It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move in one or two dimensions. In the present case, the host material is tetracene crystal in which triplet exciton diffusion is essentially two dimensional. The exciton caging is produced by introducing relatively high concentrations of a guest molecule that can reflect the triplet exciton rather than trap it; in the present case, 2,3-benzocarbazole (BC) is used. Polycrystalline mixtures of BC in tetracene were prepared in the mol fraction range 0%–50%. In tetracene, the mode of decay of the first excited singlet state is by fission into two neighboring triplet excitons that can undergo geminate recombination. The introduction of BC is found to increase the tetracene fluorescence lifetime at room temperature from 100 psec at 0% BC to 360 psec at 50% BC, while the lifetime at 77 °K is relatively constant. In addition, the characteristic increase in the tetracene fluorescence quantum efficiency with increasing excitation intensity is found to diminish as the BC concentration is increased. The lifetime and the relative fluorescence efficiency experiments are interpreted in part by invoking the concept of exciton caging, in which the geminate recombination of the original triplet exciton fission pair is enhanced by the presence of exciton reflectors that reduce the size of the domain in which the excitons can freely move. A computer simulation is made of the fission and fusion process in an ideal mixed crystal containing randomly distributed exciton reflectors, and the response of the model agrees well with the observed results.

The ionization potential (IP) and electron affinity (EA) of the isolated single dye molecule and a hypothetical isolated J-aggregated dimer are calculated as an energy difference between separately minimized ground and ionized states. Three quantum methods are employed: density functional theory (DFT) Gaussian03 B3LYP/6-311G** (++G**); DFT using Dmol(3); and a modification of CNDO/S, called CNDO/S-Deltazeta, which is developed for rapid calculation of the IP and EA. Results indicate that for the monomer, 1,1'-dimethyl-2,2'carbocyanine chloride, the vertical IP and EA are 6.2 +/- 0.1 and 1.90 +/- 0.05 eV, respectively. This is consistent with the threshold IP and EA predicted by the Yianoulis and Nelson "Statistical Model" of spectral sensitization. For the isolated J-aggregated dimer, whose configuration is consistent with being adsorbed on a dielectric substrate, the calculations predict a value of 5.2 +/- 0.2 and 2.35 +/- 0.05 eV for the IP and EA, respectively. Significant charge density is removed from the halide anion in the ionization process. The HOMO of the dye molecule is an MO associated with the halide anion. Calculation of the isolated entities is a necessary preliminary step in the study of the IP and EA of the adsorbed dye monomer and aggregate.

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AbstractsFurther developments of a recent semiempirical, variable effective charge MO theory for calculation of ionization potentials (IP) and electron affinities (EA) as energy differences between separately minimized ground and ionized states are reported. The method is extended to adiabatic as well as vertical IPS and EAS by including core repulsion and u bond compression energies in the total energy. The method is generalized to heteroatomic systems and is simplified by neglecting penetration integrals. As before, only two molecular parameters, the vertical IPS of benzene and naphthalene, are required to set the magnitude of the u changes associated with the polarization of the core during loss or gain of a ?r charge. Twenty-seven aromatic molecules are studied, including polyacenes, condensed ring compounds, nonbenzenoids with five and seven member rings, nonplanar molecules, and heteroatomics with N', as in pyridine, N+', as in pyrrole, and Of', as in furan. The results are within 0.2 eV of the photoelectron spectroscopic vertical IPS and the predicted vertical-adiabatic separation is consistent with the shape of the first band. The calculated EAS are within 0.2 eVof the observed values.The calculation is used to predict the IP and EA of the ionic photosensitizing cyanine dye, pinacyanol. The values obtained are consistent with the latest measured IP and EA of the adsorbed dye, corrected for surface and aggregation polarization effects.On presente des dtveloppements nouveaux d'une theorie de type MO semi-empirique avec des charges effectives variables pour calculer des potentiels d'ionisation (IP) et des affinites Clectroniques (EA) comme des difftrences d'tnergie entre les &tats fondamentaux et ionists, minimists stpartment. La mtthode est applicable aux IPS et EAS verticaux et adiabatiques grlce a I'inclusion de la rtpulsion des coeurs et de I'tnergie de compression de la liaison (r dans I'dnergie totale. Elle a Cte gentralisbe pour des systemes heteroatomiques et a e t t simplifite en ntgligeant les inttgrales de ptnttration. Seulement deux paramttres moltculaires, les IPS verticaux du benzene et du naphthalene, sont necessaires pour ttablir la grandeur des changements u associts a la polarisation du meur lors de la perte ou du gain d'une charge R. On a ttudit 27 molCcules aromatiques, y i n c h des polyact.nes, des composes annulaires condenses, des nonbenztnoides avec des anneaux de cinq et sept membres, des molecules non-planaires et des composts htteroatomiques avec N', mmme dans la pyridine, N'*, comme dans le pyrrole, et O", comme dans le furan. Les rtsultats different de moins de 0.2 eVdes IPS verticaux obtenus de la spectroscopie Clectronique. La stparation vertical-adiabatique est compatible avec la forme de la premiere bande. Les EAS calculees different de moins de 0.2 eV des valeurs observtes. Le calcul a t t e utilist pour prtdire I'IP et I'EA du colorant pinacyanole. Les valeurs obtenues sont en accord avec les valeurs mesurtes les plus recentes du colorant adsorbt, corrigtes pour tenir com...

The ground state configuration of the gas phase cationic dyes pinacyanol chloride and rhodamine B are optimized with HF/6-311 + G(2d,2p) method and basis set. B3PW91/6-311 + G(2df,2p) functional and basis set is used to calculate the Mulliken atom charge distribution, total molecular energy, the dipole moment, the vertical ionization potential, the adiabatic electron affinity and the lowest excited triplet state, the last three as an energy difference between separately calculated open shell and ground states. The triplet and extra electron states are optimized to find the relaxation energy. In the ground state optimization of both dyes the chloride anion migrates to a position near the center of the chromophore. For rhodamine B the benzoidal group turns perpendicular to the chromophore plane. For both dyes, the LUMO is mostly of π character associated with the aromatic part of the molecule containing the chromophore. The highest occupied MOs consist of three almost degenerate eigenvectors involving the chloride anion coordinated with σ electrons in the molecular framework. The fourth highest MO is of π character. For both molecules in the gas phase ionization process the chloride anion loses the significant fraction of electric charge. In electron capture, the excess charge goes mainly on the dye cation.