Selenorhodamine Dye-Sensitized Solar Cells - ACS Publications

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Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance Mark W Kryman, Justin N. Nasca, David F. Watson, and Michael R Detty Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04275 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance Mark W. Kryman,† Justin N. Nasca,† David F. Watson,* and Michael R. Detty* Department of Chemistry, University at Buffalo, The State University of New York Buffalo, New York 14260-3000 †

These authors contributed equally to the manuscript.

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Abstract A library of six selenorhodamine dyes (4-Se – 9-Se) was synthesized, characterized, and evaluated as photosensitizers of TiO2 in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis(julolidyl)- or bis(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO2. Absorption bands of solvated dyes 4-Se – 9-Se were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO2 as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes 4-Se and 7-Se initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65-80% under monochromatic illumination, but the dyes desorbed rapidly from TiO2 into solutions of HCl (0.1 M) in a CH3CN:H2O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes 5-Se, 6-Se, 8-Se, and 9-Se exhibited lower IPCEs (49-65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO2, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO2 in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media.

Keywords: dye sensitization, rhodamine, H-aggregation, hydroxamic acid, phosphonic acid, julolidyl


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Introduction Dye sensitization, in which electrons or holes are injected from photoexcited dyes into semiconductor substrates, is a promising strategy to convert solar irradiance to electrical power.1-4 For many years, the most efficient dye-sensitized solar cells (DSSCs) contained photoanodes consisting of ruthenium(II) polypyridyl sensitizers on nanocrystalline TiO2.2,5,6 Recently, however, power-conversion efficiencies (PCEs) rivaling or exceeding those of the best DSSCs derived from ruthenium-based sensitizers have been reported for DSSCs incorporating metalloporphyrins and various metal-free organic dyes.7-20 Organic sensitizers are potentially advantageous relative to ruthenium(II) complexes because they consist of earth-abundant elements and have high molar absorption coefficients and great structural diversity.12,21-24 However, the electronic absorption bands of organic sensitizers are often narrower than the charge-transfer bands of transition metal complexes.11,22 Some efficient DSSCs have exploited the coadsorption of different organic dyes with complementary absorption spectra to optimize absorption of the solar spectrum and improve PCEs.15,25-27 We and others have pursued an alternative approach, in which the aggregation-induced shifting and broadening of absorption spectra is exploited to maximize light harvesting. H-aggregation (planeto-plane π stacking) of dyes causes a blue shift of absorption bands, whereas J-aggregation (head-to-tail interactions) causes a red shift.21,28-30 The effects of aggregation on reported photoelectrochemical performance of DSSCs has been mixed. Aggregation of some organic dyes has led to decreased photocurrents and PCEs,31-39 whereas for other dyes aggregation has been correlated with increased or unchanged photocurrent efficiencies.40-48 We have studied chalcogenorhodamine dyes as organic sensitizers for DSSCs and photocatalysts.40,41,49-51 The incorporation of sulfur or selenium atoms into the xanthylium core red-shifts absorption bands by 0.07-0.12 eV relative to analogous oxygen-containing rhodamine dyes, increasing overlap with the solar spectrum.40,41 We have further improved lightharvesting efficiency through aggregation-induced spectral shifts. Selenorhodamine dye 1-Se (Chart 1) adsorbs to nanocrystalline TiO2 as a mixture of H-aggregated and monomeric dyes.40,41,50 Such mixtures exhibit blue-shifted and broadened absorption spectra relative to those of TiO2 films functionalized with 3 ACS Paragon Plus Environment

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2-Se (Chart 1), which aggregates to a much lesser extent on TiO2 due to the increased barrier to coplanarity of the thienyl group with the xanthylium core.40,49 Notably, H-aggregation of 1-Se and related dyes results in a 2-3-fold increase in the quantum yield of electron injection (φinj) and substantial corresponding increases in the absorbed photon-to-current efficiency (APCE) and incident photon-tocurrent efficiency (IPCE) of DSSCs.40,41,49,50 These effects probably derive from the increased electronic coupling between the xanthylium core of the dyes and the thienyl group, which are nearly coplanar within H-aggregates.51

Chart 1. Despite the potentially significant advantages associated with H-aggregation of 1-Se and related chalcogenorhodamine dyes, they have thus far suffered from two drawbacks. First, although we have measured monochromatic IPCEs exceeding 85-90% for the chalcogenorhodamine dyes at low light intensities,40,41 the PCEs of DSSCs derived from these dyes, under simulated solar illumination, are poor (1% or less), owing to rapid charge recombination and inefficient regeneration of the oxidized sensitizers following electron injection.41 Second, carboxylated dyes such as 1-Se desorb rapidly from ACS Paragon Plus Environment

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TiO2 when exposed to solutions containing small cations such as H+ and Li+, which are required to achieve efficient electron injection and high IPCEs.40,41,49 We and others have shown that phosphonic acids exhibit greater persistence and stability than corresponding carboxylic acids on TiO2.41,52-58 However, although the phosphonic acid derivative of 1-Se (3-Se, Chart 1) is indeed much more inert than 1-Se on TiO2, it exhibited approximately two-fold lower φinj and decreased IPCE, perhaps owing to decreased electronic coupling through the phosphonyl group.41,50 Notably, Crabtree and coworkers recently reported that ruthenium(II) polypyridyl dyes, metal-free dyes, and other adsorbates with hydroxamic acid surface-anchoring groups were more stable and inert on TiO2 than analogous carboxylic acid-bearing dyes while maintaining high values of φinj.59-63 Thus, hydroxamic acids are intriguing alternatives to the more-commonly employed carboxylic and phosphonic acids. We have synthesized the new selenorhodamine dyes 4-Se through 9-Se (Chart 1) and characterized their performance as sensitizers of TiO2. Dyes bearing carboxylic acid, phosphonic acid, and hydroxamic acid groups were synthesized to evaluate the influence of surface-anchoring chemistry on the stability and persistence of dyes and on electron-injection reactivity. Additionally, dyes 4-Se through 9-Se have increased steric bulk relative to 1-Se through the incorporation of bis(julolidyl) or bis(half-julolidyl) functionality at both the 3- and 6-amino positions, leading to red-shifted absorption onsets and potentially influencing the kinetics of charge recombination and dye regeneration. This article reports the syntheses of these new dyes and the influence of the structure and surface-anchoring chemistry of the dyes on photophysics, stability and persistence on TiO2, and performance of the sensitizers in DSSCs.

Experimental Section Materials and Methods. Reagents and solvents were obtained from the following sources and, unless otherwise stated, used as received: (1) Sigma-Aldrich: iodine, lithium iodide (anhydrous, beads, ~10 mesh, 99.99%), guanidinium thiocyanate, 1-methyl-3-propylimidazolium iodide (PMII), 4-tert-


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butylpyridine (4tBP); (2) JT Baker: acetonitrile (CH3CN), methylene chloride (CH2Cl2); (3) EMD Chemicals:

concentrated

HCl;

(4)

Dyesol:

cis-bis(4,4ʹ-dicarboxy-2,2ʹ-

bipyridine)dithiocyanatoruthenium(II) (N3); (5) EMD: electrochemical grade acetonitrile. Reagents and solvents used in the synthesis of the selenorhodamine dyes were purchased from Sigma-Aldrich unless otherwise noted. Tetrahydrofuran (THF), used as a solvent for syntheses of dyes, was dried over sodium/benzophenone ketyl and freshly distilled prior to use. Other solvents for syntheses were dried with activated molecular sieves. Alkyllithium reagents (n-butyl-, sec-butyl-, tert-butyl-) were titrated immediately prior to use with anhydrous diphenylacetic acid. N,N,N′,N′-Tetramethylethylenediamine (TMEDA), diisopropylamine, diethylamine, piperidine, and thiophene were distilled and stored over molecular sieves prior to use. Instrumentation and Spectroscopic Characterization. Concentration in vacuo was performed on a Büchi or Heidolph rotary evaporator. 1H NMR spectra were obtained on a 300 MHz Mercury spectrometer or a 400 or 500 MHz Varian spectrometer, referenced to residual solvent, or for chloroform, to tetramethylsilane (TMS).

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C NMR spectra were obtained on a 300 MHz Mercury

spectrometer or a 500 MHz Varian spectrometer, operating at 75.5 and 125 MHz respectively, referenced to residual solvent carbon signal. UV/Vis spectra were obtained on a Perkin Elmer Lambda 12 or Agilent 8453 spectrophotometer. Melting points were measured on a Büchi capillary melting point apparatus and are uncorrected. Infrared spectra were obtained on a Perkin Elmer Paragon 1000. General Synthetic Procedures. All reactions were performed in round bottom flasks equipped with magnetic stir bars under inert atmosphere unless otherwise stated. Selenoxanthones 10-Se and 11Se, which were intermediates in the syntheses of selenorhodamine dyes 4-Se through 9-Se, are shown in Scheme 1 and were described previously.64 Lithium 5-lithiothiophene-2-carboxylate (12, Scheme 1) was prepared as previously described.65 Thiophene-2-diethylphosphate (13) was prepared as previously described.66,67 Preparation of Bis(julolidyl) Selenorhodamine 4-Se (Chart 1). n-Butyllithium (2.22 M in hexanes, 2.47 mL, 5.47 mmol) was added dropwise to a stirred solution of N,N-diisopropylamine (791 6 ACS Paragon Plus Environment

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µL, 5.61 mmol) in THF (10 mL) at -78 °C. The resulting mixture was stirred for 45 min before it was transferred to a stirred solution of 2-thiophenecarboxylic acid (342 mg, 2.67 mmol) in THF (25 mL) at 78 °C. The resulting mixture was stirred at -78 °C for 30 min before the lithium 5-lithiothiophene-2carboxylate (12) was transferred via cannula to a stirred solution of selenoxanthone 10-Se (300 mg, 0.667 mmol) in THF (10 mL) at -78 °C. The resulting solution was stirred at -78 °C for 0.5 h before it was allowed to warm to ambient temperature and heated to 50 °C. After cooling to ambient temperature, glacial acetic acid (1 mL) was added, and the resulting mixture was poured into 200 mL of 10% (by weight) aqueous HPF6. The resulting mixture was stirred 12 h, and the precipitate was collected via filtration and then washed with water (50 mL) and diethyl ether (100 mL). The crude product was purified by recrystallization from CH3CN/Et2O, yielding 414 mg (88%) of the desired product 4-Se as a blue solid, mp >250 °C: 1H NMR (500 MHz, CD3OD) δ 7.90 (d, 1 H, J = 3.0 Hz), 7.19 (d, 1 H, J = 3.0 Hz), 7.17 (s, 2 H), 3.56–3.48 (m, 8 H), 2.86 (t, 4 H, J = 6.5 Hz), 2.71 (t, 4 H, J = 6.5 Hz), 2.20–2.14 (m, 4 H), 2.00–1.90 (m, 4 H);

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C NMR (75.5 MHz, CD3OD) δ 165.1, 149.0, 148.5, 145.6, 140.5, 136.6,

134.2, 133.8, 131.4, 125.3, 119.8, 116.8, 51.7, 50.7, 28.1, 26.2, 20.8, 20.5; λmax (MeOH) 622 nm (ε = 1.50 × 105 M-1 cm-1); λmax (CH2Cl2) 629 nm (ε = 1.63 × 105 M-1 cm-1); IR (film, NaCl) 3250, 2890, 2875, 1589, 1540, 1450, 1351, 1308 cm-1; HRMS (ESI, HRDFMagSec) m/z 561.1109 (calcd for C30H29N2O2S80Se+: 561.1109). Preparation of Bis(julolidyl) Selenorhodamine 5-Se (Chart 1). Thionyl chloride (51 µL, 0.71 mmol) was slowly added to a stirred solution of selenorhodamine 4-Se (100 mg, 0.142 mmol) in CH2Cl2 (5 mL) at 0 °C. The reaction mixture was allowed to warm to ambient temperature and stirred for 12 h. N,N-Diisopropylethylamine (252 µL, 1.42 mmol) and O-tetrahydro-2H-pyran-2yl)hydroxylamine (PHA, 166 mg, 1.42 mmol) were then added, and the reaction mixture was stirred for 1.5 h before water (50 mL) was added. The resulting mixture was extracted with CH2Cl2 (3 × 50 mL), and the combined organic fractions were dried over Na2SO4 and concentrated under reduced pressure. The crude product was recrystallized from CH3CN/Et2O and collected via filtration. The resulting blue solid was then


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dissolved in 1:1 CH2Cl2/MeOH, and p-toluenesulfonic acid monohydrate (0.1 eq) was added. The reaction mixture was allowed to stir for 16 h before being concentrated under reduced pressure, dissolved in THF (4 mL), and poured into an aqueous solution of KPF6 (5% w/w, 50 mL). The resulting solid was collected via filtration after stirring 3 h and washed with water (20 mL) and diethyl ether (100 mL). The crude product was purified by recrystallization from CH3CN/Et2O, yielding 88 mg (88%) of the desired product 5-Se as a purple solid, mp 225–227 °C: 1H NMR (500 MHz, CD2Cl2) δ 7.78 (br s, 1 H), 7.18 (s, 2 H), 7.12 (d, 1 H, J = 3.5 Hz), 3.55–3.43 (m, 8 H), 2.84 (t, 4 H, J = 6.5 Hz), 2.71 (t, 4 H, J = 6.0 Hz), 2.23–2.13 (m, 4 H), 2.02–1.92 (m, 4 H); λmax (MeOH) 621 nm (ε = 7.50 × 104 M-1 cm-1); λmax (CH2Cl2) 622 nm (ε = 7.90 × 104 M-1 cm-1); IR (film on NaCl) 3190, 2940, 2927, 1590, 1540, 1440, 1354, 1305 cm-1; HRMS (ESI, HRDFMagSec) m/z 576.1216 (calcd for C30H30N3O2S80Se+: 576.1218); Anal. Calcd for C30H30N3O2SSe.PF6: C, 50.01; H, 4.20; N, 5.83. Found: C, 50.02; H, 4.37: N, 5.73. Material was too insoluble in organic solvents for the acquisition of 13C NMR spectra. Preparation of Bis(julolidyl) Selenorhodamine 6-Se (Chart 1). n-Butyllithium (1.18 M in hexanes, 4.41 mL, 5.21 mmol) was added dropwise to a stirred solution of N,N-diisopropylamine (810 µL, 5.74 mmol) in THF (10 mL) at at -78 °C. The resulting mixture was stirred for 45 min before it was transferred to a stirred solution of thiophene-2-diethylphosphonate (13, 494 µL, 2.67 mmol) in THF (40 mL) at -78 °C. This mixture was stirred at -78 °C for 1 h before a solution of 10-Se (300 mg, 0.67 mmol) in THF (10 mL) was added. The resulting solution was stirred at -78 °C for 0.5 h before it was allowed to warm to ambient temperature and heated to 50 °C for 30 min. After cooling to ambient temperature, glacial acetic acid (2 mL) was added, and the resulting mixture was poured into 200 mL of 10% (by weight) aqueous HPF6. The resulting mixture was stirred 12 h and the precipitate was collected via filtration and then washed with water (50 mL) and diethyl ether (100 mL). The product was purified via column chromatography (SiO2, 1:9 Et2O:CH2Cl2, Rf = 0.2), followed by recrystallization from CH2Cl2/Et2O, yielding 287 mg (54%) of 14-Se as a purple solid, mp 157–159 °C: 1H NMR (500 MHz, CD2Cl2) δ 7.72 (dd, 1 H, J = 4.0, 8.5 Hz), 7.19 (t, 1 H, J = 3.5 Hz), 7.11 (s, 2 H), 4.28–4.16 (m, 4 H),


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3.55–3.44 (m, 8 H), 2.84 (t, 4 H, J = 6.5 Hz), 2.69 (t, 4 H, J = 5.5 Hz), 2.17 (quintet, 4 H, J = 6.5 Hz), 1.98 (quintet, 4 H, J = 6.5 Hz), 1.39 (t, 6 H, J = 6.5 Hz); 13C NMR (75.5 MHz, CD2Cl2) δ 148.6, 148.5, 146.0 (d, J = 6.9 Hz), 140.4, 136.2 (d, J = 10.4 Hz), 134.0, 131.6 (d, J = 17.3 Hz), 131.3 (d, J = 207 Hz), 125.1, 119.7, 116.8, 63.4 (d, J = 5.7 Hz), 51.6, 50.7, 28.1, 26.1, 20.7, 20.5, 16.5 (d, J = 7.5 Hz); λmax (MeOH) 629 nm (ε = 1.15 x 105 M-1 cm-1); λmax (CH2Cl2) 629 nm (ε = 1.39 x 105 M-1 cm-1); HRMS (ESI, HRDFMagSec) m/z 653.1508 (calcd for C33H38N2O3PS80Se+: 653.1500). Bromotrimethylsilane (300 µL, 2.3 mmol) was added to a stirred solution of NaI (169 mg, 1.13 mmol) in acetonitrile (3 mL). After 2.5 h, 14-Se (30 mg, 0.038 mmol) was added, and the resulting mixture was stirred at ambient temperature for 20 min. The reaction mixture was diluted with acetonitrile (25 mL) and methanol (10 mL) and then poured into 100 mL of 10% (by weight) aqueous HPF6. Methylene chloride (30 mL) was added and the resulting mixture was vigorously stirred for 4 h. The organic fraction was separated and stirred with an aqueous solution of KPF6 (5% w/w, 50 mL) for 24 h. The resulting precipitate was collected via filtration and washed with water (5 mL) and diethyl ether (50 mL) to give 26 mg (93%) of 6-Se as a dark blue solid, mp 177–178 °C: 1H NMR (500 MHz, 1:1 CD2Cl2/CD3OD) δ 7.66 (dd, 1 H, J = 4, 9 Hz), 7.15–7.11 (m, 3 H), 3.51–3.43 (m, 8 H), 2.81 (t, 4 H, J = 6.5 Hz), 2.65 (t, 4 H, J = 6.5 Hz), 2.14 (quintet, 4 H, J = 6 Hz), 1.93 (quintet, 4 H, J = 6 Hz); 31P NMR (121.5 MHz, CD2Cl2/CD3OD) δ 6.13; λmax (DMF) 623 nm (ε = 6.80 × 104 M-1 cm-1); HRMS (ESI, HRDFMagSec) m/z 597.0871 (calcd for C29H30N2O3PS80Se+: 597.0874). Material was too insoluble in organic solvents for the acquisition of 13C NMR spectra. Preparation of Bis(half-julolidyl) Selenorhodamine 7-Se (Chart 1). n-Butyllithium (2.22 M in hexanes, 2.44 mL, 5.42 mmol), N,N-diisopropylamine (784 µL, 5.56 mmol), 2-thiophenecarboxylic acid (339 mg, 2.65 mmol), and 11-Se (300 mg, 0.662 mmol) in THF (10 + 25 + 10 mL) were treated as described for the preparation of 4-Se. The resulting precipitate was collected via filtration after 12 h of stirring and washed with water (50 mL) and diethyl ether (100 mL). The crude product was purified by recrystallization from CH3CN/Et2O, yielding 421 mg (90%) of 7-Se as a blue solid, mp > 250 °C: 1H NMR (500 MHz, CD3OD) δ 7.89 (d, 1 H, J = 4 Hz), 7.52 (s, 2 H), 7.44 (s, 2 H), 7.24 (d, 1 H, J = 4 Hz), 9 ACS Paragon Plus Environment

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3.61 (t, 4 H, J = 5.5 Hz), 3.26 (s, 6 H), 1.78 (t, 4 H, J = 5.5 Hz), 1.14 (s, 12 H); 13C NMR (75.5 MHz, CD3OD) δ 164.3, 151.3, 151.0, 145.3, 145.0, 138.4, 136.2, 134.5, 132.4, 132.2, 121.1, 109.4, 40.2, 35.4, 32.9, 28.9; IR (film on NaCl) 3408, 2960, 1592, 1473, 1448, 1398, 1321, 1300, 1254 cm-1; λmax (MeOH) 609 nm (ε = 1.51 × 105 M-1 cm-1); λmax (CH2Cl2) 611 nm (ε = 1.45 × 105 M-1 cm-1); HRMS (ESI, HRDFMagSec) m/z 565.1420 (calcd for C30H33N2O2S80Se+: 565.1422). Preparation of Bis(half-julolidyl) Selenorhodamine 8-Se (Chart 1). Thionyl chloride (51 µL, 0.71 mmol), 7-Se (100 mg, 0.14 mmol), N,N-diisopropylethylamine (254 µL, 1.42 mmol), and Otetrahydro-2H-pyran-2yl)hydroxylamine (THA, 166 mg, 1.42 mmol) in CH2Cl2 (5 mL) were treated as described for the preparation of 5-Se. Following recrystallization from CH3CN/Et2O, the resulting blue solid was dissolved in 1:1 CH2Cl2/MeOH, and p-toluenesulfonic acid monohydrate (0.1 eq) was added. The reaction mixture was allowed to stir for 16 h before being concentrated under reduced pressure, dissolved in THF (4 mL), and poured into an aqueous solution of KPF6 (5% w/w, 50 mL). After 3 h stirring, the resulting solid was collected via filtration and washed with water (20 mL) and diethyl ether (100 mL). The crude product was purified by recrystallization from CH3CN/Et2O, yielding 89 mg (87%) of 8-Se as a purple solid, mp 239–240 °C: 1H NMR (500 MHz, CD2Cl2) δ 7.86 (s, 1 H), 7.44 (s, 2 H), 7.19 (d, 1 H, J = 3.0 Hz), 7.17 (s, 2 H), 3.56 (t, 4 H, J = 6.0 Hz), 3.24 (s, 6 H), 1.74 (t, 4 H, J = 6.0 Hz), 1.12 (s, 12 H); λmax (MeOH) 611 nm (ε = 7.20 × 104 M-1 cm-1), λmax (CH2Cl2) 612 nm (ε = 7.6 × 104 M-1 cm-1); IR (film on NaCl) 3375, 2923, 2875, 1592, 1540, 1473, 1456, 1399, 1320, 1300, 1254 cm-1; HRMS (ESI, HRDFMagSec) m/z 580.1531 (calcd for C30H34N3O2S80Se+: 580.1530); Anal. Calcd for C30H34N3O2SSe.PF6: C, 49.73; H, 4.73; N, 5.80. Found: C, 49.81; H, 4.77: N, 5.78. Material was too insoluble in organic solvents for the acquisition of 13C NMR spectra. Preparation of Bis(half-julolidyl) Selenorhodamine 9-Se (Chart 1). n-Butyllithium (1.05 M in hexanes, 4.91 mL, 5.16 mmol), N,N-diisopropylamine (803 µL, 5.69 mmol), thiophene-2diethylphosphonate (13, 490 µL, 2.65 mmol), and 11-Se (300 mg, 0.662 mmol) in THF (10 + 40 + 10 mL) were treated as described for the preparation of 6-Se. The product was purified via column


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chromatography (SiO2, 1:9 Et2O:CH2Cl2, Rf = 0.3), followed by recrystallization from CH2Cl2/Et2O, yielding 280 mg (53%) of 15-Se as a purple solid, mp 143–145 °C: 1H NMR (500 MHz, CD2Cl2) δ 7.81 (dd, 1 H, J = 3.5, 8.0 Hz), 7.40 (s, 2 H), 7.27 (t, 1 H, J = 3.5 Hz), 7.18 (s, 2 H), 4.27–4.13 (m, 4 H), 3.57 (t, 4 H, J = 6.0 Hz), 3.25 (s, 6 H), 1.77 (t, 4 H, J = 6.5 Hz), 1.37 (t, 6 H, J = 7.0 Hz), 1.12 (s, 12 H); 13C NMR (75.5 MHz, CD2Cl2) δ 150.2, 150.1, 145.6 (d, J = 8.0 Hz), 143.9, 136.8 (d, J = 11.6), 135.1, 131.8 (d, J = 204 Hz), 131.5, 131.3, 120.5, 108.2, 63.6 (d, J = 5.7 Hz), 48.7, 40.1, 34.6, 32.1, 28.5, 16.4 (d, J = 6.9 Hz); λmax (MeOH) 616 nm (ε = 1.20 x 105 M-1 cm-1); λmax (CH2Cl2) 616 nm (ε = 1.33 x 105 M-1 cm1

); HRMS (ESI, HRDFMagSec) m/z 657.1822 (calcd for C33H42N2O3PS80Se+: 657.1813). Bromotrimethylsilane (296 µL, 2.25 mmol), NaI (168 mg, 1.12 mmol), and 15-Se (30 mg, 0.037

mmol) in acetonitrile (3 mL) were treated as described for the preparation of 6-Se. The resulting precipitate was collected via filtration and washed with water (5 mL) and diethyl ether (50 mL) to give 17 mg (61%) of 9-Se as a dark blue solid, mp 170–172 °C: 1H NMR (500 MHz, 1:1 CD2Cl2/CD3OD) δ 7.71 (br s, 1 H), 7.49 (s, 2 H), 7.23–7.13 (m, 3 H), 3.59–3.50 (m, 4 H), 3.22 (s, 6 H), 1.74 (t, 4 H, J = 6.0 Hz), 1.10 (s, 12 H); 31P NMR (121.5 MHz, CD2Cl2/CD3OD) δ 5.4; λmax (DMF) 609 nm (ε = 7.20 × 104 M-1 cm-1); HRMS (ESI, HRDFMagSec) m/z 601.1184 (calcd for C29H34N2O3PS80Se+: 601.1187). Material was too insoluble in organic solvents for the acquisition of 13C NMR spectra. Preparation of TiO2 Thin Films and Sensitized TiO2 Electrodes. Nanocrystalline TiO2 films were deposited onto glass microscope slides (VWR) or fluorine-doped tin oxide (FTO)-coated glass slides (Pilkington; 12-14 Ω/square) as described previously.68,69 The films were 4.1 ± 0.9 µm thick and consisted of anatase TiO2 nanoparticles with average diameter of 36 ± 6 nm.68,69 The average projected surface area of the films was 3.5 ± 0.2 cm2. TiO2-coated FTO-on-glass slides for photoelectrochemical measurements were immersed into solutions of selenorhodamine dyes in CH2Cl2 (> 0.1 mM) or solutions of N3 in ethanol (0.75 mM) for at least 12 h, then removed and rinsed with the adsorption solvent, then stored in the dark.


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Page 12 of 37

Cyclic Voltammetry. Cyclic voltammograms of solvated dyes were acquired with a Princeton Applied Research Versastat 3 potentiostat cycling at 100-750 mV s-1, using a one-compartment cell with a Pt wire working electrode, Pt mesh counter electrode, and SCE reference electrode. The electrolyte was 0.1 M tetra-n-butylammonium perchlorate in acetonitrile that had been deaerated by purging with Ar for 10 min. Adsorption, Aggregation, and Persistence of dyes on TiO2. Kinetics of adsorption and aggregation of selenorhodamine dyes were monitored by immersing TiO2-coated glass slides in solutions of selenorhodamine dyes in CH2Cl2 for 5 s, removing the films, immersing them for 1-2 s in CH2Cl2 to remove dye-containing solution and residual dyes deposited onto TiO2 via evaporation of solvent,52 acquiring UV/Vis absorption spectra, then repeating. Concentrations of the dyes were 0.040.11 mM and are listed in the captions to Figure 1 and Figure S1 in Supporting Information. Absorption spectra of dye-functionalized TiO2 films were acquired in transmission mode. Slides were held perpendicular to the light source, which had a diameter of 0.8 cm2. Persistence of adsorbed dyes was evaluated by immersing selenorhodamine dye-functionalized TiO2-coated glass slides in a solution of HCl (0.1 M) in CH3CN:H2O (120:1 v:v). The slides were removed from the solution at various times and rinsed with CH3CN. UV/Vis absorption spectra were then acquired, and the films were immersed into fresh aliquots of acidified CH3CN. Such experiments were performed over a 48 h time period. Photoelectrochemistry. Photocurrent density-photovoltage (J-V) data under white-light illumination and short-circuit photocurrent action spectra under monochromatic illumination were acquired using instrumentation described previously.40 Two-electrode cells were assembled, consisting of a dye-functionalized TiO2-on-FTO working electrode and a platinum mesh counter electrode. Electrodes were housed within a custom-made Teflon cell. The illuminated area of the electrodes was 0.478 cm2. For DSSCs with selenorhodamine-modified TiO2 electrodes, the electrolyte consisted of I2 (0.05 M), LiI (0.5 M), 4tBP (0.1 M), and HCl (0.15 M) in CH3CN:H2O (80:1 v:v). N3-sensitized TiO2 electrodes served as controls. For N3-based DSSCs, the electrolyte consisted of I2 (0.05 M), LiI (0.1 M), guanidinium thiocyanate (0.1 M), 4tBP (0.5 M), and PMII (0.6 M) in CH3CN. For a given 12 ACS Paragon Plus Environment

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selenorhodamine-derived DSSC, short-circuit photocurrent action spectra were acquired consecutively over 100 min. J-V data were acquired using separate selenorhodamine-modified TiO2 electrodes.

Results and Discussion Syntheses of Selenorhodamine Dyes. Syntheses of carboxylic acid derivatives (4-Se, 7-Se) and hydroxamic acid derivatives (5-Se, 8-Se) are shown in Scheme 1. Deprotonation of commercially available 2-thiophenecarboxylic acid with 2.0 equivalents of LDA in THF at -78 °C afforded the lithium 5-lithiothiophene-2-carboxylate (12).65 Addition of 12 to a stirred suspension of 10-Se or 11-Se in THF and subsequent workup with aqueous HPF6 gave the desired 9-(5-carboxy-2-thienyl) selenorhodamines 4-Se and 7-Se in 88% and 90% yield, respectively, following recrystallization from CH3CN/Et2O (Scheme 1). HO

N H

OH O

O S

Li 1)

O

1) SOCl2, CH2Cl2 O 2) DiPEA, H2N

S

CO2Li

S

O

12 N

Se

2) aq HPF6

N

N

Se

10-Se

3) p-TsOH

N

N

Se

PF6

4-Se

HO

OH

O

Me

Me

Li Me 1)

S

S

CO2Li

Me

PF6

N H

O

O

Me

N

5-Se

Me

Me

Me

1) SOCl2, CH2Cl2 O 2) DiPEA, H2N

S

O Me

Me

Me

Me

12 N Me

Se 11-Se

N Me

2) aq HPF6

N Me

Se 7-Se

N PF6 Me

3) p-TsOH

N Me

Se N 8-Se PF6 Me

Scheme 1. Access to the carboxylic acid dyes allowed for the synthesis of the hydroxamic acid derivatives 5-Se and 8-Se. Selenorhodamines 4-Se and 7-Se were stirred with thionyl chloride in CH2Cl2 at ambient temperature for 12 h. Addition of diisopropylethylamine (DIPEA) and the O-tetrahydropyranyl (THP)protected hydroxylamine afforded the crude THP-protected hydroxamic acids following 1.5 h of stirring at ambient temperature. The THP-protected dyes proved to be difficult to purify, as the recovered carboxylic acid and deprotected hydroxamic acid impurities could not be removed via recrystallization, 13 ACS Paragon Plus Environment

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Page 14 of 37

and a significant amount of the THP-protected dye was found to adhere to silica upon purification via column chromatography. Due to these issues, the crude THP-protected dyes were deprotected without further purification with a catalytic amount of p-toluenesulfonic acid (0.1 eq) in a 1:1 mixture of methanol and CH2Cl2. After 16 h of stirring at ambient temperature, the crude hydroxamic acids were poured into aqueous KPF6, and the resulting solid was collected by filtration and then recrystallized from CH3CN/Et2O to give 5-Se in 88% yield and 8-Se in 87% yield (Scheme 1). Phosphonic acid derivatives 6-Se and 9-Se were synthesized as shown in Scheme 2. Thiophene2-diethylphosphonate (13) was prepared according to literature procedures.66,67 Deprotonation of 13 with excess LDA at -78 °C for 1 h gave 5-lithiothiophene-2-diethylphosphonate, which was added to stirred suspensions of 10-Se or 11-Se in THF. Subsequent workup with aqueous HPF6 gave the desired phosphonate esters 14-Se and 15-Se in 53-54% yield (Scheme 2).

Scheme 2. The phosphonate esters in Scheme 2 were deprotected with excess trimethylsilyl iodide (generated from trimethylsilyl bromide and sodium iodide) in acetonitrile and added to aqueous HPF6. A mixture of iodide and PF6 salts appeared to be present, and the isolated solids were stirred in aqueous KPF6 at 40 °C for 24 h.41 The resulting phosphonic acid dyes 6-Se and 9-Se were isolated in yields of 93% and 61%, respectively.


14

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Table 1. Photophysical properties of dyes dissolved in CH3OH. Dye 4-Se 5-Se 6-Se 7-Se 8-Se 9-Se a

λmax (nm) a

ε (M-1cm-1) a

λmax,em (nm) a

E1/2+/0 (V vs SCE) a

E1/2+/* (V vs SCE) a

622 621 629 609 611 616

1.50 × 105 7.50 × 104 1.15 × 105 1.51 × 105 7.20 × 104 1.20 × 105

648 645 643 641 639 630

0.91 0.92 0.92 1.05 1.10 1.08

-1.05 -1.04 -1.04 -0.93 -0.88 -0.92

Abbreviations are defined in the text.

Photophysical and Electrochemical Properties of Solvated Selenorhodamine Dyes. The dyes in solution exhibited visible absorption bands from approximately 525-675 nm with wavelengths of maximum absorption (λmax) of 609-629 nm and molar absorption coefficients (ε) at λmax of approximately 105 M-1cm-1, and corresponding emission bands with wavelengths at the emission maxima (λmax,em) of 630-648 nm (Table 1, Figs. 1, S1). Values of λmax for the bis(julolidyl) dyes 4-Se–6Se were red-shifted by 10–13 nm (0.033-0.043 eV) relative to the corresponding bis(half-julolidyl) analogues 7-Se–9-Se. Within a given series (4-Se–6-Se or 7-Se–9-Se), varying the anchoring group shifted λmax by ≤ 7 nm with the phosphonic acid-functionalized dyes exhibiting the longest-wavelength absorption maxima. Absorption spectral profiles of the dyes were unchanged over an approximately 30fold range of concentrations (0.003-0.1 mM) in CH2Cl2. The invariance of the relative absorbances of the low- and high-energy absorption bands with concentration suggests that the dyes existed predominantly as monomers, rather than within aggregates, in CH2Cl2 solutions with concentrations up to 0.1 mM.


16

6-Se in CH3CN

A580/Amax 1

adsorption time θinitial = 0.021, θfinal = 0.25

6-Se on TiO2

0.7

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Normalized absorbance

Page 17 of 37

0.6

0.5 0

50 100 150 200 250 300

t (s)

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

Fig. 1. Normalized absorbance and emission spectra of 6-Se (0.068 mM) in CH3CN (top) and absorbance spectra of TiO2-on-glass slides following successive 5-s immersions into CH2Cl2 solutions of 6-Se (bottom). Inset: A580/Amax as a function of immersion time. Cyclic voltammograms of the dyes exhibited reversible first oxidation waves. Ground-state oxidation potentials (E1/2+/0) were calculated as the average of half-wave potentials from data acquired at 4-6 scan rates from 100 to 750 mV s-1. Values of E1/2+/0 ranged from 1.10 to 0.91 V vs SCE and were 0.13 to 0.19 V more positive for the bis(half-julolidyl) dyes (7-Se-9-Se) than for the bis(julolidyl) dyes (4-Se-6-Se). For a given dye structure, E1/2+/0 varied minimally (< 50 mV) with surface-anchoring group. Energy differences between minima of ground- and excited-state potential energy surfaces (E00), estimated from the energy at which absorption and emission spectra intersected, ranged from 1.96 to 2.00 eV. Excited-state oxidation potentials (E1/2+/*) were calculated by subtracting E00 from E1/2+/0.70 Values of E1/2+/* ranged from -0.88 to -1.05 V vs SCE, were more positive for the bis(half-julolidyl) dyes than for the bis(julolidyl) dyes, and varied minimally with surface-anchoring group. Electrochemical data are summarized in Table 1. The potential difference between E1/2+/* and the conduction band edge potential (EC) of TiO2 determines the driving force for electron injection. The flat-band potential (EF) of nanocrystalline TiO2 films at pH 1 has been reported to be approximately -0.5 V vs SCE, and EC probably lies within 120 mV negative of EF;71,72 therefore, the driving force for


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Page 18 of 37

electron injection from the selenorhodamine dyes to TiO2, within the acidic electrolyte used for photoelectrochemical measurements, was approximately 0.3 to 0.45 V. Adsorption and Aggregation of Dyes on TiO2. To evaluate the extent of aggregation of 4-Se– 9-Se as a function of dye structure (bis(julolidyl) vs bis(half-julolidyl)) and surface-anchoring group (carboxylic vs hydroxamic vs phosphonic acid), TiO2-coated glass slides were immersed in solutions of the dyes (0.04-0.15 mM) in CH2Cl2 for 5-s time intervals. Following each immersion period, the films were removed and rinsed, then an absorption spectrum was collected. Concentrations of dyes for these experiments were lower than in the adsorption solutions used to prepare dye-functionalized TiO2 films for DSSCs, in order to ensure that adsorption was slow enough to monitor in real time. Representative spectra for 6-Se are in Figure 1, and spectra for the other dyes are in Figure S1. Absorption spectra of corresponding solvated dyes, normalized to the absorbance at λmax, are included in each figure for comparison. Amounts of the dyes per projected surface area of TiO2 (Γ) and fractional surface coverages of dyes (θ) were calculated at each adsorption time from integrated absorption coefficients and the integrated measured absorbance from 500-700 nm. The calculation of Γ and θ is described in Appendix S1 in Supporting Information, and initial and final values of θ are indicated in Figures 1 and S1. Values of θ at the earliest adsorption times ranged from 0.02 to 0.09 and increased to 0.25 to 0.66 within the 3-to-5-min duration of the experiments; thus, all dyes were present on TiO2 at sub-monolayer amounts throughout the duration of adsorption measurements. When each of the dyes was adsorbed to TiO2, the absorbance of the higher-energy visible absorption band increased relative to that of the lower-energy band, clearly indicating that the dyes formed H-aggregates on the surface.28,29,40,41,49,73 The ratio of the absorbance at 580 nm (A580), near λmax of the higher-energy band associated with H-aggregated dyes, to the absorbance at λmax (Amax) of the lower-energy band associated with monomeric dyes provides an indication of the extent of aggregation of dyes on TiO2.49 Ratios of A580:Amax are shown in the insets to Figures 1 and S1. Initial measured values of A580:Amax ranged from 0.55 to 0.85. After 4-5 min of adsorption, the values of A580:Amax for all ACS Paragon Plus Environment

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Page 19 of 37

of the dyes ranged from 0.71 to 0.86, significantly greater than the A580:Amax values of 0.22-0.40 for the solvated dyes but much less than that predicted for a monolayer consisting exclusively of H-aggregates, for which the lower-energy absorption band would disappear altogether.28,29 Therefore, the selenorhodamine dyes were present on TiO2, even at the earliest adsorption times and sub-monolayer coverages, as mixtures of H-aggregates and monomers. Ratios of A580:Amax measured at the longest adsorption times, at which the relative surface amounts of monomers and H-aggregates for most of the dyes had become constant, did not vary systematically with surface-anchoring group, dye structure, or adsorption time. For dyes 4-Se and 6-Se, the carboxylic and phosphonic acid-bearing bis(julolidyl) dyes, the ratio of A580:Amax increased by approximately 40% within the first several minutes of adsorption, indicating an increase of the extent of H-aggregation with time and increasing coverage of the dyes on TiO2. The extent of aggregation of 5-Se changed minimally over time. In contrast, each of the bis(halfjulolidyl) dyes (7-Se–9-Se) exhibited decreased H-aggregation with time, as evidenced by 8-14% decreases of A580:Amax values. These subtle differences in aggregation-deaggregation kinetics notwithstanding, all dyes 4-Se–9-Se underwent H-aggregation on TiO2 to a similar extent as our previously-reported bis(dimethylamino)selenorhodamines 1-Se and 3-Se, which exhibited equilibrium A580:Amax values of 1.2 and 0.7, respectively.41 Importantly, aggregation-induced broadening of the visible absorption bands of 4-Se–9-Se increases the light-harvesting efficiencies of the dyefunctionalized TiO2 films.

10

20

30

40

50

time (h)

0.2

1.0 0.0 0

10

20

30

40

50

time (h)

1.0

A580/Amax

0.4

9-Se

0.8

0.6

0.6 0.4 0.2 0.0 0

10

20

30

40

50

time (h)

0.5

0.5

0.5

0.0 400

(c)

time: 0-48 h

0

1.0

8-Se

0.8

Absorbance

0.0

1.0

time: 0-48 h

time: 0-48 h

0.4

Absorbance

0.6

0.2

1.0

(b) 1.5

7-Se

0.8

A580/Amax

1.0

1.5

A580/Amax

(a) Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

450

500

550

600

Wavelength (nm)

650

700

0.0 400

450

500

550

600

650

700

Wavelength (nm)

0.0 400

450

500

550

600

650

700

Wavelength (nm)

Fig. 2. Absorbance spectra of TiO2-on-glass slides functionalized with 7-Se (a), 8-Se (b), and 9-Se (c) as a function of immersion time in acidified CH3CN. Insets: A580/Amax as a function of immersion time.


19

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1.0

0.8

Normalized Amax

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

0.6

0.4

4-Se 5-Se 6-Se

0.2

7-Se 8-Se 9-Se

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

20

40

Time (h)

Fig. 3. Normalized absorbance at the wavelength maximum (Amax) of 4-Se-9-Se-functionalized TiO2on-glass slides as a function of immersion time in acidified CH3CN. Error bars represent the standard deviations relative to the average Amax values of 2 films. Persistence of Dyes on TiO2. The lability of linkages between dyes 4-Se–9-Se and TiO2 was evaluated by immersing dye-functionalized films in a solution of 0.1 M HCl in CH3CN:H2O (120:1 (v:v)) and acquiring absorption spectra as a function of time (Fig. 2 and Fig. S2 in Supporting Information). In a corresponding control experiment to evaluate the stability of dyes in acidic media, absorption spectra of solutions of the dyes (2.5 µM) dissolved in 0.15 M HCl in CH3CN:H2O (120:1 (v:v)) were acquired as a function of time. The visible absorption bands of the dyes diminished slightly, with no change in spectral profile, upon prolonged exposure to acid. The value of Amax for 4-Se decreased by 15%, whereas values of Amax for the other dyes decreased by 2-8%. The diminution of absorption indicates that dyes degraded minimally in the acidic solutions. The extent to which dyes persisted on or desorbed from TiO2 upon exposure to the acidified CH3CN:H2O solutions was quantified by plotting Amax, which is proportional to the amount of dye per projected surface area of TiO2, as a function of immersion time (Fig. 3). To ensure that values of Amax were on-scale and to minimize potentially complicating effects associated with differences in the rate and extent of desorption of dyes from monolayers and multilayers, dyes were initially adsorbed to TiO2 at sub-monolayer coverages with θ of 0.15 to 0.37. During these desorption experiments, the acidified CH3CN solution was replaced after ACS Paragon Plus Environment

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each absorption spectrum was obtained; thus, these plots are indicative of the kinetics of desorption and not the relative amounts of adsorbed dyes at equilibrium. Carboxylic acid-bearing dyes 4-Se and 7-Se desorbed by 92% or more within the first 5 min and desorbed essentially completely within 30 min. (Absorption bands diminished much more rapidly and significantly in the desorption experiments than in the control experiments in which dyes were dissolved in acidified solutions; therefore, the nearlycomplete loss of Amax within 30 min is attributed to desorption of the carboxylic acid-bearing dyes.) The lability of these dyes is consistent with our previously-reported carboxylic acid-bearing chalcogenorhodamines including 1-Se.41 Phosphonic acid-bearing dyes 6-Se and 9-Se desorbed by only 20–30% over 48 h, consistent with previous reports on the relative inertness of phosphonic acid-bearing chalcogenorhodamines and other adsorbates on TiO2.41,52,54 Hydroxamic acid-bearing dyes 5-Se and 8Se persisted on TiO2 to an intermediate extent. The value of Amax for 8-Se decreased by approximately 55% within the first 20 min of desorption experiments and by an additional 25% thereafter. Dye 5-Se desorbed more gradually over the 48 h experiment, with an overall fractional decrease in Amax similar to that of 8-Se. For the hydroxamic and phosphonic acid-bearing dyes, ratios of A580:Amax changed by only 20-30% during desorption experiments and did not vary systematically with the amount of retained dye (insets to Figs. 2 and S2), indicating that monomeric and H-aggregated dyes desorbed at similar rates from TiO2. Measured ratios of A580:Amax varied more greatly during desorption experiments for the carboxylic acid-bearing dyes 4-Se and 7-Se (insets to Figs. 2 and S2), presumably due to the much more rapid desorption of these dyes. The observed significant and relatively rapid desorption of the hydroxamic acid-bearing dyes 5Se and 8-Se stands in contrast to prior reports that hydroxamic acid-bearing terpyridyl ligands and organic dyes underwent little to no desorption into water or acidic aqueous solutions over 12-24 h time periods.59,60,62 However, our results are consistent with the more recent report by Koenigsmann et al. that a hydroxamic acid-functionalized oligothiophene dye was more inert on TiO2 than the corresponding carboxylic acid-functionalized dye upon exposure to DMF containing 0.1–1% water, but that both dyes still desorbed by 60–80% over several hours.63 Thus, the lability or inertness of 21 ACS Paragon Plus Environment

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hydroxamic acid-bearing dyes varies with structure and functionality of dyes or interfacial environment, warranting further study. Photoelectrochemical Performance of DSSCs. Short-circuit photocurrent action spectra (IPCE vs wavelength) and J-V data were acquired for DSSCs incorporating the selenorhodamine dyes to evaluate the influences of xanthylium core structure and anchoring group chemistry on photoelectrochemical performance. Dyes were loaded onto TiO2-on-FTO electrodes at higher amounts per projected surface area than for the adsorption and desorption measurements. For all dyes, the absorptance (one minus transmittance or the fraction of photons absorbed) of dye-functionalized films acquired before assembly of DSSCs was approximately 1 throughout the absorption bands associated with the monomeric and H-aggregated dyes (Fig. 4 and Fig. S3 in Supporting Information). Shortcircuit photocurrent action spectra were acquired approximately once per minute for a total of 100 min, beginning immediately after DSSCs were assembled (Figs. 4 and S3). Photoanodic currents were measured for all dyes, and profiles of photocurrent action spectra corresponded closely to profiles of absorptance spectra, indicating that the dyes sensitized TiO2 through electron injection.


22

Page 23 of 37

1.0

100

4-Se 80 Absorptance, t = 0 IPCE, t = 0 IPCE, t = 100 min

0.6

60

0.4

40

0.2

20

0.0 400

450

500

550

600

650

100

5-Se

0.8

Absorptance, t = 0 IPCE, t = 0 IPCE, t = 100 min

0.6

0 700

40

0.2

20

0.0 400

450

500

80

Absorptance, t = 0 IPCE, t = 0 IPCE, t = 100 min

60

0.4

40

0.2

20

0.0 400

450

500

550

600

650

0 700

(d) Normalized IPCE @ λ max

6-Se

IPCE (%)

Absorptance

100

0.6

550

600

0 700

650

Wavelength (nm)

1.0

0.8

60

0.4

Wavelength (nm)

(c)

80

IPCE (%)

Absorptance

0.8

1.0

(b) Absorptance

(a)

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0

0.8

0.6

0.4

4-Se 5-Se 6-Se

0.2

0.0 0

10

20

30

40

50

60

70

80

90

100

Time (min)

Wavelength (nm)

Fig. 4. Absorptance and short-circuit photocurrent action spectra of FTO/TiO2 electrodes functionalized with 4-Se (a), 5-Se (b), and 6-Se (c). Absorptance spectra were acquired before the assembly of DSSCs. IPCE data were acquired immediately after and 100 min after the assembly of DSSCs. Normalized IPCE values at λmax are plotted in (d) as a function of time during successive measurements. Error bars represent the standard deviations relative to the average IPCE values of 2 DSSCs. Within the bis(julolidyl) series of dyes (4-Se–6-Se), IPCEs increased slightly during the first 215 min of successive short-circuit photocurrent measurements, then decreased to varying degrees over the duration of measurements (Fig. 4). The maximum initial value of IPCE at λmax (IPCEmax,i), measured after the brief induction period, was greatest for carboxylic acid-bearing 4-Se (IPCEmax,i of (79 ± 8)% at 602 nm) (Fig. 4a). Values of IPCEmax,i were lower and similar for hydroxamic acid-bearing 5-Se ((58 ± 11)%) and phosphonic acid-bearing 6-Se ((52 ± 3)%) (Fig. 4b,c). The values of IPCEmax,i for 4-Se and 6-Se

are

similar

to

(within

10-12%

of)

our

previously-reported

values

for

the

bis(dimethylamino)selenorhodamines 1-Se and 3-Se.40,41 IPCE equals the product of absorptance, electron-injection yield (φinj), and charge-collection efficiency (ηel).74 Because absorptances were initially approximately 1 for electrodes functionalized with each of the dyes (Figs. 4, S3), the measured ACS Paragon Plus Environment

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trend in IPCEmax,i values indicates that φinj and/or ηel were greatest for carboxylic acid-bearing 4-Se. The persistence of sensitized photocurrent over time for the bis(julolidyl) dyes varied with surface-anchoring group. The IPCE at λmax of carboxylic acid-bearing 4-Se decreased to 40–50% of IPCEmax,i after 100 min, reaching a final IPCE at λmax (IPCEmax,f) of (37 ± 12)% at 602 nm (Fig. 4a,d). Measurable sensitized photocurrent from 4-Se persisted at times after essentially all 4-Se had desorbed from TiO2 during desorption experiments (Figs. 3 and S2), suggesting either that desorption into the I-/I3-containing CH3CN electrolyte of DSSCs was slower than into the acidified CH3CN solution from desorption measurements or that desorbed dye contributed to sensitized photocurrent. The IPCE at the band maximum of H-aggregated 4-Se (600 nm) decreased more rapidly than the IPCE at the band maximum of monomeric 4-Se (634 nm) (Fig. 4a). IPCE values decreased to a much lesser extent for hydroxamic acid-bearing 5-Se and phosphonic acid-bearing 6-Se, for which IPCEmax,f values were 80– 90% of IPCEmax,i (Fig. 4b-d). Thus, while 4-Se exhibited the highest IPCEmax,i, the sensitized photocurrents from 5-Se and 6-Se were more persistent over time. Among the bis(julolidyl) series of dyes, hydroxamic acid-bearing 5-Se maintained the highest IPCE after 100 min (IPCEmax,f of (57 ± 9)%) with minimal degradation of IPCE over the duration of the experiment. The bis(half-julolidyl) dyes 7-Se–9-Se exhibited IPCEmax,i values of 49–65% (Fig. S3). For carboxylic acid- and hydroxamic acid-bearing dyes 7-Se and 8-Se, values of IPCEmax,i ((65 ± 3)% and (49 ± 1)%, respectively) were 80-85% of those of the corresponding bis(julolidyl) dyes, whereas for phosphonic acid-bearing 9-Se, IPCEmax,i ((64.5 ± 0.3)%) was approximately 25% greater than for its bis(julolidyl) analogue 6-Se. Thus, the structure of the xanthylium core (bis(julolidyl) vs bis(halfjulolidyl)) did not greatly or systematically influence IPCEmax,i. IPCE values of the bis(half-julolidyl) dyes persisted to varying degrees during successive measurements. The IPCE at λmax of carboxylic acidbearing 7-Se decreased most significantly to IPCEmax,f of (27 ± 1)%. IPCE values within the absorption band associated with H-aggregated 7-Se decreased to a greater extent than IPCE values within the band associated with the monomeric dye (Fig. S3a). Similarly to 4-Se, the IPCE of 7-Se decreased more slowly than the amount of adsorbed dye in desorption experiments. IPCE values for hydroxamic acid24 ACS Paragon Plus Environment

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bearing 8-Se and phosphonic acid-bearing 9-Se decreased to 75% and 80% of IPCEmax,i, respectively, reaching IPCEmax,f of (37 ± 1)% for 8-Se and (51 ± 1)% for 9-Se (Fig. S3b,c). In summary, values of IPCEmax,i were greatest for the carboxylic acid derivatives 4-Se and 7-Se, due to greater φinj and/or ηel. An increase of φinj may have been correlated with enhanced electronic coupling and faster electron transfer through the carboxylate linkage.50,75-77 However, measured IPCEs for 4-Se and 7-Se decreased most rapidly and to the greatest extent over the 100 min duration of measurements, presumably due to the greater lability of carboxylic acid-functionalized dyes on TiO2. DSSCs incorporating the hydroxamic acid- and phosphonic acid-bearing derivatives 5-Se, 6-Se, 8-Se, and 9-Se performed similarly to each other in terms of IPCEmax,i and the persistence of IPCE, with hydroxamic acid-bearing dyes 5-Se and 8-Se exhibiting slightly higher IPCEmax,i. Interestingly, the IPCE decreased the least during successive measurements for 5-Se, despite the hydroxamate-TiO2 linkages being more labile than phosphonate-TiO2 linkages in desorption experiments.

Table 2. Averaged photoelectrochemical data for DSSCs with dyes 4-Se-9-Se. Dye Voc (mV) a Jsc (mA cm-2) a 4-Se 186 ± 3 1.4 ± 0.3 5-Se 197 ± 11 2.9 ± 0.2 6-Se 3.3 ± 0.1 207 ± 7 7-Se 338 ± 6 2.61 ± 0.04 8-Se 235 ± 13 2.9 ± 0.4 9-Se 238 ± 8 3.67 ± 0.01 a Values are averages of two successive deviations are relative to average values.

ff a

PCE (%) a

0.280 ± 0.004 0.15 ± 0.03 0.273 ± 0.001 0.32 ± 0.03 0.269 ± 0.002 0.36 ± 0.02 0.40 ± 0.05 0.71 ± 0.03 0.29 ± 0.02 0.41 ± 0.08 0.280 ± 0.003 0.47 ± 0.01 measurements each from two different DSSCs for each dye; standard

Photocurrent-photovoltage data were acquired under white-light illumination. Representative JV curves are plotted in Figure S4 in Supporting Information; average values of parameters obtained from measurements on multiple DSSCs per dye are summarized in Table 2. Under our conditions, the average PCE of N3-sensitized cells, which served as a reference, was (5.4 ± 0.1)%, which is lower than the originally reported value of approximately 10%.5 Measured PCEs for the selenorhodamine dyes were 0.15-0.71% with the carboxylic acid-bearing bis(half-julolidyl) dye 7-Se exhibiting the highest PCE (Table 2). The relatively low PCEs of the selenorhodamine-derived DSSCs arose from decreased 25 ACS Paragon Plus Environment

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short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (ff) relative to the N3-sensitized cells (Table S1). The inefficient power conversion of the selenorhodamine-derived DSSCs stands in contrast to their relatively high monochromatic IPCEs and suggests that the regeneration of dyes was inefficient at higher irradiance under white-light illumination. To regenerate the chalcogenorhodamine dyes more efficiently is a goal of our ongoing research. Additionally, the presence of both H+ (0.15 M) and Li+ (0.5 M) in the electrolyte probably shifted EC of TiO2 positively, decreasing the maximum attainable Voc. This effect may also have contributed to the low values of Voc and PCE. Despite the low PCEs of the selenorhodamine-derived DSSCs, the J-V data reveal that the bis(half-julolidyl) dyes (7-Se-9-Se) exhibited higher values of Voc and PCE than the corresponding bis(julolidyl) dyes (4-Se-6-Se). We speculate that this difference, which was most significant for the carboxylic acid-bearing dyes, may have arisen from the influence of the gem-dimethyl groups of the half-julolidyl moieties on the recombination of electrons in TiO2 with I3-. It is well-established that peripheral alkyl groups on dyes can slow the recombination of injected electrons with the oxidized half of electrolyte redox couples;78-81 a similar effect may have occurred with these bis(half-julolidyl) selenorhodamine dyes. Conclusions. Selenorhodamine dyes 4-Se-9-Se were synthesized by reaction of a lithiated thiophene derivative with the appropriate selenoxanthone. Incorporation of the bis(half-julolidyl) and bis(julolidyl) moieties into the periphery of the xanthylium cores red-shifted the dyes’ absorption bands relative to the analogous dimethylamino-containing selenorhodamines. Dyes 4-Se-9-Se adsorbed to nanocrystalline TiO2 films as mixtures of H-aggregated and monomeric dyes, irrespective of their structure and surface-anchoring group. The carboxylic acid-bearing dyes 4-Se and 7-Se desorbed rapidly and almost completely from TiO2, whereas the hydroxamic acid-bearing dyes (5-Se, 8-Se) and phosphonic acid-bearing dyes (6-Se, 9-Se) were significantly more inert. DSSCs incorporating 4-Se and 7-Se exhibited the highest initial IPCEs of 65-80% under monochromatic illumination, but the IPCE decreased by approximately 2-fold with time and successive measurements, due to the lability of the dyes. The hydroxamic acid- and phosphonic acid-bearing dyes initially exhibited lower IPCEs (4926 ACS Paragon Plus Environment

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65%); however, the IPCE degraded to a much lesser extent than for the corresponding carboxylic acidbearing dyes, particularly for the hydroxamic acid-bearing bis(julolidyl) dye 5-Se. The vastly greater persistence of the hydroxamic acid- and phosphonic acid-bearing selenorhodamine dyes on TiO2, together with their H-aggregation-induced spectral broadening and increased light-harvesting efficiency, render them attractive photosensitizers for DSSCs. PCEs of selenorhodamine-derived DSSCs under white-light illumination were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given surface-anchoring group, both the Voc and PCE of DSSCs incorporating the bis(half-julolidyl) dye were greater than those of the bis(julolidyl) dye, suggesting that functionalization of the periphery of the xanthylium core may ultimately enable improved dyeregeneration and power-conversion performance for DSSCs incorporating selenorhodamine sensitizers.

Acknowledgments. This material is based upon work supported by the National Science Foundation under Grant No. CHE-1151379.

Supporting information available: Absorption spectra of solvated and TiO2-adsorbed selenorhodamine dyes, data from adsorption and desorption experiments, and photoelectrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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1h-Cyclopentapyrimidin-2,4(1h,3h)-Dione:

Synthesis,

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Langmuir

Table of contents graphic: Anchor =

O C

; OH

O C

N H

O OH ; HO P OH

Anchor

Anchor

S

S Me

N

Se 4-Se, 5-Se, 6-Se

N PF6

Me

N Me

Me

Me

Se

N PF6 Me 7-Se, 8-Se, 9-Se


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Selenorhodamine Dye-Sensitized Solar Cells - ACS Publications

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