Vibronic coupling in indole: I. Theoretical description of the 1La–1Lb interaction and the electronic spectrum

Abstract

The properties of the three lowest singlet electronic states (ground, (1)L(b), and (1)L(a) states) of indole (C(8)H(7)N) have been calculated with second-order approximate coupled-cluster theory (CC2) within the resolution-of-the-identity approximation. Refined electronic energies at the CC2 optimized structures and transition dipole moments were calculated using a density functional theory multi-reference configuration-interaction (DFT/MRCI) approach. Structures, energies, and dipole moments are reported for all three states and compared to experimental values. From the optimized structures and calculated transition dipole moments, we predict that pure (1)L(b) bands will have positive signs for both the axis reorientation angle theta(T) and the angle theta of the transition dipole moment with respect to the inertial a axis. For (1)L(a) bands the signs of both angles will be reversed. Vibronically coupled bands can exhibit opposite signs for theta and theta(T). The absorption and emission spectra of indole are calculated based on the Franck-Condon Herzberg-Teller approximation using numerical transition dipole moment derivatives at the DFT/MRCI level of theory. Implications for the experimentally observed vibronic spectra are discussed. Predictions are made for rotationally resolved spectra of various rovibronic bands. A conical intersection, connecting the (1)L(b) and (1)L(a) states, which can be accessed to varying extents via different Herzberg-Teller active modes is found approximately 2000 cm(-1) above the (1)L(b) minimum.

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

“…Our own calculations, as well as those of Sobolewski and Domcke [24][25][26] conclude, however, that the 1 L a state is strongly bound along the N-H coordinate. On the basis of this evidence, and also the fact that both our calculations and those of Schmitt and co-workers 21 predict a conical intersection between the 1 L b and 1 L a states, we therefore suggest that, at 249 nm excitation, the τ 3 DAS originates from the 1 L b state following extremely rapid internal conversion from 1 L a .…”

“…19,20 The lack of observable vibronic structure in the 1 L a state may be attributed to its short lifetime and this may be rationalised, at least in part, on the basis of results from a recent comprehensive theoretical and experimental study by Schmitt and co-workers. 18,21 These authors conclude that the 1 L a state origin is very close to a conical intersection that connects to the 1 L b state, mediated by Herzberg-Teller active modes. This process is essentially barrierless and so vibrationally excited 1 L a states that are accessed radiatively, funnel directly through into the 1 L b minimum.…”

Explaining Level Inversion of the La and Lb States of Indole and Indole Derivatives in Polar Solvents

“…The optimized structures are planar, in agreement with experimental [28] and theoretical findings. [18,29] Figure 1 shows the atom labeling that was used for the geometrical data given in Table 1. The optimized ground-state structure of indole is compared with the crystal structure of l-tryptophan.…”

Bidimensional electronic spectroscopy on indole in gas phase and in water from first principles

“…for the ground state) of the L a state we observe a red-shift of the ground state (GS) ? L a transition by $0.10 eV in aqueous solution, even though less pronounced than the shift of 0.17 eV observed experimentally [34]. Due to the small energy gap between L b and L a their vibronic coupling is predicted to be very strong [34].…”

“…L a transition by $0.10 eV in aqueous solution, even though less pronounced than the shift of 0.17 eV observed experimentally [34]. Due to the small energy gap between L b and L a their vibronic coupling is predicted to be very strong [34]. Hence, one can assume that both states are nearly simultaneously populated at sufficiently high excitation energies.…”