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The Elusive Nature of Excited States in Singlet Fission Materials
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The Elusive Nature of Excited States in Singlet Fission Materials
will feature ground-state molecules closest to the excited-state geometry, therefore requiring the least reorganization. This remarkable observation could be facilitated by a relatively sterically unhindered bridge between the two pentacenes, allowing them to adopt a wide range of mutual orientations.
Samuel N. Sanders1 and Matthew Y. Sfeir2,* In this issue of Chem, Basel et al. describe solvent-induced changes in photophysics for a non-conjugated pentacene dimer. The authors clearly demonstrate dynamic relaxation of the excited singlet exciton and find that singlet fission rates can be enhanced in more polar media and are particularly rapid in benzonitrile. Spin-conversion processes are important to future molecular-based energy devices such as organic light-emitting diodes and photovoltaics.1 For example, the singlet exciton fission process utilizes molecules with a large singlet-triplet energy gap to generate two charge carriers (in the form of a triplet pair) from one absorbed photon (multiple-exciton generation).2 A key challenge has been exploring ways to control this process and develop schemes by which the extra charge carriers can be exploited in next-generation technologies, such as third-generation solar cells.3 Crucially, the overall singlet fission (SF) process is not well understood, partly because of an incomplete understanding of the character of the excited states in SF materials and the many interrelated chemical, electronic, and structural factors that affect the dynamical process.4 The character of the singlet excited states will directly determine the mechanism by which the conversion of the singlet state to the coupled triplet pair occurs. As exemplified by Basel et al. in this issue of Chem, this topic represents a contemporary scientific problem that combines synthetic chemistry, time-resolved charge and spin probes, and theoretical quantum chemistry approaches capable of investigating systems with strong exchange interactions.5
In this work, the authors studied a bipentacene linked by a non-conjugated bicyclooctane bridge. This system featured sufficiently weak coupling between chromophores such that SF occurred on a timescale similar to that of other singlet decay channels. The similarity of these rates allowed for amplification of small changes in the electronic coupling, such that changes affected not only the SF rate but also the triplet yield. The slow SF rate also enabled the authors to study the equilibration dynamics of the photoexcited singlet exciton in a variety of solvents with varied polarity, as well as interrogate the spin dynamics in the system by time-resolved electron paramagnetic resonance (EPR). By correlating a red shift in steady-state absorption and fluorescence to singlet features that appear in the nearinfrared region (1,300 nm) of the transient absorption spectroscopy, the authors paint a convincing picture that the excited singlet state features a relatively larger dipole moment than the ground state. Therefore, the singlet excited state is more effectively stabilized by more polar solvents, resulting in a dynamic blue shift in its photoinduced absorption. A further corroboration of this concept is that photoexcitation into the tail of absorption onset does not display this dynamic blue shifting. The lowest energy absorption
Furthermore, the authors employed transient absorption to demonstrate singlet exciton fission in these systems and demonstrate the effect of solvent on the relaxation dynamics. Interestingly, the lifetime of the TIBS (6,13-bis(tri- isobutylsilylethynyl)pentacene) monomer showed a large solvent dependence even in the absence of a SF decay channel, indicating that a polarity change between the ground and excited states in these compounds affects non-radiative decay. However, the reduction of the singlet lifetime was more pronounced in the dimer and led to a higher yield of triplet excitons. This suggests that the increase in the SF rate constant was the dominant effect, such that SF is able to successfully outcompete both radiative and nonradiative decay. This effect was particularly striking in benzonitrile, which exhibited the highest dielectric constant and index of refraction. Timeresolved EPR measurements definitively resolved a population of quintet states that subsequently evolved to triplet states. This observation of coupled triplet pair states is consistent with previous EPR measurements of SF and is powerful proof that SF (as opposed to intersystem crossing) is responsible for triplet formation in these slow SF systems.6–9
1Department
of Chemistry, Columbia University, New York, NY 10027, USA
2Center
for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.04.017
Chem 4, 931–942, May 10, 2018 ª 2018 Elsevier Inc.
935
Fellowship Program (DGE 11-44155). This research used resources of the Center for Functional Nanomaterials, which is a US Department of Energy Office of Science facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. 1. Rao, A., Chow, P.C.Y., Ge´linas, S., Schlenker, C.W., Li, C.-Z., Yip, H.-L., Jen, A.K.Y., Ginger, D.S., and Friend, R.H. (2013). The role of spin in the kinetic control of recombination in organic photovoltaics. Nature 500, 435–439. 2. Smith, M.B., and Michl, J. (2010). Singlet fission. Chem. Rev. 110, 6891–6936.
Figure 1. The Proposed Mechanism of Singlet Fission Solvent polarity is shown to have a weak effect on changing the singlet exciton energy but strongly alters the energy of the CT state, potentially accelerating the SF channel that proceeds through a virtual CT intermediate.
Calculations suggested that the origin of rate enhancement in polar solvents originates from stronger coupling between singlet and charge-transfer (CT) states and between CT states and the multiexcitonic state. Furthermore, Basel et al. showed a significant lowering of the CT state energy from 1.3 eV above the singlet to 0.7 eV above the singlet when they included a benzonitrile solvation effect in their model. Combined, these theoretical results support the idea that the lowering of CT energies could be involved in facilitating faster SF in more polar solvents, a concept summarized in Figure 1. These results are an interesting addendum to previous theoretical studies suggesting that a vibrationally mediated resonance between the singlet and triplet-pair state is sufficient to mediate SF.10 Here, the authors showed that enhanced CT coupling can further serve to accelerate the SF process, especially when electronic coupling is weak. This result suggests that polar media could be used to enhance SF in otherwise inoperative systems. Importantly for the field, this work reinforces the strong connection between linear optical properties and carrier dynamics and highlights the continual
936
Chem 4, 931–942, May 10, 2018
lack of understanding of the singlet exciton character and excited-state potential energy surface in SF materials. Although SF can be incoherent in intramolecular systems (as exemplified by the long timescales), vibrational and other dynamic nuclear processes appear to play a large role in mediating tripletpair formation. Further progress on this topic will certainly require a more detailed treatment of the role of static and dynamic nuclear effects, including vibrations and nuclear reorganization, as well as the role of bond hybridization in coupling SF chromophores. This work clearly shows the need for future efforts designed to better resolve competing structural and electronic effects, such as decoupling geometric changes and environmental polarity effects. The authors make a compelling case that further insight would be obtained from the reduction of inhomogeneous broadening in ensembles of these materials or singlechromophore studies.
ACKNOWLEDGMENTS S.N.S. thanks the National Science Foundation for the Graduate Research
3. Xia, J., Sanders, S.N., Cheng, W., Low, J.Z., Liu, J., Campos, L.M., and Sun, T. (2017). Singlet fission: Progress and prospects in solar cells. Adv. Mater. 29, 1601652. 4. Sanders, S.N., Kumarasamy, E., Pun, A.B., Appavoo, K., Steigerwald, M.L., Campos, L.M., and Sfeir, M.Y. (2016). Exciton correlations in intramolecular singlet fission. J. Am. Chem. Soc. 138, 7289–7297. 5. Basel, B.S., Zirzlmeier, J., Hetzer, C., Reddy, S.R., Phelan, B.T., Kryzaniak, M.D., Volland, M.K., Coto, P.B., Young, R.M., Clark, T., et al. (2018). Evidence for CT mediation in the primary events of singlet fission in a weakly coupled pentacene dimer. Chem 4, this issue, 1092–1111. 6. Kumarasamy, E., Sanders, S.N., Tayebjee, M.J.Y., Asadpoordarvish, A., Hele, T.J.H., Fuemmeler, E.G., Pun, A.B., Yablon, L.M., Low, J.Z., Paley, D.W., et al. (2017). Tuning singlet fission in p-bridge-p chromophores. J. Am. Chem. Soc. 139, 12488–12494. 7. Tayebjee, M.J.Y., Sanders, S.N., Kumarasamy, E., Campos, L.M., Sfeir, M.Y., and McCamey, D.R. (2017). Quintet multiexciton dynamics in singlet fission. Nat. Phys. 13, 182–189. 8. Weiss, L.R., Bayliss, S.L., Kraffert, F., Thorley, K.J., Anthony, J.E., Bittl, R., Friend, R.H., Rao, A., Greenham, N.C., and Behrends, J. (2017). Strongly exchange-coupled triplet pairs in an organic semiconductor. Nat. Phys. 13, 176–181. 9. Basel, B.S., Zirzlmeier, J., Hetzer, C., Phelan, B.T., Krzyaniak, M.D., Reddy, S.R., Coto, P.B., Horwitz, N.E., Young, R.M., White, F.J., et al. (2017). Unified model for singlet fission within a non-conjugated covalent pentacene dimer. Nat. Commun. 8, 15171. 10. Fuemmeler, E.G., Sanders, S.N., Pun, A.B., Kumarasamy, E., Zeng, T., Miyata, K., Steigerwald, M.L., Zhu, X.Y., Sfeir, M.Y., Campos, L.M., and Ananth, N. (2016). A direct mechanism of ultrafast intramolecular singlet fission in pentacene dimers. ACS Cent Sci 2, 316–324.
The Elusive Nature of Excited States in Singlet Fission Materials
will feature ground-state molecules closest to the excited-state geometry, therefore requiring the least reorganization. This remarkable observation could be facilitated by a relatively sterically unhindered bridge between the two pentacenes, allowing them to adopt a wide range of mutual orientations.
Samuel N. Sanders1 and Matthew Y. Sfeir2,* In this issue of Chem, Basel et al. describe solvent-induced changes in photophysics for a non-conjugated pentacene dimer. The authors clearly demonstrate dynamic relaxation of the excited singlet exciton and find that singlet fission rates can be enhanced in more polar media and are particularly rapid in benzonitrile. Spin-conversion processes are important to future molecular-based energy devices such as organic light-emitting diodes and photovoltaics.1 For example, the singlet exciton fission process utilizes molecules with a large singlet-triplet energy gap to generate two charge carriers (in the form of a triplet pair) from one absorbed photon (multiple-exciton generation).2 A key challenge has been exploring ways to control this process and develop schemes by which the extra charge carriers can be exploited in next-generation technologies, such as third-generation solar cells.3 Crucially, the overall singlet fission (SF) process is not well understood, partly because of an incomplete understanding of the character of the excited states in SF materials and the many interrelated chemical, electronic, and structural factors that affect the dynamical process.4 The character of the singlet excited states will directly determine the mechanism by which the conversion of the singlet state to the coupled triplet pair occurs. As exemplified by Basel et al. in this issue of Chem, this topic represents a contemporary scientific problem that combines synthetic chemistry, time-resolved charge and spin probes, and theoretical quantum chemistry approaches capable of investigating systems with strong exchange interactions.5
In this work, the authors studied a bipentacene linked by a non-conjugated bicyclooctane bridge. This system featured sufficiently weak coupling between chromophores such that SF occurred on a timescale similar to that of other singlet decay channels. The similarity of these rates allowed for amplification of small changes in the electronic coupling, such that changes affected not only the SF rate but also the triplet yield. The slow SF rate also enabled the authors to study the equilibration dynamics of the photoexcited singlet exciton in a variety of solvents with varied polarity, as well as interrogate the spin dynamics in the system by time-resolved electron paramagnetic resonance (EPR). By correlating a red shift in steady-state absorption and fluorescence to singlet features that appear in the nearinfrared region (1,300 nm) of the transient absorption spectroscopy, the authors paint a convincing picture that the excited singlet state features a relatively larger dipole moment than the ground state. Therefore, the singlet excited state is more effectively stabilized by more polar solvents, resulting in a dynamic blue shift in its photoinduced absorption. A further corroboration of this concept is that photoexcitation into the tail of absorption onset does not display this dynamic blue shifting. The lowest energy absorption
Furthermore, the authors employed transient absorption to demonstrate singlet exciton fission in these systems and demonstrate the effect of solvent on the relaxation dynamics. Interestingly, the lifetime of the TIBS (6,13-bis(tri- isobutylsilylethynyl)pentacene) monomer showed a large solvent dependence even in the absence of a SF decay channel, indicating that a polarity change between the ground and excited states in these compounds affects non-radiative decay. However, the reduction of the singlet lifetime was more pronounced in the dimer and led to a higher yield of triplet excitons. This suggests that the increase in the SF rate constant was the dominant effect, such that SF is able to successfully outcompete both radiative and nonradiative decay. This effect was particularly striking in benzonitrile, which exhibited the highest dielectric constant and index of refraction. Timeresolved EPR measurements definitively resolved a population of quintet states that subsequently evolved to triplet states. This observation of coupled triplet pair states is consistent with previous EPR measurements of SF and is powerful proof that SF (as opposed to intersystem crossing) is responsible for triplet formation in these slow SF systems.6–9
1Department
of Chemistry, Columbia University, New York, NY 10027, USA
2Center
for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.04.017
Chem 4, 931–942, May 10, 2018 ª 2018 Elsevier Inc.
935
Fellowship Program (DGE 11-44155). This research used resources of the Center for Functional Nanomaterials, which is a US Department of Energy Office of Science facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. 1. Rao, A., Chow, P.C.Y., Ge´linas, S., Schlenker, C.W., Li, C.-Z., Yip, H.-L., Jen, A.K.Y., Ginger, D.S., and Friend, R.H. (2013). The role of spin in the kinetic control of recombination in organic photovoltaics. Nature 500, 435–439. 2. Smith, M.B., and Michl, J. (2010). Singlet fission. Chem. Rev. 110, 6891–6936.
Figure 1. The Proposed Mechanism of Singlet Fission Solvent polarity is shown to have a weak effect on changing the singlet exciton energy but strongly alters the energy of the CT state, potentially accelerating the SF channel that proceeds through a virtual CT intermediate.
Calculations suggested that the origin of rate enhancement in polar solvents originates from stronger coupling between singlet and charge-transfer (CT) states and between CT states and the multiexcitonic state. Furthermore, Basel et al. showed a significant lowering of the CT state energy from 1.3 eV above the singlet to 0.7 eV above the singlet when they included a benzonitrile solvation effect in their model. Combined, these theoretical results support the idea that the lowering of CT energies could be involved in facilitating faster SF in more polar solvents, a concept summarized in Figure 1. These results are an interesting addendum to previous theoretical studies suggesting that a vibrationally mediated resonance between the singlet and triplet-pair state is sufficient to mediate SF.10 Here, the authors showed that enhanced CT coupling can further serve to accelerate the SF process, especially when electronic coupling is weak. This result suggests that polar media could be used to enhance SF in otherwise inoperative systems. Importantly for the field, this work reinforces the strong connection between linear optical properties and carrier dynamics and highlights the continual
936
Chem 4, 931–942, May 10, 2018
lack of understanding of the singlet exciton character and excited-state potential energy surface in SF materials. Although SF can be incoherent in intramolecular systems (as exemplified by the long timescales), vibrational and other dynamic nuclear processes appear to play a large role in mediating tripletpair formation. Further progress on this topic will certainly require a more detailed treatment of the role of static and dynamic nuclear effects, including vibrations and nuclear reorganization, as well as the role of bond hybridization in coupling SF chromophores. This work clearly shows the need for future efforts designed to better resolve competing structural and electronic effects, such as decoupling geometric changes and environmental polarity effects. The authors make a compelling case that further insight would be obtained from the reduction of inhomogeneous broadening in ensembles of these materials or singlechromophore studies.
ACKNOWLEDGMENTS S.N.S. thanks the National Science Foundation for the Graduate Research
3. Xia, J., Sanders, S.N., Cheng, W., Low, J.Z., Liu, J., Campos, L.M., and Sun, T. (2017). Singlet fission: Progress and prospects in solar cells. Adv. Mater. 29, 1601652. 4. Sanders, S.N., Kumarasamy, E., Pun, A.B., Appavoo, K., Steigerwald, M.L., Campos, L.M., and Sfeir, M.Y. (2016). Exciton correlations in intramolecular singlet fission. J. Am. Chem. Soc. 138, 7289–7297. 5. Basel, B.S., Zirzlmeier, J., Hetzer, C., Reddy, S.R., Phelan, B.T., Kryzaniak, M.D., Volland, M.K., Coto, P.B., Young, R.M., Clark, T., et al. (2018). Evidence for CT mediation in the primary events of singlet fission in a weakly coupled pentacene dimer. Chem 4, this issue, 1092–1111. 6. Kumarasamy, E., Sanders, S.N., Tayebjee, M.J.Y., Asadpoordarvish, A., Hele, T.J.H., Fuemmeler, E.G., Pun, A.B., Yablon, L.M., Low, J.Z., Paley, D.W., et al. (2017). Tuning singlet fission in p-bridge-p chromophores. J. Am. Chem. Soc. 139, 12488–12494. 7. Tayebjee, M.J.Y., Sanders, S.N., Kumarasamy, E., Campos, L.M., Sfeir, M.Y., and McCamey, D.R. (2017). Quintet multiexciton dynamics in singlet fission. Nat. Phys. 13, 182–189. 8. Weiss, L.R., Bayliss, S.L., Kraffert, F., Thorley, K.J., Anthony, J.E., Bittl, R., Friend, R.H., Rao, A., Greenham, N.C., and Behrends, J. (2017). Strongly exchange-coupled triplet pairs in an organic semiconductor. Nat. Phys. 13, 176–181. 9. Basel, B.S., Zirzlmeier, J., Hetzer, C., Phelan, B.T., Krzyaniak, M.D., Reddy, S.R., Coto, P.B., Horwitz, N.E., Young, R.M., White, F.J., et al. (2017). Unified model for singlet fission within a non-conjugated covalent pentacene dimer. Nat. Commun. 8, 15171. 10. Fuemmeler, E.G., Sanders, S.N., Pun, A.B., Kumarasamy, E., Zeng, T., Miyata, K., Steigerwald, M.L., Zhu, X.Y., Sfeir, M.Y., Campos, L.M., and Ananth, N. (2016). A direct mechanism of ultrafast intramolecular singlet fission in pentacene dimers. ACS Cent Sci 2, 316–324.