Evidence for significant through-space and through-bond electronic coupling in the 1,4-diphenylcyclohexane-1,4-diyl radical cation gained by absorption spectroscopy and DFT calculations.

Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.

Spectroscopic and density functional theory studies of the molecular geometry and electronic structure of classical and nonclassical radical ions derived from 7-benzhydrylidenenorbornene analogues.

A spectroscopic study, using nanosecond time-resolved laser flash photolysis and gamma-irradiation of low-temperature matrices, was undertaken along with a theoretical study using density functional theory (DFT) and time-dependent (TD)-DFT calculations to gain insight into the molecular geometry and electronic structure of radical cations and radical anions of 7-benzhydrylidenenorbornene (4) and its derivatives 6-8. The radical ions 4(.+), 6(.+), 7(.+), 8(.+), 4(.-), 6(.-), 7(.-), and 8(.-) exhibited clear absorption bands in the 350-800 nm region, which were reproduced successfully from the electronic transitions calculated with TD-UB3LYP/cc-pVDZ. Radical cations 4(.+) and 8(.+) are consistent with a bent structure having a delocalized electronic state where the spin and charge are delocalized not only in the benzhydrylidene subunit but also in the residual subunit. In contrast, 6(.+) and 7(.+) have nonbent structures with a localized electronic state where their spin and charge are localized in the benzhydrylidene subunit only. Therefore, 4(.+) and 89(.+) have a nonclassical nature, with 6(.+) and 7(.+) possessing a classical nature. In contrast, in the radical anion system, 7(.-) and 8(.-) are considered nonclassical, and 4(.-) and 6(.-) are classical. Orbital interaction theory and DFT calculations can account fully for the spectroscopic features, molecular geometries, and electronic structures of the radical ions. For example, the shift of the absorption bands and the nonclassical nature of 4(.+) are due to the antibonding character of the highest occupied molecular orbital (HOMO) of 4, and those of 7(.-) arise from the bonding character of the lowest unoccupied molecular orbital (LUMO) of 7. A topological agreement of p-orbitals at C-2, C-3 (or C-5, C-6), and C-7 produces strong electronic coupling with an antibonding or a bonding character in the frontier orbitals. It is the ethylene and butadiene skeleton at C-2-C-3 (or C-5-C-6), with its contrasting topology in the HOMO and LUMO of the neutral precursor, that holds the key to deducing the nonclassical nature of the 7-benzhydrylidenenorbornene-type radical cation and radical anion systems.

Thermoluminescence and a new organic light-emitting diode (OLED) based on triplet-triplet fluorescence of the trimethylenemethane (TMM) biradical.

The results of an investigation of the thermoluminescence (TL) and electroluminescence (EL) of arylated methylenecyclopropanes 1, systems whose photoinduced electron-transfer (PET) chemistry has been thoroughly studied, are described. In both the TL and EL experiments with 1, electronically excited triplet trimethylenemethane (TMM) biradicals (3)2** are generated by back electron transfer (charge recombination) of a TMM radical cation (hole) 2*+, formed by isomerization of the substrate radical cation (hole, 1*+). The application of this chemistry to the design of new organic light-emitting diodes (OLEDs) is described. The mechanistic features of this reaction system have the potential of overcoming significant problems (e.g., quantum efficiency, difficulty obtaining long wavelength emission, and device durability) normally associated with OLEDs that rely on the use of organic closed-shell hydrocarbons.

Substituent effects on the energies of the electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations. Experimental and theoretical evidence for a twisted molecular and localized electronic structure.

Substituent effects on the energies (Eob) of electronic transitions of geminally diphenyl-substituted trimethylenemethane (TMM) radical cations 5a-k*+ and those of structurally related 1,1-diarylethyl cations 7a-k+ were determined experimentally by using electronic transition spectroscopy. In addition, transition energies of these radical cations were determined by using density functional theory (DFT) and time-dependent (TD)-DFT calculations. The electronic transition bands of 5a-k*+ and 7a-k+ have maxima (lambdaob) that appear at 500-432 and 472-422 nm, respectively. A Hammett treatment made by plotting the Eob values relative to that of the diphenyl-TMM radical cation 5d*+ (DeltaEob) vs the cationic substituent parameter sigma+ give a favorable correlation with a boundary point at sigma+ = 0.00 and a positive rho for sigma+ < 0 and a negative rho for sigma+ > 0. A comparison of the lambdaob and rho values for 5a-k*+ and 7a-k+ suggests that the chromophore of 5*+ is substantially the same as that of 7+. The results of TD-DFT calculations, which reproduce the experimental electronic transition spectra and relationships between DeltaEob and sigma+, and suggest that the absorption band of 5*+ is associated with the SOMO-X --> SOMO transition, while that of 7+ is due to the HOMO --> LUMO transition. Another interesting observation is that Cl and Br substituents in the diphenyl-substituted TMM radical cations and 1,1-diarylethyl cations 7a-k+ act as electron-donating groups in terms of their effect on the corresponding electronic transitions. The results show that the molecular structure of 5*+ is a considerably twisted and that 5*+ has a substantially localized electronic state in which the positive charge and odd electron are localized in the respective diarylmethyl and the allyl moieties.

An evolved explanation for the molecular geometry and electronic structure of diphenyl-substituted cyclic trimethylenemethane in the ground state: a nearly planar conformation with a considerably localized electronic state.

[structures: see text] We reinvestigated the molecular geometry and electronic structure of the diphenyl-substituted, five-membered cyclic trimethylenemethane (TMM) diradical (Berson's TMM, 3**) using UV/VIS absorption and emission spectroscopy combined with density functional theory (DFT) and time-dependent (TD)-DFT calculations. Two intense absorption bands, A and B, with lambda(ab) at 298 and 328 nm, respectively, a weak absorption band C, with lambda(ab) at 472 nm, and an intense emission band D, with lambda(em) at 491 nm, were observed for 3**. By comparing the spectrum of 3** with those of the 1,1-diphenylethyl (7*) and cyclopent-2-en-1-yl (9*) radicals, it was found that bands B, C, and D originated from the diphenylmethyl radical moiety (subunit I), while band A should most likely be assigned to an electronic transition related to an interaction between subunit I and residual subunit II, the cyclopentenyl radical moiety. An UB3LYP/cc-pVDZ calculation indicated that, in the ground state, the two unpaired electrons of 3** are mainly localized in subunits I and II, respectively, and the interaction between them is inefficient, despite the nearly planar conformation (theta = +23.5 degrees). Furthermore, a TD-UB3LYP/cc-pVDZ calculation suggested that absorption band A is assigned to an electronic transition involved with enhancement of the electron density of the C-2-C-3 bond. Substituent effects on the absorption and emission spectra of 3** using 11** and 13** support the conclusion based on the experiments and calculations. Therefore, we propose an evolved explanation for the molecular geometry and electronic structure of the ground state of 3** in a low-temperature matrix, a nearly planar conformation with a considerably localized electronic state, which alone accounts for the spectroscopic characteristics.