Walking the Emission Tightrope: Spectral and Computational Analysis

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Walking the Emission Tightrope: Spectral and Computational Analysis of Some Dual-Emitting Benzothiadiazole Donor-Acceptor Dyes Jonathan E. Barnsley, Georgina E. Shillito, Joseph I. Mapley, Christopher B. Larsen, Nigel T. Lucas, and Keith C. Gordon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05361 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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The Journal of Physical Chemistry

Walking the Emission Tightrope: Computational Analysis of Some Benzothiadiazole Donor-Acceptor Dyes

Spectral and Dual-Emitting

Jonathan E. Barnsley, Georgina E. Shillito, Joseph I. Mapley, Christopher B. Larsen, Nigel T. Lucas*, Keith C. Gordon* Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand

Supporting Information Placeholder ABSTRACT: The synthesis, spectroscopic characterization and computational modelling of seven benzo[c][2,1,3]thiadiazole-based donor-acceptor dyes is reported. Using a range of linker units it is possible to alter the lowest energy transition in terms of intensity (from 8,000 to 25,000 L mol-1 cm-1) and wavelength (from 350 to 430 nm). Resonance Raman spectroscopy was used in concert with DFT calculations to indicate that the linker unit participates in charge transfer processes. In each compound the excited state behavior appears to be primarily described by a BTD.--Linker-TPA.+ state. Stokes shift versus solvent parameter gradients are on the order of 15,000 cm-1 indicating ∆µ values are large. Dual emission is observed in six of the seven compounds and it can be modulated as a function of solvent. TD-DFT calculations, including excited state optimizations (linear response and state specific), indicate that the lowest energy emission is charge transfer in character. The high energy emissive state is assigned as n-π*. In non-polar solvents, only the low energy charge transfer emission band is observed and this band generally has a high quantum yield (Φ ≈ 0.9). For compounds with phenyl and triazolyl linkers, in polar solvents only the high energy n-π* emission is observed. The high energy n-π* emission has a low quantum yield regardless of solvent.

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INTRODUCTION Organic photo-active materials of donor-acceptor (D-A) character continue to be of interest in chemical research.1-3 D-A systems offer distinct and highly tunable optical properties for applications in organic photovoltaics, dyesensitized solar cells and non-linear optics.4-16 Furthermore, small molecule D-A designs are of interest due to a comparative ease of purification and well defined structure when compared to their polymer counterparts.9-16 Commonly used donors include fluorenes,17 carbazoles17-18 and modified triphenylamines;19-20 while heterocycle acceptor units such as pyrazine,21 quinoxaline,21-24 25-30 dipyridophenazine and benzothiadiazole (BTD) have been of interest. The BTD acceptor has been used here as it is electron deficient, hence is a good low-energy electron acceptor.9, 17, 31-34 The BTD unit has been used with a triphenylamine donor and is connected through a range of linkers (Figure 1). D-A optical properties are typically defined by charge transfer (CT) processes, where electron density is donated from an electron rich unit (the donor) to an electron withdrawing unit (the acceptor).35-37 Control of D-A coupling has been achieved through geometric and electronic constraints, which allows tuning of the photophysical properties.34-50 More specifically, D-A torsion angle (φD-A),34, 39-42 donor energy,35-37 donor additivity,49 and linker length43-47, 51 have been investigated as methods to tune D-A coupling. A number of studies have probed the influence of the linker nature;43, 48 and how this conjugated unit either enhances or inhibits D-A connectivity. The ‘zwitterionic’ excited state that is formed through CT is inherently sensitive to environmental effects. This is especially true for the solvent environment, and changes of excited state energies due to solvent is common.52 This sensitivity to solvent environment can further tune the emission energies, which can make D-A materials useful in sensor applications.53-56 Excited state computational modelling has progressed in recent years and has been shown to be an effective tool for excited state energetics.5761 Linear response (LR) and state-specific (SS) formalisms have been utilized to model absorption and emission energies of compounds in solution. These methods differ in how equilibrium between the solute and solvent is established. The LR method avoids explicit calculation of the excited state wavefunction and thus only the response of the ground state reaction field to the excitation is calculated.57, 59, 62-63 In contrast, the SS method explicitly calculates the electron density of the specified excited state and allows the solvent to directly respond to it.57, 59, 62-63 The emission energy is then calculated as a difference between the solute-solvent energies. The SS method typically models the solute-solvent interaction more accurately and tends to give excitation energies closer to experimental data.58-59, 64-65 The LR method can provide the energies of multiple excited states, however the SS method requires each excited state to be calculated separately and is more computationally intensive.58 The LR method is the

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default method applied for electronic absorption TDDFT calculations and is usually suitable for strong absorptions.63 In order to accurately model emission energies the SS method is usually required.58-59, 64-65 Herein we report the synthesis and photophysical properties of a number of BTD-based D-A dyes with varying linker units. The effect of the linker units was monitored using a suite of techniques including electrochemical studies, electronic absorption, emission, FT-Raman and resonance Raman spectroscopies and time-dependent density functional theory methods. Relative to the parent BTDTPA, which lacks a linker unit, structural modification achieved a variation in the CT absorption energy of ~0.65 eV and a variation of extinction coefficient around threefold. Strong emission (Φ ≈ 0.9) is observed for most compounds in non-polar solvents such as toluene. For six of the seven compounds, increasing the solvent polarity results in the appearance of a second higher energy emission band. For example, compounds using triazolyl and phenyl linkers, emission could be completely switched from the low energy emission (500-700 nm, Elow) to the higher energy emission (~450 nm, Ehigh) by increasing the solvent polarity from toluene to acetonitrile. TD-DFT calculations using both LR and SS formalisms were run in order to simulate the emission energies, employing both toluene and acetonitrile solvent fields.

EXPERIMENTAL SECTION Reagents and solvents were purchased from commercial suppliers and used as received. Spectroscopic or HPLC grade solvents were used for all measurements. Data was analyzed using GRAMS32 A/I (Galactic Industries) software. The atom numbering scheme for NMR assignments is presented in Figure S1. Synthesis. Reagents and solvents were purchased from commercial suppliers and used as received. 5Bromobenzo[c][2,1,3]thiadiazole (BTD-Br),66 5-(4iodophenyl)benzo[c][2,1,3]thiadiazole (BTD-Ph-I),27 5-(4diphenylaminophenyl)benzo[c][2,1,3]thiadiazole (BTD-TPA)34 and 5-(4-(4-diphenylaminophenyl)-1,2,3-triazol-1-yl)benzo[c][2,1,3]thiadiazole (BTD-TRZ-TPA) were prepared using literature procedures.34 1

H NMR spectra were recorded at 500 MHz and 13C at 126 MHz on a Varian 500AR spectrometer. All samples were recorded at 25 °C in 5 mm diameter tubes. Chemical shifts were referenced internally to CHCl3 (1H: δ 7.26 ppm) or CDCl3 (13C: δ 77.16 ppm). Assignment of signals was assisted through the use of 2D NMR techniques (COSY, NOESY, 1H-13C HSQC and 1H-13C HMBC), recorded on a Varian 500AR spectrometer using standard pulse sequences. MALDI-TOF mass spectra were recorded on an Applied Biosystems 4800 Tandem TOF mass spectrometer with external calibration to within m/z ±0.08 using a TCNQ matrix. Analysis of elemental composition was made by the Campbell Microanalytical Laboratory at the University of Otago, using a Carlo Erba 1108 CHNS combustion analyser. The estimated error is the measurements in ±0.4%. 5-(Diphenylamino)benzo[c][2,1,3]thiadiazole (BTD-NPh2). A mixture of BTD-Br (0.476 g, 2.21 mmol), N,N-diphenylamine


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(0.818 g, 4.83 mmol) and tBuOK (0.812 g, 7.24 mmol) in toluene (20 mL) was purged with argon for 15 min. [tBu3PH]BF4 (0.134 g, 0.462 mmol) and Pd2(dba)3 (0.199 g, 0.217 mmol) were added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CHCl3, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (basic Al2O3, CH2Cl2) to afford BTD-NPh2 (0.523 g, 78%) as a yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.13 (d, J = 9.5 Hz, 1H, H7), 7.79 (dd, J = 9.5, 2.3 Hz, 1H, H6), 7.69 (m, 5H, H4,3′), 7.53 (d, J = 8.5 Hz, 4H, H2′), 7.51 (t, J = 7.4 Hz, 2H, H4′) ppm. 13 C NMR (126 MHz, CDCl3): δ 156.32 (C3a), 151.78 (C7a), 149.55 (C5), 146.84 (C1′), 129.78 (C3′), 128.19 (C6), 125.82 (C2′), 124.73 (C4′), 121.04 (C7), 109.02 (C4) ppm. MS (MALDI-TOF) calcd for C18H13N3S ([M]+): m/z 303.08. Found: m/z 303.05. Elemental analysis calcd for C18H13N3S: C, 71.26; H, 4.32; N, 13.85. Found: C, 71.12; H, 4.40; N, 13.66. 5-(4-Diphenylaminophenyl)ethynylbenzo[c][2,1,3]thiadiazole (BTD-CC-TPA). A mixture of BTD-Br (0.248 g, 1.15 mmol) and 4-ethynyl-N,N-diphenylaniline (0.368 g, 1.37 mmol) in NEt3 (20 mL) was purged with argon for 15 min. CuI (0.016 g, 0.084 mmol) and PdCl2(dppf) (0.050 g, 0.068 mmol) were added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CHCl3, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford BTD-CC-TPA (0.272 g, 58%) as a yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.13 (dd, J = 1.5, 0.8 Hz, 1H, H4), 7.95 (dd, J = 9.1, 0.8 Hz, 1H, H7), 7.67 (dd, J = 9.0, 1.5 Hz, 1H, H6), 7.42 (d, J = 8.8 Hz, 2H, H2″), 7.30 (dd, J = 8.5, 7.4 Hz, 4H, H3‴), 7.14 (dd, J = 8.5, 1.1 Hz, 4H, H2‴), 7.09 (tt, J = 7.4, 1.1 Hz, 2H, H4‴), 7.03 (d, J = 8.8 Hz, 2H, H3″) ppm. 13C NMR (126 MHz, CDCl3): δ 154.86 (C3a), 154.25 (C7a), 148.72 (C4″), 147.16 (C1‴), 132.92 (C2″), 132.72 (C6), 129.61 (C3‴), 125.38 (C5,2‴), 123.99 (C4‴), 123.85 (C4), 122.02 (C3″), 121.34 (C7), 115.07 (C1″), 93.69 (C2′), 87.95 (C1′) ppm. MS (MALDI-TOF) calcd for C26H17N3S ([M]+): m/z 403.11. Found: m/z 403.09. Elemental analysis calcd for C26H17N3S: C, 77.39; H, 4.25; N, 10.41. Found: C, 77.62; H, 4.15; N, 10.01. 5-(Thien-2-yl)benzo[c][2,1,3]thiadiazole . A mixture of BTD-Br (0.500 g, 2.33 mmol), 2-thienylboronic acid (0.365 g, 2.85 mmol) and K2CO3 (1.78 g, 12.8 mmol) in toluene (15 mL), water (10 mL) and EtOH (5 mL) was purged with argon for 15 min. PdCl2(dppf) (0.164 g, 0.224 mmol) was added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CH2Cl2, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford 5(thien-2-yl)benzo[c][2,1,3]thiadiazole (0.474 g, 93%) as a yellow crystalline solid. 1H NMR (500 MHz, CDCl3): δ 8.18 (d, J = 1.7 Hz, 1H, H4), 7.98 (d, J = 9.1 Hz, 1H, H7), 7.89 (dd, J = 9.1, 1.7 Hz, 1H, H6), 7.49 (dd, J = 3.7, 1.0 Hz, 1H, H3′), 7.40 (dd, J = 5.1, 1.0 Hz, 1H, H5′), 7.15 (dd, J = 5.1, 3.7 Hz, 1H, H4′) ppm. 13C NMR (126 MHz, CDCl3): δ 155.44 (C3a), 154.29 (C7a), 142.71 (C2′), 135.66 (C5), 129.13 (C6), 128.62 (C4′), 126.82 (C5′), 125.21 (C3′), 121.76 (C7), 116.59 (C4) ppm. MS (MALDI-TOF) calcd for C10H6N2S2 ([M]+): m/z 218.00. Found: m/z 217.96. Elemental analysis calcd for C10H6N2S2: C, 55.02; H, 2.77; N, 12.83. Found: C, 55.15; H, 2.71; N, 12.95.

5-(5-Bromothien-2-yl)benzo[c][2,1,3]thiadiazole . A solution of N-bromosuccinimide (0.689 g, 3.87 mmol) in DMF (20 mL) was added dropwise to a solution of 5-(thien-2yl)benzo[c][2,1,3]thiadiazole (0.724 g, 3.32 mmol) in DMF (20 mL), and the reaction mixture heated at 60 °C for 15 h. The reaction mixture was allowed to cool to rt and excess Br2 quenched with aq. Na2S2O5 (sat.). The product was extracted into CH2Cl2, washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford 5-(5-bromothien-2-yl)benzo[c][2,1,3]thiadiazole (0.835 g, 85%) as a yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.10 (dd, J = 1.8, 0.8 Hz, 1H, H4), 8.00 (dd, J = 9.2, 0.8 Hz, 1H, H7), 7.81 (dd, J = 9.2, 1.8 Hz, 1H, H6), 7.25 (d, J = 3.9 Hz, 1H, H3′), 7.11 (d, J = 3.9 Hz, 1H, H4′) ppm. 13C NMR (126 MHz, CDCl3): δ 155.34 (C3a), 154.39 (C7a), 144.12 (C2′), 134.83 (C5), 131.50 (C4′), 128.44 (C6), 125.44 (C3′), 122.09 (C7), 116.62 (C4), 113.77 (C5′) ppm. MS (MALDI-TOF) calcd for C10H5BrN2S2 ([M]+): m/z 295.91. Found: m/z 295.89. Elemental analysis calcd for C10H5BrN2S2: C, 40.42; H, 1.70; N, 9.43. Found: C, 40.43; H, 1.65; N, 9.59. 5-(5-(4-Diphenylaminophenyl)thien-2yl)benzo[c][2,1,3]thiadiazole (BTD-Thio-TPA). A mixture of 5(5-bromothien-2-yl)benzo[c][2,1,3]thiadiazole (0.506 g, 1.70 mmol), 4-diphenylaminophenylboronic acid (0.679 g, 2.35 mmol) and K2CO3 (1.74 g, 12.6 mmol) in toluene (40 mL), water (20 mL) and EtOH (10 mL) was purged with argon for 15 min. PdCl2(dppf) (0.162 g, 0.221 mmol) was added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CH2Cl2, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford BTD-Thio-TPA (0.558 g, 71%) as an orange solid. 1H NMR (500 MHz, CDCl3): δ 8.18 (s, 1H, H4), 7.99 (d, J = 9.2 Hz, 1H, H7), 7.92 (dd, J = 9.2, 1.5 Hz, 1H, H6), 7.52 (d, J = 8.5 Hz, 2H, H2″), 7.47 (d, J = 3.8 Hz, 1H, H3′), 7.29 (t, J = 7.5 Hz, 4H, H3‴), 7.27 (d, J = 3.6 Hz, 1H, H4′), 7.14 (d, J = 7.9 Hz, 4H, H2‴), 7.09 (d, J = 8.6 Hz, 2H, H3″), 7.07 (t, J = 7.3 Hz, 2H, H4‴) ppm. 13C NMR (126 MHz, CDCl3): δ 155.57 (C3a), 154.32 (C7a), 147.98 (C4″), 147.49 (C1‴), 145.87 (C5′), 140.73 (C2′), 135.71 (C5), 129.52 (C3‴), 128.80 (C6), 127.76 (C1″), 126.72 (C2″), 126.31 (C3′), 124.88 (C2‴), 123.56 (C4′), 123.50 (C3″), 123.47 (C4‴), 121.74 (C7), 115.92 (C4) ppm. MS (MALDI-TOF) calcd for C28H19N3S2 ([M]+): m/z 461.10. Found: m/z 461.08. Elemental analysis calcd for C28H19N3S2: C, 72.86; H, 4.15; N, 9.10. Found: C, 72.62; H, 4.00; N, 9.05. 5-(4-(4-Diphenylaminophenyl)phenyl)benzo[c][2,1,3]thiadiazole (BTD-Ph-TPA). A mixture of BTD-Ph-I (0.756 g, 2.24 mmol), 4-diphenylaminophenylboronic acid (0.850 g, 2.94 mmol) and K2CO3 (1.54 g, 11.2 mmol) in toluene (20 mL), water (10 mL) and EtOH (5 mL) was purged with argon for 15 min. PdCl2(dppf) (0.145 g, 0.198 mmol) was added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CH2Cl2, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford BTD-Ph-TPA (0.857 g, 84%) as a yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.22 (dd, J = 1.6, 0.6 Hz, 1H, H4), 8.07 (dd, J = 9.1, 0.4 Hz, 1H, H7), 7.94 (dd, J = 9.1, 1.7 Hz, 1H, H6), 7.78 (d, J = 8.4 Hz, 2H, H2′), 7.72 (d, J = 8.4 Hz, 2H, H3′), 7.55 (d, J = 8.6 Hz, 2H, H2″), 7.29 (dd, J = 8.3, 7.4 Hz, 4H, H3‴), 7.17 (d, J = 8.6 Hz, 2H, H3″), 7.16 (dd, J = 8.4, 0.9 Hz, 4H, H2‴), 7.06



(tt, J = 7.3, 1.0 Hz, 2H, H4‴) ppm. 13C NMR (126 MHz, CDCl3): δ 155.63 (C3a), 154.39 (C7a), 147.74 (C4″), 147.71 (C1‴), 142.16 (C5), 140.89 (C4′), 137.92 (C1′), 134.03 (C1″), 130.19 (C6), 129.47 (C3‴), 127.98 (C2′), 127.82 (C2″), 127.36 (C3′), 124.74 (C2‴), 123.81 (C3″), 123.27 (C4‴), 121.66 (C7), 118.37 (C4) ppm. MS (MALDI-TOF) calcd for C3OH21N3S ([M]+): m/z 455.15. Found: m/z 455.19. Elemental analysis calcd for C3OH21N3S: C, 79.09; H, 4.65; N, 9.22. Found: C, 79.12; H, 4.75; N, 9.00. 5-(4-(4-Trimethylsilylphenyl)phenyl)benzo[c][2,1,3]thiadiazole . A mixture of BTD-Ph-I (1.29 g, 3.83 mmol), 4-trimethylsilylphenylboronic acid (4.10 g, 9.80 mmol) and K2CO3 (2.60 g, 24.5 mmol) in toluene (40 mL), water (20 mL) and EtOH (10 mL) was bubbled with argon for 15 min. PdCl2(dppf) (0.209 g, 0.286 mmol) was added and the reaction mixture heated at reflux for 15 h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CH2Cl2, and washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford 5(4-(4-trimethylsilylphenyl)phenyl)benzo[c][2,1,3]thiadiazole (0.921 g, 67%) as a white solid. 1H NMR (500 MHz, CDCl3): δ 8.23 (s, 1H, H4), 8.08 (d, J = 9.1 Hz, 1H, H7), 7.94 (dd, J = 9.1, 1.4 Hz, 1H, H6), 7.80 (d, J = 8.3 Hz, 2H, H2′), 7.76 (d, J = 8.3 Hz, 2H, H3′), 7.66 (m, 4H, H2″,3″), 0.33 (s, 9H, SiMe3) ppm. 13C NMR (126 MHz, CDCl3): δ 155.60 (C3a), 154.42 (C7a), 142.13 (C5), 141.38 (C4′), 140.78 (C1″), 139.98 (C4″), 138.59 (C1′), 134.10 (C2″), 130.18 (C6), 128.03 (C2′), 127.94 (C3′), 126.53 (C3″), 121.70 (C7), 118.53 (C4), 0.94 (SiMe3) ppm. MS (MALDI-TOF) calcd for C21H20N2SSi ([M]+): m/z 360.11. Found: m/z 360.08. Elemental analysis calcd for C21H20N2SSi: C, 69.96; H, 5.59; N, 7.77. Found: C, 70.03; H, 5.89; N, 7.84. 5-(4-(4-Iodophenyl)phenyl)benzo[c][2,1,3]thiadiazole. A solution of 5-(4-(4-trimethylsilylphenyl)phenyl)benzo[c][2,1,3]thiadiazole (0.727 g, 2.02 mmol) in CH2Cl2 (200 mL) was cooled to −78 °C using a dry ice/acetone bath under an argon atmosphere. ICl (4.5 mL, 1 M in CH2Cl2) was added dropwise and the resultant mixture stirred at −78 °C under an argon atmosphere for 2 h. The dry ice/acetone bath was removed, excess ICl quenched by dropwise addition of aqueous Na2S2O4 (sat., 20 mL), and the mixture allowed to warm to rt. The product was extracted into CH2Cl2 and washed with aqueous NH4Cl (sat.), then water. The organic extract was dried over MgSO4, and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford 5(4-(4-iodophenyl)phenyl)benzo[c][2,1,3]thiadiazole (0.807 g, 97%) as a white solid. 1H NMR (500 MHz, CDCl3): δ 8.22 (dd, J = 1.8, 0.8 Hz, 1H, H4), 8.09 (dd, J = 9.1, 0.8 Hz, 1H, H7), 7.93 (dd, J = 9.1, 1.8 Hz, 1H, H6), 7.81 (d, J = 8.5 Hz, 2H, H3″), 7.80 (d, J = 8.5 Hz, 2H, H2′), 7.71 (d, J = 8.5 Hz, 2H, H3′), 7.41 (d, J = 8.5 Hz, 2H, H2″) ppm. 13C NMR (126 MHz, CDCl3): δ 155.58 (C3a), 154.46 (C7a), 141.97 (C5), 140.26 (C4′), 139.97 (C1″), 139.04 (C1′), 138.17 (C3″), 130.12 (C6), 129.03 (C2″), 128.18 (C2′), 127.71 (C3′), 121.80 (C7), 118.66 (C4), 93.64 (C4″) ppm. MS (MALDI-TOF) calcd for C18H11IN2S ([M]+): m/z 413.97. Found: m/z 413.92. Elemental analysis calcd for C18H11IN2S: C, 52.19; H, 2.68; N, 6.76. Found: C, 52.27; H, 2.85; N, 6.99. 5-(4-(4-(4-Diphenylaminophenyl)phenyl)phenyl)benzo[c][2,1,3]thiadiazole (BTD-Ph2-TPA). A mixture of 5-(4-(4iodophenyl)phenyl)benzo[c][2,1,3]thiadiazole (0.807 g, 1.95 mmol), 4-diphenylaminophenylboronic acid (1.03 g, 3.58 mmol) and K2CO3 (1.95 g, 14.1 mmol) in toluene (60 mL), water (30 mL) and EtOH (15 mL) was bubbled with argon for 15 min. PdCl2(dppf) (0.141 g, 0.193 mmol) was added and the

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reaction mixture heated at reflux for 15h under an argon atmosphere. The reaction mixture was allowed to cool to rt, the product extracted into CH2Cl2, washed with NH4Cl solution (sat.), then water. The organic extract was dried over MgSO4 and the solvent removed under reduced pressure. The residue was purified using preparative column chromatography (SiO2, CH2Cl2) to afford BTD-Ph2-TPA (0.836 g, 81%) as a yellow solid. 1H NMR (500 MHz, CDCl3): δ 8.24 (dd, J = 1.7, 0.8 Hz, 1H, H4), 8.09 (dd, J = 9.1, 0.8 Hz, 1H, H7), 7.96 (dd, J = 9.1, 1.8 Hz, 1H, H6), 7.82 (d, J = 8.7 Hz, 2H, H2′), 7.80 (d, J = 8.8 Hz, 2H, H3′), 7.74 (d, J = 8.5 Hz, 2H, H2″), 7.69 (d, J = 8.6 Hz, 2H, H3″), 7.54 (d, J = 8.7 Hz, 2H, H2‴), 7.29 (dd, J = 8.5, 7.4 Hz, 4H, H3′′′′), 7.17 (d, J = 8.7 Hz, 2H, H3‴), 7.16 (dd, J = 8.6, 1.1 Hz, 4H, H2′′′′), 7.05 (tt, J = 7.4, 1.2 Hz, 2H, H4′′′′) ppm. 13C NMR (126 MHz, CDCl3): δ 155.63 (C3a), 154.44 (C7a), 147.78 (C1′′′′), 147.55 (C4‴), 142.15 (C5), 140.96 (C4′), 140.15 (C4″), 138.73 (C1″), 138.51 (C1′), 134.44 (C1‴), 130.21 (C6), 129.46 (C3′′′′), 128.08 (C2′), 127.80 (C3′), 127.75 (C2‴), 127.53 (C2″), 127.23 (C3″), 124.67 (C2′′′′), 123.94 (C3‴), 123.19 (C4′′′′), 121.73 (C7), 118.52 (C4) ppm. MS (MALDI-TOF) calcd for C36H25N3S ([M]+): m/z 531.18. Found: m/z 531.16. Elemental analysis calcd for C36H25N3S: C, 81.33; H, 4.74; N, 7.90. Found: C, 81.05; H, 4.79; N, 7.56. Crystallography. A single crystal was attached with Paratone N to a fiber loop supported in a copper mounting pin, and then quenched in a cold nitrogen stream. Data were collected at 100 K using Cu-Kα radiation (micro-source, mirror monochromated) using an Agilent Supernova, Dual, Cu at zero diffractometer with an Atlas detector. Data processing was undertaken with CrysAlisPro.67 A multiscan absorption correction was applied to the data. Structures were solved by direct methods with SHELXS97, and extended and refined with SHELXL-9768-69 using the XSeed interface.70 The non-hydrogen atoms in the asymmetric unit were modelled with anisotropic displacement parameters and a riding atom model with group displacement parameters used for the hydrogen atoms. Crystal data for BTD-Ph-TPA: C30H21N3S, M = 455.56, yellow plate, 0.70 × 0.06 × 0.04 mm3, monoclinic, a = 25.8443(13) Å, b = 9.1900(3) Å, c = 19.4946(7) Å, α = 90.00°, β = 106.700(5)°, γ = 90.00°, V = 4434.9(3) Å3, space group P21/c (#14), Z = 8, µ(Cu-Kα) = 1.479 mm–1, 2θmax = 149.76°, 2θfull = 134° (99.8% complete), 29935 reflections measured, 8897 independent reflections (Rint = 0.0671). The final R1(F) = 0.0870 (I > 2σ(I)); 0.0993 (all data). The final wR2(F2) = 0.2230 (I > 2σ(I)); 0.2338 (all data). GoF = 1.031. CCDC 1435219. Electrochemical Methods. The electrochemical cell for cyclic voltammetry was made up of a glassy carbon working electrode, a platinum auxiliary electrode and a Ag/AgCl reference electrode. The potential of the cell was controlled by an ADI Powerlab 4SP potentiostat. Solutions were typically about 10-3 M in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) as a supporting electrolyte, and were purged with argon for approximately 5 min prior to measurement. The scanning speed was 100 mV s-1, and the cyclic voltammograms were calibrated against the decamethylferrocenium/decamethylferrocene (Fc*+/Fc*) couple (-0.012 V vs Ag/AgCl in CH2Cl2) and are reported relative to the saturated calomel electrode (SCE) for comparison with other data by subtracting 0.045 V.71 Anodic and cathodic peak separations for all oxidation and reduction processes (including the Fc*+/Fc* internal standard) are approximately 110 mV. Computational Methods. Density functional theory calculations were performed using the Gaussian09 program suite.72 Calculations were performed using both B3LYP73-75 and CAMB3LYP functionals76, with a 6-31G(d) basis set.77-78 Calculated Raman vibrational frequencies were generated using GaussSum v2.2.5 software,79 and scaled by a factor of 0.975 and 0.95


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(B3LYP and CAM-B3LYP respectively) to generate the lowest mean absolute deviation (MAD) from experimental spectra. Electronic transitions and MOs were predicted using timedependent DFT (TD-DFT) calculations. A Mulliken charge analysis was applied to the TD-DFT calculations. A representation of how the excited states are accessed has been given in Figure S2. The vibrational modes of the optimized structures were visualized using Molden80 and MO diagrams were generated using GaussView v5.0.8 (Gaussian Inc).72 Onsager radii used for Lippert Mataga analysis were taken as recommended by the polarizable continuum model (PCM) using the volume key word. With the introduction of a linker unit some possible conformations were considered (Figure S3). Conformations which were within 5 kJ mol-1 were used to provide average φD-A, φA-L, φL-L, φL-D and dD-A values. These are noted with a *. No appreciable change was noted in predicted Raman or absorption data as has been seen in the literature.81 Due to this, the lowest energy structure for each compound was used for further calculations. Spectroscopic Methods. All spectroscopic measurements were made with Sigma-Aldrich spectroscopic or HPLC grade solvents. Spectral data was analyzed using GRAMS A/I (ThermoScientific) and OriginPro v9.0 (Origin Lab Corporation). Electronic absorption spectra were measured using an OceanOptics USB2000 spectrometer. FT-Raman Methods FT-Raman spectra were recorded using a Bruker MultiRAM spectrometer implementing a 1064 nm excitation at 50 mW power, a resolution of 1 cm-1 and 32 coadded scans. Resonance Raman spectra were collected using an excitation beam and collection lens in a 135° backscattering arrangement. Scattered photons were focused on the entrance slit of an Acton Isoplane 320 spectrograph with a 1200 grooves/mm grating, which disperses the radiation in a horizontal plane on a Princeton Instruments PyLoN 400BR liquid-nitrogen-cooled CCD detector. A Coherent Innova I-302 krypton ion laser was used to provide excitation wavelengths (λex) of 350.7, 406.7 and 413.1 nm, a solid-state CrystaLaser was used for 448.0 nm and solid state Cobolt lasers provided 458 and 491 nm. Notch filters (Kaiser Optical, Inc.) or long-pass filters (Semrock, Inc.) matched to these wavelengths were used to remove the laser excitation line. Sample concentrations were typically 1x10-3 mol L-1. Emission spectra were recorded on a Princeton Instruments SP2150i spectrograph with 300 groove mm–1 grating and Pixis 100B CCD. The samples were excited by a 20mW 355nm diodepumped solid state laser from Cobolt. Emission maps and TCSPC lifetime measurements were carried out on an Edinburgh Instruments FS5 Fluorimeter. The excitation for mapping measurements was a Xe source, while the TCSPC measurements used an EPLED 320nm (325.4 nm center) laser. In each case a R928P photomultiplier detected the emission signal. Bandwidths and steps of 3-5 nm were used to attain sufficient emission signal, with dwell times from 0.1-1 seconds. 40% colloidal silica in H2O was run at 325.4 nm to provide an instrument response function for lifetime measurements.

RESULTS & DISCUSSION Compounds Investigated

We report studies with various linker units as shown in Figure 1. These include alkyne (BTD-CC-TPA), thiophene (BTD-Thio-TPA), phenyl (BTD-Ph-TPA) and diphenyl (BTD-Ph2-TPA). The use of these linkers result in a change of D-A distance (dD-A) from 1.5 to 10.1 Ǻ and

result in changes of D-A torsion angle (φD-A) from 2o to 47o, see Table 1. These parameters have been defined in Figure S4. Changes of dD-A will alter the excited state dipole (Onsager radius), and both dD-A and φD-A may alter the coupling between donor and acceptor units.

Figure 1. Compound series studied in this work and nomenclature used.

The BTD-NPh2 compound was also included. In this compound an altered NPh2 donor unit was used which allows for a more direct linkage between donor and acceptor. Due to the changed donor, direct comparison between this compound and the other compounds of this series may not be suitable. Some data for BTD-TPA and BTD-TRZ-TPA has been presented previously which is included here for completeness and comparision.34 These data are augmented with new results and interpretation of these compounds. Synthesis

Target compounds BTD-TPA and BTD-TRZ-TPA have been previously reported.34 Synthetic routes to the new compounds are shown in Scheme 1. BTD-NPh2 was obtained through Buchwald-Hartwig coupling between previously reported 5bromobenzo[c][2,1,3]thiadiazole (BTD-Br) and N,Ndiphenylamine in 78% yield, and BTD-CC-TPA through Sonogashira-Hagihara coupling between BTD-Br and 4-ethynyl-N,N-diphenylaniline in 58% yield. BTD-Thio-TPA was prepared in three steps from BTD-Br; the first step involves Suzuki-Miyaura coupling with 2-thienylboronic acid, followed by bromination with N-bromosuccinimide, which occurs selectively at the 5-position of the thienyl group, and subsequent Suzuki-Miyaura coupling with 4diphenylaminophenylboronic acid to afford BTDThio-TPA in an overall yield of 56% (3 steps). BTD-Ph-TPA was prepared in one step (84% yield) from previously reported BTD-Ph-I27 using a SuzukiMiyaura coupling with 4-diphenylaminophenylboronic acid. BTD-Ph2-TPA was prepared in three steps from BTD-Ph-I; the first step involves Suzuki-Miyaura coupling with 4-trimethylsilylphenylboronic acid. The trimethylsilyl group acts as a ‘protected halide’, and is iododesilylated with ICl to afford 5-(4iodo)benzo[c][2,1,3]thiadiazole, which is subsequently coupled with 4-diphenylaminophenylboronic acid in a



Suzuki-Miyaura coupling to afford BTD-Ph2-TPA in an overall yield of 53% (3 steps).

Page 6 of 18

Ph-TPA in the P21/c space group (Figure 2). The BTD units are completely planar, as are the TPA units with the amine nitrogen atoms (N3, N53) lying only 0.016/0.020 Å above the plane defined by the ipsocarbons. The overall twist between the BTD benzoring and TPA phenyl is 31.6° for molecule 1 (N1N3/S1) and 56.4° for molecule 2 (N51-N53/S51).

Crystals of BTD-Ph-TPA suitable for a single-crystal X-ray diffraction study were obtained by recrystallization from ethanol. The crystal structure obtained revealed two independent molecules of BTDScheme 1. Synthesis of the target compounds.a

N

N S N

BTD-NPh2 N

(i) Br

N

(ii)

N

S

S N

N

BTD-CC-TPA

BTD-Br

(iii)

N

N

(iv)

S

S

S

S

Br

N

(v)

N

N

N

N

S

S

BTD-Thio-TPA

SiMe 3

I

I N

(vii)

N

S N

N

N

(viii)

S

S

N

BTD-Ph-I

(ix) (vi)

N N

N

N

S N

S

BTD-Ph-TPA

N

a

BTD-Ph2-TPA

Reagents and conditions: (i) N,N-Diphenylamine, tBuOK, [tBu3PH]BF4, [Pd2(dba)3], toluene, reflux, 15 h, 78%, (ii) 4-ethynyl-N,Ndiphenylaniline, CuI, [PdCl2(dppf)], NEt3, reflux, 15 h, 58%; (iii) 2-thienylboronic acid, K2CO3, [PdCl2(dppf)], toluene, water, EtOH, reflux, 15 h, 93%; (iv) NBS, DMF, 60 °C, 15 h, 85%; (v) 4-diphenylaminophenylboronic acid, K2CO3, [PdCl2(dppf)], toluene, water, EtOH, reflux, 15 h, 71%; (vi) 4-diphenylaminophenylboronic acid, K2CO3, [PdCl2(dppf)], toluene, water, EtOH, reflux, 15 h, 84%; (vii) 4-


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trimethylsilylphenylboronic acid, K2CO3, [PdCl2(dppf)], toluene, water, EtOH, reflux, 15 h, 67%; (viii) ICl, CH2Cl2, -78 °C, 2 h, 97%; (ix) 4-diphenylaminophenylboronic acid, K2CO3, [PdCl2(dppf)], toluene, water, EtOH, reflux, 15 h, 81%.

Table 1 shows calculated parameters for the compound series. The linear alkyne linker in BTD-CC-TPA increased dD-A from a single C-C bond length (1.48 Å in BTD-TPA) to 4.06 Å. This caused a reduction of steric constraints, hence a φD-A of 2° was predicted compared to 35° in BTDTPA. Cyclic linkers also increased dD-A, for example BTDTRZ-TPA at 5.0 Å and BTD-Ph2-TPA at 10.1 Å. In each case variations in φD-A were also seen. The five-membered thiophene and triazolyl linkers exhibit smaller φD-L and φL-A values than phenyl linked compounds. This can again be assigned to changes in steric bulk and H-H interactions. Table 1 DFT-predicted (B3LYP/6-31G(d)) geometry data for compounds in vacuo. Angles have been averaged for compounds which possess +/- conformations. These are marked with *. The full data set is shown in Table S3. φD-L

Figure 2. X-ray crystal structure obtained for BTD-Ph-TPA. The two independent molecules of the asymmetric unit are shown along with selected atom labelling and torsion angles (in red) involving the phenylene linker. Ground State Geometries

Density functional theory (DFT) and TD-DFT can model the electronic structure of molecules, and give predictive power for geometries and various photophysical properties.82-85 Previous work has indicated the need for complementary DFT methods to better investigate BTD donor-acceptor dyes.34 In this study B3LYP, a standard hybrid (20% Hartree Fock exchange), is utilized for the modelling of ground state geometries, vibrational spectra and electrochemical potentials.86 CAM-B3LYP, a rangecorrected hybrid (using 19 % HF at short range and 65 % HF at long range) is used for the modelling of absorption spectra and excited state characteristics. Each functional offers advantages in performance. For example B3YLP generally models short-range electron-electron interactions more effectively.87 The attenuating CAM-B3YLP functional can approximate long range interactions more effectively and is especially relevant for CT processes in this study.87-89 Both data sets have been generated and are displayed where appropriate. DFT optimized structures are validated through the calculation of mean absolute deviations (MAD) which compare experimental and calculated Raman frequencies.90 MADs less than 15 cm-1 are considered satisfactory and signify that the geometry and bonding network is sufficiently predicted. All compounds give MADs less than 12 cm-1 for B3LYP and 15 cm-1 for CAM-B3LYP. These data are reported in Table S1. Ground state FT-Raman spectra are shown in Figure S5 and FT-Raman data with assignments are given in Table S2.

BTD-TPA BTD-CC-TPA BTD-Thio-TPA* BTD-TRZ-TPA* BTD-Ph-TPA* BTD-Ph2-TPA*

24.1 25.8 36.3 36.6

Torsional Angle / ° φL-L φL-A φD-A 36.0

23.6 5.4 34.9 35.5

Distance / Å dD-A

34.8 2.3 28.4 24.0 36.6 47.1

1.48 4.06 5.37 4.99 5.81 10.15

Interestingly, BTD-TRZ-TPA was twisted between TPA and the triazolyl linker (φD-L = 25.8°) but close to planar between the triazolyl and BTD (φL-A = 5.4°). All other compounds with linker units showed φD-L values comparable with φL-A. BTD-NPh2 has a direct linkage between the electron rich nitrogen and the BTD moiety. For this reason it is not directly comparable to the other compounds. Electrochemistry

Electrochemical parameters for the compound series were analyzed using both differential pulse voltammetry (DPV) and cyclic voltammetry (CV), data are shown in Table 2. Table 2 Electrochemical data in CH2Cl2. Values given are relative to SCE and referenced to [Fc*]+/0 = 0.057 V. Data collected at solute concentrations of 1 × 10-3 M with a concentration of 0.1 M for Bu4NPF6 as supporting electrolyte. * BTD-TPA and BTDTRZ-TPA data reported in literature.34

BTD-TPA* BTD-CC-TPA BTD-Thio-TPA BTD-TRZ-TPA* BTD-Ph-TPA BTD-Ph2-TPA BTD-NPh2

BTD0/-1.45

-1.35 -1.40 -1.30 -1.44 -1.44 -1.53

E° / V Donor+/0 1.03

1.05 0.91 1.00 0.98 0.98 1.20

The first oxidation and first reduction potentials, which are primarily TPA- and BTD-based processes respectively, vary with changes in linker. For example, for BTD-CC-TPA the first reduction potential is +0.1



V compared to the unmodified BTD-TPA. This indicates an increase in electron affinity in the BTD region. Conversely, the close proximity of donor and acceptor in BTD-NPh2 indicates a decrease of BTD electron affinity (-0.08 V compared with BTD-TPA). BTD-Thio-TPA is the only compound to significantly modify the TPA oxidation potential (-0.12 V), i.e. making TPA more susceptible to oxidation. Electrochemical results are used to infer the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular (LUMO) energies using a method of Li et. al.86 These data are shown in Figure S6 and compared with B3LYP predicted orbital energies. The electrochemically evaluated energies and DFT energies correlate well (within 0.3 eV). FMO diagrams are shown in Table S4. The HOMO and LUMO orbitals of BTD-TPA are localized over the donor and acceptor units respectively. This indicates the lowest energy electronic transition will likely be of CT nature. The HOMO-1 and LUMO+1 orbitals, however, are delocalized, opening the possibility for a close lying π-π* state involving the linker unit. For compounds which have a cyclic linker (Thio, TRZ, Ph and Ph2) the HOMO orbital is located on both TPA and the linker. This correlates with of easier oxidation of TPA. For BTD-TPA and BTD-CC-TPA the HOMO orbital is located almost entirely on the TPA unit. Electronic Absorption Spectroscopy

Electronic absorption data are shown in Figure 3 and tabulated in Tables S5-S7. Two strong absorption features are observed for all compounds. The high energy transition is assigned as π-π*. The low energy band is predicted to be of CT character by both B3LYP and CAM-B3LYP (Table S6 and S7). The energy and intensity of both bands are modulated as dye structure is modified

Page 8 of 18

Figure 3. UV-Vis absorption spectra for all compounds in CH2Cl2. BTD-TPA and BTD-TRZ-TPA data reported in literature.34

Compounds BTD-CC-TPA and BTD-Thio-TPA, with relatively small torsions of around 2° and 25° (BTD-TPA φD-A = 35°), show red-shifted transitions (413 and 432 nm respectively compared to 409 nm). Phenyl compounds exhibit blue-shifted transitions (381 and 352 nm for BTDPh-TPA and BTD-Ph2-TPA respectively compared to 409 nm for BTD-TPA). This can rationalized by larger dD-A values and decreases of D-A communication. The triazolyl linker is known to disrupt communication between donor and acceptor.50, 91 In this study, a decrease of intensity for the lowest energy band is observed in both experimental and calculated data when a triazolyl linker is used. For example, the observable ε and f (CAM-B3LYP) values decrease by around 40% compared to the unmodified BTD-TPA. Whilst this represents a significant decrease of intensity, triazolyl linkers have been shown to cause even greater disruption of D-A communication.50 For BTD-NPh2, which differs to other compounds as it lacks a full TPA group, the λmax shifts to the red by 0.1 eV and exhibits a 30% decrease in intensity. This compound directly couples donor and acceptor moieties, which eliminates steric and distance considerations. In this case, limited CT is observed as evidenced by low Mulliken charge values (Table S6-S7). This data suggests that BTDNPh2 has lost the ‘Goldilock balance’ between donor and acceptor; that is the donor and acceptor units have become so directly connected that the ‘individuality’ of the donor and acceptor units has become compromised. This is consistent with the donor electron density becoming delocalized over both BTD and NPh2. Indeed the HOMO orbital for BTD-NPh2 does show delocalization over the BTD unit (Table S4). Furthermore, electrochemical data does show the most negative BTD0/- of -1.53 V and also the most positive Donor +/0 of 1.20 V (note NPh2 does have a different oxidation energy from TPA).


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Figure 4. Mulliken charge analysis of the lowest energy transition for compounds with linker units. Dotted lines indicate contribution from the linker that is less than 10%.

The contribution of the linker unit, from the Mulliken charge analysis, to the lowest energy transition has been illustrated in Figure 4. Alkyne and triazolyl linkers are not predicted to participate much in the lowest energy CT transition (0.8) were observed for a number of compounds (Table S9). In more polar solvents such as dimethylformamide and acetonitrile emission intensity is much weaker with quantum yields less than 0.1. Two further effects are noted with changes of solvent polarity, 1) the low energy band (denoted Elow) shows positive solvatochromisim and 2) a second higher energy band (denoted Ehigh) gains intensity. This is shown for the unmodified BTD-TPA in Figure 6. The solvatochromic effect of Elow can be exemplified by BTD-TPA which shows shifts of 0.6 eV between toluene (504 nm) and acetonitrile (672 nm). This is consistent with a CT excited state. The degree of excited-state stabilization is proportional to the solvent polarity parameter (∆f); in this case, a bathochromic shift indicates a more polar excited state, relative to the ground state.52 Furthermore, the decreased quantum yields in polar solvents can be rationalized when considering positive solvatochromism in the context of the energy gap law.95 In a polar solvent the energy gap between the lowest excited electronic state and the ground electronic state is decreased and thus the rate of nonradiative decay (knr) increases. This leads to a decrease in the quantum yield.

BTD-TPA BTD-CC-TPA BTD-Thio-TPA BTD-TRZ-TPA BTD-Ph-TPA BTD-Ph2-TPA BTD-NPh2

Page 10 of 18 Onsager Radii / Å

∆µ (D)

5.23 6.02 6.16 6.09 6.10 6.27 5.34

15 ± 2 20 ± 1 18 ± 2 20 ± 2 21 ± 2 25 ± 3 11 ± 2

Figure 7. Lippert-Mataga plot for all compounds. Error bars are included for Stokes shift (νA-νF) with 5% error.

The dual emission behavior has been as observed for BTDTPA in previous work.34 In the current study, six of the seven compounds show dual emission. Interestingly, the relative intensity of the Ehigh and Elow changes with ∆f. In non-polar solvents only Elow is appreciably observed. Conversely, in more polar solvents Ehigh becomes increasingly dominant. Materials with triazolyl and phenyl linkers show a complete switching of emissive excited state in response to solvent polarity (Figure 8). The emission wavelength of Ehigh does not shift appreciably with solvent.

Figure 6. Emission spectra for BTD-TPA in a number of solvents with an excitation wavelength of 355 nm. Solvents are colored with respect to solvent parameter (∆f), where toluene (0.01) is red and acetonitrile (0.31) is blue.

A Lippert-Mataga analysis was used to determine the change in dipole between the electronic ground and excited state (∆µ); the findings are shown in Figure 7 and Table 3.52 Compounds with longer linker units appear to show larger ∆µ values. BTD-Ph2-TPA, in this case, shows the largest value (25 D). Standard error in ∆µ values was largest for BTD-TRZ-TPA and BTD-Ph2-TPA (±2 and ±6 D, respectively), which was partly due to the lower number of solvents which these compounds were sufficiently soluble in. Table 3. Lippert-Mataga analysis for the compound series implementing the lowest energy absorption and emission data. Recommended Onsager radii were generated at a B3LYP/631G(d) level of theory using the volume keyword.

Figure 8. High to low energy emission (high energy to CT) ratios over a number of solvents. Data collected at 1x10-5 M and with an excitation of 355 nm. Fitting has been included for indicative purposes only and assumes complete S1CT emission at f(x) = -∞,


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and complete high energy emission at f(x) = ∞. Error bars are included for the relative intensity ratio with 5% error.

Dual emission behavior may be attributed to formation of a twisted-intramolecular charge transfer (TICT) state.56 In the TICT model, two states are considered; the first is a local excited state (S1b, emission peak Eb) which then twists to form the TICT state (S1a, emission peak Ea). In non-polar solvents the charge transfer TICT state is strongly disfavored so only the strong higher energy emission (Eb) is observed. In polar solvents, the TICT state is stabilized and in this case, the lowest energy state Ea is observed. A classic example of this behavior is observed for N,N– dimethylaminobenzonitrile (DMABN).56 Boron dipyrromethene (BODIPY) analogues of BTD-TPA and BTD-Ph-TPA have also shown TICT behaviors.96 However, for this series of compounds, the TICT model does not satisfactorily explain the switching behavior. In non-polar solvents only the low energy emission is observed (Elow). As the solvent polarity is increased, Elow shifts to lower energy consistent with a CT state. Ehigh is only observed in polar solvents like dimethlyformamide and acetonitrile, and it is higher in energy than Elow in the most polar solvent (toluene). This behavior runs completely contrary to the TICT model. Excited State Calculations To explore the dual-emission behavior, a number of excited state calculations were run. Using well established methods the three lowest energy states (n = 1, 2 and 3) were examined.58 These states are denoted from low energy to high energy respectively. The associated transitions are given in Figure 9. TD-DFT and the associated frontier molecular orbitals indicate that state 1 is CT in nature, while states 2 and 3 are delocalized π-π* in nature. In some cases, the state 3 optimizations converge to state 2. This exemplifies one of the issues with excited state

optimizations: where the optimization of a higher energy state (state 3) may meet a conical intersection of a lower energy excited state (state 2).97-98 Further optimization steps can result in the ‘collapse’ from the state 3 potential energy surface to the state 2 potential energy surface. Avoiding this issue can be difficult. State 3 for BTD-TPA and BTDPh-TPA could not be reached despite repeated attempts.

Figure 9. Experimental and calculated absorption characteristics for BTD-CC-TPA in toluene (CAM-B3LYP/6-31G(d)). Black vertical lines indicate transitions, and States 1, 2 and 3 investigated.

Excited state geometry parameters are shown in Table 4. Predicted dipole moments for state 1 are larger than states 2, 3 and the ground state by about 20%. This supports the assignment of state 1 as CT. States 2 and 3 are similar to the ground state structure. These are π- π* in nature. State 1 also shows decreased φD-A values, i.e. it is distinct from the other states and appears to become more planar when compared to the ground state. An acetonitrile solvent field accentuates both of these trends.

Table 4. Calculated excited state geometry parameters collected at a CAM-B3LYP/6-31G(d) level of theory. *For BTD-TPA and BTDPh-TPA state 3 was unable to be attained, and repeatedly collapsed to a state of equivalent energy and nature to state 2. Geometric Parameters

BTD-TPA* BTD-CC-TPA

BTD-Thio-TPA

BTD-TRZ-TPA

BTD-Ph-TPA* BTD-Ph2-TPA

State 1 2 1 2 3 1 2 3 1 2 3 1 2 1 2 3

φD-L 1.7 2.3 6.1 0.5 0.8 6.4 15.4 25.7 18.6 19.1 23.7

Toluene φL-L φL-A 0.0 8.2 35.2 20.5 19.7 32.5 15.6 31.8 12.9 16.4 29.1 30.4 32.5 39.9

φD-A 17.6 36.5 0.8 1.5 1.1 2.0 12.5 50.4 22.4 24.2 30.5 0.3 6.0 11.9 40.6 48.3

µ 5.3 3.3 6.1 3.8 4.6 6.4 5.1 4.4 5.8 4.9 4.8 5.7 3.9 5.5 4.0 4.4

φD-L 1.5 1.7 6.7 0.9 0.7 7.0 12.8 22.6 18.2 16.4 26.4


φL-L 7.9 28.1 31.3

Acetonitrile φL-A φD-A 12.1 38.9 0.6 1.1 1.6 0.1 1.7 1.6 4.1 26.7 24.4 1.2 0.3 18.9 23.3 39.7 38.0 12.7 0.3 30.3 7.6 14.5 12.1 28.8 15.7 33.2 25.1

µ 9.2 5.2 10.6 6.5 7.3 10.9 7.4 8.5 9.6 6.5 6.3 10.7 6.5 11.5 7.6 5.4

The Journal of Physical Chemistry Bond length alternation (BLA) analysis is used to explore subtle changes in the geometry.99-101 This technique uses calculated geometries and assigns a bond numbering system (Figure S8). Bond lengths of the excited states are compared to the equivalent bond lengths of the ground state, and the difference (∆ bond length) is shown. BLA for BTD-CC-TPA is given in Figure 10 and is representative for most other compounds. The remaining compounds are given in the appendix (Figure S9). State 3 shows the greatest bond length changes in the BTD region (bonds 1-10); i.e. the electronic transition from the ground state mostly rearranges the BTD region. State 1 also shows a bond length change in the BTD region but this is accompanied by unique changes in the linker and part of TPA region (bonds 12-16). State 2 appears to be intermediate, with similarities to both states 1 and 3. Changing the solvent field, i.e. toluene or acetonitrile, does not alter BLA trends. Another advantage of optimizing the excited state structures is that transitions from the excited state to the ground state can be calculated, i.e. a prediction of emission energies for the lowest energy state (state 1). This can be done in two ways, either using the LR or a SS formalism (see introduction). Comparison has been made between the emission energies predicted using the LR and SS formalisms. Experimental and calculated data for the lowest energy emissive state (Elow, state 1) has been tabulated in Tables 5 and 6 for the toluene and acetonitrile solvent fields, respectively. 0.08

BTD S1 S2 S3

0.06 0.04

CC

∆ Bond Length / Å

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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Regrettably, the decrease of error for SS calculations is met with a decrease of trend prediction. This may make this formalism less useful when trying to study similar D-A materials systematically. It is worth noting that BTD-TRZTPA exhibits poor predictions for both methods. In acetonitrile, similar results are observed for compounds which do not show emission switching, i.e. still show an Elow band (Table 6). LR predicted emission energies are within approximately 0.8 eV of experimental values. SS emission energies are within 0.1 eV. Table 5. Absorption and emission data for the Elow band and state 1 in toluene. Calculated data was collected using a CAMB3LYP/6-31G(d) level of theory.

BTD-TPA BTD-CC-TPA BTD-Thio-TPA BTD-TRZ-TPA BTD-Ph-TPA BTDPh2-TPA BTD-NPh2

λmax / nm Exp. Cal. (LR) 407 352 412 369 429 370 387 336 383 333 360 324 425 371

Exp. 504 500 535 538 496 480 529

Cal. (LR) 418 428 471 399 415 408 447 Mean

λemission/ nm ∆ / eV 0.51 0.42 0.31 0.80 0.49 0.46 0.43 0.49

Cal. (SS) 493 489 509 870 510 506 529 Mean

∆ / eV 0.05 0.06 0.12 0.88 0.07 0.13 0.00 0.19

Interestingly, for compounds which only show Ehigh in acetonitrile (linker =TRZ, Ph, Ph2), unreasonably low emission energies are predicted when using the SS method (from 1000-12000 nm). Although the emission energies are far smaller than what would be expected experimentally, this exaggerated stabilization effect is consistent with a CT state and the general trends seen experimentally. It appears that this stabilization is great enough in high polarity solvents that for certain compounds (linker =TRZ, Ph, Ph2), the Elow band is no longer visible as decay to the ground state is dominated by non-radiative decay processes.

TPA Table 6. Absorption and emission data for the Elow band and state 1 in acetonitrile. Calculated data was collected using a CAMB3LYP/6-31G(d) level of theory.

0.02 0.00 -0.02 -0.04 -0.06 5

10

15

20

25

30

BTD-TPA BTD-CC-TPA BTD-Thio-TPA BTD-NPh2

λmax / nm Exp. Cal. (LR) 399 354 402 369 420 372 420 373

Bond #

Figure 10. Bond length alternation patterns for BTD-CC-TPA relative to the optimized ground state structure. Data collected at CAM-B3LYP/6-31G(d) level of theory. Dotted lines indicate an acetonitrile solvent field, solid lines indicate toluene.

In toluene, LR predicted emission energies are within approximately 0.5 eV of the experimental values. SS emission energies lie within 0.2 eV of the experimental data. This is perhaps unsurprising given the CT nature of the transition. Solvent reorganization in response to electron density change becomes important with a dipolar excited state. With the LR approach, the solute-solvent stabilization is underestimated leading to larger energy gaps, as has been observed previously for CT states.58-59, 64,

Exp. 672 678 678 617

Cal. (LR) 449 459 513 459 Mean

λemission / nm ∆ Cal. / eV (SS) 0.92 707 0.87 718 0.59 699 0.69 654 0.77 Mean

∆ / eV 0.09 0.10 0.05 0.11 0.09

Emission Mapping Emission maps were measured for BTD-TPA in acetonitrile (Figure 11a) and in dimethylformamide (Figure S10a). Excitation wavelengths of 300 to 330 nm correspond to a strong Elow feature around 720 nm. Excitation at these wavelengths populate a π→π* excited state. This state must then deactivate to the Elow CT state. Excitation at wavelengths of 370 nm to 450 nm also result in Elow. Excitation at these wavelengths populate the CT excited state. Excitation around 350 nm results in emission from the Ehigh (λemission ≈ 420 nm) band. This state is unable to decay to the lower energy state.

102


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Lifetimes for the respective emissive states are shown in Table S9. The emissive lifetimes of both the Ehigh (for linkers TRZ, Ph and Ph2) and Elow bands are 10 ns or shorter and show minimal sensitivity to degassing under argon. For all compounds, quantum yields decrease drastically in these polar solvents. Decreases quantum

yields provides evidence that non-radiative decay rates increase for the Elow state when polar solvents are used. As mentioned earlier, this can be rationalized by decreases of the energy gap in polar solvents.95

Figure 11. a) Emission versus excitation map of BTD-TPA in acetonitrile at around 2x10-6 M and b) the absorption spectrum for BTDTPA in acetonitrile.

The Elow exhibits solvatochromism and decreases in intensity in more polar solvents. This is consistent with a CT excited state which undergoes solvent stabilization in polar solvents. Hence, the mechanism for ‘switching’ between Elow and Ehigh arises from increasing non-radiative decay from the Elow state in polar solvents. Overall, this has afforded control of dominant emissive state by manipulating solvent polarity.

state has a very low extinction coefficient in the absorption spectrum and 4) the Ehigh state does not decay to the Elow state. These results are not inconsistent with a Ehigh that is n→π* in nature.103 Indeed, for each compound a very weak BTD localized n→π* transition is predicted by TD-DFT. These transitions and the HOMO orbital responsible are included in Tables S10-12. The possibility that this state is a triplet is discarded because the lifetimes are short. Thus, an energy diagram for this system is proposed in Figure 12.

CONCLUSION

Figure 12. The energy diagram proposed to explain the dual emission behavior.

The data collected in regard to Ehigh suggests the following: 1) the excited state associated with Ehigh is a singlet, 2) the quantum yield for this state is very low regardless of solvent, 3) the excitation wavelength that populated this

In this study we have synthesized and characterized seven benzo[c][2,1,3]thiadiazole -based donor-acceptor compounds, which are systematically altered with respect to linker unit. A broad absorption band is observed between 350 and 440 nm, and assigned as charge-transfer in nature using resonance Raman spectroscopy and TD-DFT calculations. The energy and intensity of this transition is successfully tuned via subtle interplay of torsion angle, donor-acceptor distance and inclusion various linker units. Dual emission characteristics were observed for six of the seven compounds compared. ‘Insulating’ linkers such as triazolyl and phenyl showed switching from the Elow state in non-polar solvents to a Ehigh state in polar solvents. The BTD-NPh2 compound did not show dual emission



characteristics. These states can be rationalized as a high energy n→π* (Ehigh) and a low energy CT state (Elow). The sensitivity of these materials to the environment may be useful for sensor applications. Excited state calculations were used to support the assignment of the three lowest energy excited states (accessed through electronic absorption). Using a state specific solvation approach improved emission energies, and response to solvent polarity, was achieved.

ASSOCIATED CONTENT Supporting Information NMR compound number schemes; an illustration of excited state formalisms; geometry parameter definitions, conformation definitions with two examples, mean absolute deviations between FT and calculated Raman spectra; calculated data for the various conformations; frontier molecular orbital energies; frontier molecular orbital diagrams; FT-Raman vibrations and assignments; TPA vibrational eigenvector diagrams; experimental and calculated electronic absorption data; resonance Raman data collected at a number of excitation wavelengths; experimental and calculated emission data; emission quantum yields and lifetimes bond length alternation numbering scheme; bond length alternation plots; emission mapping; calculated electronic absorption data for n→π* transitions and the associated HOMO orbital diagrams; indicative relationship between band gaps, extinction coefficients and D-A distance; variable temperature emission. This supporting information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

Correspondence should be addressed to Keith Gordon ([email protected]) and Nigel T. Lucas ([email protected]); fax: +64 3 479 7906, phone: +64 3 479 7908. Notes

Authors declare no competing financial interest.

ACKNOWLEDGMENT JEB, GES, JIM and CBL thank the University of Otago for a PhD scholarship. The MacDiarmid Institute for Advanced Materials and Nanotechnology is gratefully acknowledged for support. The New Zealand eScience Infrastructure (NeSI) is thanked for computational time.


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