From torsional spectra to Hamiltonians and dynamics: effects of coupled bright and dark states of 9-(N-carbazolyl)-anthracene

Abstract

Abstract. The torsional dynamics of the 9-(N-carbazolyl)-anthracene (C9A) molecule is investigated by means of time-independent (1) and time-dependent (2) quantummechanical simulations in adiabatic representation. The study includes effects of surface crossing of the bright St state with a dark state. (1) The intensity pattern of the So ~ St fluorescence excitation spectrum is used to fit an effective one-dimensional Hamiltonian with a single-minimum potential for the dark state together with diabatic couplings to the double well potential of the bright state.(2) Based on this Hamiltonian, first predictions for a pump-probe scheme are made. In the pump process the molecules are excited to the $1 state followed by competing torsions in the bright state and diabatic curve crossings to the dark state, depending on the pump frequency. Assuming the probe process to be an ionization from the bright state, the interfering effects of the dark state on the dynamics in the bright state can be monitored in a directly time-dependent way on a fs-ps time scale.

Twist dynamics of 9-(N-carbazolyl)-anthracene: effects of Intramolecular Vibrational Redistribution and non-adiabatic transitions in coupled bright and dark states

“…This "dark" state S X is likely a TICT (Twisted Intramolecular Charge Transfer) state whose existence in an isolated molecule is of particular interest in photochemistry. In the sequel the three state model has been refined [2,3], yielding an overall satisfactory agreement of the calculated and the experimental fluorescence emission spectra. In a further contribution [4] the fluorescence lifetimes given in [1] could be fitted based on a simple quantum mechanical model where the remaining vibrational degrees of freedom of C9A are treated as a thermal bath coupled to the twist motion.…”

“…We thus describe the way from continuous wave laser induced fluorescence spectroscopy via model Hamiltonians and IVR rates to time dependent dissipative dynamics, extending previous approaches from cw spectroscopy to ultrafast dynamics, (see e.g. [2,[18][19][20][21]). In the following section we present the model hamiltonian from which the dissipative dynamics and relaxation rates are calculated by the procedure outlined in Sect.…”

“…In a further contribution [4] the fluorescence lifetimes given in [1] could be fitted based on a simple quantum mechanical model where the remaining vibrational degrees of freedom of C9A are treated as a thermal bath coupled to the twist motion. According to this model, the twist dynamics of C9A in the S 1 and S X states should depend not only on the diabatic transitions induced by the couplings of S 1 and S X [2], but also on competing IVR processes induced by different couplings of the twist and complementary vibrations in the S 1 and S X states.…”

“…Summarizing, we present an investigation of the twist dynamics of C9A on the fs-ps timescale, taking into account the effects of IVR as well as the diabatic transition from the S 1 to the S X state, based on all the information given in the articles [2,3,4]. We thus describe the way from continuous wave laser induced fluorescence spectroscopy via model Hamiltonians and IVR rates to time dependent dissipative dynamics, extending previous approaches from cw spectroscopy to ultrafast dynamics, (see e.g.…”

“…The resulting time dependent density matrix yields nearly coherent twist dynamics of this rather large molecule, at least on a timescale of several ps. The corresponding pump-probe spectra are predicted following the approach proposed by Manz et al [2].…”