Effect of chromophore elongation on linear and nonlinear optical properties of merocyanines derivatives of diethylaminocoumarin

• A series of merocyanines based on aminocoumarin core with different length of the polymethine chain were studied.
• Spectral and quantum-chemical analysis of their electronic structures and lowest electron transitions have been performed.
• The chromophore elongation of studied dyes produces the bathochromic shift and enhancement of the third hyperpolarizability.
• The structure of the lowest electron transitions of neutral merocyanines are the same to well-known cationic cyanine dyes.
• The nonlinear optical response enhancement with the conjugated chain elongation was experimentally confirmed.

A series of the merocyanines containing the aminocoumarin as an acceptor terminal group and typical donor residues with different length of the polymethine chain were studied in detail. Joint spectral and quantum-chemical analysis of their molecular geometries and electronic structures as well as origin of their lowest electron transitions have been performed. It was shown that the electronic and spectral properties of the neutral merocyanines are similar to the well-known cationic cyanine dyes, being determined by the same structure of the lowest electron transitions in molecule. The elongation of the chromophore chain of the studied merocyanines leads to the significant bathochromic shift of the long wavelength band in their absorption spectra, approximately to that in the symmetrical cyanines. The calculated third hyperpolarizability increases for the higher merocyanine vinylog due to the dependence of the energy of the excited state on the elongation of the conjugated chain. It was confirmed by nonlinear optical (NLO) study performed within the picosecond range pulsed laser excitation at 532 nm.