Non-linear optical properties of organic molecules. Part 2. Effect of conjugation length and molecular volume on the calculated hyperpolarisabilities of polyphenyls and polyenes

Abstract

The calculated hyperpolarisabilities of the symmetrical polyphenyls, containing an electron-donating dimethylamino group and an electron-attracting nitro group positioned at opposite ends of the conjugated system, slowly increase with an increasing number of phenyl units, but the effect per unit volume is a maximum for 4-dimethylamino-4′-nitroterphenyl. In contrast, the calculated values for polyenes containing the same donor and attractor increase rapidly with an increasing number of ethenyl units, and the effect per unit volume is a maximum for 20 units. Overall, the polyene system shows an effect which is at least 20 times that of the polyphenyl system and 10 times that of any other known system. A similar effect is also found in the dimethylaminopolyenals, though a comparison between calculated and experimental dipole moments and electronic transition energies suggests that their hyperpolarisabilities may be somewhat overestimated at the CNDO level of approximation.