Localized states and electronic transport in single component organic solids with diagonal disorder

The optical absorption spectra of the first excited singlet states of the benzene, toluene, and toluene–benzene dimers, created in a supersonic molecular jet, are reported. The absorption spectra are detected through two‐color time of flight mass spectroscopy; this method eliminates fragmentation of dimers and higher clusters and the dimer spectra are uniquely observed. The benzene dimer observed in this experiment is suggested to have a parallel stacked and displaced configuration of C2h symmetry. Both the toluene and toluene–benzene dimers have two configurations: parallel stacked and displaced [based on (benzene)2] and perpendicular. (Benzene)2, (toluene)2, and toluene–benzene form excimers in the excited state for the parallel stacked displaced configurations. The transformation of (benzene)2 to the excimer takes place at the 00 with a ∼0 cm−1 barrier while the excimer is formed for toluene–benzene with a ∼900 cm−1 barrier. An exciton analysis of the (benzene)2 000 and 610 yields M12, the excitation e...

Dimers of aromatic molecules: (Benzene)2, (toluene)2, and benzene–toluene

An experimental study is presented that leads to the elucidation of the whole band structure for the triplet exciton state of 1,4‐dibromonaphthalene. We show that the band states are essentially those of a linear chain characterized by nearest neighbor interactions along the c crystallographic axis. Although the crystal is topologically a three‐dimensional network, for the practical purpose of energy transfer and trapping the crystal behaves as a set of linear chains. Heavy doping of DBN‐h6 (host) with up to 18% DBN‐d6 (guest) yielded the expected discrete spectra of a random linear array corresponding to a nearest neighbor interaction of 7.4± 0.1 cm−1 and total bandwidth of 29.6± 0.4  cm−1 for the zero‐zero transition. A symmetric mode at 0+520 cm−1 was shown to have an exciton bandwidth of 15 cm−1, and an unsymmetric vibrational level at 0+250  cm−1 was shown to have a vanishing bandwidth in accordance with expectations from weak coupling theory. The quasiresonance interactions of host and guest were significant and were included in establishing the agreement between experiment and calculation. The total bandwidth is estimated to be 90± 20  cm−1. This type of doping has an effect on line shape even at very low concentration and even in undoped crystals the line shape is asymmetrical, perhaps due to impurities and imperfections that have small zero‐order shifts from the band center. The intensity of transitions to the carbon‐13 isotopic traps is shown to be distributed over the band states in the origin and is clearly visible in the narrow band nontotally symmetric vibrational state. The vibrational analysis of the crystal spectrum (electronic or vibrational states) is shown to be dependent on exciton effects even in the case where no Davydov splitting is observed. Spectra of DBN‐h6 (guest, H) in DBN‐d6 (host, D) were also studied and resonance multiplets were seen and identified in absorption and emission. We identified ··· DHD···,  ··· DHHD···,  ··· DHHHD···, and probably ··· DHDHD··· in the phosphorescence spectra at 2°K. We present a brief discussion of the effect of dimensionality on the dynamical features of energy transfer and trapping, and we show experimentally, that heavy doping (d6 in h6) enhances the phosphorescence of the crystal about 60‐fold. In addition the existence at 2°K of a Boltzmann distribution over resonance multiplets leads us to propose a long‐range trap‐to‐trap migration as part of the thermalizing process.

Exciton Band Structure and Properties of a Real Linear Chain in a Molecular Crystal

We explore the relationship between the electronic-nuclear level structure, the electronic couplings, and the dynamics of hole hopping transport in DNA. We utilized the electronic coupling matrix elements for hole transfer between nearest-neighbor nucleobases in DNA (Voityuk, A. A.; Jortner, J.; Bixon, M.; Rosch, N.J. Chem. Phys. 2001, 114, 5614) to evaluate intrastrand and interstrand superexchange electronic couplings, which determine hole hopping rates within the framework of a semiempirical quantum mechanical-kinetic model. Calculations of the exponential distance (R) dependence of the superexchange mediated intrastrand electronic couplings jVsuperj 2 ∝ exp(-‚R) between guanines (G) in "short" G + (T-A)n G( n j 3) duplexes result in ‚ ) 0.8-0.9 A -1 . We interpret the experimental data on time-resolved hole transport in the presence of a site-specifically bound methyl transferase mutant in DNA (Wagenknecht, H.-A.; Rajski, S. R.; Pascally, M.; Stemp, E. D. A.; Barton, J. K. J. Am. Chem. Soc. 2001, 123, 4400) in terms of composite sequential, interstrand and intrastrand superexchange mediated, and direct interstrand hole hopping. This mechanism accounts for the rate determining step, for the weak duplex size dependence of the rate, and for the long- range charge transport induced by interstrand superexchange via short (T-A) bridges, containing a single mediating nucleobase. For hole transfer via longer (T-A)n (n J 3) bridges, the superexchange mechanism is replaced by the parallel mechanism of thermally induced hole hopping (TIH) via long (A)n chains. A kinetic analysis of the experimental data for hole transport through seven GG pairs separated by (T-A)n (n ) 2-5) bridges across the 3'-5' strand of the DNA duplex (Sartor, V.; Boone, E.; Schuster, G. B. J. Phys. Chem. B, 2001, 105, 11057) reveals that the superexchange-TIH crossover occurs at n ) nx ) 3. The explorations of the range of applicability and the breakdown of the superexchange mechanism in DNA lay the foundations for the scrutiny of the universality and system specificity of this mechanism in large-scale chemical and biophysical systems.

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Superexchange Mediated Charge Hopping in DNA

Isotopically mixed jets of benzene produced the various possible isotopic benzene dimers in a supersonic jet. Mass coincidence in time‐of‐flight multiphoton mass spectra separated the spectra due to the various dimers produced into distinguishable spectra even in the presence of overlap. From the many different isotopic spectral shifts a detailed model of interaction between the two halves of the dimer can be established revealing detailed new information on this interaction. In particular a splitting in the 0–0 transition is reported for the first time indicating an exciton interaction in the dimer of isotropic identical benzene molecules in the gas phase.

Spectra of isotopically mixed benzene dimers: Details on the interaction in the vdW bond

The phosphorescence, fluorescence, and absorption spectra of the isotopic benzenes C6H6, C6H5D, p‐C6H4D2, and sym‐C6H3D3, present as dilute guests in a C6D6 host crystal at 4.2°K, are obtained with sufficient spectral resolution to ascertain the magnitude of the crystalline site effects. The relative vibronic intensities in fluorescence and phosphorescence are compared. Two site effects are emphasized: site splitting of degenerate fundamentals and orientational effects. The former can occur for the isotopes C6H6 and sym‐C6H3D3, while the latter is possible only for isotopes with less than a molecular threefold rotation axis. The observations show that both site‐splitting and orientational effects do occur as a general rule on vibronic and vibrational states in benzene isotopic mixed crystals. An empirical correlation between the magnitudes of the site splitting, orientation effect and site (gas‐to‐crystal) shifts for in‐plane and out‐of‐plane modes is noted. The phosphorescence of C6H6 and sym‐C6H3D3 has ...

Static Crystal Effects on the Vibronic Structure of the Phosphorescence, Fluorescence, and Absorption Spectra of Benzene Isotopic Mixed Crystals

The purpose of this paper is to lay a consistent theoretical framework for the discussion of a series of forthcoming experimental papers on the absorption and emission of light by benzene crystals. Emphasis will be placed not only on the usual Davydov splittings but also on k≠O states and band‐to‐band transitions in neat (pure) crystals, and on orientational effects, site splittings, shifts, and resonance pair spectra in isotopic mixed crystals. The role of translationally equivalent interactions will be discussed. The interchange‐group concept is used in order to simplify the theoretical analysis. From the four possible interchange groups D2, C2υa, C2υb, C2υc, the D2 group is found to be the most convenient for the classification of benzene exciton functions. A differentiation between static and dynamic interactions is made in the limit of Frenkel excitons, and the concepts of site distortion energy and the ideal mixed crystal are introduced to aid in this distinction. Data pertaining to site shifts and ...

Electronic and Vibrational Exciton Structure in Crystalline Benzene

Perturbation shifts of guest levels in isotopic mixed crystals of naphthalene have been determined. These data verify that the density function for exciton states is almost symmetric over the entire exciton band of the first singlet state. This result agrees with the entire band structure as observed in band‐to‐band transitions and disagrees with the band structure calculated from the octopole model for the inter‐molecular interaction. The transition energies of guest levels also disagree with a model for the exciton splitting involving configuration interaction with charge transfer states. A possible source of the exciton splitting is suggested, and the available data for evaluating theoretical descriptions are outlined.

Information on the Exciton Band Structure of the 1B2u State of Crystalline Naphthalene from the Variation of Energy Denominators Method

Three types of dipole selection rules are proved for k≠0 in a restricted Frenkel exciton limit. They are: (1) Δk=0; (2) general selection rules based on interchange symmetry; and (3) the g↔u selection rule for centrosymmetric crystals. Even though the latter two selection rules are generally valid only for special values of k, and in particular for k=0, they can be shown to be valid for all k in crystals where restrictions are placed on the magnitude of interactions between certain translationally equivalent molecules. For benzene and naphthalene, and probably for many other organic crystals, the translationally equivalent interactions that ruin the selection rules are nonnearest neighbors. Evidence suggests these in some cases are small, in which event the selection rules are excellent approximations. It is also shown that the transition matrix element in this restricted Frenkel limit is k‐independent, a fact that is important in analyzing the intensity distribution of band←band transitions in terms of the density‐of‐states function. Inability to detect a u←u, band←band transition in crystalline benzene is consistent with the theoretical results.

G. W. Robinson

Papers

Static Crystal Effects on the Vibronic Structure of the Phosphorescence, Fluorescence, and Absorption Spectra of Benzene Isotopic Mixed Crystals

Electronic and Vibrational Exciton Structure in Crystalline Benzene

Information on the Exciton Band Structure of the 1B2u State of Crystalline Naphthalene from the Variation of Energy Denominators Method

Frenkel Exciton Selection Rules for k≠0 Transitions in Molecular Crystals

Erratum: Frenkel Exciton Selection Rules for k≠0 Transitions