The role of intramolecular charge transfer and symmetry breaking in the photophysics of pyrrolo[3,2-b]pyrrole-dione†
A three-step synthetic route to a structurally unique π-expanded pyrrolo[3,2-b]pyrrole derived bis-ketone has been developed. In contrast to all previous ladder-type pyrrolopyrroles, the new dye exhibits a low-energy absorption band in the visible region which is responsible for its red-purple color. Interestingly, even though the compound is centrosymmetric, this band coincides with the lowest energy two-photon absorption (TPA) transition. This non-typical behaviour has been computationally rationalized by finding two close lying excited states, one of which (S1) is active for OPA and the other (S2) for TPA processes, which arise from the mixing of two symmetric partial charge-transfer states. The ultrafast excited-state dynamics was characterized by means of transient absorption analysis. A relaxation process involving S1 symmetry breaking occurs in a few ps, leading to the formation of the lowest energy charge-transfer state. This is weakly emitting, with a measured lifetime in the order of tens of picoseconds. Interestingly, two-photon polymerization has been achieved using this new ketone. The high yield of radical photo-initiation upon two-photon excitation was demonstrated by the fabrication of woodpile photonic crystal templates by direct laser writing using a zirconium–silicon hybrid composite.