THE introduction of optical detection methods for observing magnetic resonance transitions in metastable paramagnetic states1-4 has contributed enormously to our understanding of the properties of... Show moreTHE introduction of optical detection methods for observing magnetic resonance transitions in metastable paramagnetic states1-4 has contributed enormously to our understanding of the properties of photoexcited molecules in condensed phases. In such experiments the luminescence intensity is recorded as a function of the frequency of an applied microwave field. At resonance with transitions between sublevels of a metastable paramagnetic state, the lifetime of the metastable state is altered and a consequent change in the luminescence intensity is observed. Here we report the observation of such optically detected magnetic resonance transitions for the triplet state of a single pentacene molecule embedded in a p-terphenyl host crystal. This result has been obtained by combining the conventional optical detection technique for observing magnetic resonance transitions1-4 with the new single-molecule optical detection methods developed recently5,6. This observation opens the way for magnetic resonance studies in condensed phases with single-molecule sensitivity. Show less
Ab-initio calculations at the UHF and CASSCF levels have been performed in which the geometry of pyrimidine in the B-3(1)(npi*), 3A2(npi*), and 3A1(pipi*) states has been optimized. The... Show moreAb-initio calculations at the UHF and CASSCF levels have been performed in which the geometry of pyrimidine in the B-3(1)(npi*), 3A2(npi*), and 3A1(pipi*) states has been optimized. The calculations clearly demonstrate the influence of vibronic coupling on the molecular and electronic structure of the molecule in these states. For the npi* states, conformations corresponding to minimum energy are found that deviate significantly from planar. For the 3A1(pipi*) state, vibronic coupling similar to that between the B-3(1u) and 3E1u states of benzene has been found. From the calculations, we deduce a dynamic out-of-plane distortion of the molecule in the B-3(1u)(npi*) state, which provides for a coherent description of the results of optical and magnetic-resonance data available for the lowest triplet state of pyrimidine. The out-of-plane distortions occurring in the B-3(1)(npi*) and 3A2(npi*) states conform to the increased antibonding character of the pi-electron system upon npi* excitation. Show less
Donckers, M.C.J.M.; Schwencke, A.M.; Groenen, E.J.J.; Schmidt, J. 1992
The hyperfine and quadrupole tensors of the two nitrogen atoms and the hyperfine tensors of the four hydrogen atoms of pyrazine in the lowest triplet state T0 are obtained from optically-detected... Show moreThe hyperfine and quadrupole tensors of the two nitrogen atoms and the hyperfine tensors of the four hydrogen atoms of pyrazine in the lowest triplet state T0 are obtained from optically-detected electron-nuclear double resonance (ENDOR) experiments on pyrazine-h4 in a single crystal of benzene-d6 at 1.2 K. Analysis of these tensors shows that pyrazine is in good approximation a planar molecule of D2h, symmetry in the lowest triplet state. The in-plane structure changes significantly upon excitation into T0. An increase of the C-N-C angles is observed and a reduction of the C-N bond lengths. From the hyperfine tensors the distribution of the electron-spin density is derived. The nitrogen nonbonding orbitals together carry 36% of the total spin density and it is shown that the lowest triplet state of pyrazine is appropriately described in terms of a 3n-pi* excitation. The results of the optically-detected ENDOR experiments give no indication of vibronic coupling of the lowest 3n-pi* state with nearby 3-pi-pi* states. Show less
