An inquisitive study on photophysical properties of few modified indole derivatives

Indole is an interesting molecule that exists in nature as a part of several important biomolecules including melatonin — a chemical that maintains the circadian rhythm in human body, serotonin — a chemical carrying messages through nerve cells, and tryptophan — an amino acid. It has a benzene ring fused with a pyrrole ring. Similar molecules containing cycloheptene ring fused with pyrrole — called azulene, cyclohepta[b]pyrrole and dihydrocyclohepta[b]pyrrole — also exist. It would be very interesting to see the effect on different properties of the above-mentioned biomolecules, if indole is replaced by azulene or cyclohepta[b]pyrrole or dihydro-cyclohepta[b]pyrrole. This paper precisely investigates such interesting substitutions. We replaced indole in the three biomolecules by azulene, cyclohepta[b]pyrrole, and dihydrocyclohepta[b]pyrrole and studied the corresponding change in acid-dissociation constant, static polarizabilities, first and second hyperpolarizabilities, absorption spectra, fluorescence, charge-transfer, and nature of the first two singlet, and the first two triplet states, using the state-of-the-art (time-dependent) density functional theory. The results are compared with the values obtained for the indole analogues calculated at the same level of theory.