Photoinduced electron transfer in malachite green lactone

Abstract

The spectroscopy and photophysics of malachite green lactone (MGL), a lactonic form of the well-know malachite green dye, have been studied as functions of solvent polarity in aprotic and protic solvents at different temperatures in solution and in glass. It has been found that MGL photophysics substantially differs from that of other malachite green leucoderivatives (e.g. leuconitrile, leucohydroxide or halides). In aprotic solvents ultrafast intramolecular electron transfer (estimated τ_ET ≤ 130 fs) between MGL structural components (from initially excited insulated dimethylaniline, DMA, to phthalide, Pd) results in formation of a highly polar (25.0 D) intramolecular CT state which then emits fluorescence. The dynamics of the primary ET are determined mainly by intramolecular vibrational motions and not by the solvation process. The polar nature of the emitting state was verified with the solvatochromic method, and S_n ← S₁ transient absorption spectra prove that charge separation results in formation of DMA radical cation in MGL. The separation of charges is maintained in the charge transfer triplet state, which lies below the local triplet levels of MGL structural components, making MGL a unique candidate for studying both CT triplet state and CT-singlet–triplet dynamics. The electronic structure of the ¹CT state stabilizes charge separation in moderately polar solvents (e.g.Φ_fl and τ_fl in butyl acetate (BA) are 0.12 and 23.8 ns, respectively) and no evidence has been found for photodissociation of C–O bond in lactone ring in aprotic solvents, which is the dominant photoprocess in malachite green leuconitrile or halides. Further increase of solvent polarity results in dramatic enhancement of MGL nonradiative deactivation from the ¹CT state (increase in k_nr by two orders of magnitude on going from BA to ACN) pointing to a solvent polarity-driven deactivation channel. The strong dependence of CT fluorescence quantum yield on CT transition energy found in supercooled and in glassy butyronitrile (BTN), where conformational motions are restricted, suggests that the nonradiative decay is (i) not related to conformational changes and (ii) consists in enhancement of nonradiative return electron transfer (direct radiationless charge recombination), which is proven by the good linear correlation between ln k_nr and _fl in more polar solvents (energy gap law). The large temperature-dependent blue shift of the fluorescence maximum in BTN below the melting point (rigidochromism) makes MGL a very sensitive probe for studying reorientation polarization in rigid and semi-rigid supercooled media.