Ultrafast Photoinduced Electron Transfer in Spiro-type Donor- Acceptor Systems

Perpendicular orientation of the electron donor in relation to the acceptor in the ground state seems to contribute significantly to high rate constants of the photoinduced intramolecular electron transfer (PET) in electron donor-acceptor (DA) type molecules [1]. A very interesting class of molecules in this respect are systems build around a spiro-type carbon atom, separating mutually orthogonal donor and acceptor; such as lactone forms of rhodamines and leuco forms of triphenylene dyes [2]. Due to a weak coupling between structural components, the UV absorption spectrum of the latter systems corresponds essentially to the sum of the spectra of individual subunits. Recently it has been found that malachite green lactone (MGL), a leuco form of the well-known triphenylene dye undergoes ultrafast (< 130 fs) charge separation from dimethylaniline (DMA) donor to phthalide (Pd) acceptor and a highly polar CT state charge transfer state (intramolecular “ion pair”, dipole moment 25 D) is populated [3]. Substitution of the phthalide moiety in MGL with dimethylamino group (crystal violet lactone, CVL) creates a new chromophore (dimethylaminophthalide, DMAPd) which has its own D-A structure [4]. The lowest excited state of CVL reached upon excitation is a CT state localised on the DMAPd unit but under sufficiently high solvent polarity CVL relaxes to a lower lying CT state with the electronic structure resembling that of the highly polar CT state in MGL. The relaxation to the two CT states is manifested by dual luminescence displayed by CVL in highly polar solvents [5]. O