Speaker
Description
In a network of chromophores (i.e., light-absorbers) such as that in photosynthetic proteins, the chromophores interact strongly at short distances where orbital overlap is significant, and new quantum states arise after photoexcitation. These states are no longer associated with one chromophore but spatially delocalized over two or more chromophores. On the other hand, when the chromophores are more separated and the electronic coupling weaker, we can to a good approximation talk about a “locally” excited chromophore that can transfer its energy to a nearby chromophore in a process called FRET (Förster Resonance Energy Transfer). In the case of ionic chromophores, there is an additional form of communication that occurs even in the ground state when the electric field from one chromophore is strong enough to polarize another and vice versa. This interaction affects the transition energy of the chromophore (i.e., Stark shift) and as a result has an impact on the exciton states and FRET. To reveal the maximum effect of a charged chromophore on another charged one, it is advantageous to study the systems isolated in vacuo where there are no solvent molecules to screen the charges, an extreme case so to speak. Our approach is gas-phase fluorescence spectroscopy based on the custom-made LUNA/2 (LUminescence iNstrument in Aarhus) setups where ions produced by electrospray ionization (ESI) are mass-selected (and cooled in LUNA2) and photoexcited in a cylindrical Paul trap followed by collection of emitted photons. We have chosen systems composed of rhodamine dyes for several reasons: They are strongly fluorescent, each dye carries a positive charge and has no dipole moment along its long axis in the absence of electric fields, and they are easily formed by ESI. Recent results on dyads and triads will be presented.
