Study on the prediction of maximum absorption wavelength for conjugated alkenes

The optimized geometry and maximum absorption wavelength of conjugated alkenes were calculated by semi-empirical PM3 and ZINDO/S method in HyperChem, the results showed that based on the optimized geometry obtained from PM3 method, the maximum absorption wavelength of conjugated alkenes could be predicted precisely by setting OWFπ–π (one of the adjustable parameters in ZINDO/S method) at an appropriate value. It was found that there existed an excellent linear relationship between OWFπ–π and n (the number of conjugated double-bonds in the conjugated chain), the value of OWFπ–π decreased as n increased. The relationship could be extrapolated to the calculation of maximum absorption wavelength for β-carotene and lycopene, and the agreement with experiment was excellent. At the same time, the research on molecular orbital showed that the maximum absorption wavelength of conjugated alkenes originated from electron transition from HOMO to LUMO — a transition weakened the bonding of double-bonds while enhanced the bonding of single-bonds.