Even flow: Photoinduced symmetry-breaking charge separation takes place in a few picoseconds in a 1,3-bis(perylene)propane dyad in polar solvents. Polarized transient absorption measurements show that the direction of the charge flow is random and entirely governed by the fluctuations of the solvent orientation around the dyad

The excited-state dynamics of covalently linked electron donor-acceptor systems consisting of N,Ndimethylaniline (DMA) as electron donor and either perylene (Pe) or cyanoperylene (CNPe) as acceptor has been investigated in a large variety of solvents, including a room-temperature ionic liquid, by using femtosecond time-resolved fluorescence and absorption spectroscopy. The negligibly small solvent dependence of the absorption spectrum of both compounds and the strong solvatochromism of the fluorescence are interpreted by a model where optical excitation results in the population of a locally excited state (LES) and emission takes place from a charge-separated state (CSS). This interpretation is supported by the fluorescence upconversion and the transient absorption measurements that reveal substantial spectral dynamics in polar solvents only, occurring on time scales going from a few hundreds of femtoseconds in acetonitrile to several tens of picoseconds in the ionic liquid. The early transient absorption spectra are similar to those found in nonpolar solvents and are ascribed to the LES absorption. The late spectra due to CSS absorption show bands that are red-shifted relative to those of the radical anion of the acceptor moiety by an amount that depends on solvent polarity, pointing to partial charge separation. Global analysis of the time-resolved data indicates that the charge separation dynamics in PeDMA is essentially solvent controlled, whereas that in CNPeDMA is faster than diffusive solvation, this difference being accounted for by a larger driving force for charge separation in the latter. On the other hand, the CSS lifetime of PeDMA is of the order of a few nanoseconds independently of the solvent, whereas that of CNPeDMA decreases with increasing solvent polarity from a few nanoseconds to a few hundreds of picoseconds. Comparison of these results with previously published data on the fluorescence quenching of Pe and CNPe in pure DMA shows that the charge separation and the ensuing charge recombination occur on similar time scales independently of whether these processes are intra-or intermolecular.

In polaren Lösungsmitteln kommt es in einer 1,3‐Bis(perylen)propan‐Diade binnen weniger Pikosekunden zu einer lichtinduzierten Ladungstrennung unter Symmetriebruch. Polarisierte Transienten‐Absorptionsmessungen zeigen, dass die Ladung dabei in zufälliger Richtung fließt, nur gesteuert durch Fluktuationen der Lösungsmittel‐Orientierung in der Umgebung der Diade.

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The primary physical and chemical processes which follow photoexcitation of the covalently linked systems, 1- [(4-azido-2,3,5,6-tetrafluorobenzoyloxy)methyl]pyrene (2) and 1-[3-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)propyl]pyrene (3), have been studied using femto-and nanosecond transient absorption spectroscopy and computational chemistry. Excitation of 2 and 3 at 336 nm results in the population of the second excited singlet state of the pyrene moiety (S 2 ). Internal conversion to the lowest excited singlet state of the pyrene moiety (S 1 ) occurs with a time constant of ∼140 fs, a value which is similar to that of unsubstituted pyrene. The S 1 local pyrene state, initially formed with excess vibrational energy, undergoes vibrational cooling with a time constant ∼2 ps. The decay of the local pyrene S 1 state measured in the pump-probe experiments was described by a two exponential function with time constants which agree well with the values obtained previously from the fluorescence decay kinetics (Barabanov, I. I.; Pritchina, E. A.; Takaya, T.; Gritsan, N. P. MendeleeV Commun. 2008, 18, 273). In both systems decay of the local pyrene S 1 state on the time scale of tens of picoseconds is accompanied by formation of a product with a narrow band at ∼460 nm. This transient absorption was assigned to the radical cation of the pyrene moiety. Calculations predict very fast dissociation of the counterpart aryl azide radical anion with formation of the corresponding arylnitrene radical anion. Decay of the local pyrene S 1 state on the longer time scale (hundreds of ps) is not accompanied by formation of a noticeable amount of pyrene radical cation. Most likely, both electron transfer and energy transfer from pyrene to aryl azide moiety are responsible for the photosensitization of the perfluorinated aryl azide decomposition. † Part of the "Hiroshi Masuhara Festschrift".