Accessibility of the lowest quintet state of organic molecules through triplet-triplet annihilation; an INDO CI study

By the spin-allowed annihilation of two metastable triplet states (triplet-triplet annihilation-TTA) one electronic ground state (S0) and one electronically excited singlet (Si) or triplet (Ti) or quintet (Qk) state are created, provided the sum of the excitation energies of the two metastable triplet states is sufficient for the creation of the particular excited state. On the basis of semi-empirical calcns. of the excitation energies of T1 and Q1 of forty-six conjugated org. compds. it is shown that Q1 of benzene and some other compds. should be accessible through annihilation of like triplets (homo-TTA), and that Q1 of many compds. should be accessible through annihilation of unlike triplets (hetero-TTA). The population of Q1, competing with that of S1, should cause an unusual magnetic-field dependence of the delayed fluorescence S1 -> S0. In favorable cases, the population of Q1 should lead to an inverse (pos.) magnetic high-field effect on the delayed fluorescence

Importance of initial and final states as intermediate states in two-photon spectroscopy of polar molecules

Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. This mechanism may account for the largest contribution to the 2-photon cross section in polar mols. if the dipole moment strongly alters with excitation. Possible consequences for the 2-photon detection of forbidden states in polar polyene systems are discussed

Two-photon spectroscopy of dipole-forbidden transitions. The low-lying singlet states of anthracene

The 2-photon excitation spectrum of anthracene in soln. from 29,000 to 47,000 cm-1 is reported. Two-photon allowed transitions to 1B3g(35,800 cm-1), 2Ag (38,000 cm-1), and 3Ag (43,000 cm-1) were assigned by polarization measurements. These results, with those from 1-photon spectroscopy, agree with calcns. Theor. data suggest assignment of a B3g state to a shoulder at 39,000 cm-1 in the 2-photon spectrum

Two-photon spectroscopy of dipole-forbidden transitions. I. Dipole-forbidden transitions and double excited configurations in the CNDO-CI methods

The applicability was investigated of CNDO/S-type methods for the calcn. of optical spectra of mols. with the special implication that the calcns. should not only describe the intense, dipole-allowed transitions which dominate the 1-photon absorption spectrum but also those transitions which are 1-photon forbidden in 1st order approxn. Such a method is well suited to predict dipole allowed and dipole forbidden transitions at a similar level of accuracy if doubled excited configurations are taken into account. In spite of the lack of perfect pairing in NDO methods there are still 2 types of states which exhibit a different sensitivity towards correlation effects. Therefore, the approxn. used to describe the R-dependence of the Coulomb repulsion gains much more importance than in cases where mainly dipole allowed transitions are of interest. These findings confirm results obtained earlier from theories for which the pairing theorem is valid. The calcd. data show an excellent stability with respect to further increase of the no. of configurations if at least ~200 energy selected configurations are taken into account

Calculation of transition metal compounds using an extension of the CNDO formalism. II. Metal to metal bonding in binuclear transition metal compounds

A recently developed extension of the CNDO-method (Freund and Hohlneicher, 1979) is used to study the electronic structure of a no. of binuclear transition metal carbonyls and carboxylates with 4-fold or quasi-4-fold symmetry. The results are compared to those available from nonempirical calcns. Special attention is paid to the nature of the metal-metal bond. Connections with qual. MO-considerations allow a fairly general discussion of metal-metal bonding in binuclear transition metal complexes with basic 4-fold symmetry. A few, up to now unknown, but possibly existing, complexes are considered

The T1 state of p-nitroaniline and related molecules: a CNDO/S study

The nature of the lowest energy triplet state (T1) of p-nitroaniline (PNA), N,N-dimethyl-p-nitroaniline (DMPNA) and nitrobenzene (NB) is reexamd. using the semiempirical CNDO/S-CI method with selected parameter options. In the case of the unperturbed mols. the short-axis polarized p* A- singlet excitation. Computations suggest, however, that polar solvents strongly stabilize the PNA and DMPNA p* <- p charge-transfer triplet relative to other excitations, whereas specific solvent hydrogen-bonded interactions stabilize the p* <- n(s) triplet of NB below those of p* <- p character. These assignments allow a rationalization of phosphorescence lifetime data, Tn <- T1 absorption measurements and relative photochem. behavior

Spectroscopic and line-narrowing properties of resonant sum and difference frequency generation

Three wave mixing was investigated with respect to its spectroscopic applications under fully resonant conditions. A theor. study of the appropriate 2nd order susceptibility c(2) reveals interesting features both for sum and difference frequency generation. For difference frequency generation, 2 different mechanisms are possible, 1 of which allows the observation of resonances between excited states and the study of dephasing induced coherent emission. The coherence decay rate between the 2 excited states may be measured and also its pure dephasing rate. The line-narrowing characteristics of the fully resonant c(2) were studied with respect to correlation between the inhomogeneous distribution for different levels. The line-narrowing capabilities are partly complementary for the sum-and the 2 difference frequency resonance configurations. This leads to the suggestion of new line-narrowing techniques which could provide information about type and amt. of correlation in inhomogeneously broadened systems

Resonant nonlinear spectroscopy in strong fields

A method is presented to describe multiple resonant nonlinear spectra in the presence of strong laser fields. The Liouville equation for the d. operator of the mol. system is transformed to a time-independent linear equation system. This can be easily solved rigorously by numerical methods or, after partitioning into a strong-field part and a perturbation, the soln. can be obtained anal. by a novel perturbative approach. The results account for power broadening. Rabi splitting of signals, and power-induced extra resonances, the latter being related to the pure dephasing-induced resonances in the weak-field limit. The method can be applied to a large no. of multiple resonant nonlinear spectroscopies, esp. CARS, CSRS, coherent Rayleigh scattering and sum- or difference-frequency generation

How Is CYP17A1 Activity Altered in Autism? A Pilot Study to Identify Potential Pharmacological Targets.

Background: Increasing evidence exists that higher levels of androgens can be found in individuals with autism. Evidence yields to a susceptible role of Cytochrome P450 17A1 (CYP17A1) with its catalyzation of the two distinct types of substrate oxidation by a hydroxylase activity (17-alpha hydroxylase) and C17/20 lyase activity. However, to what extent steps are altered in affected children with autism versus healthy controls remains to be elucidated. Methods: Urine samples from 48 boys with autism (BMI 19.1 ± 0.6 kg/m2, age 14.2 ± 0.5 years) and a matched cohort of 48 healthy boys (BMI 18.6 ± 0.3 kg/m2, 14.3 ± 0.5 years) as well as 16 girls with autism (BMI 17.5 ± 0.7 kg/m2, age 13.8 ± 1.0 years) and a matched cohort of 16 healthy girls (BMI 17.2 ± 0.8 kg/m2, age 13.2 ± 0.8 years) were analyzed for steroid hormone metabolites by gas chromatography-mass spectrometry. Results: The activity of 17-alpha Hydroxylase increased by almost 50%, whereas activity of 17/20 Lyase activity increased by around 150% in affected children with autism. Furthermore, the concentration of Cortisol was higher as compared to the average increase of the three metabolites TH-Corticosterone, 5α-TH-Corticosterone and TH-11β-DH-Corticosterone, indicating, in addition, a stimulation by the CRH-ACTH system despite a higher enzymatic activity. Discussion: As it was shown that oxidative stress increases the 17/20-lyase activity via p38α, a link between higher steroid hormone levels and oxidative stress can be established. However, as glucocorticoid as well as androgen metabolites showed higher values in subjects affected with autism as compared to healthy controls, the data indicate, despite higher CYP17A1 activity, the presence of increased substrate availability in line with the Cholesterol theory of autism