Study on the prediction of electronic absorption spectrum for cyanine compounds

The optimized geometry and electronic absorption spectrum of cyanine compounds were calculated by semi-empirical PM3 and ZINDO/S method in HyperChem 7, the results show that based on the optimized geometry obtained from PM3 method, the electronic absorption spectrum of cyanine compounds could be predicted precisely by setting OWFπ-π (one of the adjustable parameters in ZINDO/S method) at an appropriate value. It is found that there is an excellent linear relationship between OWFπ-π and n (the number of conjugated double-bonds within the conjugated chain) exists, the value of OWFπ-π decreases as n increases. At the same time, the research on molecular orbital indicates that the maximum absorption wavelength of cyanine compounds originates from the electron transition from HOMO to LUMO, a transition enhances the bonding of central carbon-atom with neighboring atoms when n is an odd number while weakens the bonding of central carbon-atom with neighboring atoms when n is an even number.