Electron Donor-Acceptor-Donor (DAD) molecular systems are of interest due to their two-photon absorption, solvatochromism and potential application as photoinitiators for polymerisation or fluorescent probes. We are currently investigating a series of quadrupolar DAD molecules that, in addition to being of technological interest, show large structural reorganisation in the excited state.
Here, we have used broadband fluorescence upconversion in order to probe the early excited-state dynamics of a DAD in an apolar and a polar solvent. Despite an excited-state lifetime of several hundreds of picoseconds, substantial changes are seen in polar solvent within a 30 ps time window.
In the apolar solvent, the fluorescence remains virtually unchanged after excitation (case a). However, in the polar solvent (case b) the fluorescence intensity decreases by around a factor of c. 1.5 within the first ten picoseconds, and then remains unchanged.
This change can be seen more easily if the fluorescence intensity vs time is plotted, as in case c. This shows that the fluorescence intensity decreases in c. 10 ps, pointing to a large reorganisation of the electronic distribution occurring in the excited state during that time.
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