Third Order Nonlinear Optical Properties of Squaraine Dyes Having Absorption below 500 nm: A Combined Experimental and Theoretical Investigation of Closed Shell Oxyallyl Derivatives

Abstract

Nonlinear optical (NLO) activity of a molecule in general is connected to fundamental issues like charge transfer, conjugation, polarization, and recently suggested and less investigated diradical character. Extensively studied molecules for negative second order hyperpolarizabilities are the centrosymmetric squaraine derivatives (SQ) which have absorption in the region greater than 600 nm. These have no formal electronic structure but can be represented as a mixture of zwitterionic and diradicaloid valence bond resonance structures. In this work we have designed and synthesized arylamino SQ, 1−10, which bias the valence bond resonance picture completely to the zwitterionic type and totally eliminates the diradicaloid contribution (estimated using computational studies to be zero). Due to this, the lowest energy transitions are blue-shifted to the region less than 500 nm. We have carried out degenerate four wave mixing (DFWM) studies for estimation of γ values of these molecules using a 800 nm wavelength laser. The γ values are measured under nonresonant conditions and also at intensities where two photon absorption does not play any role. Nonlinear absorption properties of these molecules are studied through Z scan technique. The γ values, though smaller than the SQ having absorption in the red region, are reasonably large and range from −1.2 to −6.9 × 10−33 esu. High level computational techniques and model molecules have been utilized to understand the transition and the NLO activity in these molecules.