Fig. 1 SF in a pentacene film: 2D electronic spectroscopy and correlation analysis indicate a vibrationally driven primary step, PSF. (A) Ground-state absorption spectrum of a pentacene film on quartz substrate (red dots) and laser spectrum is used in this measurement (light-blue shadow). (B) Calculated site energies of the ground, singlet excited states (S0, S1, and Sn), and triplet-pair states [1(T1T1) and 1(T1T2)]. The selected 2D electronic spectra (real part) for selected waiting times are shown from (C to F). Positive diagonal and negative off-diagonal features denote the GSB and ESA, respectively. (G) 2D correlation map obtained from a correlation analysis along the diagonal direction. Two negative peaks are shown in the region of the GSB (the 2D spectrum at T = 500 fs is shown as a white contour), which indicates the vibrational origin of the oscillations.
Fig. 2 Vibrational coherence mediating the PSF process is captured by TG measurements. (A) Measured TG spectrum of pentacene film with a time step of 2 fs. The GSB (positive) and the ESA (negative) in the spectrum originate from the corresponding transitions in Fig. 1B, as red (S0 → S1) and blue arrows [1(T1T1) → 1(T1T2)]. The averaged kinetics of the GSB [685 to 690 nm, marked by black dashed lines in (A)] and ESA [700 to 705 nm, marked by green dashed lines in (A)] are shown as red and blue dashed lines in (B) and (C), respectively. The associated exponential fitting curves are presented as black dashed lines. The obtained residuals of the GSB and the ESA bands are magnified and shown as red and blue solid lines in (B) and (C), respectively. A Fourier transform has been performed to examine the vibrational dynamics, the obtained power spectra of GSB and ESA are shown as red and blue lines in (D) and (E), respectively. In (D), the marked dashed lines indicate the vibrational frequencies at 140, 218, 460, 953, 1030, 1206, 1300, and 1370 cm−1, respectively. In addition, the marked dashed lines in ESA show the frequencies of 140, 276, 460, 1027, 1206, 1300, and 1370 cm−1.
Fig. 3 Time evolution of vibrational coherences shows the structural evolution by low-frequency modes. (A) Magnified residuals of the GSB band. The dynamics of each vibrational mode is revealed by the wavelet analysis. The high- and low-frequency ranges are shown in (B) and (C), respectively. In (B), we show the vibrational mode at frequency of ∼1300 cm−1 with a lifetime of 150 fs, which is highlighted by the dashed arrow. In (C), we initially find a strong vibration at 170 cm−1, which reduce its frequency to 140 cm−1 on a time scale of 100 fs (marked in magenta box). Moreover, one new frequency at 250 cm−1 is gradually generated within the initial 500 fs. (D) Magnified residuals of the ESA. The high- and low-frequency ranges of the vibrations are shown in (E) and (F), respectively. In (E), a broadband vibration (∼1300 cm−1) shows weak oscillations decaying on a time scale of 150 fs. In addition, one low-frequency mode at 140 cm−1 shows strong oscillation in the ESA band, which is highlighted by a dashed arrow.
Fig. 4 Calculated vibrational modes and PESs. (A and B) Few key modes identified from the calculations. The low-frequency mode (177 cm−1 in A) is associated with the intermolecular rocking motion of two pentacene molecules along the longitudinal molecular axis, which serves as the intermolecular vibration. The calculated high-frequency mode of 1013 cm−1 (in B) corresponds to the intramolecular vibration. Constructed PES along the coupling (C) and the tuning (D) modes, which are based on quantum chemistry calculations for the Huang-Rhys factor between the singlet excited and triplet-pair states. The calculated Huang-Rhys factors of the low- and high-frequency modes are shown in (E) and (F), respectively. Red and blue bars correspond to the modes of the singlet and triplet-pair states.
Supplementary Materials
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/38/eabb0052/DC1
Additional Files
Supplementary Materials
Intermolecular vibrations mediate ultrafast singlet fission
Hong-Guang Duan, Ajay Jha, Xin Li, Vandana Tiwari, Hanyang Ye, Pabitra K. Nayak, Xiao-Lei Zhu, Zheng Li, Todd J. Martinez, Michael Thorwart, R. J. Dwayne Miller
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- Sections S1 to S10
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- References
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