Vibrational spectra and static vibrational contribution to first hyperpolarizability of naphthopyrans—A combined experimental and DFT study

•Vibrational spectra of two pairs of photochromic naphthopyrans were investigated for the first time.•Important vibrational frequencies that are affected by the mode of pyran ring fusion to the naphthalene unit are discussed.•The βv/βe ratio increases when the geminal 4-methoxyphenyl groups are replaced by a phenyl and a morpholinophenyl group. Two pairs of isomeric photochromic naphthopyrans were investigated by FT-IR and FT-Raman spectroscopy and by density functional theory (DFT) methods. The vibrational frequencies and contribution to first hyperpolarizabilities of the four naphthopyrans were determined at the B3LYP/6-31G (d,p) level. Comparison of the experimental and theoretical vibrational frequencies of the naphthopyrans was made. The assignments of the vibrational spectra have been carried out with the help of normal co-ordinate analysis (NCA). These results show that the ratio between pure vibrational contribution to the electronic contribution of first hyperpolarizability (βv/βe) increases when the 4-methoxyphenyl groups in the symmetrical isomers is replaced by 4-morpholinophenyl and phenyl group resulting in an asymmetrically substituted isomer pair. At the static limit, the low vibrational frequency modes have dominating contributions to the vibrational dipole polarizability and first hyperpolarizability.