PUB - Publikationen an der Universität Bielefeld

The ultrafast photo-induced dynamics of the E-isomers of four selected photochromic fulgides with distinct structural motifs have been elucidated by femtosecond broadband transient absorption spectroscopy in n-hexane as solvent. E -> C and E -> Z isomerisations, respectively, with time constants of similar to 0.12 +/- 0.02 ps and similar to 0.34 +/- 0.03 ps taking place in parallel were found for derivatives with a methyl substituent at the central hexatriene (HT) unit. In contrast, fulgides with increased steric constraints by an iso-propyl substituent or by intramolecular bridging displayed virtually zero E -> Z isomerisation, but instead a desired accelerated and more efficient ring closure in a reaction time of only similar to 50 +/- 10 fs. Both photoisomerisations appear to follow excited-state pathways with distinctive conical intersections. For the ring closure, direct barrierless pathways with steep downhill gradients are likely. Furthermore, the results indicate conformer-specific reactions, with ring closure exclusively by the E-alpha conformer and E -> Z isomerisation predominantly by the E-beta conformer, because the E-alpha -> Z channel is unfavoured by the faster and kinetically more competitive E-alpha -> C reaction. DFT calculations of the equilibrium structures showed that the sterically demanding groups at the HT unit shift the conformer equilibria towards the E-alpha conformers. At the same time, they appear to cause a favourable pre-orientation of the furyl unit that accelerates the conrotatory ring closure in the E-alpha -> C reaction. Benzo-annulation of the furyl unit has little effect on the observed dynamics. Overall, the results demonstrate how the excited-state dynamics and thereby the photoswitching properties of fulgides can be successfully tuned and improved by structural modifications at the chromophores.