LUMO Excitation Research Articles

Effects of Substituents and Solvents on the Electronic Spectra of 9,10-Dihydro-9,10-o-benzenoanthracene-1,4-diones: Intramolecular Charge Transfer

Abstract Electronic spectra of substituted 9,10-dihydro-9,10-o-benzenoanthracene-1,4-diones (triptycenequinones) in various solvents and that of benzo- and dibenzotriptycenequinones were investigated in this paper. Intramolecular charge-transfer (CT) bands ware observed in triptycenequinone system as a result of intramolecular interaction between benzene ring and benzoquinone moiety. Substituents on the benzene rings strongly affected the CT bands. Electron-donating groups gave absorption maxima at long wavelength. Naphthalene ring gave similar but stronger CT bands than benzene ring. Hammett σ+ value of substituents gave good linear relationship with the energy of the CT bands. Calculations of reduced charge matrix of triptycenequinones by extended Hückel theory showed that the charge of aromatic ring(s) were transferred to benzoquinone moiety accompanying HOMO–LUMO excitation. Especially, absorption maxima of the methyl-substituted triptycenequinones gave good correlation with the amounts of charge transferred from benzene ring to benzoquinone moiety. These analysis confirmed clearly CT character of the absorption maxima of triptycenequinones. However, solvent effect of these CT bands maxima is not so clear as the substituent effect.

A MNDO study of the structural and electronic properties of polysilane model compounds

AbstractThe results of MNDO geometry optimizations on selected H(SiH2)nH polysilane model compounds are presented. Near energetic degeneracy is indicated for all‐trans(T), alternating gauche–trans (GT), and all‐gauche (G+G+) models (n = 10). The most stable (T) and least stable (G+G+) conformations are separated by only ca. 0.11 eV. The existence of low‐energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground‐state electronic distribution of the saturated all‐trans silane chains are calculated to be considerably more polarizable than the fully conjugated H(CH)nH π‐electron framework of comparable length. The one‐electron HOMO → LUMO excitation can be viewed essentially as an in‐plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.

A theoretical study of the radical cations of carbazole and N-alkylcarbazoles: Optical and photoelectron spectra

The nature of the lower-lying radical cation states of carbazole and typical N-alkyl substituted analogs are addressed within the semiempirical open-shell RHF-CNDO/S(Cl) formalism. Modified atomic parameters suitable for the N(pyrrole)-type nitrogen center are first identified by noting common deficiencies in previous computations and by drawing parallels between the PPP π-electron and CNDO/S all-valence-electron approaches. Computations indicate that the apparent contradictory experimental results concerning the absorption characteristics of carbazole-based cations can be attributed to media effects. Particularly, protonation of the heteroatom is found to induce a level reversal between the two closely spaced (Δ E≲ 0.4 eV) highest occupied orbitals of the unperturbed parent system — the 4 b 1 (π) orbital localized on the nitrogen atom and the 3 a 2(π) “biphenyl-like” orbital, respectively. Ionization of the model compound exhibiting such level inversion yields optical transitions consistent with previously recorded spectra taken in solutions found to promote strong intermolecular interactions. Comparisons between the experimental absorption spectra and CNDO/S(CI) results on protonated carbazole and the carbazyl radical are included to support assignments of the various cation species. Photoelectron spectral assignments based on doublet-state computations yield final-state energy and projected intensity distributions in excellent agreement with experiment. The considerably reduced intensity observed for the fifth ionization in N-alkylcarbazoles (≈10.0 → 11.0 eV) is shown to be consistent with an unusually large non-Koopmans' contribution due to strong coupling between the fifth π −1 primary hole-state and the one-electron HOMO → LUMO excitation.

Spectroscopic and theoretical studies of metal cluster complexes. Part 2. X.alpha. calculations and spectroscopic studies of triruthenium and triosmium dodecacarbonyls

Non-empirical calculations of the X..cap alpha.. type are repoted for Ru/sub 3/(CO)/sub 12/ and Os/sub 3/(CO)/sub 12/. Two procedures were used to model the molecular charge density for calculating the self-consistent potential: an approximate self-consistent charge scheme and the more rigorous superposition of multipolar densities. This latter type of procedure accurately calculates the splitting of occupied d orbitals in Ru/sub 3/(CO)/sub 12/ and Os/sub 3/(CO)/sub 12/. Centrally directed metal-metal sigma and ..pi..-bonding orbitals are occupied as well as their antibonding counterparts. Stability of the latter levels results from metal-carbon monoxide ..pi.. bonding. A filled degenerate cyclopropane-like edge-bounding orbital furnishes four bonding electrons to the cluster framework. Difference electron density maps aid the analysis the two lowest energy electronic transitions. The dipole-allowed transition (15e' ..-->.. 6a/sub 2/') should be strongly metal-metal antibonding, and the dipole-forbidden HOMO ..-->.. LUMO excitation (10a/sub 1/' ..-->.. 6a/sub 2/') is predominantly transition-metal-CO antibonding. Raman spectra for RU/sub 3/(CO)/sub 12/ exhibit resonance enhancement of both the 185 (a/sub 1/') and 130-cm/sup -1/ (e') Ru-Ru stretching vibrations. This behavior confirms the predicted antibonding nature of the lowest allowed transition and suggests a Jahn-Teller distortion in the excited state or B-term scattering. Spectrophotometric titrations show the metal center inmore » Os/sub 3/(CO)/sub 12/ to be approximately 5 times more basic toward the proton than that in Ru/sub 3/(CO)/sub 12/.« less