Quantum chemical calculations, synthesis and properties of novel organic molecules with photoinduced intramolecular electron transfer and large change in electric dipole moment in the excited state

Abstract

The topic of this paper is bipolar organic compounds containing both charged electron donor and electron acceptor groups interconnected by various kinds of bridges (–D–X–A+). Such betaines are subject to photoinduced intramolecular electron transfer (PIET), large change in dipole moment in the excited state and considerable hyperpolarisability, which causes large non-linear optical effects in solutions and Longmuir–Blodgett (LB) films. Quantum chemical calculations (CNDO/S, ZINDO/S) of some types of betaines containing a 1,3-indandione anion or diazole anion electron donor part and an N-pyridinium cation electron acceptor part show that the HOMO and LUMO are strongly localized and an effective PIET takes place. The calculated change in dipole moment in the excited state, δμ=μg−μex , is substantial (8–20 D) and in many cases the direction of μ is reversed. The character of the bridge X in betaines shows the largest effect on μg and δμ, especially in the case of X=p-phenylene. The pyrazole anion is a better electron donor than the 1,3-indandione anion. The synthesis of betaines containing a 1,3-indandione anion part of an N-pyridinium cation part interconnected directly or through a p-phenylene bridge has been achieved and their electron absorption spectra have been investigated. Preliminary experiments confirm NLO effects in solutions and LB films. The synthesis of novel substituted betaines is unlimited and the synthesis of polymer-bonded or surface-bonded betaines is possible. Copyright © 1999 John Wiley & Sons, Ltd.