands at 558 and 588 nm in DMF. The ratio of the two bands changed with concentration, with the 558 nm band increasing at higher concentrations, which is consistent with H aggregation. In contrast, the absorption spectrum of 3-O in 0.01 M HCl in CH3OH was consistent with a monomeric species (λmax 594 nm, ε = 5.6 × 104 M−1 cm−1) over a concentration range of 6.7 × 10−6 to 2.7 × 10−5 M. Dye 3-Se in DMF displayed a shoulder at 590 nm with the primary absorbance at 629 nm. In 0.01 M HCl in CH3OH, dye 3-Se behaved as a monomeric species, but the molar absorption coefficient was too low (ε < 2 × 104 M−1cm−1) on the basis of the oscillator strength of diethylphosphate ester 7-Se (622 nm, ε = 1.08 × 105 M−1cm−1). For 3-Se in a protic solvent, the phosphonic acid was acting as a buffer, and the addition of a negatively charged hydroxide or phosphonate to the selenoxanthylium core was in equilibrium with the cationic selenoxanthylium core. In 0.1 M HPF6 in 25% aqueous CH3CN, 3-Se gave a constant chromophore (unchanging with additional acid) with λmax of 629 nm and ε = 5.6 × 104 M−1 cm−1. Dyes 4-E did not show the same propensity to form H aggregates in polar aprotic solvents such as DMF and did not appear to undergo nucleophilic addition as readily in protic solvents. Photophysical Properties of Solvated Dyes. Absorption spectra of dyes 1-E, 2-E, 3-E, and 4-E dissolved in CH3OH are shown in Figure S1 in the Supporting Information. Photophysical and adsorption data are summarized in Table 1. All dyes exhibited intense charge-transfer bands with absorption maxima (λmax) of 550−600 nm (in CH3OH) and higher-energy shoulders. Values of εmax for solvated dyes were 104−105 M−1 cm−1. For a given dye structure, increasing the mass of the chalcogen heteroatom red shifted λmax (for CH3OH solutions) by 19−35 nm (600−1030 cm−1), consistent with our previous reports on 1-E and related dyes.32,95 The absorption maxima of phenyl-substituted dyes 2-E and 4-E were blue-shifted by 7076

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reports that carboxylate−TiO2 linkages are susceptible to hydrolysis under acidic conditions,59,99 whereas phosphonate− TiO2 linkages are not.59,61,62,68,69 The persistence of dyes 3-E and 4-E on TiO2 is advantageous for DSSCs. The lower- and higher-energy absorption bands of TiO2adsorbed 3-E and 4-E red shifted initially upon exposure to acidified CH3CN, and values of R increased throughout the experiments (Figure 2 and Figure S3 in the Supporting Information). (The absorbances of 4-O-modified TiO2 films were off-scale, precluding the precise determination of R.) The increase in R indicates that nonaggregated dyes preferentially desorbed and/or that the dyes underwent additional H aggregation during exposure to acidified CH3CN. Photoelectrochemical Performance of DSSCs. Shortcircuit photocurrent action spectra (IPCE vs wavelength) and white-light J−V data were acquired to evaluate the influence of structure and the surface-attachment functionality on the photoelectrochemical performance. IPCE and J−V measurements were acquired successively for 20−80 min after DSSCs were assembled. IPCE is defined as follows

structure or surface-attachment functionality of dyes did not greatly alter their propensity to aggregate on TiO2. All dyes underwent H aggregation. Persistence of Dyes on TiO2. The lability of dye−TiO2 linkages was characterized by immersing dye-functionalized films in acidified CH3CN. Absorption spectra were acquired as a function of the immersion time (Figure 2 and Figure S3 in

IPCE = α × ϕinj × ηel = α × APCE

(1)

where α is the absorptance {(one minus transmittance), which equals the fraction of photons absorbed}, ηel is the chargecollection efficiency, and APCE is the absorbed photon-tocurrent efficiency.6 For all dye-functionalized TiO2 electrodes, the maximum absorptance of films prior to the assembly of DSSCs was approximately 1; thus, APCE equaled IPCE in the absence of desorption. Photocurrent action spectra for all dyes corresponded closely to the absorptance spectra (Figure 3 and Figure S4 in the Supporting Information). Values of APCE were similar within the higher-energy band of H-aggregated dyes and the lower-energy band of nonaggregated dyes. Therefore, H aggregation did not deleteriously affect ϕinj or ηel under monochromatic illumination. This result is consistent with our previously reported measurement of H-aggregationinduced increases in ϕinj and ηel for dyes 1-E.32,40 Average values of the maximum IPCE in the first photocurrent action spectrum acquired after the assembly of each DSSC (IPCEmax,i) and in the final spectrum acquired after 20− 80 min of successive IPCE and J−V measurements (IPCEmax,f)

Figure 2. Normalized absorbance at λmax as a function of the time that TiO2 films functionalized with dyes 1-O, 1-Se, 3-O, and 3-Se were immersed in 0.1 M HCl in CH3CN. (Inset) Temporal evolution of the absorption spectrum of a 3-O-modified TiO2 film.

Supporting Information). For carboxylic acid-functionalized dyes 1-E and 2-E, 80−95% of dyes desorbed from TiO2 within 5 min, and desorption was essentially complete after 30 min. In contrast, the surface coverages of phosphonic acid-functionalized dyes 3-E and 4-E decreased by only 10−20% within the first 4 h of the immersion of films in acidified CH3CN. Little or no additional desorption occurred during the ensuing 2 days of measurements. During the desorption experiments, we replaced the surrounding HCl/CH3CN solution after each data point was obtained; therefore, differences in the extent of desorption of carboxylic and phosphonic acid-functionalized dyes reflect differences in lability. Our results are consistent with prior

Figure 3. Absorptance and short-circuit photocurrent action spectra of FTO/TiO2 electrodes functionalized with (a) 1-O and (b) 3-O. Absorptance spectra were acquired before the assembly of DSSCs. IPCE data were acquired immediately after and 20 min after the assembly of DSSCs. Error bars represent the standard deviations relative to the average IPCE values of 2 DSSCs. 7077

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Figure 4. J−V data for DSSCs with dyes (a) 1-O and (b) 3-O; arrows indicate the changes in Jsc and Voc with successive measurements.

the average η for N3-sensitized TiO2 was 4.9 ± 0.6 %, which is lower than the originally reported value of approximately 10%.10 Our measured values of Voc were comparable to reported values for N3; however, our average values of Jsc (6.6 ± 0.9 mA cm−2) and f f (52 ± 5 %) were lower than for fully optimized DSSCs. The highest values of η that we achieved for our chalcogenorhodamine dyes ranged from 0.4 to 0.8% (Figure S7 and Table S3 in the Supporting Information); values of Voc, Jsc, and f f were all significantly lower than for N3 under our conditions. The poor J−V performance of our dyes under white-light illumination, despite their high monochromatic IPCEs, suggests that the regeneration of dyes was inefficient at high irradiance. Rapid charge recombination and the inefficient regeneration of dyes often limit η of organic DSSCs.100−104 In ongoing research, we aim to characterize charge recombination and dye regeneration at TiO2−chalcogenorhodamine−electrolyte interfaces. In this article, we focus on the influence of the surfaceattachment functionality of chalcogenorhodamines on the temporal evolution of the performance of DSSCs. Data are summarized in Figure 4 and Figures S8 and S9 in the Supporting Information. For all of our dyes, Voc initially increased by 25−85 mV (10−40%) before leveling off after 30−50 min of successive measurements. We speculate that this effect was associated with the diffusion of I− and/or 4tBP, the concentrations of which may influence dye regeneration and charge recombination,105,106 into the pore structure of TiO2 films. For carboxylic acid-functionalized dyes 1-E and 2-E, Jsc decreased by 50−70% during the course of our experiments (20−80 min). Values of Jsc decreased more rapidly for thiophene-substituted dyes 1-E than for phenyl-substituted dyes 2-E. Values of η for 1-E and 2-E decreased by 15−25% with successive measurements. (An exception was 2-S, for which η did not decrease significantly.) Decreases in Jsc were offset partially by increases in f f; therefore, η did not fall off as rapidly as Jsc. In contrast, for phosphonic acid-functionalized dyes 3-E and 4-E, values of Jsc remained constant and values of η increased by 30−100% during the course of our experiments. The increases in η arose primarily from increases in Jsc and Voc. In summary, for a given structure and chalcogen heteroatom, the initial values of Jsc, Voc, f f, and η were similar. With time, however, the performances of DSSCs incorporating carboxylic acid-functionalized dyes 1-E and 2-E deteriorated significantly, primarily because of the desorption of the dyes. Alternatively,

are compiled in Table S2. Using our instrumentation and cell architecture, we achieved an average IPCEmax,i of 77.4 ± 8.1% for the N3 dye, which is similar to the originally reported value.10 For a given structure of our chalcogenorhodamine dyes, values of IPCEmax,i were generally 25−75% greater for O-containing dyes than for S- or Se-containing dyes. Exceptions were 1-O and 1-Se, for which values of IPCEmax,i were similar. Thiophene-substituted dyes generally exhibited 20−75% greater IPCEmax,i than did the corresponding phenyl-substituted dyes, with the exceptions of 1-O and 2-O, which had similar values. Finally, for a given structure and chalcogen heteroatom, values of IPCEmax,i were 10−40% greater for carboxylic acidfunctionalized dyes (1-E and 2-E) than for phosphonic acidfunctionalized dyes (3-E and 4-E). Decreased electronic coupling via the phosphonic acid relative to the carboxylic acid has been correlated with decreased ϕinj and IPCE.64,73−77 Short-circuit photocurrent action spectra of DSSCs with carboxylic acid-containing dyes decayed rapidly with successive measurements, whereas those of DSSCs with phosphonic acidcontaining dyes were essentially invariant over time (Figure 3 and Figures S4 and S5 and Table S2 in the Supporting Information). Within the time frame of our experiments (20− 80 min), values of IPCEmax for carboxylic acid-containing dyes 1-E and 2-E decreased to 20−60% of IPCEmax,i. Absorption spectra of dye-functionalized FTO/TiO2 working electrodes were acquired after photoelectrochemical measurements (Figure S6 in the Supporting Information). Absorptances at λmax for 1-E and 2-E decreased to 45−75% of the initial values. Therefore, while the decreased IPCEs of carboxylic acidfunctionalized dyes were correlated primarily to the desorption of the dyes and the corresponding decreased visible absorptances of working electrodes, prolonged exposure to electrolytes and illumination also resulted in decreased ϕinj and/or ηel. Photocurrent−photovoltage data were acquired under whitelight illumination, and global energy-conversion efficiencies (η) were calculated as follows η=

Voc × Jsc × ff Pin

(2)

where Voc is the open-circuit voltage, Jsc is the short-circuit photocurrent density, f f is the fill factor, and Pin is the power density of incident light.3 Averaged data are summarized in Table S2 in the Supporting Information. Under our conditions, 7078

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(4) Gerischer, H. Semiconductor Electrochemistry. In Physical Chemistry: An Advanced Treatise; Eyring, H., Henderson, D., Jost, W., Eds.; Academic Press: New York, 1970; Vol. 9A, pp 463−542. (5) Gerischer, H. Electrochemical Techniques for the Study of Photosensitization. Photochem. Photobiol. 1972, 16, 243−260. (6) Watson, D. F.; Meyer, G. J. Electron Injection at Dye-Sensitized Semiconductor Electrodes. Annu. Rev. Phys. Chem. 2005, 56, 119−156. (7) Listorti, A.; O’Regan, B.; Durrant, J. R. Electron Transfer Dynamics in Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3381−3399. (8) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt(III/II)-Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (9) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (10) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of Light to Electricity by cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) Charge Transfer Sensitizers (X = Cl−, Br−, I−, CN−, and SCN−) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382−6390. (11) Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C.; Grätzel, M. Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2-Based Solar Cells. J. Am. Chem. Soc. 2001, 123, 1613−1624. (12) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell. J. Phys. Chem. B 2003, 107, 8981−8987. (13) Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.; Wang, P. Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)Thiophene Conjugated Bipyridine. J. Phys. Chem. C 2009, 113, 6290−6297. (14) Ooyama, Y.; Harima, Y. Molecular Designs and Syntheses of Organic Dyes for Dye-Sensitized Solar Cells. Eur. J. Org. Chem. 2009, 2903−2934. (15) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. A Coumarin-Derivative Dye Sensitized Nanocrystalline TiO2 Solar Cell Having a High Solar-Energy Conversion Efficiency of up to 5.6%. Chem. Commun. 2001, 569−570. (16) Koops, S. E.; Barnes, P. R. F.; O’Regan, B. C.; Durrant, J. R. Kinetic Competition in a Coumarin Dye-Sensitized Solar Cell: Injection and Recombination Limitations Upon Device Performance. J. Phys. Chem. C 2010, 114, 8054−8061. (17) Hagberg, D. P.; Yum, J.-H.; Lee, H. J.; De Angelis, F.; Marinado, T.; Karlsson, K. M.; Humphry-Baker, R.; Sun, L.; Hagfeldt, A.; Grätzel, M.; Nazeeruddin, M. K. Molecular Engineering of Organic Sensitizers for Dye-Sensitized Solar Cell Applications. J. Am. Chem. Soc. 2008, 130, 6259−6266. (18) Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M.-H.; Lee, W.; Park, J.; Kim, K.; Park, N.-G.; Kim, C. A Highly Efficient Organic Sensitizer for Dye-Sensitized Solar Cells. Chem. Commun. 2007, 4887−4889. (19) Tian, H.; Yang, X.; Chen, R.; Zhang, R.; Hagfeldt, A.; Sun, L. Effect of Different Dye Baths and Dye Structures on the Performance of Dye-Sensitized Solar Cells Based on Triphenylamine Dyes. J. Phys. Chem. C 2008, 112, 11023−11033. (20) Zhang, G.; Bala, H.; Cheng, Y.; Shi, D.; Lv, X.; Yu, Q.; Wang, P. High Efficiency and Stable Dye-Sensitized Solar Cells with an Organic Chromophore Featuring a Binary π-Conjugated Spacer. Chem. Commun. 2009, 2198−2200. (21) Im, H.; Kim, S.; Park, C.; Jang, S.-H.; Kim, C.-J.; Kim, K.; Park, N.-G.; Kim, C. High Performance Organic Photosensitizers for DyeSensitized Solar Cells. Chem. Commun. 2010, 46, 1335−1337.

the performance of DSSCs incorporating phosphonic acidfunctionalized dyes 3-E and 4-E persisted or improved with time and successive measurements.

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CONCLUSIONS Phosphonic acid-functionalized chalcogenorhodamine dyes 3-E and 4-E were synthesized and compared with analogous carboxylic acid-functionalized dyes 1-E and 2-E as sensitizers of TiO2. Each class of dyes underwent H aggregation on TiO2, resulting in broadened absorption spectra and increased lightharvesting efficiencies. Dyes 3-E and 4-E exhibited slightly lower surface coverages on TiO2, and H aggregated to a slightly lesser extent than did dyes 1-E and 2-E. However, 3-E and 4-E resisted desorption from TiO2 into acidified CH3CN and the iodide/triiodide electrolyte of DSSCs under conditions in which 1-E and 2-E desorbed rapidly. Initial values of monochromatic IPCE and of Jsc, Voc, f f, and η obtained under whitelight illumination were essentially independent of surfaceattachment group, clearly indicating that neither ϕinj nor ηel decreased significantly upon replacement of the carboxylic acids with phosphonic acids. Notably, however, the decreased lability of dyes 3-E and 4-E gave rise to the vastly improved stability of the performance of DSSCs. Thus, our results indicate that tethering chalcogenorhodamines to TiO2 via phosphonic acids can greatly improve the stability and inertness of interfaces without deleteriously affecting H aggregation, the interfacial electron-transfer reactivity, or the photoelectrochemical performance of DSSCs.

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ASSOCIATED CONTENT

* Supporting Information S

Absorption spectra of solvated and TiO2-adsorbed dyes. Photocurrent action spectra and J−V data of DSSCs. Temporal evolution of surface coverages and photoelectrochemical data. Photophysical and photoelectrochemical data. NMR spectra of dyes 3-Se, 3-O, 4-Se, 4-O, 7-Se, 7-O, 8-Se, 8-O, 9-Se, and 9-O. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their partial support of this research. This work was also supported in part by the National Science Foundation (CHE-1151379). K.R.M. acknowledges the financial support provided by the Silbert Fellowship in Chemistry, Department of Chemistry, University at Buffalo.

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