The calculated vibrationally resolved absorption and fluorescence spectra are shown in Fig. . In our calculations, the adiabatic energy gap between S 0 and S 1 states (0-0 transition) is set to be nearly same with the calculated value by TDA-CAM-B3LYP*. The theoretical calculation with the vertical gradient (VG) approximation produces a narrow-band absorption and emission with very small full-width at half-maximum (FWHM), which coincides with the experimental measurements. The spectral lineshapes indicate that this material possesses a rigid nature which can significantly reduce the vibrational motion to obtain narrow-band emission with very small FWHM. The VG approximation assumes that the S 1 potential energy surface (PES) is just a shifted PES of the S 0 , meaning that the electronic excitation doesn't change the shape of PES much. The theoretical calculation without VG approximation accounts for the difference between S 0 and S 1 PESs, and largely overestimates FWHM compared to the experimental value, attributed to the inaccurate S 1 PES calculated by the TDDFT method. The HTVC effect on the absorption and emission spectra is negligibly small.