The Key Role of Solvation Dynamics in Intramolecular Electron Transfer: Time-Resolved Photophysics of Crystal Violet Lactone

Abstract

The intramolecular electron-transfer reaction in crystal violet lactone in polar aprotic solvents is studied with femtosecond transient absorption spectroscopy. The initially excited charge transfer state (1)CT A is rapidly converted into a highly polar charge transfer state (1)CT B. This ultrafast electron transfer is seen as a solvent-dependent dual fluorescence in steady-state spectra. We find that the electron-transfer process can be followed by a change from a double-peaked transient absorption spectrum to a single-peak one in the low picosecond range. The transient absorption kinetic curves are multiexponential, and the fitted time constants are solvent dependent but do not reproduce the known solvation times. For 6-dimethylaminophthalide, the optically active constituent of crystal violet lactone, only a small temporal evolution of the spectra is found. To explain these findings, we present a model that invokes a time-dependent electron-transfer rate. The rate is determined by the instantaneous separation of the two charge-transfer states. Because of their differing dipole moments, they are dynamically lowered to a different extent by the solvation. When they temporarily become isoenergetic, equal forward and backward transfer rates are reached. The intrinsic electron-transfer ( (1)CT A --> (1)CT B) reaction is probably as fast as that in the structurally analogous malachite green lactone (on the 100 fs time scale). The key element for the dynamics is therefore its control by the solvent, which changes the relative energetics of the two states during the solvation process. With further stabilization of the more polar state, the final equilibrium in state population is reached.

Tuneable white fluorescence from intramolecular exciplexes

“…17 The solvent polarity dynamically determines the relative energetics of both CT states and controls the availability of the ET process and various deactivation pathways, thus effectively shaping the fluorescence spectrum displayed by CVL. 20 The ultrafast photoinduced ET process reversibly converting 1 CT A to 1 CT B occurs when the two states become temporarily isoenergetic which happens when the medium surrounding the molecule provides the energy stabilization required for the 1 CT B state down to the 1 CT A level, i.e., in solvents above certain threshold dielectric constant. The energies of the 1 CT B state in different media are essentially equal under isopolar conditions, and the same holds for 1 CT A .…”

“…3), is of essential importance for the threshold of medium dielectric constant required for appearance of the 1 CT B fluorescence, and conclusively, for the shape of the fluorescence spectrum displayed by CVL-like molecules, as lower DE requires lower e to stabilize 1 CT B to the energy equal to that of 1 CT A . 17,[19][20] DE can be changed by changing either the energy of 1 CT A or that of 1 CT B . The energy of the S 1 state in aminophthalides (i.e., of the 1 CT A state in a CVL-like structure) can be easily changed by modifying the substituent pattern at the amino nitrogen; taking 6-DMAPd as a reference, it is higher in 6-methylamino-and 6-aminophthalide and lower in aminophthalides with the amino group linked to an aromatic moiety by saturated six-membered rings.…”

A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

“…Such a solvent-controlled electron transfer has been documented in crystal violet lactone. [82] We believe that the radical pair state is best described as metastable. Once the ion pair state is accessed it is immediately lowered sufficiently that back electron transfer is highly unlikely.…”

New frontiers in ultrafast broadband pump-probe spectroscopy for the investigation of electron transfer processes

“…The new frontier in ultrafast pump-probe spectroscopy is to extend the white light probe further into the UV and into the near IR as well as the kHz broadband detection of these continua with diode arrays or CCDs. These advancements give the spectroscopist broader and more unambiguous information about the dynamics of the system and hence aid in the interpretation of the complex mechanisms behind ultrafast chemical and physical processes [1,2]. In a first set of experiments on a flavine dissolved in methoxybenzyl alcohol we show the importance of continua ranging further into the UV and of the multichannel detection.…”