The structure and electronic properties of the electronic ground and the lowest excited singlet states of 5-cyanoindole (5CI) were determined using rotationally resolved spectroscopy of the vibrationless electronic origin of 5CI. In contrast to most other indole derivatives, the lowest excited state of 5CI is determined to be of L(a) character. The conventional approximate coupled cluster singles and doubles model (CC2) fails to describe the geometry of the excited state correctly. Nevertheless, scaling the spin components of equal and opposite spins within the CC2 model as proposed by Hellweg et al. (Phys. Chem. Chem. Phys., 2008, 10, 1159) resulted in very good geometry parameters for the excited state.

Rotationally resolved electronic spectra of the vibrationless origin and of eight vibronic bands of 5-methoxyindole (5MOI) have been measured and analyzed using an evolutionary strategy approach. The experimental results are compared to the results of ab initio calculations. All vibronic bands can be explained by absorption of a single conformer, which unambiguously has been shown to be the anti-conformer from its rotational constants and excitation energy. For both anti- and syn-conformers, a (1)L(a)/(1)L(b) gap larger than 4000 cm(-1) is calculated, making the vibronic coupling between both states very small, thereby explaining why the spectrum of 5MOI is very different from that of the parent molecule, indole.

The electronic origin bands A and B of 5-hydroxyindole were measured using rotationally resolved electronic spectroscopy. From comparison of the experimental rotational constants to the results of ab initio calculated structures, we could make the assignment of band A being due to the syn conformer and of band B being due to the anti conformer. These conformers, which differ in the orientation of the hydroxy group with respect to the rest of the molecule, have considerably different S1 state life times. The most probable explanation for this surprising finding is a different conical intersection of the ππ* states of both conformers with the repulsive πσ* state.

The structure and electronic properties of the electronic ground state and the lowest excited singlet state (S(1)) of 5-fluoroindole (5FI) were determined by using rotationally resolved spectroscopy of the vibration-less electronic origin of 5FI. From the parameters of the axis reorientation Hamiltonian, the absolute orientation of the transition dipole moment in the molecular frame was determined and the character of the excited state was identified as L(b).

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