ELECTRONIC-TRANSITION OSCILLATOR STRENGTH BY THE EXTENDED HUCKEL MOLECULAR-ORBITAL METHOD

Calculations of the oscillator strength of electronic dipole-induced transitions (EDiTs) based on EHMO wave functions including all transition matrix elements offer a generally applicable method for computing intensities of all types of transitions found in molecules, clusters, and complexes. Application of this EHMO-EDiT procedure to formaldehyde, MnO4-, p-(N,N-dimethylamino)benzonitrile (DMABN), 2,2'-bipyridyl (bpy), Ru(bpy)(3)(2+), and [CO(CO)(4)(H7Si8O12)] illustrates the versatility of this procedure and offers new insight into some ''old problems''. We find that the TICT state of DMABN is expected to live long enough to relax to a state described by the (b(1))(1)(b(2))(1) configuration. Emission of a photon in this state is forbidden by symmetry. However, a small twist of the dialkylamino group by only 15 degrees increases this oscillator strength along the z axis enormously. TICT emission has empirically been shown to be z-polarized and strong in intensity. The emission rates can be thermally activated. This is in good qualitative agreement with the oscillator strength calculation. We also find that the intensities of the two z-polarized pi* <-- pi transitions depend relatively little on the amino group torsional angle. This dependence is indeed characteristic for the TICT state. EDiT calculations on bpy as a function of the torsional angle theta lead to a satisfactory interpretation of the two prominent pi* <-- pi transitions of this very often used ligand. The 2 pi* <-- 2n oscillator strength is very small for cis. However, the cis (2n)(1)(2 pi*)(1) configuration correlates with the trans (1 pi*)(1)(1 pi*)(1) which bears a large 1 pi* <-- 1 pi oscillator strength. The cis 2 pi* <-- 1 pi transition retains its pi* <-- pi character but loses intensity with increasing theta and becomes symmetry-forbidden at theta = 180 degrees. The first intense band which is broad, featureless, and very similar in different organic solvents is the result of a superposition of bands arising from an equilibrium Boltzmann distribution over the whole range of angles theta from 0 degrees to 180 degrees. This causes a hypsochromic shift of the maximum of the first intense band, because the 1 pi* <-- 1 pi transition energies of the species with angles different from theta approximate to 180 degrees and theta approximate to 0 degrees appear at larger energy. The bathochromic shift of the first 1 pi* <-- 1 pi transition of the protonated bpy is due to the predominance of the cis isomer and hence a narrow Boltzmann distribution of the theta region close to 0 degrees. The number of possible RU(bpy)(3)(2+) excited-state configurations in the HOMO/LUMO region give rise to 143 different one-electron spin-allowed transitions. Even though a few of them are forbidden by symmetry, most are allowed, but many are of low intensity. They can be grouped according to the usual classification MLCT(pi* <-- d), MC(d* <-- d), LC(pi* <-- pi,pi* <-- n), and LMCT(d* <-- pi, d* <-- n), which is based on the orbitals engaged and on the specific parts of the complex involved. However; the absorption around 40000 cm(-1) is composed of a LC(pi* <-- pi)-type and a LMCT(d-n* <-- 1 pi)-type transition. Thus, the often encountered opinion that this region has to be attributed to a MLCT transition should be revised. The HOMO of [Co(CO)(4)(H7Si8O12)] consists Of oxygen lone pairs localized on H7Si8O12, and the LUMO is identical with the LUMO of Co(CO)(4). The first electronic transitions observed in the near-UV are of the H7Si8O12 (oxygen lone pair) to Co(CO)(4) fragment charge-transfer type.