Prof. Michael Schmitt
Room 186 (first floor), Ugo Schiff Chemistry Dept., Polo Scientifico di Sesto Fiorentino
Published on-line at 05:27:47 PM on Wednesday, November 28th, 2012
Manipulation of electronically excited states
Manipulation of the energetics of the excited states: the interactions between chromophoric system and solvent molecules.
Electronically excited states are subject to shifts in energy upon solvation. In some cases even the energy order of some excited states is changed. We focus to well-defined interactions between the chromophoric system and the solvent molecules which manipulate the energetics of the excited states.
A very prominent example of close-lying electronically excited states which change their energy order is the La/Lb system of indole. It has long been known, that polar solvents stabilize the more polar La state with respect to the Lb state. For the isolated molecule, the Lb state is generally the more stable one. Even complexation with one water molecule is in some cases (depending on the size of energy gap and the relative polarity of the states) enough to draw the La state below the Lb. Examples for this behaviour are the 7-azaindole(H2O)n clusters. The lowest escited singlet state in the monomer ist the Lb, while even for the cluster with one water molecule, the La state absorbs below the Lb.
The range of energy gaps (ΔE = E(La) - E(Lb)) for monomer moieties with the indole chromophore is enourmous. They range from more the +4000 cm-1 for methoxyindole over a few hundred cm-1 for tryptamine and indole to about -1000 cm-1 for tetrahydrocarbazole. A special case is 2,3-dimethylindole. Here, the energetic ordering of the La and Lb states is subject of a long-lasting debate. It is now commonly accepted, that in the gas phase, the La origin lies 342 cm-1 above the Lb origin. However, the extreme dependence of the relative positions of La and Lb states in 2,3-dimethylindole on the local surrounding is illustrated by the fact, that even in a very non-polar media, the energetic order of La and Lb states is interchanged, as determined via fluorescence anisotropy of 2,3-dimethylindole in an argon matrix, and in cyclohexane solution, where the La state is found to be shifted below the Lb state.
The relative position of these states influences considerably the photophysics and dynamics of the chromophores, since potentially repulsive states, like the pi sigma* states are differently influenced by microsolvation or the local surrounding. The different couplings of excited states and energetic positions of conical intersections between them and also between excited states and the ground state lead to strongly varying magnitudes and orientations of transition dipoles and of the excited state life times. The determination of the electronic nature of various indole derivatives, using rotationally resolved spectroscopy will be discussed.
For further informations, please contact Dr. Maurizio Becucci.
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