Absorption and Fluorescence of 3-Methylindole:  A Theoretical Study, Including H₂O Interactions

Abstract

The paper presents a computational study of the ground and excited states of 3-methylindole. The ground-state geometry is optimized using the DFT B3LYP potential in conjunction with a 6-31+G(d) basis set. In addition the ground and first and second excited singlet states are optimized, for the first time, using the CASSCF method with an ANO−S basis set. The results are compared with experiment and with the experimental and theoretical literature data for the parent indole. The calculated excitations are in agreement with the absorption data. The comparison also shows that the 3-methylindole relaxation of the second excited-state terminates in an avoided crossing. The relaxed geometry of the second excited 3-methylindole state is therefore of mixed 1L a /L b nature. The interaction of 3-methylindole with water is also examined, revealing three stable chromophore−water complexes. The calculations are extended to the more stable chromophore−(water)2 complexes. The excitation energies of these complexes are calculated using the CASPT2 method (ANO−S type basis). Calculations using ESP-charges instead of water molecules demonstrate the usefulness of such approximation for the calculation of excitation energy shifts. Analogous results are obtained using Amber5.0 point charges.

Study of the Light‐Induced Spin Crossover Process of the [FeII(bpy)3]2+ Complex

“…This combined strategy, consisting of starting with a DFT-optimized structure and subsequently reoptimizing the crucial structure parameters at the CASPT2 level, has been successfully applied in other studies. [32,36,37] All calculations have been performed within the abelian C 2 point group symmetry. For all atoms, atomic natural orbital basis sets, specifically designed to include relativistic effects, have been used.…”

How Do Rotameric Conformations Influence the Time-Resolved Fluorescence of Tryptophan in Proteins? A Perspective Based on Molecular Modeling and Quantum Chemistry

“…The CASPT2 study on indole by SerranoAndrés and Roos indicated that upon relaxation the 1 L b remains the lowest excited state, and will be the emitting state, as is confirmed by matrix isolation spectroscopy [139]. For 3-methylindole, where the splitting between the states is lower, the CASPT2 results point out that the optimization which starts at the 1 L a Franck-Condon state induces a state crossing between with the 1 L b state [137]. Nonetheless also in this chromophore the lowest excited energy level is the relaxed 1 L b state, and the emission maximum is calculated at 4.02 eV.…”

“…These transitions are immediately followed by a Rydberg excitation from the HOMO to MO33. In Table 4 we present calculated and experimental transition energies both for indole [135] and its homologue 3-methylindole [137]. In the latter model system the excitations are slightly red-shifted and the separation between the radiative states is lower than in indole, making it an even better model of the actual tryptophan chromophore.…”

Towards an insight on photodamage in hair fibre byUV‐light: An experimental and theoretical study

“…The humidity control was rigorous as the Trp photoluminescence (PL) is quite sensitive to moisture variations. Somers and Ceulemans [47] studied the conformational changes in the Trp molecule when hydrated with water. The Trp degradation kinetics is also altered as a function of the amount of moisture because photolytic processes are usually accelerated in water.…”