J. Prochorow

Equilibration of an excited charge-transfer state of molecular electron-donor—acceptor systems in liquid solution

Abstract It is demonstrated that in the molecular electron-donor—acceptor system of sym-tetracyanobenzene with para-xylene, both ground-state and stable excited-state complexes can be formed. Measurements of the decay times of fluorescence and investigation of the influence of solvent polarity on the fluorescence spectra have revealed the identity of fluorescent states of complexes of both kinds.

Radiative and radiationless processes in charge-transfer complexes

Abstract The measurements of absorption and fluorescence spectra, fluorescence quantum yields and decay times for charge-transfer complexes of tetrachlorophthalic anhydride-hexamethylbenzene and pyromellitic dianhydride-hexamethylbenzene, in different solvents at room temperature, were made. The results have been discussed in terms of changes of the radiative and radiationless transition probabilities.

Fluorescence and phosphorescence emission and nonradiative relaxation of acridine in a crystalline matrix of 2,3-dimethylnaphthalene

Abstract The results of experimental studies of fluorescence and phosphorescence of acridine, selectively excited within its absorption band, in a crystalline host lattice of 2,3-dimethylnaphthalene, are reported. The energies of the first excited singlet and triplet states of acridine in this crystalline matrix were determined. The spectra of both emissions display a broadening caused by the orientational disorder of the host lattice. The intensity and the decay time of the fluorescence and phosphorescence show moderate temperature dependence (in the 4.2–300 K temperature range). It is concluded that over the whole temperature range the most important mode of the nonradiative relaxation of the excited acridine molecule is that of the (nearly) temperature-independent intersystem crossing from the S 1 (π, π ∗ ) singlet to the T 3 (π, π ∗ ) and/or T 2 ( n , π ∗ ) triplet states.

Optical emission and ODMR of the biphenyl-TCNB 1:1 CT complex in single crystal☆

Abstract Optical emission and high-field ODMR spectra of biphenyl-1,2,4,5-tetracyanobenzene crystal have been investigated in a wide range of temperatures (4.2–300 K). At room temperature the triplet-triplet annihilation dominates the exciton decay accompanied with a thermally activated energy transfer back from triplet to the excited singlet. On cooling an efficient trapping of the excitons (X traps) occurs and the ODMR spectra are attributed to the annihilation between excitons and traps. At 4.2 K the optical emission and the ODMR signal monitor the phosphorescence from the X traps.

Fluorescence excitation and fluorescence spectra of jet-cooled acridine molecules: acridine dimer formation and structure

Abstract The fluorescence excitation and fluorescence spectra from a supersonic helium jet seeded with acridine molecules have been obtained. The presence of low-frequency vibrational transitions in the fluorescence excitation spectrum, which can be assigned as intermolecular vibrational modes, clearly indicates that acridine dimers are formed under jet-cooling conditions. The acridine dimer ground-state geometry determined by AM1 calculations is a head-to-tail-like structure with an almost collinear arrangement of the short molecular axes of both molecular components. The results of calculations of spectral characteristics predict a large enhancement of the oscillator strength for the transition from the ground to first excited singlet state of the dimer (as compared to acridine molecule). Such enhancement is entirely connected with structure of the dimer (which presumably is also stabilized by the formation of a hydrogen bond).

The analysis of excited-state equilibria of weak electron donor–acceptor systems:: Tetracyanobenzene–toluene complex

Abstract The electron donor–acceptor system of 1,2,4,5-tetracyanobenzene–toluene is a prime example of a weak charge-transfer complex which is stable in the ground state and which is also capable of forming an exciplex in diffusion-controlled reaction in liquid solution (in non- or low-polarity solvents). The potential energy surfaces calculated for the ground and excited state of this system reveal the presence of different orientational isomers of the isolated complex (of charge-transfer and of van der Waals character). An extended photokinetic scheme which includes excitation of these isomers as well as the diffusion-controlled formation of an exciplex has been proposed and analyzed. Computer simulations of the time profiles of emission of the investigated system performed within the framework of such a photokinetic scheme were in satisfactory agreement with experimental observations. This leads to the conclusion about the existence of different Franck–Condon excited states of the TCNB–toluene system in liquid solutions with relaxation pathways strongly controlled by the physical conditions of the solution.

Heavy-atom effect on the phosphorescence of charge-transfer complexes

Abstract Fluorescence and phosphorescence spectra, decay times of phosphorescence and quantum-yield ratio of phosphorescence to fluorescence, for a series of charge-transfer complexes of hexamethylbenzene (an electron donor) with phthalic anhydride and its monohalo- and tetrahalo-derivatives, have been measured at 90 K. It is shown that changes of decay times and quantum-yield ratio of phosphorescence to fluorescence are due to the heavy-atom effect of the charge-transfer phosphorescence.

Migration of electronic excitation energy in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene

Abstract Singlet and triplet exciton energy migration in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene, undoped and doped with anthracene (10 −4 −10 −2 M/M), have been studied in the 5–300 K temperature range. At low temperatures singlet and triplet excitons are self-trapped. They become mobile with increasing temperature with activation energies of 380 ± 70 and 400 ± 80 cm −1 for singlet and triplet excitons, respectively. The motion of triplet excitons in the temperature range 50–100 K can be described by one-dimensional hopping within cages, limited by structural defects of the host crystal. Mobile singlet excitons are observed for temperatures above 100 K and their motion is well described by a time-independent excitation transfer rate constant.

On the role of excitation energy in the formation of a molecular exciplex under supersonic jet conditions

Abstract Molecular exciplex formation under supersonic jet conditions is studied as a transition from the excited state of the van der Waals complex to a charge-transfer state. For a typical model exciplex system the role of excess excitation energy in the formation process is investigated.

Heavy-atom effect on radiative and radiationless transitions in charge-transfer complexes

Abstract The quantum yields of fluorescence and phosphorescence and decay times of fluorescence were measured for a series of charge-transfer complexes of hexamethylbenzene (an electron donor) with various electron acceptors containing heavy (halogen) atoms and the rate constants of different radiative and radiationless transitions were determined. It was found that radiative T1 → S0 transition, i.e. the phosphorescence, is strongly enhanced by the heavy-atom effect. The mechanism of the heavy-atom enhancement of charge-transfer phosphorescence is discussed and it is concluded that increasing spin- orbit mixing of radiative T1 → S0 charge-transfer transition with singlet-singlet, locally excited transition of acceptor molecule is responsible for the observed effects.