Lowest Nπ Research Articles

On the propensity of formation of cyclobutane dimers in face-to-face and face-to-back uracil stacks in solution.

UV irradiation of RNA leads to the formation of intra- and inter-strand crosslinks of cyclobutane type. Despite the importance of this reaction, relatively little is known about how the mutual orientation of the two bases affects the outcome of the reaction. Here we report a comparative nonadiabatic molecular dynamics study of face-to-back (F2B) and face-to-face (F2F) stacked uracil-water clusters. The computations were performed using the second-order algebraic-diagrammatic-construction (ADC(2)) method. We found that F2B stacked uracil-water clusters either relax non-reactively to the ground state by an ethylenic twist around the CC bond or remain in the lowest nπ* state in which the two bases gradually move away from each other. This finding is consistent with the low propensity for the formation of intra-strand cyclobutane dimers between adjacent RNA bases. On the contrary, in F2F stacked uracil-water clusters, in addition to non-reactive deactivation, we found a pro-reactive deactivation pathway, which may lead to the formation of cyclobutane uracil dimers in the electronic ground state. On a qualitative level, the observed photodynamics of F2F stacked uracil-water clusters explains the greater propensity of RNA to form inter-strand cyclobutane-type crosslinks.

Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10-Phenanthrenequinone. A Time-Resolved Resonance Raman Study.

The photochemistry of aromatic ketones plays a key role in various physicochemical and biological processes, and solvent polarity can be used to tune their triplet state properties. Therefore, a comprehensive analysis of the conformational structure and the solvent polarity induced energy level reordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) was carried out using nanosecond-time-resolved absorption (ns-TRA), time-resolved resonance Raman (TR(3)) spectroscopy, and time dependent-density functional theory (TD-DFT) studies. The ns-TRA of PQ in acetonitrile displays two bands in the visible range, and these two bands decay with similar lifetime at least at longer time scales (μs). Interestingly, TR(3) spectra of these two bands indicate that the kinetics are different at shorter time scales (ns), while at longer time scales they followed the kinetics of ns-TRA spectra. Therefore, we report a real-time observation of the thermal equilibrium between the two lowest triplet excited states of PQ, assigned to nπ* and ππ* of which the ππ* triplet state is formed first through intersystem crossing. Despite the fact that these two states are energetically close and have a similar conformational structure supported by TD-DFT studies, the slow internal conversion (∼2 ns) between the T(2)(1(3)nπ*) and T(1)(1(3)ππ*) triplet states indicates a barrier. Insights from the singlet excited states of PQ in protic solvents [ J. Chem. Phys. 2015 , 142 , 24305 ] suggest that the lowest nπ* and ππ* triplet states should undergo hydrogen bond weakening and strengthening, respectively, relative to the ground state, and these mechanisms are substantiated by TD-DFT calculations. We also hypothesize that the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states of PQ could influence its ISC mechanism.

Direct observation of the solvent effects on the low-lying nπ* and ππ* excited triplet states of acetophenone derivatives in thermal equilibrium.

Low-lying excited triplet states of aromatic carbonyl compounds exhibit diverse photophysical and photochemical properties of fundamental importance. Despite tremendous effort in studying those triplet states, the effects of substituents and solvents on the energetics of the triplet manifold and on photoreactivity remain to be fully understood. We have recently studied the ordering of the low-lying nπ* and ππ* excited triplet states and its substituent dependence in acetophenone derivatives using nanosecond time-resolved near-IR (NIR) spectroscopy. Here we address the other important issue, the solvent effects, by directly observing the electronic bands in the NIR that originate from the lowest nπ* and ππ* states of acetophenone derivatives in four solvents of different polarity (n-heptane, benzene, acetonitrile, and methanol). The two transient NIR bands decay synchronously in all the solvents, indicating that the lowest nπ* and ππ* states are in thermal equilibrium irrespective of the solvent polarity studied here. We found that the ππ* band increases in intensity relative to the nπ* band as solvent polarity increases. These results are compared with the photoreduction rate constant for the acetophenone derivatives in the solvents to which 2-propanol was added as a hydrogen-atom donor. Based on the present findings, we present a comprehensive, solvent- and substituent-dependent energy level diagram of the low-lying nπ* and ππ* excited triplet states.

Theoretical study of the ground and excited state properties of newly designed size-expanded adenine analogue x-adenine

Recently, creating unnatural fluorescent nucleobase analogues has gained increasing attention. In this work, a detailed computational investigation on the photophysical properties of the fluorescent adenine analogue x-adenine (xA) is carried out. The ten lowest low-lying exited states are analyzed and assigned. The effects of methanol solution, linking to deoxyribose, and base pairing on its absorption and emission processes are considered. The calculated excitation and emission energies are in good agreement with the measured data available. It is found that linking to deoxyribose and methanol solution have a hyperchromic effect on xA. Also it is found that methanol solution and linking to deoxyribose will lead to the red shift of fluorescence, while base pairing does not have obvious effects on the lowest ππ* state and fluorescence emission but produces the blue shift of lowest nπ* to some extent.

The Different Photoisomerization Efficiency of Azobenzene in the Lowest nπ* and ππ* Singlets: The Role of a Phantom State

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.

Vertical excitation energies for ribose and deoxyribose nucleosides

Vertical excitation energies for DNA and RNA nucleosides are determined with electron structure calculations using the time-dependent density functional theory (TDDFT) method at the B3LYP/6-311++G(d,p) level for nucleoside structures optimized at the same level of theory. The excitation energies and state assignments are verified using B3LYP/aug-cc-pVDZ level calculations. The nature of the first four excited states of the nucleosides are studied and compared with those of isolated bases. The lowest npi* and pipi* transitions in the nucleoside remain localized on the aromatic rings of the base moiety. New low-energy npi* and pisigma* transitions are introduced in the nucleosides as a result of bonding to the ribose and deoxyribose molecules. The effect on the low-lying excited state transitions of the binding to phosphate groups at the 5'- and 3',5'-hydroxyl sites of the uracil ribose nucleoside are also studied. Some implications of these calculations on the de-excitation dynamics of nucleic acids are discussed.

Optical properties of two types of sex hormones of the cyclopentenephenanthrene series

The spectral and luminescent characteristics of estradiol and testosterone—two basic sex hormones of the cyclopentenephenanthrene series—are calculated by employing quantum-chemical methods. The results of calculations are in good agreement with experimental data. It is shown that fluorescence observed in estrogens is associated with the occurrence of the lowest ππ state, while the absence of fluorescence in androgens is attributed to the existence of the lowest nπ state, from which fluorescence is forbidden.

Ab initio study of the potential energy functions relevant for hydrogen transfer in formamide, its dimer and its complex with water

The potential energy (PE) functions of the electronic ground state and the lowest nπ ∗ and ππ ∗ excited singlet states of formamide, its dimer and its complex with water have been characterized theoretically along the proton transfer (PT) reaction coordinate. The calculations were performed using the ab initio complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory employing the CASSCF wavefunction as the reference state. The results suggest that the photoinduced PT reaction in an isolated molecule occurs via successive dissociation and association of the hydrogen atom, whereas in hydrogen-bonded complexes, the PE functions along the PT reaction coordinate are qualitatively the same as for the PT reaction along the intramolecular hydrogen bond. Some implications concerning biological molecules containing peptide linkages are discussed.

Spectroscopic and structural features of small thiocarbonyl molecules in low‐lying excited states: Further applications of a variant of the orthogonal gradient method

AbstractStructural parameters of a set of five thiocarbonyl molecules in the lowest nπ* states are calculated by using a generalized orbital optimization algorithm (a variant of the orthogonal gradient method) in an INDO MCSCF framework. Transition energies, singlet‐triplet splittings, planar inversion barriers, and dipole moments in nπ* states of different spin multiplicities are reported. Predicted structural features agree reasonably well with available experimental or theoretical data. Some interesting trends are noted in the computed inversion barrier heights, singlet‐triplet splittings, and dipole moments in nπ* states.

Optically detected electron—nuclear double resonance and electron—nuclear—nuclear triple resonance study of 17O hyperfine coupling in the lowest nπ * triplet state of benzil

Optically detected ENDOR and electron—nuclear—nuclear triple resonance of 17O were measured via phosphorescence from 3(nπ *) benzil in benzophenone- d 10 crystals at high magnetic field. The n and π * spin densities on the oxygen atom are 0.201 and 0.092, respectively, the angle between the two CO bonds being 150°.

Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR, LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A 1 = A 2 = 34.4 MHz and A 3 = −53.7 MHz. The tunneling frequency (3 J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.

Strucrure and dynamics of the lowest triplet state in p-benzoquinone IV. The effect of mild substitution on the proton endor spectra

The effect of mild (deuterium and methyl) substitution on the proton ENDOR spectra in the lowest nπ* triplet state of p-benzoquinone is reported and analyzed. The conclusion is that, as recently suggested by Merienne-Lafore and Trommsdorff, p-benzoquinone in its lowest triplet state is best described as a dimer.

Relative magnitudes of singlet and triplet state dipole moments: the lowest nπ* excited states of p-methylbenzaldehyde and p-chlorobenzaldehyde

The molecular dipole moment changes upon 1nπ* and 3nπ* excitation of p-methylbenzaldehyde and p-chlorobenzaldehyde isolated in p-dimethoxybenzene host crystals have been determined from measurements of the Stark splittings in phosphorescence excitation and phosphorescence spectra. Within the experimental uncertainty, the changes are identical for the corresponding triplet and singlet states. This result contrasts with similar determinations for formaldehyde, benzophenone, and 4,4′-dichlorobenzophenone and with predictions based on simple molecular orbital and electron correlation arguments. The result is considered to be a consequence of a relatively large mixing of 3nμ* and 3μμ* states.

Optical nuclear polarization in the excited state through cross-relaxation ant its use in the study of the carbon-13 hyperfine coupling in the lowest triplet state of 1- 13C- p-benzoquinone

In this paper the phenomenon of optical nuclear polarization in the excited state through cross-relaxation is described. It is shown that when the populating and depopulating rates of the triplet spin sublevels are known the absolute nuclear polarization can be calculated and that optical detection of the nuclear alignment in at least one electron spin state is quite often feasible. The paper further deals with the magnetic resonance properties of the lowest triplet state in 1- 13C- p-benzoquinone whereof the zero-field splitting parameters and carbon-13 hyperfine data are reported. The carbon-13 hyperfine data, obtained by optical detection of ENDOR and CRENDOR, indicate that in the lowest nπ * triplet state of p-benzoquinone the π-electron spin density at the carbonyl carbon oxygen are about equal while the remaining carbon atoms carry about half this spin density.

Vibronic coupling of pyridine in the nπ* excited state

The electronic absorption spectra and Raman spectra of pyridine and pyridine-d5 were measured. The vibrational analysis of the nπ* absorption spectrum and the preresonance Raman effect showed that the CH out-of-plane bending vibration ν10a(a2) (of the ground and excited-state frequencies 886 and 331 cm−1 for pyridine) is responsible for the vibronic coupling between the lowest nπ* state and the second lowest ππ* state.

Correspondences of the vibrational frequencies of the ground and electronic excited states of quinoxaline as revealed by the Raman intensities

The intensities of the Raman lines of quinoxaline were measured at different excitation wavelengths. The matrix element ratios of the vibronic couplings between the two lowest electronic excited states of the molecule were evaluated from the Raman intensities of the b 1 vibrations, and they were compared with the matrix element ratios obtained from the vibrational frequencies of the ground and electronic excited states of the molecule. It was suggested that the ground state frequency of the b 1 vibration at 867 cm −1 decreases greatly to 425 cm −1 in the lowest nπ ∗ excited state.

Mechanism of thiobenzoic acid O-ester photolysis

Nanosecond flash photolysis, quenching, and phosphorescence studies show that the photoreactions of thiobenzoate O-esters proceed mainly via the lowest nπ* triplet state; chemical evidence for the formation of a 1,4-diradical intermediate is presented.

Photosensitized Dehydrogenation of Crystalline Durene: Mechanism of Duryl Radical Formation

The photosensitized dehydrogenation of durene doped with 2,4,5-trimethylbenzaldehyde and naphthalene-d8 gives rise to duryl (2,4,5-trimethylbenzyl) radicals. The rate of production of these radicals was studied as a function of the guest, of photolyzing light intensity, and of temperature. The experimental data show that the mechanism of radical formation is dependent upon the nature of the particular guest involved. In naphthalene-d8/durene mixed crystals, duryl radical is probably formed by a stepwise biphotonic absorption process involving the first triplet state of the guest as an intermediate. The results show that radical formation is not responsible for the temperature dependence of the naphthalene-d8 phosphorescence lifetime in durene host at T &amp;gt; 220°K. In 2,4,5-trimethylbenzaldehyde/durene mixtures, the formation of duryl radical is a monophotonic process, and the over-all reaction is the hydrogen abstraction of aromatic carbonyl compounds with a lowest nπ* triplet state, a process which has been extensively studied in liquid solutions. However, the use of the solid state together with low temperature techniques has revealed the intervention of a new transient species as a precursor to matrix dehydrogenation.