On low temperature fluorescence of perylene crystals

At low temperatures, the broad excimer fluorescence band of α-perylene crystals is replaced by a weakly structured emission at higher energy. This emission originates from a new crystal state (Y-state) which is populated independently of the high temperature excimer (E-state). Due to the temperature dependence of its first order decay rate and due to the thermally activated formation of the E-state, the Y-emission grows rapidly at temperatures below 90 K. The Y-emission differs from the fluorescence of the monomeric β-perylene at 5.5 K by its Stokes shift of 1300 cm^?1, the lack of vibronic structure, the long first order decay time of 40 ns and the absence of bimolecular annihilation indicating a localized state. The Y-state is attributed to a less relaxed pair state formed upon contraction of the dimeric crystal lattice.     

Magnetic field effects on the recombination of radical ions in reaction centers of photosynthetic bacteria

The amount of triplet products formed upon recombination of the bacteriochlorophyll dimer cation and the bacteriopheophytin anion in modified bacterial reaction centres can be manipulated by external magnetic fields. Making use of this effect, two methods provide information about structural and dynamic properties of the reaction centre. The two methods differ in that one (MARY) uses low static magnetic fields (0 ? B₀ < 1 kG) whereas the other (RYDMR) uses microwaves and high static fields (B₀ > 1 kG). As is shown here the two methods are equivalent from a theoretical point of view, the interpretation of experimental spectra being equally involved in both cases.     

Magnetic field and temperature dependencies shed light on the recombination kinetics of a transition metal donor/acceptor system

The radical pair recombination of an intramolecular electron-transfer system containing a transition metal moiety has been addressed by femtosecond spectroscopy. The radical pair is formed by ultrafast electron transfer (90 fs) from a ferrocene residue to a photoexcited Nile blue moiety. Its recombination proceeds on the picosecond time scale in a multiexponential fashion. The kinetic pattern is a manifestation of spin processes competing with electron transfer. Magnetic field effects on these kinetics allow one to disentangle the two contributions. Their temperature dependencies yield the activation parameters of the two processes. The discussion focuses on the mechanism of electron spin relaxation. Strong evidence for the Orbach/Kivelson mechanism will be given.     

Distance-dependent activation energies for hole injection from protonated 9-amino-6-chloro-2-methoxyacridine into duplex DNA

The recent investigation of the apparently anomalous attenuation factor (beta > 1.5 A(-1)) for photoinduced hole injection into DNA duplexes modified by protonated 9-amino-6-chloro-2-methoxyacridine (X+) led to the conclusion that in addition to the electronic couplings, the activation energy must also be distance-dependent. In this communication we report the verification of this postulate by direct measurements of the activation energies for a series of (X+)-modified DNA duplexes which sample an appreciable range of donor-acceptor distances (approximately 4-11 A). The resulting changes in thermal activation energy can be explained within the framework of a distance-dependent reorganization energy.     

Pair-states in α-perylene crystal. A theoretical study

The low- and high-temperature emissions from α-perylene are interpreted as originating in, respectively, loosely- and tightly-bound pair complexes. The change-over from one to the other results from thermal expansion and deformation of the lattice and the resulting change in contact distances between the complexing pair and the surrounding molecules. With this model the binding energies, Stokes shifts, lifetimes, and emissive bandwidths associated with the complexes are estimated, as is the activation energy for excimer-exciton migration; the values agree well with the available experimental data.     

Singlet fission in rubrene single crystal: direct observation by femtosecond pump-probe spectroscopy

The excited state dynamics of rubrene in solution and in the single crystal were studied by femtosecond pump-probe spectroscopy under various excitation conditions. Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution. Upon 500 nm excitation, triplet excitons form on the picosecond time scale via fission from the lowest excited singlet state. Upon 250 nm excitation, fission from upper excited singlet states is observed within 200 fs.     

Exciton fission in excimer-forming crystal dynamics of excimer build-up

The optically induced fission of singlet excitons in both α- and β-perylene crystals is investigated at room temperature. In the monomeric β-perylene the fission thresold coincides with the energy of two triplet excitons whereas it is blueshifted by ～3500 cm^-1 in the excimer forming α-crystal. This indicates that the excimer is formed prior to the fission of the initially excited monomer state and it implies that the rate of excimer formation is exceeding 10¹² s^-1. Theoretical estimates led to k_exc ≈ 10¹²-10¹³ s^-1. Analysis of the experimental data shows also that two adjacent perylene molecules in the α-crystal, each of them in the T₁ electronic state, are bound with the binding energy B_2T1 ≈ 350 cm^-1.