Highly Polarized 1,4-Dihydropyrrolo[3,2-b]Pyrroles: From Synthesis to Photophysics and Two-Photon Induced Polymerization.

A series of ladder-type, highly polarized functional dyes possessing 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) scaffold was synthesized in a straightforward manner. All dyes have a quadrupolar, centrosymmetric architecture and two five-membered rings adjacent to a central electron-rich DHPP core, but they differ in the strength of peripheral electron-withdrawing functionalities and degree of deviation from planarity. Ketones exhibit charge-transfer (CT) bands located between 500-600 nm and vanishingly weak emission, whereas dyes possessing dicyanovinylidene functionality and π-expanded chromophore feature complex absorption spectra with CT bands beyond 700 nm. Computational studies enabled assignment of absorption bands to intramolecular charge-transfer and higher-energy π-π* transitions. The lack of emission was rationalized by invoking efficient nonradiative decay arising from internal conversion and/or intersystem crossing (DHPP possessing four bromine atoms). The two-photon absorption cross-section reaches up to 240 GM for the most strongly polarized and twisted donor-acceptor DHPP system. All the synthesized dyes proved to be good photosensitizers in two-photon induced polymerization, with the twisted DHPP derivative exhibiting a polymerization threshold as low as 130 GW/cm2, nearly an order of magnitude lower than those of conventional photosensitizers.