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

ABSTRACT

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/cm ² , nearly an order of magnitude lower than those of conventional photosensitizers.