NEW PHOTOCHROMIC AND ELECTROOPTICALLY ACTIVE DERIVATIVES OF TCNQ - CALCULATED SHG PARAMETERS OF MODEL ZWITTERIONIC AND QUINONOIDAL STRUCTURES

The calculated values of the vector average of beta -2 omega, omega, omega, beta -vec, for two pyridinium alpha -cyano-4-styryldicyanomethanides, are shown to be dominated by a diagonal contribution from the first excited (intramolecular charge transfer) electronic state. However, there are non-negligible off-diagonal contributions from an interaction between the first and third excited electronic states. In all cases, by far the most dominant component of beta -vec is beta xxx. The values of beta Xand beta xxxfor the zwitterionic and quinonoidal forms of both the 2-pyridinium and 4-pyridinium compounds are large and negative in the pre-resonant region due to the fact that the intramolecular charge-transfer transition is accompanied by a large and negative change in molecular electric dipole moment in the x (long-axis) direction. © 1990 IOP Publication Ltd.