Effect of Selenium Substitution on Intersystem Crossing in π

Letter pubs.acs.org/JPCL

Effect of Selenium Substitution on Intersystem Crossing in π‑Conjugated Donor−Acceptor−Donor Chromophores: The LUMO Matters the Most Rajendra Acharya,† Seda Cekli,† Charles J. Zeman, IV,† Rashid M. Altamimi,‡ and Kirk S. Schanze*,† †

Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611-7200, United States ‡ Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, Riyadh 11442, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: This study explores the effect of substitution of selenium (Se) for sulfur (S) on the photophysical properties of a series of π-conjugated donor−acceptor−donor chromophores based on 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT). The effect of Se substitution is studied systematically, where the substitution is in the thiophene donors only, the benzothiadiazole acceptor only, and in all of the positions. The fluorescence quantum yield decreases with an increase in Se substitution. Nanosecond−microsecond transient absorption and singlet oxygen sensitization experiments show that the effect of Se is due to an increase in the rate and efficiency of intersystem crossing with increased Se substitution. The relationship between intersystem crossing efficiency and heteroatom substitution pattern shows that the effects are largest when the heavy atom Se is in the acceptor benzothiadiazole unit. DFT calculations support the hypothesis that the effect arises because the LUMO is concentrated in the acceptor moiety, enhancing the spin−orbit coupling effect imparted by the Se atom.

P

or polymers,6−9,19 few studies have reported details such as the effect of Se or Te on the ISC dynamics and efficiency in πconjugated systems.25 Herein, we report a detailed study of the photophysics of a set of donor−acceptor−donor (DAD) chromophores in which the number and position of Se atoms is varied. In particular, we compare the properties of 4,7-bis(2thienyl)-2,1,3-benzothiadiazole, Se-0 (also referred to as TBT),26,27 and its Se-substitution congeners, Se-1, Se-2, and Se-3, Chart 1.16,19,28−30 The TBT chromophore is important because it serves as a model for the repeat structure in many important low-band-gap conjugated polymers that feature the DAD architecture.28−32

olymers featuring alternating electron-rich donor (D) and electron-deficient acceptor (A) repeating units have drawn attention in organic electronics as an approach to control and reduce the HOMO−LUMO energy gap.1,2 D−A-type conjugated low-band-gap polymers effectively harvest solar radiation and contribute to exciton dissociation in organic photovoltaics.3,4 Recently, the use of heavy atoms to tune the band gap in conjugated polymers has also gained attention.5−7 One approach is to substitute selenium or tellurium for sulfur in the heterocyclic polymer repeat units.8−11 Selenium and sulfur have similar electronegativity and chemical properties; however, the atomic radius of Se is larger, and it is more polarizable. Organoselenium compounds have larger dipole moments and are more polarizable than the structurally analogous sulfur compounds,12,13 and consequently, the band gap and optical properties can be tuned by replacing sulfur with selenium.14−16 Selenium compounds also exhibit weak Se−Se or Se−N interactions in the solid state that are important for inducing cofacial interactions and uniform packing structures.17−19 As a result, Se-substituted polymers often exhibit improved exciton dissociation and charge-transport properties.20−23 Substitution of Se or Te within heterocyclic units in conjugated polymers also reduces the fluorescence quantum yields due to enhanced singlet−triplet intersystem crossing (ISC).11,24,25 This effect was highlighted in a recent ultrafast spectroscopy study of Se and Te derivatives of poly(3hexylthiophene).25 While there have been several studies that have reported qualitative results concerning the effect of Se or Te on the optical properties of DA conjugated small molecules © XXXX American Chemical Society

Chart 1. Structure of TBT Derivativesa

a

The structures are drawn to represent the minimum-energy conformation as calculated by DFT.33

Received: December 30, 2015 Accepted: January 28, 2016

693

DOI: 10.1021/acs.jpclett.5b02902 J. Phys. Chem. Lett. 2016, 7, 693−697

Letter

The Journal of Physical Chemistry Letters

Figure 1. Absorption (A) and emission (B) spectra of Se-0−Se-3 in THF.

Figure 2. Photophysical data for Se-0−Se-3. (A) Optical energy gap, Eg; (B) fluorescence and singlet oxygen quantum yields (Φfl and ΦΔ, respectively); (C) fluorescence lifetime, τ; and (D) radiative decay rate constant, kf and ISC rate constant, kisc. Black and orange filled circles in the structures shown at the top represent sulfur and selenium atoms, respectively. Rates are calculated according to expressions in ref 35.

1B). Optical energy gaps calculated from the intersection of the absorption and emission spectra range from 2.19 to 2.45 eV. The overall optical gap varies in the sequence Se-3 < Se-1 < Se2 < Se-0, reinforcing the notion that the effect of Se on decreasing the HOMO−LUMO gap is greatest when the Se is in the acceptor moiety. The relationship between structure and position of the Se substitution and the optical gap is illustrated in Figure 2A. Selenium has a larger spin−orbit coupling constant than sulfur,36 and therefore, ISC is expected to be more rapid and efficient in the Se-substituted TBT analogues due to greater spin−orbit coupling. In order to quantify the effect, the excitedstate dynamics were probed for the series by measurement of the fluorescence quantum yields, lifetimes, and singlet oxygen quantum yields (Φfl, τ, and ΦΔ, respectively). The singlet oxygen quantum yield reflects the relative triplet yields for the Se-substituted TBTs and quantitatively affords the lower limit for the triplet yield.37 Interestingly, as seen in Figure 2B, there is an inverse correlation between Φfl and ΦΔ across the series. In particular, relative to the parent chromophore Se-0, Φfl decreases while ΦΔ increases when Se is substituted for S. The inverse correlation between Φfl and ΦΔ suggests that the variation is mainly the result of an increase in ISC efficiency and triplet yield when Se is present in the structures (Se-1−Se-3). Noteworthy is the fact that the effect is most pronounced when

Steady-state and time-resolved spectroscopy were applied to the set of TBT chromophores to elucidate the positional effect of Se substitution on the excited state dynamics, including the triplet yield and ISC rate. The results clearly show systematic variation in the ISC efficiency with the Se atom substitution pattern, with the most efficient ISC arising when Se replaces S in the benzothiadiazole acceptor moiety. The results reveal that the heavy atom effect is most pronounced when Se makes a substantial contribution to the LUMO of the acceptor moiety in the D−A chromophore structure. As shown in Figure 1A, absorption spectra of Se-0−Se-3 feature two bands typical of DAD chromophores associated with a high-energy π,π* transition and a lower-energy chargetransfer band.27,34 These compounds show a relatively intense π,π* band at ∼330−340 nm, while a broad charge transfer band is present at around 400−550 nm. The λmax of both absorption bands varies in the sequence Se-0 < Se-2 < Se-1 < Se-3, showing that in general the HOMO−LUMO gap decreases with Se substitution and the effect is larger when the Se is located in the acceptor moiety (Se-1 and Se-3). (These effects are also borne out by electrochemical determination of the HOMO and LUMO levels; see the Supporting Information). Se substitution also influences the fluorescence λmax in the same sequence, with the most pronounced red shifts observed for Se1 and Se-3 (49 and 81 nm, respectively, relative to Se-0, Figure 694

DOI: 10.1021/acs.jpclett.5b02902 J. Phys. Chem. Lett. 2016, 7, 693−697

Letter

The Journal of Physical Chemistry Letters

with the inverted ground-state absorption spectra for comparison. Notably, for all of the Se-substituted complexes, weak to moderate TA was observed, with lifetimes ranging from 0.5 to 4 μs. The transients are quenched by oxygen and as such are attributed to the triplet excited states. The T 1−Tn absorption of the Se-substituted TBT derivatives features broad absorption throughout the visible and near-infrared region, with notable bands at λmax ≈ 400 nm and a broad, weaker band with λmax ≈ 750 nm. (The apparent absorption at 500 nm results from the ground-state bleach that gives rise to a “dip” in the spectra near 450 nm.) Given that the TA spectra were obtained under identical conditions, the intensity of the TA qualitatively reflects the triplet yields for the Se-substituted TBTs. The fact that only very weak TA was observed for Se-0 is consistent with the notion that this compound is the least efficient sensitizer of singlet oxygen (ΦΔ ≈ 0.08) and by inference has a low triplet yield. In this regard, it is significant that the TA observed for Se-1 and Se-3 is greater than that of Se-2, even though the latter has two selenium atoms. This observation reinforces the hypothesis that ISC is most efficient when Se is in the acceptor moiety. It is also noteworthy that the triplet lifetimes vary in the sequence Se-3 < Se-1 < Se-2 (0.48, 0.68, and 4.05 μs, respectively). This trend in lifetimes indicates that the triplet decay rate is largest for the chromophores with Se in the acceptor unit. The effect of Se on the triplet lifetime arises because of the heavy atom effect on the (reverse) ISC that occurs upon triplet → singlet decay. Again, it is clearly seen that the effect of the Se is larger (i.e., fastest triplet decay) when Se is located in the acceptor moiety. Quantitative insight regarding the effect of seleno-subsitution on the excited-state dynamics can be seen from the trends in the radiative and nonradiative (ISC) rate constants (kf, and kisc, respectively, Figure 2D). Here, it is clearly seen that, in general, selenium substitution lowers the radiative rate and increases the rate of ISC. A familiar trend is seen in the way that kisc varies across the series, that is, Se-0 ≪ Se-2 < Se-1 < Se-3. Noteworthy is the fact that, compared to Se-0, kisc is 3- and 5fold larger for Se-1 and Se-3, whereas it is only 2 times greater for Se-2. Interestingly, kisc in Se-3 is the largest among the three, suggesting that the effect of Se substitution on the spin− orbit coupling and the ISC rate is approximately additive. Taken together, the results presented here show that the ISC rate and yield depend on the number and position of the Se atoms. Moreover, the effect is more significant when Se is located in the acceptor unit. Clearly, the interaction between the heavy atom (Se) and the frontier orbitals of the

the Se atom is present in the acceptor moiety. In particular, even though Se-2 has a greater number of Se atoms than Se-1, the singlet oxygen yield (and hence triplet yield) for the latter is larger. In order to provide more insight regarding the triplet excited states and their dynamics, nanosecond transient absorption (TA) spectroscopy was carried out. These experiments utilized 355 nm (5 ns) excitation pulses, and for all of the samples, the absorption at 355 nm was the same and the laser pulse energies were matched (∼8 mJ-pulse−1). The resulting TA difference spectra obtained for Se-0−Se-3 are shown in Figure 3, along

Figure 3. TA difference spectra of Se-0−Se-3 in deoxygenated tetrahydrofuran solvent. Spectra were obtained at 60 ns delay following a 5 ns, 355 nm excitation pulse. Triplet decay lifetimes (τT) are shown in the inset of each panel. The gray plots show the ground-state absorption spectra of the chromophores normalized and inverted in sign. This facilitates assignment of the bleaching bands in the excited-state difference spectra.

Table 1. Contribution of S or Se to the HOMO and LUMO of Se-0−Se-3 Obtained from MO Composition Analysisa,b

a

The box at the right shows representative structures, showing the positions of S and Se in Se-0−Se-3 and density maps of the HOMO and LUMO in Se-3 calculated at the B3PW91/6-311G(2df,p) level. bThe MO composition was obtained by applying the Multiwfn program to the NAO calculations. Contributions are indicated as percentages (out of 100% total). 695

DOI: 10.1021/acs.jpclett.5b02902 J. Phys. Chem. Lett. 2016, 7, 693−697

The Journal of Physical Chemistry Letters

■

chromophore determines the strength of spin−orbit coupling. Early investigations of the position dependence of heavy atom substitution on ISC in aromatic hydrocarbons indicate that to first-order, the heavy-atom-induced spin−orbit coupling is proportional to the extent of overlap between the LUMO (which is singly occupied in the triplet state) and the heavy atom.38,39 Evidence for a similar effect was uncovered in a recent study from our lab on a series of platinum(II)substituted DAD chromophores.40 In these systems, it was found that spin−orbit coupling induced by the heavy atom (platinum) decreased as the LUMO became localized in the acceptor unit of the DAD chromophores,40 which decreased the contribution from orbitals localized on the platinum atoms.41 In order to explore the relationship between the Se positional dependence, the ISC efficiency and dynamics, and the structure of the frontier orbitals in the DAD chromophores, density functional theory (DFT) calculations were carried out for Se0−Se-3. Details of the computational results are in the Supporting Information, and Table 1 provides a summary of molecular orbital (MO) composition analysis showing the contribution of the heteroatoms (S or Se) to the HOMO and LUMO of the chromophores (as a percentage contribution). Here, it is evident that the heteroatoms contribute little to the HOMO, which is largely delocalized across the entire DAD skeleton. However, the LUMO, which is strongly localized in the acceptor moiety, features a significant contribution from the S (or Se) heteroatoms. This is in direct accordance with the experimental results, which reveal that the most important position for substitution of S by Se to have an effect on spin− orbit coupling is within the acceptor unit, and this is because of the very large contribution of the acceptor S (or Se) to the LUMO. In essence, these results confirm that positioning the heavy atom within the acceptor portion of a D−A-type chromophore will give rise to the largest spin−orbit coupling and therefore have the greatest effect on ISC.42,43 In summary, this study confirms that heavy atom substitution within a model π-conjugated DAD chromophore increases the rate and efficiency of ISC to a triplet excited state. Study of the positional dependence of the photophysics clearly reveals that the effect is greatest when the heavy atom is contained within the acceptor heterocycle unit. Interestingly, ISC is 2.5 times more efficient when a single Se atom is contained within the acceptor compared to when two Se atoms are in the peripheral donor heterocycle units. MO calculations indicate that the effect is most pronounced because the contribution of the heavy atom Se to the LUMO is substantial when it is localized in the acceptor moiety. Experiments in progress seek to explore whether these effects are generalized to D−A-type π-conjugated polymers of interest for organic electronics.

■

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant No. CHE-1504727). We acknowledge the University of Florida Research Computing (http://researchcomputing.ufl. edu) for providing computational resources and support that have contributed to the research results reported in this publication. Partial support from King Abdulaziz City of Science and Technology is acknowledged.

■

REFERENCES

(1) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (2) Zhang, Q. T.; Tour, J. M. Alternating Donor/Acceptor Repeat Units in Polythiophenes. Intramolecular Charge Transfer for Reducing Band Gaps in Fully Substituted Conjugated Polymers. J. Am. Chem. Soc. 1998, 120, 5355−5362. (3) Beaujuge, P. M.; Subbiah, J.; Choudhury, K. R.; Ellinger, S.; McCarley, T. D.; So, F.; Reynolds, J. R. Green DioxythiopheneBenzothiadiazole Donor-Acceptor Copolymers for Photovoltaic Device Applications. Chem. Mater. 2010, 22, 2093−2106. (4) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher, D.; Scharber, M.; Brabec, C. Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications. Macromolecules 2007, 40, 1981−1986. (5) Kronemeijer, A. J.; Gili, E.; Shahid, M.; Rivnay, J.; Salleo, A.; Heeney, M.; Sirringhaus, H. A Selenophene-Based Low-Bandgap Donor-Acceptor Polymer Leading to Fast Ambipolar Logic. Adv. Mater. 2012, 24, 1558−1565. (6) Patra, A.; Wijsboom, Y. H.; Leitus, G.; Bendikov, M. Tuning the Band Gap of Low-Band-Gap Polyselenophenes and Polythiophenes: The Effect of the Heteroatom. Chem. Mater. 2011, 23, 896−906. (7) Gibson, G. L.; Seferos, D. S. “Heavy-Atom” Donor−Acceptor Conjugated Polymers. Macromol. Chem. Phys. 2014, 215, 811−823. (8) Lee, W.-H.; Son, S. K.; Kim, K.; Lee, S. K.; Shin, W. S.; Moon, S.J.; Kang, I.-N. Synthesis and Characterization of New SelenopheneBased Donor−Acceptor Low-Bandgap Polymers for Organic Photovoltaic Cells. Macromolecules 2012, 45, 1303−1312. (9) Kim, B.; Yeom, H. R.; Yun, M. H.; Kim, J. Y.; Yang, C. A Selenophene Analogue of PCDTBT: Selective Fine-Tuning of LUMO to Lower of the Bandgap for Efficient Polymer Solar Cells. Macromolecules 2012, 45, 8658−8664. (10) Jahnke, A. A.; Djukic, B.; McCormick, T. M.; Buchaca Domingo, E.; Hellmann, C.; Lee, Y.; Seferos, D. S. poly(3-Alkyltellurophene)s Are Solution-Processable Polyheterocycles. J. Am. Chem. Soc. 2013, 135, 951−954. (11) Jahnke, A. A.; Seferos, D. S. Polytellurophenes. Macromol. Rapid Commun. 2011, 32, 943−951. (12) Smyth, C.; Lewis, G.; Grossman, A.; Jennings, F., III The Dipole Moments and Structures of Certain Compounds of Sulfur, Selenium and Phosphorus. J. Am. Chem. Soc. 1940, 62, 1219−1223. (13) Kamada, K.; Ueda, M.; Nagao, H.; Tawa, K.; Sugino, T.; Shmizu, Y.; Ohta, K. Molecular Design for Organic Nonlinear Optics: Polarizability and Hyperpolarizabilities of Furan Homologues Investigated by Ab Initio Molecular Orbital Method. J. Phys. Chem. A 2000, 104, 4723−4734. (14) Zhuang, W.; Zhen, H.; Kroon, R.; Tang, Z.; Hellström, S.; Hou, L.; Wang, E.; Gedefaw, D.; Inganäs, O.; Zhang, F.; et al. Molecular Orbital Energy Level Modulation through Incorporation of Selenium and Fluorine into Conjugated Polymers for Organic Photovoltaic Cells. J. Mater. Chem. A 2013, 1, 13422−13425.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02902. Experimental methods, synthetic schemes, procedures, and spectral data, computational data, cyclic voltammetry, fluorescence lifetime kinetic traces, triplet−triplet absorption decay kinetics, and normalized absorption and emission plots (PDF) 696

DOI: 10.1021/acs.jpclett.5b02902 J. Phys. Chem. Lett. 2016, 7, 693−697

Letter

The Journal of Physical Chemistry Letters (15) Jiang, J.-M.; Raghunath, P.; Lin, H.-K.; Lin, Y.-C.; Lin, M.; Wei, K.-H. Location and Number of Selenium Atoms in Two-Dimensional Conjugated Polymers Affect Their Band-Gap Energies and Photovoltaic Performance. Macromolecules 2014, 47, 7070−7080. (16) Alghamdi, A. A.; Watters, D. C.; Yi, H.; Al-Faifi, S.; Almeataq, M. S.; Coles, D.; Kingsley, J.; Lidzey, D. G.; Iraqi, A. Selenophene vs. Thiophene in Benzothiadiazole-Based Low Energy Gap Donor− Acceptor Polymers for Photovoltaic Applications. J. Mater. Chem. A 2013, 1, 5165−5171. (17) Hollinger, J.; Jahnke, A. A.; Coombs, N.; Seferos, D. S. Controlling Phase Separation and Optical Properties in Conjugated Polymers through Selenophene-Thiophene Copolymerization. J. Am. Chem. Soc. 2010, 132, 8546−8547. (18) Palermo, E. F.; McNeil, A. J. Impact of Copolymer Sequence on Solid-State Properties for Random, Gradient and Block Copolymers Containing Thiophene and Selenophene. Macromolecules 2012, 45, 5948−5955. (19) Pati, P. B.; Zade, S. S. Benzoselenadiazole Containing Donor− Acceptor−Donor Small Molecules: Nonbonding Interactions, Packing Patterns, and Optoelectronic Properties. Cryst. Growth Des. 2014, 14, 1695−1700. (20) Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.; Gordeyev, S.; Hamilton, R.; Higgins, S. J.; McCulloch, I.; Skabara, P. J.; Sparrowe, D.; et al. Regioregular poly(3-Hexyl) Selenophene: A Low Band Gap Organic Hole Transporting Polymer. Chem. Commun. 2007, 47, 5061−5063. (21) Pang, H.; Skabara, P. J.; Gordeyev, S.; McDouall, J. J.; Coles, S. J.; Hursthouse, M. B. Poly (3,4-Ethylenediselena)thiophene the Selenium Equivalent of PEDOT. Chem. Mater. 2007, 19, 301−307. (22) Saadeh, H. A.; Lu, L.; He, F.; Bullock, J. E.; Wang, W.; Carsten, B.; Yu, L. Polyselenopheno [3,4-b]selenophene for Highly Efficient Bulk Heterojunction Solar Cells. ACS Macro Lett. 2012, 1, 361−365. (23) Wang, Y.; Masunaga, H.; Hikima, T.; Matsumoto, H.; Mori, T.; Michinobu, T. New Semiconducting Polymers Based on Benzobisthiadiazole Analogues: Tuning of Charge Polarity in Thin Film Transistors Via Heteroatom Substitution. Macromolecules 2015, 48, 4012−4023. (24) Gibson, G. L.; Gao, D.; Jahnke, A. A.; Sun, J.; Tilley, A. J.; Seferos, D. S. Molecular Weight and End Capping Effects on the Optoelectronic Properties of Structurally Related Heavy Atom DonorAcceptor Polymers. J. Mater. Chem. A 2014, 2, 14468−14480. (25) Pensack, R. D.; Song, Y.; McCormick, T. M.; Jahnke, A. A.; Hollinger, J.; Seferos, D. S.; Scholes, G. D. Evidence for the Rapid Conversion of Primary Photoexcitations to Triplet States in Selenoand Telluro-Analogues of poly(3-Hexylthiophene). J. Phys. Chem. B 2014, 118, 2589−2597. (26) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. New Narrow-Bandgap Polymer Composed of benzobis(1,2,5-Thiadiazole) and Thiophenes. J. Am. Chem. Soc. 1995, 117, 6791−6792. (27) Pina, J.; de Melo, J. S.; Breusov, D.; Scherf, U. Donor− Acceptor−Donor Thienyl/Bithienyl-Benzothiadiazole/Quinoxaline Model Oligomers: Experimental and Theoretical Studies. Phys. Chem. Chem. Phys. 2013, 15, 15204−15213. (28) Cihaner, A.; Algi, F. A Novel Neutral State Green Polymeric Electrochromic with Superior n-and p-Doping Processes: Closer to Red-Blue-Green (RGB) Display Realization. Adv. Funct. Mater. 2008, 18, 3583−3589. (29) Yang, R.; Tian, R.; Yan, J.; Zhang, Y.; Yang, J.; Hou, Q.; Yang, W.; Zhang, C.; Cao, Y. Deep-Red Electroluminescent Polymers: Synthesis and Characterization of New Low-Band-Gap Conjugated Copolymers for Light-Emitting Diodes and Photovoltaic Devices. Macromolecules 2005, 38, 244−253. (30) Pati, P. B.; Senanayak, S. P.; Narayan, K.; Zade, S. S. Solution Processable Benzooxadiazole and Benzothiadiazole Based Dad Molecules with Chalcogenophene: Field Effect Transistor Study and Structure Property Relationship. ACS Appl. Mater. Interfaces 2013, 5, 12460−12468. (31) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J.; Hummelen, J. C.; Kroon, J. M.; Inganäs, O.; Andersson, M. R. High-

Performance Polymer Solar Cells of an Alternating Polyfluorene Copolymer and a Fullerene Derivative. Adv. Mater. 2003, 15, 988− 991. (32) Kitamura, C.; Tanaka, S.; Yamashita, Y. Design of Narrow Bandgap Polymers. Syntheses and Properties of Monomers and Polymers Containing Aromatic-Donor and o-Quinoid Acceptor Units. Chem. Mater. 1996, 8, 570−578. (33) Single-crystal X-ray analysis of Se-0 and Se-1 reveals that the thiophene rings are in the syn−anti conformation, whereas the selenophene rings in Se-2 and Se-3 are in the syn−syn conformation; see refs 19 and 30. (34) Qian, G.; Dai, B.; Luo, M.; Yu, D.; Zhan, J.; Zhang, Z.; Ma, D.; Wang, Z. Y. Band Gap Tunable, Donor-Acceptor-Donor ChargeTransfer Heteroquinoid-Based Chromophores: Near Infrared Photoluminescence and Electroluminescence. Chem. Mater. 2008, 20, 6208− 6216. (35) Rate constants calculated according to kf = Φfl/τ and kisc = ΦΔ/ τ, where τ is the fluorescence lifetime. The latter expression assumes that Φisc ≈ ΦΔ. (36) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of Photochemistry; CRC Press: Boca Raton, FL, 2006. (37) Phosphorescence emission was not observed for any of the chromophores at 77 K over the wavelength range of 800−1500 nm. (38) Steiner, U.; Winter, G. Position Dependent Heavy Atom Effect in Physical Triplet Quenching by Electron Donors. Chem. Phys. Lett. 1978, 55, 364−368. (39) Miller, J. C.; Meek, J. S.; Strickler, S. Heavy Atom Effects on the Triplet Lifetimes of Naphthalene and Phenanthrene. J. Am. Chem. Soc. 1977, 99, 8175−8179. (40) Cekli, S.; Winkel, R. W.; Alarousu, E.; Mohammed, O. F.; Schanze, K. S. Triplet Excited State Properties in Variable Gap πConjugated Donor−Acceptor−Donor Chromophores. Chem. Sci. 2016, in press. (41) Pandey, L.; Risko, C.; Norton, J. E.; Brédas, J.-L. Donor− Acceptor Copolymers of Relevance for Organic Photovoltaics: A Theoretical Investigation of the Impact of Chemical Structure Modifications on the Electronic and Optical Properties. Macromolecules 2012, 45, 6405−6414. (42) A reviewer pointed out that spin−orbit coupling for S1 to T1 intersystem crossing for states based on the HOMO to LUMO transition is forbidden in C2v symmetry, which is the case when Se-0− Se-3 are in the planar anti−anti or syn−syn coformations. However, the barrier to rotation around the aryl−aryl bonds is low (