Ultrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridine

Abstract

We investigated the excited-state dynamics of bridged and non-bridged chiral helicene-like bidibenzo[c,h]acridines deposited as a thin film. Broadband UV/VIS−NIR transient absorption spectroscopy from the femtosecond to the microsecond time-scale and transient fluorescence measurements were employed to understand the photoinduced processes in these organic thin-film layers. Photoexcitation at 370 nm populates the S1 state, which has a lifetime of 15 and 36 ps for the non-bridged and bridged derivative, respectively. In both cases, the decay is governed by internal conversion (78% and 84%), whereas intersystem crossing represents only a smaller channel (21% and 15%), and the remaining 1% accounting for fluorescence. Fast repopulation of the S 0 state leads to the generation of hot ground state molecules (S 0*), with characteristic finger-type transient absorption features. The T 1 state decays back to S 0 by intersystem crossing with a time constant of 775 and 5500 ps, respectively. Even at high excitation fluence, there are no indications for higher-order processes, such as singlet–singlet annihilation or Förster energy transfer. Photoexcitation also launches coherent acoustic phonons in these films, appearing as damped oscillations in the transient absorption kinetics on the red and blue edge of each S 0 absorption band, with a period of about 190 ps.