Abstract
The functional properties of organic semiconductors are defined by the interplay between optically bright and dark states. Organic devices require rapid conversion between these bright and dark manifolds for maximum efficiency, and one way to achieve this is through multiexciton generation (S1→1TT). The dark state 1TT is typically generated from bright S1 after optical excitation; however, the mechanistic details are hotly debated. Here we report a 1TT generation pathway in which it can be coherently photoexcited, without any involvement of bright S1. Using <10-fs transient absorption spectroscopy and pumping sub-resonantly, 1TT is directly generated from the ground state. Applying this method to a range of pentacene dimers and thin films of various aggregation types, we determine the critical material properties that enable this forbidden pathway. Through a strikingly simple technique, this result opens the door for new mechanistic insights into 1TT and other dark states in organic materials.
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Data availability
The online version of this paper includes Supplementary Information, including synthetic and experimental details, figures and text. All data are available from the corresponding authors upon reasonable request. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition nos. CCDC 2353076 (M1), 722606 (M3) and 2353077 (M4). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. Source data are provided with this paper.
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Acknowledgements
This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC programme (DMR-1719875). This work was supported by the College of Arts and Sciences at Cornell University (A.J.M.), the US Department of Energy, Office of Science, Basic Energy Sciences, Condensed Phase and Interfacial Molecular Science, Early Career Research Program DE-SC0021941 (A.J.M.), the Alfred P. Sloan Foundation (A.J.M.), Cornell Atkinson Center for Sustainability (A.J.M.), the National Research Foundation of Korea funded by the Ministry of Education 2022R1A6A3A03072477 (J.K.), a National Research Foundation of Korea Grant funded by the Korean Government RS-2023-00210400 (W.K.), National Science Foundation grant no. DMR-1627428 (J.A.), and the Science and Engineering Research Board (SERB), India, through IRHPA grant IPA/2020/000033 and core research grant CRG/2022/004523 (S.P.).
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A.J.M. and W.K. conceived the project. K.M., D.W., J.F., J.W., S.P. and J.A. synthesized the samples and D.C.B. and V.D. prepared thin films using these materials. J.K. and D.C.B. designed the experiments and performed the narrowband and broadband TA measurements. J.K., D.C.B., W.K. and A.J.M. led the analysis of the datasets and wrote the paper with input from all authors. All authors contributed to discussions.
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Extended data
Extended Data Fig. 1 TA of film M1.
(a) Absorbance spectrum of M1 compared to the ultrafast resonant and sub-resonant excitation pulses. Broadband ultrafast TA data for M1 using resonant (b) and sub-resonant (c) excitation pulses along with comparison of respective kinetics (d). Results show direct 1TT excitation under sub-resonant excitation in agreement with the narrowband TA presented in Fig. 3. Kinetics in D were extracted by integrating from 480–505 nm for 1TT and 530–580 nm for S1. (e) `1TT kinetic extracted at single wavelengths rather than by integration as done in Fig. 3b. The kinetic trace extracted at 488 nm shows a rise with similar time constant to the S1 decay in Fig. 3b (309 vs. 260 fs), whereas a kinetic extracted where there is more spectral overlap with S1 shows nearly no rise. (f and g) `TA spectra for M1 zoomed on the stimulated emission peak along with its respective kinetic showing similar time constant (248 fs) to the above 1TT rise and corresponding 1TT decay from main text.
Extended Data Fig. 2 TA of film M2.
(a) Absorbance spectrum of M2 compared to the ultrafast resonant and sub-resonant excitation pulses. The absorbance spectrum of M2 has too much overlap with the sub-resonant excitation pulse to selectively excite just 1TT. Broadband ultrafast TA data for M2 using resonant (b) and sub-resonant (c) excitation pulses along with comparison of respective kinetics (d). Full narrowband TA data under resonant (e) and sub-resonant (f) excitation for M2 for which spectral slices are presented in Fig. 3 along with comparison of respective kinetics (g). Kinetics in D and G were extracted by integrating from 515–540 nm for 1TT and 560–620 nm for S1.
Extended Data Fig. 3 TA of film M3.
(a) Absorbance spectrum of M3 compared to the ultrafast resonant and sub-resonant excitation pulses. The absorbance spectrum of M3 has too much overlap with the sub-resonant excitation pulse to selectively excite just 1TT. Broadband ultrafast TA data for M3 using resonant (b) and sub-resonant (c) excitation pulses along with comparison of respective kinetics (d). Full narrowband TA data under resonant (e) and sub-resonant (f) excitation for M3 for which spectral slices are presented in Fig. 3 along with comparison of respective kinetics (g). Kinetics in D and G were extracted by integrating from 550–580 nm for 1TT and 480–520 nm for S1.
Extended Data Fig. 4 TA of film M4.
(a) Absorbance spectrum of M4 compared to the ultrafast resonant and sub-resonant excitation pulses. The absorbance spectrum of M4 has too much overlap with the sub-resonant excitation pulse to selectively excite just 1TT. Broadband ultrafast TA data for M4 using resonant (b) and sub-resonant (c) excitation pulses along with comparison of respective kinetics (d). Full narrowband TA data under resonant (e) and sub-resonant (f) excitation for M4 for which spectral slices are presented in Fig. 3 along with comparison of respective kinetics (g). Kinetics in D and G were extracted by integrating from 520–540 nm for 1TT and 620–640 nm for S1.
Extended Data Fig. 5 TA of film M5.
Full narrowband TA data under resonant (a) and sub-resonant (b) excitation for M5 for which spectral slices are presented in Fig. 3 along with comparison of respective kinetics (c). There is no evidence for direct 1TT excitation in this sample. Kinetics in C were extracted by integrating from 480–520 nm for 1TT and 560–660 nm for S1.
Extended Data Fig. 6 TA of dimer D1.
(a) Broadband ultrafast TA measurements on D1 in polystyrene matrix. 2D contour maps (left) and temporal profiles (right) are plotted. (b) Narrowband TA measurements on D1 in polystyrene matrix. Representative TA spectra in visible region (left) & near-infrared region (middle) and decay profiles (right) are plotted.
Extended Data Fig. 7 TA of dimer D2.
(a) Broadband ultrafast TA measurements on D2 in polystyrene matrix. 2D contour maps (left) and temporal profiles (right) are plotted. (b) Narrowband TA measurements on D2 in polystyrene matrix. Representative TA spectra in visible region (left) & near-infrared region (middle) and decay profiles (right) are plotted.
Extended Data Fig. 8 TA of dimer D3.
(a) Broadband ultrafast TA measurements on D3 in polystyrene matrix. 2D contour maps (left) and temporal profiles (right) are plotted. (b) Narrowband TA measurements on D3 in polystyrene matrix. Representative TA spectra in visible region (left) and decay profiles (right) are plotted.
Extended Data Fig. 9 TA of dimer D4.
(a) Broadband ultrafast TA measurements on D4 in polystyrene matrix. 2D contour maps (left) and temporal profiles (right) are plotted. (b) Narrowband TA measurements on D4 in polystyrene matrix. Representative TA spectra in visible region (left) & near-infrared region (middle) and decay profiles (right) are plotted.
Extended Data Fig. 10 TA of dimer D5.
(a) Broadband ultrafast TA measurements on D5 in polystyrene matrix. Representative TA spectra (left) and temporal profiles (right) are plotted. (b) Narrowband TA measurements on D5 in polystyrene matrix. Representative TA spectra in visible region (left) & near-infrared region (middle) and decay profiles (right) are plotted.
Supplementary information
Supplementary Information
Supplementary Figs. 1–41, Table 1, methods and discussion.
Supplementary Crystallography Data 1
Crystallographic data of M1, registered CCDC 2353076.
Supplementary Crystallography Data 2
Structure factors for M1.
Supplementary Crystallography Data 3
Crystallographic data of M4, CCDC 2353077.
Supplementary Crystallography Data 4
Structure factors for M4.
Source data
Source Data Fig. 1
Raw absorption spectra of all samples.
Source Data Fig. 2
Raw TA maps and extracted spectra/kinetics.
Source Data Fig. 3
Raw TA maps and extracted spectra/kinetics.
Source Data Fig. 4
Extracted TA spectral components.
Source Data Extended Data Fig. 1
Full raw TA data for film M1.
Source Data Extended Data Fig. 2
Full raw TA data for film M2.
Source Data Extended Data Fig. 3
Full raw TA data for film M3.
Source Data Extended Data Fig. 4
Full raw TA data for film M4.
Source Data Extended Data Fig. 5
Full raw TA data for film M5.
Source Data Extended Data Fig. 6
Full raw TA data for dimer D1.
Source Data Extended Data Fig. 7
Full raw TA data for dimer D2.
Source Data Extended Data Fig. 8
Full raw TA data for dimer D3.
Source Data Extended Data Fig. 9
Full raw TA data for dimer D4.
Source Data Extended Data Fig. 10
Full raw TA data for dimer D5.
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Kim, J., Bain, D.C., Ding, V. et al. Coherent photoexcitation of entangled triplet pair states. Nat. Chem. (2024). https://doi.org/10.1038/s41557-024-01556-3
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DOI: https://doi.org/10.1038/s41557-024-01556-3
