Controlled Intramolecular Charge Transfer Using a Sulfur-Containing

Dec 30, 2015 - Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada. J. Phys. Chem. C , 2016, 120 (1), pp 70–77...
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Controlled Intramolecular Charge Transfer Using a Sulfur-Containing Acceptor Group Paniz Pahlavanlu, Peter R. Christensen, Jeffrey A. Therrien, and Michael O. Wolf* Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada S Supporting Information *

ABSTRACT: A series of donor−acceptor (D−A) and donor−acceptor−donor (D−A−D) molecules containing anthracene chromophores paired with a sulfur-containing acceptor group have been prepared. The oxidation state of the acceptor sulfur atom controls the degree of charge transfer character in the excited state wave function. Steady-state photoluminescence spectroscopy shows more pronounced solvatochromism with increasing sulfur oxidation state. Computational methods (DFT/TD-DFT) are used to show that orbital mixing between donor and acceptor is facilitated as the oxidation state of the acceptor sulfur is increased.

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INTRODUCTION The formation of charge transfer (CT) or charge separated states in organic materials is central to the function of both organic photovoltaics (OPVs)1−3 and light-emitting diodes (OLEDs).4 In OPVs, the binding energy of an electron/hole pair (Frenkel exciton) determines the ability to separate charges. In OLEDs, electrons and holes are injected from opposite electrodes and combine in the emitting layer of these devices to form luminescent (Frenkel) excitons.5 At separations greater than the capture radius (typically >100 Å), the polarons have uncorrelated spin states. However, upon capture, an initially formed CT state results which possesses either singlet (1CT) or triplet (3CT) character. In most systems the 1CT and 3 CT excitons decay to their respective Frenkel excitons in a 1:3 singlet/triplet statistical distribution. With fluorescent materials this statistical branching ratio results in an upper limit for electroluminescence efficiency of 25% as triplet excitons typically decay nonradiatively without the presence of strong spin−orbit coupling.6 Recently, two separate mechanisms, (1) thermally activated delayed fluorescence (TADF)7 and (2) “extra-fluorescence”,8 have been proposed for deliberately controlling the singlet/ triplet branching ratio in an electroluminescent device. In the case of TADF it has been demonstrated that by introducing charge transfer in the excited state, the singlet/triplet energy gap (ΔEST) can be reduced. By reducing ΔEST, triplet excitons can be thermally promoted to the emissive singlet state, enabling >25% internal quantum efficiency. Extra-fluorescence, much like TADF, is a photophysical process by which >25% singlet exciton formation is achieved by controlling the decay of singlet and triplet charge transfer states (1CT/3CT) to their respective singlet and triplet Frenkel excitons. For instance, if the energy difference between 1CT and 3CT is small, then the rate of singlet (ks) and triplet (kt) formation (from 1CT and 3 CT states) will control the singlet exciton fraction. For both of these methods, deliberate engineering of CT states in the emitting molecules is used to control the singlet/triplet exciton © XXXX American Chemical Society

ratio. Accordingly, methods to deliberately interrogate the effect of CT in organic chromophores are of interest. Control over CT states is also expected to be important in singlet fission, a photophysical process by which an excited singlet state can decay into two triplet excitons.9 This process has the potential to overcome the Shockley−Queisser limit of ∼34% to ∼50% in solar cells.10 After initial excitation to the singlet state, a CT state mediates the formation of the geminate triplet pair (fission). Recently, single molecules (dimers) that exhibit singlet fission have been prepared, but few methods exist whereby the intermediate CT state can be systematically controlled.11−14 Donor−acceptor (D−A) molecules and polymers have been extensively explored for their ability to readily form CT states.15 In most D−A systems the HOMO is localized on the electronrich donor while the LUMO is localized on the electrondeficient acceptor. The spatial separation of HOMO and LUMO in D−A molecules leads to the formation of polar, charge separated or charge transfer excited states. Careful selection of donor and acceptor groups enables tuning of HOMO and LUMO energy levels, which is essential for controlling absorption energies11−14 and matching a material with neighboring layers in OPVs and OLEDs.2,16,17 A wide variety of donor and acceptor groups have been employed in order to tune the optoelectronic properties of D− A compounds.15,18−24 Typical donor groups include aromatic amines, thiophenes, and acene derivatives while quinoxalines, diimides, and thiadiazoles are commonly used as acceptors.16,25 The most common method employed to tune charge transfer between donors and acceptors is through the incorporation of additional electron donating and withdrawing groups.16 Creating a series of molecules or materials with variable electronic properties, however, typically necessitates the Received: October 7, 2015 Revised: December 11, 2015

A

DOI: 10.1021/acs.jpcc.5b09826 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthesis of Sulfur-Bridged D−A (1SOn) and D−A−D (2SOn) Molecules (n = 0, 1, 2)

Experimental Methods. Nuclear magnetic resonance (NMR) spectra were obtained using Bruker AV-300 or AV400 MHz spectrometers and were referenced to either chloroform or dichloromethane. All compounds were characterized by 1H NMR spectroscopy, and compounds 1SOn and 2SOn were further characterized by 2D NMR spectroscopy (COSY). Infrared spectroscopy was performed using an attenuated total reflection (ATR) crystal on a PerkinElmer Spectrum 100 FT-IR spectrometer. Photoluminescence lifetimes (τPL) were measured using a HORIBA Jobin Yvon Fluorocube with a 370 nm excitation wavelength. Absorbance spectra were obtained using a Varian Cary 5000 spectrophotometer, and photoluminescence spectra were measured with a PTI QuantaMaster 50 fluorometer using λex = 350 nm. Extinction coefficients (ε) were calculated from the linear regression of a plot of absorption at λmax as a function of concentration. Three samples were weighed out for each compound, and each sample was diluted to approximately 10−5 M. Stokes shifts were calculated as the difference in energy (in cm−1) between the lowest energy absorption maximum and the emission energy with the greatest intensity. Photoluminescence quantum yields (ΦPL) were calculated relative to a standard, 9-cyanoanthracene (λex = 355 nm, ΦPL = 0.8 in methanol33), using eq 1.26 The calculated area of the photoluminescence spectrum for λex = 350 nm (F) was compared to absorption at that wavelength (A) for each sample (x) and the standard (s). Absorption and photoluminescence data were collected for at least three different concentrations of each analyte, and the average ratios of A to F were calculated. Standard deviations were calculated from the averaged data. The index of refraction (η) of each solvent is included in eq 1 to account for solvent differences between the samples and the standard.

synthesis of a broad set of donor and acceptor molecules, each with a different donor or acceptor. Previously, we reported increased photoluminescence in symmetrical sulfur-bridged dimers.26 The observed systematic increase in photoluminescence with sulfur oxidation state (S < SO < SO2) is due to an increased contribution of CT in the excited state wave function which reduces intersystem crossing to nonradiative triplet states.27 Likewise, others have reported changes in the optoelectronic properties of thiophenes upon oxidation to the S,S-dioxides.28−30 Thiophene dioxides, compared to unoxidized thiophenes, have much lower-lying LUMO levels and thus behave as efficient electron acceptors. The incorporation and sequential oxidation of sulfur-containing acceptor groups could enable control over the degree of charge transfer in D−A systems. Furthermore, the incorporation of a second donor group in D−A−D chromophores is expected to further promote CT.15 Here, we demonstrate a new approach to tuning intramolecular charge transfer in a series of donor−acceptor molecules by controlled chemical oxidation of the acceptor molecule (Scheme 1). Anthracene was chosen as an electron donor because it has unique and distinguishing absorption and photoluminescence properties. Additionally, controlling CT in acene dimers is relevant for optimizing singlet fission.11,31,32 Dibenzothiophene (DBT) was selected as a sulfur-containing acceptor molecule because, unlike thiophene, the sulfur atoms in DBT are easily oxidized allowing access to both sulfoxide and sulfone derivatives. The CT states of these sulfur-bridged donor−acceptor pairs were studied using steady-state absorption and photoluminescence spectroscopy as well as using density functional theory (DFT) modeling.

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EXPERIMENTAL SECTION Materials. 9-Anthraceneboronic acid was purchased from Tokyo Chemical Industry Co., Ltd. Sodium thiosulfate, sodium bicarbonate, HPLC-grade acetonitrile, methanol and reagentgrade cyclohexane were purchased from VWR. Potassium carbonate, magnesium sulfate, and HPLC-grade dichloromethane and chloroform were purchased from Fisher Scientific. Tetrahydrofuran (THF) was purchased from Sigma-Aldrich and distilled over sodium and benzophenone. All other reagents and solvents were purchased from Sigma-Aldrich and used as received. Schlenk techniques were employed for experiments conducted under nitrogen.

⎛ A ⎞⎛ F ⎞⎛ η ⎞ Φx = ⎜ s ⎟⎜ x ⎟⎜⎜ s ⎟⎟Φs ⎝ Ax ⎠⎝ Fx ⎠⎝ ηx ⎠

(1)

Radiative rate constants (kr) were calculated from photoluminescence quantum yields (ΦPL) and photoluminescence lifetimes (τPL) (eq 2).34,35 Experimental radiative rate constants are used as an indicator of the theoretical oscillator strength of an electronic transition.34,35 B

DOI: 10.1021/acs.jpcc.5b09826 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C kr =

ΦPL τPL

1SO2. Compound 1S (0.1 g, 0.3 mmol) was dissolved in CH2Cl2 in an aluminum foil covered flask, protected from light. A solution of m-CPBA (0.1 g, 0.5 mmol) in CH2Cl2 (10 mL) was added dropwise to the reaction flask, and the reaction was monitored by thin layer chromatography. After 1 h, the organic solution was washed with a saturated solution of sodium bicarbonate and concentrated under vacuum. Purification by column chromatography afforded a pale yellow solid. Yield: 0.04 g, 36%. 1H NMR (400 MHz, CD2Cl2): δ 8.61 (s, 1H), 8.12 (d, J = 8.5 Hz, 2H), 8.04 (d, J = 7.8 Hz, 1H), 7.92 (s, 1H), 7.89 (d, J = 7.5 Hz, 1H), 7.78 (d, J = 7.6 Hz, 1H), 7.59−7.67 (m, 5H), 7.48−7.55 (m, 2H), 7.37−7.45 (m, 2H). Highresolution EI-MS: mass calcd for C26H16O2S: 392.08710; found: 392.08728. IR (neat) cm−1: 1159 (νS=O, sym), 1290 (νS=O, asym). UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 476 nm. 2S. Compound 2S was synthesized through an adapted literature procedure.38 A flask containing 2,8-dibromodibenzothiophene (1.5 g, 4.4 mmol), 9-anthraceneboronic acid (2.4 g, 10.9 mmol), and potassium carbonate (9.9 g, 71.5 mmol) in toluene and ethanol (230 mL, 12:1 v/v) was sparged with nitrogen for 45 min. Pd(PPh3)4 (0.5 g, 0.4 mmol) was added, and the mixture was heated to reflux under nitrogen for 3 days. After washing with water, the organic phase was concentrated in vacuo. The resulting black solid was washed with diethyl ether and filtered through silica with CH2Cl2 to remove starting materials and catalyst. The eluent was concentrated, and yellow crystals were collected and washed with diethyl ether. Yield: 0.4 g, 18%. 1H NMR (400 MHz, CD2Cl2): δ 8.51 (s, 2H), 8.19 (s, 2H), 8.18 (d, J = 8.4 Hz, 2H), 8.04 (d, J = 8.5 Hz, 4H), 7.69 (d, J = 8.8 Hz, 4H), 7.57 (dd, J = 8.1, 1.6 Hz, 2H), 7.41−7.47 (m, 4H), 7.30−7.35 (m, 4H). High-resolution EI-MS: mass calcd for C40H24S: 536.15987; found: 536.15974. UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 407 nm. 2SO. Compound 2S (0.050 g, 0.09 mmol) was dissolved in CH2Cl2 (100 mL) in an aluminum foil covered flask, protected from light. A solution of m-CPBA (0.023 g, 0.09 mmol) in CH2Cl2 (5 mL) was added dropwise to the reaction flask, and the reaction was monitored by thin layer chromatography. After 22 h, more m-CPBA (0.0147 g, 0.06 mmol) in CH2Cl2 (3 mL) was added in 0.5 mL aliquots to the reaction mixture. After washing with a saturated solution of sodium bicarbonate, the organic phase was concentrated under vacuum, and purification by column chromatography afforded a pale yellow solid. Yield: 0.0085 g, 16%. 1H NMR (400 MHz, CD2Cl2): δ 8.53 (s, 2H), 8.28 (d, J = 7.9 Hz, 2H), 8.05 (d, J = 8.5 Hz, 4H), 7.86 (d, J = 1.1 Hz, 2H), 7.69 (d, J = 8.8 Hz, 2H), 7.63 (d, 2H), 7.63 (d, 2H), 7.46 (m, 4H), 7.34−7.40 (m, 4H). High-resolution EIMS: mass calcd for C40H24OS: 552.15479; found: 552.15492. IR (neat) cm−1: 1030 (νS=O). UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 470 nm. 2SO2. Compound 2S (0.050 g, 0.09 mmol) and m-CPBA (0.064 g, 0.26 mmol) were mixed in CH2Cl2 (5 mL) in an aluminum foil covered flask, protected from light. More mCPBA was added after 8 h (0.020 g, 0.08 mmol) and 25 h (0.021 g, 0.09 mmol), and the reaction mixture was fully dissolved with additional CH2Cl2. After 3 days, the organic solution was washed with a saturated solution of sodium bicarbonate and concentrated under vacuum. Purification by column chromatography afforded a white-yellow solid. Yield: 0.020 g, 38%. 1H NMR (400 MHz, CD2Cl2): δ 8.54 (s, 2H), 8.12 (d, J = 7.9 Hz, 2H), 8.05 (d, J = 8.5 Hz, 4H), 7.85 (s, 2H), 7.67 (dd, J = 7.8, 1.3 Hz, 2H), 7.63 (d, J = 8.7 Hz, 4H), 7.44−

(2)

Synthesis. 2-Bromodibenzothiophene. 2-Bromodibenzothiophene was synthesized through adapted literature procedures.36,37 Bromine (1.3 mL, 25.2 mmol) in chloroform (50 mL) was added dropwise over 1.5 h to a stirring solution of dibenzothiophene (4.6 g, 25.0 mmol) in chloroform (150 mL) at 0 °C. The resulting solution was allowed to stir at room temperature for 10 days. The organic solution was washed with saturated solutions of sodium bicarbonate and sodium thiosulfate and then dried over anhydrous magnesium sulfate before concentration in vacuo. A crude mixture of starting material (∼20%), 2-bromodibenzothiophene (∼75%), and 2,8dibromodibenzothiophene (∼5%) (as determined by 1H NMR analysis) was obtained and used as is in subsequent procedures. 2,8-Dibromodibenzothiophene. 2,8-Dibromodibenzothiophene was synthesized through an adapted literature procedure.36 Bromine (3.4 mL, 66.0 mmol) in chloroform (30 mL) was added dropwise over 1 h to a stirring solution of dibenzothiophene (3.1 g, 16.6 mmol) in chloroform (100 mL) at 0 °C. The resulting solution was allowed to stir at room temperature for 6 days. The organic solution was washed with saturated solutions of sodium bicarbonate and sodium thiosulfate before concentration in vacuo to afford an offwhite solid. 2,8-Dibromodibenzothiophene was obtained in >90% purity and was used as is in subsequent procedures. Yield: 5.1 g, 90%. 1S. Compound 1S was synthesized through an adapted literature procedure.38 A flask containing 2-bromodibenzothiophene (2.4 g, ∼ 75% pure, 6.9 mmol), 9-anthraceneboronic acid (2.2 g, 9.8 mmol), and potassium carbonate (14.9 g, 107.6 mmol) in toluene and ethanol (230 mL, 12:1 v/v) was sparged with nitrogen for 45 min. Pd(PPh3)4 (0.8 g, 0.7 mmol) was added, and the mixture was heated to reflux under nitrogen for 2 days. After washing with water, the organic phase was concentrated in vacuo. The resulting black solid was filtered through silica with CH2Cl2 to remove catalyst and further purified by column chromatography to afford a pale yellow solid. Yield: 0.4 g, 18%. 1H NMR (400 MHz, CD2Cl2): δ 8.58 (s, 1H), 8.25 (s, 1H), 8.06−8.14 (m, 4H), 7.96 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 8.8 Hz, 2H), 7.42−7.58 (m, 5H), 7.29−7.38 (m, 2H). High-resolution EI-MS: mass calcd for C26H16S: 360.09727; found: 360.09715. UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 406 nm. 1SO. Compound 1S (0.4 g, 1.1 mmol) was dissolved in CH2Cl2 in an aluminum foil covered flask, protected from light. A solution of m-chloroperoxybenzoic acid (m-CPBA, 0.2 g, 0.9 mmol) in CH2Cl2 (10 mL) was added dropwise to the reaction flask, and the reaction was monitored by thin layer chromatography. After 15 min, the organic solution was washed with a saturated solution of sodium bicarbonate, dried over anhydrous magnesium sulfate, and concentrated under vacuum. Purification by column chromatography afforded a pale yellow solid. Yield: 0.2 g, 50%. 1H NMR (400 MHz, CD2Cl2): δ 8.60 (s, 1H), 8.20 (d, J = 7.8 Hz, 1H), 8.11 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 7.2 Hz, 1H), 7.93 (s, 1H), 7.79 (d, J = 7.1 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H), 7.64 (d, J = 7.9 Hz, 1H), 7.55−7.64 (m, 3H), 7.48−7.54 (m, 2H), 7.36−7.43 (m, 2H). High-resolution EI-MS: mass calcd for C26H16OS: 376.09219; found: 376.09194. IR (neat) cm−1: 1030 (νS=O). UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 443 nm. C

DOI: 10.1021/acs.jpcc.5b09826 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Normalized absorption spectra for 1SOn (a) and 2SOn (b) compared to unsubstituted anthracene. The arrows highlight the broadening of the spectra as the oxidation state is increased.

Figure 2. Normalized photoluminescence spectra for the sulfides (a), sulfoxides (b), and sulfones (c) of 1SOn (- - -) and 2SOn () in solvents of increasing polarity (cyclohexane, dichloromethane, and acetonitrile).

absorption profiles of 1SOn and 2SOn are shifted bathochromically by ∼9 nm. Similar bathochromic shifts in the structured absorption profiles have been observed for other substituted anthracenes (e.g., 9-phenylanthracene).25 Interestingly, neither the oxidation state of the bridging sulfur in the acceptor nor the presence of an additional donor group affects the absorption energies. The only difference observed in the absorption spectra for both 1SOn and 2SOn is a slight broadening as the oxidation state of the sulfur is increased (Figure 1a,b, arrows). Compound 1S has a similar molar extinction coefficient to that of anthracene (Table S1), while the molar extinction coefficient of 2S is approximately 3 times that of anthracene. The increased absorption of 2S is attributed to the presence of the additional anthracene chromophore. Furthermore, the absorption spectra are insensitive to solvent polarity, indicating that the absorbing state (Franck−Condon) is localized and anthracene-based, with overall neutral character. Emitting State. The emitting states of 1SOn and 2SOn were probed by measuring the photoluminescence (PL) spectra in solvents of increasing polarity (Figure 2a−c). While the absorbing states exhibit neutral, anthracene-based (π−π*) character, the emitting states exhibit substantially different behavior. Similar to unsubstituted anthracene, vibrational fine structure is present in the photoluminescence spectra for all species in nonpolar solvent (cyclohexane). However, with increasing solvent polarity (dichloromethane and acetonitrile) the sulfoxides (SO) and sulfones (SO2) exhibit broad, featureless photoluminescence profiles that are shifted to lower energies. The degree of bathochromic (Stokes) shift was quantified using the difference in energy between the lowest energy absorption maximum (385 nm) and the emission band with the greatest intensity (Table S1). While only very slight changes in the photoluminescence spectra are observed for both 1S and 2S (Figure 2a), the photoluminescence of 1SO, 1SO2, 2SO, and 2SO2 (Figure 2b,c) is strongly solvent dependent, exhibiting a significant (up to ∼7000 cm−1, 2SO2) Stokes shift as the solvent polarity is increased. More

7.49 (m, 4H), 7.35−7.41 (m, 4H). High-resolution EI-MS: mass calcd for C40H24O2S: 568.14970; found: 568.14976. IR (neat) cm−1: 1160 (νS=O, sym), 1292 (νS=O, asym). UV−vis (CH2Cl2): λmax = 367 nm. Photoluminescence (CH2Cl2): λmax = 491 nm. Computational Methods. DFT and TD-DFT calculations were performed with Gaussian 09 (Revision D.01) using the long-range and dispersion-corrected ωB97xD hybrid functional.39 The double-ζ def2-SVP basis set40 with auxiliary Coulomb fitting41 was employed in a self-consistent reaction field of dichloromethane using the integral equation formalism polarizable continuum model (IEFPCM).42 Frequency calculations were performed on geometry optimized ground state structures to ensure that energy minima were achieved. An ultrafine integration grid was required to achieve convergence in the geometry optimizations of 2S in the ground state and 2S and 2SO in the excited state.

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RESULTS AND DISCUSSION Synthesis. A series of D−A (1SOn) and D−A−D (2SOn) compounds incorporating anthracene donor groups and dibenzothiophene acceptor groups of varying sulfur oxidation state were synthesized (Scheme 1). Structural characterization of all new compounds is detailed in the Supporting Information. Notably, NMR analysis indicates that the protons on the anthracene rings closest to dibenzothiophene (positions 1 and 8) are inequivalent in 1SO and 2SO (Figures S3 and S7). This is attributed to the loss of C2 symmetry about the central sulfur atom for both sulfoxides due to restricted rotation about the C−C bond linking the donor and acceptor. Similar locked conformations may be expected for the sulfides (S) and sulfones (SO2); however, the protons in these molecules are in identical chemical environments and are indistinguishable by NMR. Absorbing State. The absorption spectra of 1SOn (Figure 1a) and 2SOn (Figure 1b) exhibit similar fine structure to that observed in unsubstituted anthracene except that the overall D

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Table 1. Fluorescence Quantum Yields (ΦPL, ± Standard Deviation from 3 Runs), Fluorescence Lifetimes (τPL), and Radiative Rate Constants (kr) for 1SOn and 2SOn in Cyclohexane (CHx), Dichloromethane, and Acetonitrile; Change in Dipole (Δμ) as a Function of Oxidation State Is Also Shown ΦPL

τPL (ns)

kr (10−8 s−1)

Δμ (D)

solvent

1S

1SO

1SO2

2S

2SO

2SO2

CHx CH2Cl2 MeCN CHx CH2Cl2 MeCN CHx CH2Cl2 MeCN

0.52 (0.08) 0.7 (0.1) 0.43 (0.05) 4.1 5.0 4.5 1.4 1.5 1.0

0.31 (0.03) 0.31 (0.03) 0.16 (0.02) 2.4 3.3 3.5 1.3 0.9 0.5 4.2

0.36 (0.08) 0.24 (0.02) 0.14 (0.06) 3.3 3.4 3.8 1.1 0.7 0.4 4.5

0.47 (0.06) 0.57 (0.07) 0.50 (0.05) 3.9 4.7 4.3 1.2 1.2 1.2

0.31 (0.03) 0.27 (0.03) 0.14 (0.01) 2.2 3.6 3.7 1.4 0.8 0.4 5.3

0.39 (0.04) 0.24 (0.02) 0.13 (0.01) 3.2 3.7 4.3 1.2 0.6 0.3 5.7

importantly, as the sulfur oxidation state increases, a larger Stokes shift is observed for both 1SOn and 2SOn. The incorporation of a second donor group (2 vs 1) also results in a slightly larger Stokes shift as the oxidation state of the bridging sulfur is increased. The observed solvatochromism in the PL spectra of 1SO, 1SO2, 2SO, and 2SO2 indicates a polar emitting state, likely resulting from the contribution of an intramolecular charge transfer (CT) state in the excited state wave function. Charge transfer states exhibit broad photoluminescence spectra due to the large number of structural configurations available to them compared to localized excited (LE) states, resulting in a broader energy distribution of emitting states.35 Charge transfer states are often stabilized relative to LE states due to the distribution of charge over two molecular fragments resulting in a red-shift in the photoluminescence as the degree of CT character increases. Furthermore, the solvent dependence of photoluminescence spectra involving CT states is associated with the ability of polar solvents to better stabilize the polar character of the CT state.18,35 Thus, for both 1SOn and 2SOn, the solvatochromism observed in the photoluminescence indicates an increase in the contribution of CT to the excited state wave function as the oxidation state of the dibenzothiophene acceptor is increased (S < SO < SO2). The effect of solvent polarity on the excited state can be analyzed using the Lippert−Mataga relationship (eq 3).43,44 The change in dipole moment from the ground state to the excited state of the solute (Δμ) is estimated by treating the solvated molecule as a polarized spherical cavity with radius a and considering the relationship between the measured Stokes shift (Δv) and the solvent polarizability (Δf). The solvent polarizability is a unitless constant calculated as a function of the refractive index (η) and the dielectric constant (ϵ) of the medium (eq 4). Figure S11 shows the linear relationship between Δv and Δf for both 1SOn and 2SOn. From eq 3, the change in dipole moment is related to the slope of the linear fit of Δv versus Δf by the solvent cavity (a), Planck’s constant (h), the speed of light (c), and the dielectric permittivity of vacuum (ϵ0). The spherical solvent cavity (Onsager radius)45 is estimated using the molecular weight (M) of the solute as well as Avogadro’s number (N) and an assumed molecular density (d) of 1 g/cm3 (eq 5). All of the sulfoxides (SO) and sulfones (SO2) exhibit excellent linear relationships (R ≥ 0.95) while both 1S and 2S show no dependence on solvent polarity (R ≤ 0.82). Accordingly, the linear fits for 1SO, 2SO, 1SO2, and 2SO2 were used to calculate the change in dipole (Δμ). For both the

1SOn and 2SOn series, as the oxidation state of the sulfur is increased, the calculated Δμ value also increases. The similar values of Δμ for both 1SOn and 2SOn further indicates that a similar CT state is formed for both D−A and D−A−D systems. Δv =

Δf =

a=

2(Δμ)2 Δf + Δv° ϵ0hca3

(3)

η2 − 1 ϵ−1 − 2 2ϵ + 1 2η + 1

(4)

⎛ 3M ⎞1/3 ⎜ ⎟ ⎝ 4Nπd ⎠

(5)

To further probe the emitting state, photoluminescence quantum yields (ΦPL), lifetimes (τPL), and radiative rate constants (kr) were obtained (Table 1). For both 1SOn and 2SOn, as the oxidation state of the sulfur is increased, a decrease in fluorescence quantum yield and an increase in fluorescence lifetime are observed. Concordantly, the resulting radiative rate constants, calculated by taking ΦPL/τPL, also decrease as the oxidation state of the sulfur is increased. A lower radiative rate is expected between an excited state with polar character (CT) and a ground state with predominantly neutral character.46,47 These data further indicate that the contribution of CT in the excited state can be controlled by changing the oxidation state of the DBT acceptor. DFT Calculations. Further information about the nature of the nuclear and electronic structure of 1SOn and 2SOn was obtained using density functional theory (DFT) and timedependent (TD) DFT calculations. Molecular Structure. In the energy-minimized ground state structures, the anthracene (D) and dibenzothiophene (A) fragments are approximately orthogonal for all 1SOn and 2SOn species, with calculated dihedral angles (θDA) of 85°−90° (Figure 3, Tables S2 and S3). These calculated structures are in agreement with the NMR analyses that indicate restricted rotation around the C−C bond connecting donor and acceptor. The geometry relaxed nuclear configurations of 1SOn in their excited states exhibit D−A dihedral angles that are twisted away from orthogonality (θDA ≈ 57°). Similar twists are observed for 2SOn, but with only one of the donor−acceptor linkages exhibiting a twist in the geometry relaxed excited state and the second anthracene remaining nearly orthogonal to dibenzothiophene. Both the ground and excited state nuclear configurations of 1SOn and 2SOn are much like that of 9,9′bianthryl, where symmetry breaking CT from one anthracene E

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Equation 6 is a simplified first-order approximation representing the overall excited state wave function (ΨS1) as a linear combination of donor (ΨLE*) and acceptor (ΨCT*) orbitals. The degree of acceptor or CT* contribution to ΨS1 is given by the mixing coefficient (λ, eq 7) which is a zeroth-order approximation accounting for the magnitude of the matrix element mixing these two states (H) and the energy difference (E) between LE* and CT* states. Accordingly, as the energy difference between LE* and CT* (donor/acceptor) orbitals decreases, the probability of these two states mixing will increase.35,49

Figure 3. Geometry-optimized ground and excited state nuclear structures for 1SO2 and 2SO2.

ψS1 = ψLE + λψCT * *

to the other results in an excited state twisting (TICT) of the two anthracene units toward planarity.48 Since both 1SOn and 2SOn exhibit nearly identical trends in photoluminescence, it is more likely that CT is occurring from donor to acceptor (anthracene to dibenzothiophene) as intramolecular CT between neighboring anthracenes is not possible for 1SOn. Additionally, the rotation of a single anthracene in 2SOn upon excitation would explain the similarity in experimental electronic behavior of both the D−A and D−A−D molecules. Electronic Structure. Using 1SO2 as an example, a summary of the excited state dynamics for 1SOn and 2SOn is presented in Figure 4. In the ground state geometry, the major

(6)

Figure 4. Major contributing frontier molecular orbitals (MOs) for 1SO2 illustrating the electronic contribution in the both the ground and excited state geometries. The electronic excited state in both geometries is indicated using an asterisk.

⟨ψLE |H |ψCT ⟩ * * E LE − ECT (7) * * In general, a decrease in ΔE*CT−LE is calculated for all 1SOn and 2SOn with increasing sulfur oxidation state and to a greater extent with the second donor group. These data suggest that as the oxidation state of the acceptor sulfur is increased, the degree of CT* contribution (ΨCT*) to the excited state wave function (ΨS1) increases. In other words, the magnitude of the mixing coefficient λ increases as S < SO < SO2. Since the 1S and 2S photoluminescence spectra indicate localized π*−π photoluminescence in all solvents, this suggests that ΔE*CT−LE for the sulfides is too large to allow significant mixing of donor and acceptor orbitals (i.e., λ is small). Additional support for the contribution of charge transfer is provided by the calculated difference between LE and CT (ΔECT−LE) in the ΨS0 wave function (Figure 4, Figures S12 and S13). As photoluminescence is most likely to occur between states with identical geometries, the overlap between ΨS1 and ΨS0 will determine the oscillator strength of photoluminescence.35 For all 1SOn and 2SOn, with increasing sulfur oxidation state the acceptor localized MO decreases in energy, resulting in an increase in ΔECT−LE. Accordingly, a decrease in mixing between LE/CT is likely in the ΨS0 wave function. As a result, overlap between the excited state (ΨS1) and ground state (ΨS0) wave functions decreases, and the oscillator strength of a transition from ΨS1 to ΨS0 (emission) decreases. It is not, therefore, unexpected that the experimentally observed quantum yields and radiative rate constants would decrease with increasing sulfur oxidation state (Table 1).

contributing molecular orbitals (MOs) are localized almost entirely on the anthracene π-framework (donor). For both 1SOn and 2SOn, the dominant electronic transitions (from TDDFT) for all molecules exhibit anthracene localized π−π* character. The assignment of a π−π* absorption is further evidenced by the structured absorption profiles and the overall neutral character of the absorbing state, very similar to that of unsubstituted anthracene (Figure 1). As the oxidation state of the acceptor sulfur is increased, the emitting state (geometry relaxed excited state) becomes increasingly polar, as evidenced by the significant solvatochromism observed in the photoluminescence (Figure 2). In the geometry relaxed excited state (twisted anthracene), the MOs of 1SOn and 2SOn exhibit contribution from both localized donor (LE*) and acceptor (CT*). For all 1SOn and 2SOn, the calculated energy difference between LE* and CT* MOs (ΔE*CT‑LE) decreases as the oxidation state of the bridging sulfur is increased (Figure 4, Figures S12 and S13).

CONCLUSION We demonstrate that the degree of excited state charge transfer (CT*) in D−A and D−A−D chromophores can be controlled through sequential oxidation of a sulfur-containing acceptor. As the sulfur group is oxidized, the excited state molecular orbital for the acceptor is lowered in energy and is able to more efficiently mix with the donor localized molecular orbital. We propose that the incorporation and subsequent oxidation of a sulfur-containing acceptor is a facile method for tuning the degree of charge transfer character in the excited state wave function. Specifically, this synthetic approach enables one to first assemble a D−A or D−A−D molecule and modify the degree of charge transfer in a controlled manner. This approach may also obviate the need for multiple donors and acceptors to access a range of optoelectronic properties. In the context of OPVs using a sulfur-containing acceptor could enable control over the HOMO−LUMO gap with simultaneous control over the excited state lifetime. For application in light-emitting

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(12) Busby, E.; Xia, J. L.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X. Y.; Campos, L. M.; Sfeir, M. Y. A Design Strategy for Intramolecular Singlet Fission Mediated by Charge-Transfer States in Donor-Acceptor Organic Materials. Nat. Mater. 2015, 14, 426−433. (13) Johnson, J. C.; Akdag, A.; Zamadar, M.; Chen, X. D.; Schwerin, A. F.; Paci, I.; Smith, M. B.; Havlas, Z.; Miller, J. R.; Ratner, M. A.; Nozik, A. J.; Michl, J. Toward Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled Covalent Dimers of 1,3Diphenylisobenzofuran. J. Phys. Chem. B 2013, 117, 4680−4695. (14) Smith, M. B.; Michl, J. Recent Advances in Singlet Fission. Annu. Rev. Phys. Chem. 2013, 64, 361−386. (15) Qian, G.; Dai, B.; Luo, M.; Yu, D. B.; Zhan, J.; Zhang, Z. Q.; Ma, D. G.; Wang, Z. Y. Band Gap Tunable, Donor-Acceptor-Donor Charge-Transfer Heteroquinoid-Based Chromophores: Near Infrared Photoluminescence and Electroluminescence. Chem. Mater. 2008, 20, 6208−6216. (16) Ajayaghosh, A. Donor-Acceptor Type Low Band Gap Polymers: Polysquaraines and Related Systems. Chem. Soc. Rev. 2003, 32, 181− 191. (17) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. Design Rules for Donors in Bulk-Heterojunction Solar Cells - Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (18) Clarke, T. M.; Gordon, K. C.; Kwok, W. M.; Phillips, D. L.; Officer, D. L. Tuning from π,π* to Charge-Transfer Excited States in Styryl-Substituted Terthiophenes: An Ultrafast and Steady-State Emission Study. J. Phys. Chem. A 2006, 110, 7696−7702. (19) Hiruta, Y.; Koiso, H.; Ozawa, H.; Sato, H.; Hamada, K.; Yabushita, S.; Citterio, D.; Suzuki, K. Near IR Emitting Red-Shifting Ratiometric Fluorophores Based on Borondipyrromethene. Org. Lett. 2015, 17, 3022−3025. (20) Lu, X. F.; Fan, S. H.; Wu, J. H.; Jia, X. W.; Wang, Z. S.; Zhou, G. Controlling the Charge Transfer in D-A-D Chromophores Based on Pyrazine Derivatives. J. Org. Chem. 2014, 79, 6480−6489. (21) Pron, A.; Zhou, G.; Norouzi-Arasi, H.; Baumgarten, M.; Mullen, K. Controlling the Charge Transfer in Phenylene-Bridged BoryleneAmine π-Conjugated Systems. Org. Lett. 2009, 11, 3550−3553. (22) Tao, T.; Ma, B. B.; Peng, Y. X.; Wang, X. X.; Huang, W.; You, X. Z. Asymmetrical/Symmetrical D-π-A/D-π-D Thiazole-Containing Aromatic Heterocyclic Fluorescent Compounds Having the Same Triphenylamino Chromophores. J. Org. Chem. 2013, 78, 8669−8679. (23) Wu, Y. Z.; Zhu, W. H. Organic Sensitizers from D-π-A to D-Aπ-A: Effect of the Internal Electron-Withdrawing Units on Molecular Absorption, Energy Levels and Photovoltaic Performances. Chem. Soc. Rev. 2013, 42, 2039−2058. (24) Ying, W. J.; Yang, J. B.; Wielopolski, M.; Moehl, T.; Moser, J. E.; Comte, P.; Hua, J. L.; Zakeeruddin, S. M.; Tian, H.; Gratzel, M. New Pyrido 3,4-B Pyrazine-Based Sensitizers for Efficient and Stable DyeSensitized Solar Cells. Chem. Sci. 2014, 5, 206−214. (25) Kitamura, N.; Sakuda, E.; Iwahashi, Y.; Tsuge, K.; Sasaki, Y.; Ishizaka, S. Linkage-Structure Dependences of the Spectroscopic and Photophysical Properties of Anthracene Derivatives: Tri(9-anthryl)benzene and Tri(9-anthryl)borane. J. Photochem. Photobiol., A 2009, 207, 102−108. (26) Christensen, P. R.; Nagle, J. K.; Bhatti, A.; Wolf, M. O. Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores. J. Am. Chem. Soc. 2013, 135, 8109−8112. (27) Cruz, C. D.; Christensen, P. R.; Chronister, E. L.; Casanova, D.; Wolf, M. O.; Bardeen, C. J. Sulfur-Bridged Terthiophene Dimers: How Sulfur Oxidation State Controls Interchromophore Electronic Coupling. J. Am. Chem. Soc. 2015, 137, 12552−12564. (28) Camaioni, N.; Ridolfi, G.; Fattori, V.; Favaretto, L.; Barbarella, G. Oligothiophene-S,S-Dioxides as a Class of Electron-Acceptor Materials for Organic Photovoltaics. Appl. Phys. Lett. 2004, 84, 1901−1903. (29) Dell, E. J.; Campos, L. M. The Preparation of Thiophene-S,Sdioxides and Their Role in Organic Electronics. J. Mater. Chem. 2012, 22, 12945−12952.

devices (OLEDs) controlling the degree of charge transfer in the excited state wave function could prove useful for designing molecules that exhibit singlet exciton fractions greater than 25% (e.g., delayed fluorescence and extra-fluorescence). Finally, for the rational design of molecular dimers that exhibit singlet fission (SF), using a sulfur-containing linker between chromophores may prove useful for finding the ideal amount of charge transfer necessary for efficient SF.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09826. NMR and IR spectra and descriptions, tabulation of molar extinction coefficients and Stokes shifts, Lippert− Mataga plots, and supplementary DFT/TD DFT data and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel 1 604 822 1702; e-mail [email protected] (M.O.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Natural Sciences and Engineering Research Council (NSERC) of Canada and the Peter Wall Institute (Major Thematic Grant) are acknowledged for funding.

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REFERENCES

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