Azinium-(π-bridge)-pyrrole NLO-phores: Influence of heterocycle acceptors on chromophoric and self-assembled thin-film properties

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.