Daniel Krügler

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

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.

High-Resolution and Dispersed Fluorescence Examination of Vibronic Bands of Tryptamine: Spectroscopic Signatures for La/Lb Mixing near a Conical Intersection

The vibronic spectrum of tryptamine has been studied in a molecular beam up to an energy of 930 cm(-1) above the S(0)-S(1) electronic origin. Rotationally resolved electronic spectra reveal a rotation of the transition dipole moment direction from (1)L(b) to (1)L(a) beginning about 400 cm(-1) above the (1)L(b) origin. In this region, vibronic bands which appear as single bands at low resolution contain rotational structure from more than one vibronic transition. The number of these transitions closely tracks the total vibrational state density in the (1)L(b) electronic state as a function of internal energy. Dispersed fluorescence spectra show distinct spectroscopic signatures attributable to the (1)L(b) and (1)L(a) character of the mixed excited-state wave functions. The data set is used to extrapolate to a (1)L(a) origin about 400 cm(-1) above the (1)L(b) origin. DFT-MRCI calculations locate a conical intersection between these two states at about 900 cm(-1) above the L(a) origin, whose structure is located along a tuning coordinate which is close to a linear interpolation between the two excited-state geometries. Along the branching coordinate, there is no barrier from (1)L(a) to (1)L(b). A two-tier model for the vibronic coupling is proposed.

Determination of the excited state structure of 7-azaindole using a Franck-Condon analysis

Abstract The change in structure of 7-azaindole upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of six different vibronic bands. A total of 107 emission band intensities were fit, together with the changes in the rotational constants of four 7-azaindole isotopomers. The geometry change of the ring framework upon electronic excitation from the electronic ground state to the 1Lb state (ππ*) can be described by an overall expansion of the pyridine ring of 7-azaindole, with minor changes of the pyrrole ring. The resulting geometry changes are interpreted on the basis of ab initio calculations.

Geometry change of simple aromatics upon electronic excitation obtained from Franck-Condon fits of dispersed fluorescence spectra.

The S(1) state geometries of benzonitrile, p-cyanophenol, o-cyanophenol, chlorobenzene, and p-chlorophenol were determined by Franck-Condon simulations and a fit of the geometry to the vibronic intensities and effective rotational constants in the harmonic limit based on ab initio force constants.

A genetic algorithm based determination of the ground and excited (1Lb) state structure and the orientation of the transition dipole moment of benzimidazole

The structure of benzimidazole has been determined in the electronic ground and excited states using rotationally resolved electronic spectroscopy. The rovibronic spectra of four isotopomers and subsequently the structure of benzimidazole have been automatically assigned and fitted using a genetic algorithm based fitting strategy. The lifetimes of the deuterated isotopomers have been shown to depend on the position of deuteration. The angle of the transition dipole moment with the inertial a-axis could be determined to be -30 degrees. Structures and transition dipole moment orientation have been calculated at various levels of theory and were compared to the experimental results.

Determination of the excited-state structure of 7-azaindole-water cluster using a Franck-Condon analysis

The change of the 7-azaindole-water cluster structure upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of five different vibronic bands. A total of 105 emission band intensities were fitted, together with the changes of rotational constants of one isotopomer. These rotational constants have been obtained from a fit to the rovibronic contour of the cluster. The geometry change upon electronic excitation to the pi pi* state can be described by a strong and asymmetric shortening of the hydrogen bonds and a deformation of both the pyridine and the pyrrole rings of 7-azaindole. The resulting geometry changes are interpreted on the basis of ab initio calculations.