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Intermolecular vibrations mediate ultrafast singlet fission

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Science Advances  18 Sep 2020:
Vol. 6, no. 38, eabb0052
DOI: 10.1126/sciadv.abb0052
  • 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 (S0S1) 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 cm1, respectively. In addition, the marked dashed lines in ESA show the frequencies of 140, 276, 460, 1027, 1206, 1300, and 1370 cm1.

  • 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 cm1 with a lifetime of 150 fs, which is highlighted by the dashed arrow. In (C), we initially find a strong vibration at 170 cm1, which reduce its frequency to 140 cm1 on a time scale of 100 fs (marked in magenta box). Moreover, one new frequency at 250 cm1 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 cm1) shows weak oscillations decaying on a time scale of 150 fs. In addition, one low-frequency mode at 140 cm1 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 cm1 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 cm1 (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.

  • Fig. 5 Calculated wave-packet dynamics in the vicinity of the CI.

    The calculated wave-packet dynamics of S1 and T1T1 along the tuning mode are shown in (A) and (B). The wave-packet dynamics along coupling mode are shown in (C) and (D), respectively.

Supplementary Materials

  • 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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