Azinium−(π-Bridge)−Pyrrole NLO-Phores:  Influence of Heterocycle Acceptors on Chromophoric and Self-Assembled Thin-Film Properties

Abstract

Novel heterocycle-based azine/azinium−(π-bridge)−pyrrole systems, [(1-(pyrid-4-yl)-2-( N -methylpyrrol-2-yl)]ethene ( 1 ), 5-[( N -methylpyrrol-2-yl)azo]quinoline ( 2 ), 5-[( N -methylpyrrol-2-yl)azo]isoquinoline ( 3 ), and the corresponding N -methotriflates 4 − 6 were synthesized and characterized. Chromophore precursors 1 − 3 react with iodobenzyl-functionalized surfaces ( 10 ) affording polar-ordered σ-bonded thin films 7 − 9, respectively. All systems were studied by optical (UV−visible, photoluminescence) spectroscopies, electrochemical (CV), and thermal (TGA, DSC) techniques. Self-assembled chromophore monolayers 7 − 9 on glass, quartz, and silicon substrates have been characterized by a full complement of physicochemical techniques: optical spectroscopy, aqueous advancing contact angle measurements, specular X-ray reflectivity, atomic force microscopy, and angle-dependent polarized second harmonic generation. Film second harmonic generation responses χ (2) zzz vary more than 1 order of magnitude ranging from 1.3 and 1.6, to 34 × 10 - 8 esu for 9 -, 8 -, and 7 -based monolayers, respectively. This study demonstrates the following: (i) SA monolayers can be prepared using various types of azine-containing precursors; (ii) chromophore and SA film properties are influenced predominantly by the nature of the π-deficient azinium acceptor; (iii) unsubstituted, π−excessive pyrrol-2-yl rings can act as primary donor groups in push−pull conjugated systems.