Effects of alkyl substituents on the excited states of naphthalene: Semiempirical study

The effects of the successive addition of alkyl substituents (methyl and re duced rings) on the excited states of naphthalene are reported. The calcula ted electronic states of all the reduced derivatives with two, three, and f our substituents are compared with the excited states of their methylated a nalogues. The excited states of several reduced derivatives with seven and eight substituents are also studied. The AMI method was used to optimize th e geometry of 57 naphthalene derivatives, and excited states were calculate d with the ZINDO/S (INDO/S) method. ZINDO/S calculations on naphthalene gav e excited states in better agreement with experimental results than with re sults of other semiempirical (CNDO/S and CNDOL) and ab initio (CIS, TD-HF, and TD-DFT) methods. Successive alkyl substitutions cue accompanied by bath ochromic displacements of the UV-visible bands, since the occupied orbitals are raised in energy more than the unoccupied orbitals. However, not all a vailable substituent positions in napthalene alter its orbital energy distr ibution in the same way when they are occupied by alkyl substituents. Disto rtion from planarity of the naphthalene skeleton of some reduced derivative s is the cause of "anomalous" bathochromic displacements of the absorption bands.