Mihajlo Etinski

Intersystem crossing and characterization of dark states in the pyrimidine nucleobases uracil, thymine, and 1-methylthymine.

The ground and low-lying excited states of the pyrimidine nucleo bases uracil, thymine, and 1-methylthymine have been characterized using ab initio coupled-cluster with approximate doubles (CC2) and a combination of density functional theory (DFT) and semiempirical multireference configuration interaction (MRCI) methods. Intersystem crossing rate constants have been determined perturbationally by employing a nonempirical one-center mean-field approximation to the Breit-Pauli spin-orbit operator for the computation of electronic coupling matrix elements. Our results clearly indicate that the S(2)((1)pi-->pi*)-->T(2)((3)n-->pi*) process cannot compete with the subpicosecond decay of the S(2) population due to spin-allowed nonradiative transitions, whereas the T(1)((3)pi-->pi*) state is populated from the intermediate S(1)((1)n-->pi*) state on a subnanosecond time scale. Hence, it is very unlikely that the S(1)((1)n-->pi*) state corresponds to the long-lived dark state observed in the gas phase.

Time-dependent approaches for the calculation of intersystem crossing rates

We present three formulas for calculating intersystem crossing rates in the Condon approximation to the golden rule by means of a time-dependent approach: an expression using the full time correlation function which is exact for harmonic oscillators, a second-order cumulant expansion, and a short-time approximation of this expression. While the exact expression and the cumulant expansion require numerical integration of the time correlation function, the integration of the short-time expansion can be performed analytically. To ensure convergence in the presence of large oscillations of the correlation function, we use a Gaussian damping function. The strengths and weaknesses of these approaches as well as the dependence of the results on the choice of the technical parameters of the time integration are assessed on four test examples, i.e., the nonradiative S(1) ⇝ T(1) transitions in thymine, phenalenone, flavone, and porphyrin. The obtained rate constants are compared with previous results of a time-independent approach. Very good agreement between the literature values and the integrals over the full time correlation functions are observed. Furthermore, the comparison suggests that the cumulant expansion approximates the exact expression very well while allowing the interval of the time integration to be significantly shorter. In cases with sufficiently high vibrational density of states also the short-time approximation yields rates in good agreement with the results of the exact formula. A great advantage of the time-dependent approach over the time-independent approach is its excellent computational efficiency making it the method of choice in cases of large energy gaps, large numbers of normal modes, and high densities of final vibrational states.

A Theoretical Investigation of the Geometries and Binding Energies of Molecular Tweezer and Clip Host-Guest Systems.

A quantum chemical study of host-guest systems with dimethylene-bridged clips and tetramethylene-bridged tweezers as host molecules and six different aliphatic and aromatic substrates as guests is presented. The geometries and binding energies of the complexes are investigated using the recently developed density functional theory with empirical corrections for dispersion interactions (DFT-D) in combination with the BLYP functional and basis sets of TZVP quality. It is found that the DFT-D method provides accurate geometries for the host-guest complexes that compare very favorably to experimental X-ray data. Without the dispersion correction, all host-guest complexes are unbound at the pure DFT level. Calculations of the clip complexes show that the DFT-D binding energies of the guests agree well with those from a more sophisticated SCS-MP2/aug-cc-pVTZ treatment. By a partitioning of the host into molecular fragments it is shown that the binding energy is clearly dominated by the aromatic units of the clip. An energy decomposition analysis of the interaction energies of some tweezer complexes revealed the decisive role of the electrostatic and dispersion contributions for relative stabilities. The calculations on the tweezer complexes show that the benzene spaced tweezer is a better receptor for aliphatic substrates than its naphthalene analogue that has a better topology for the binding of aromatic substrates. The tweezer with a OAc substituent in the central spacer unit is found to favor complex formation with both aliphatic and aromatic substrates. The theoretical results are qualitatively in very good agreement with previous experimental findings although direct comparison with experimental binding energies which include solvent effects is not possible. The good results obtained with the DFT-D-BLYP method suggest this approach as a standard tool in supramolecular chemistry and as the method of choice for theoretical structure determinations of large complexes where both electrostatic and dispersive interactions are crucial.

Time-dependent approach to spin-vibronic coupling: Implementation and assessment

In this work, we present the generalization of a time-dependent method for the calculation of intersystem crossing (ISC) rates in the Condon approximation. When ISC takes place between electronic states with the same orbital type, i.e., when the transition is forbidden according to the El-Sayed rules, it is necessary to go beyond the Condon approximation. Similar to the Herzberg-Teller expansion of the vibronic interaction, the electronic spin-orbit matrix elements are assumed to depend linearly on the nuclear coordinates. The ISC rate is then a sum of three contributions: a direct, mixed direct-vibronic, and vibronic term. The method, presented in this work, is based on the generating function formalism and the multi-mode harmonic oscillator approximation. In addition to the zero-temperature case, we implemented formulae for finite-temperature conditions assuming a Boltzmann population of vibrational levels in the initial state. Tests have been carried out for a variety of molecules for which literature data were available. We computed vibronic one-photon spectra of free-base porphyrin and free-base chlorin and calculated ISC rates for xanthone, thioxanthone, thionine, as well as free-base porphyrin and found excellent agreement with previous results. Quantitative rates for triplet formation in rhodamine A have been determined theoretically for the first time. We find the S1↝ T2 channel to be the major source of triplet rhodamine formation in the gas phase.

Electronic and Vibrational Spectroscopy of 1‐Methylthymine and its Water Clusters: The Dark State Survives Hydration

Electronic and vibrational gas phase spectra of 1-methylthymine (1MT) and 1-methyluracil (1MU) and their clusters with water are presented. Mass selective IR/UV double resonance spectra confirm the formation of pyrimidine-water clusters and are compared to calculated vibrational spectra obtained from ab initio calculations. In contrast to Y. He, C. Wu, W. Kong; J. Phys. Chem. A, 2004, 108, 94 we are able to detect 1MT/1MU and their water clusters via resonant two-photon delayed ionization under careful control of the applied water-vapor pressure. The long-living dark electronic state of 1MT and 1MU detected by delayed ionization, survives hydration and the photostability of 1MT/1MU cannot be attributed solely to hydration. Oxygen coexpansions and crossed-beam experiments indicate that the triplet state population is probably small compared to the (1)n pi* and/or hot electronic ground state population. Ab initio theory shows that solvation of 1MT by water does not lead to a substantial modification of the electronic relaxation and quenching of the (1)n pi* state. Relaxation pathways via (1)pi pi*(1)-n pi*(1) and (1)pi pi*-S(0) conical intersections and barriers have been identified, but are not significantly altered by hydration.

Photophysics of Xanthone: A Quantum Chemical Perusal

A theoretical analysis of the chromophore xanthone has been carried out in various cases, starting from the isolated molecule and going on to the extremely polar-protic solvated chromophore. In the gas phase, we find that an El-Sayed forbidden channel with a rate constant of kISC = 2 × 10(11) s(-1) is one of the trend-setting factors in the photophysical kinetics. The fluorescence is found to be quenched in the gas phase and polar-aprotic solvents. The analysis of xanthone in aqueous solution supports the delayed fluorescence model, which has been suggested to explain the high fluorescence quantum yield observed in water. Besides, we present a detailed analysis of the photophysics of xanthone in acetonitrile and methanol.

Overruling the energy gap law: fast triplet formation in 6-azauracil

The photophysical properties of 6-azauracil were studied by means of ab initio quantum chemical methods. On the basis of our calculations we propose here the following mechanism for the lack of fluorescence and the high triplet quantum yield that was observed experimentally after irradiation of this compound with UV light [Kobayashi et al., J. Phys. Chem. A, 2008, 112, 13308]. Multiple potential energy surface crossings between excited singlet states of π→π* and n→π* character lead to an ultrafast transfer of the S(2) ((1)π→π*) population to the lower-lying S(1) ((1)n→π*) state. This state acts as a doorway state from which the T(1) ((3)π→π*) state is formed approximately within 125 ps in the isolated 6-azauracil and within 30 ps in acetonitrile solution according to our calculations. The enhancement of the S(1)[radiolysis arrow - arrow with voltage kink] T(1) intersystem crossing in acetonitrile solution is noteworthy as it goes along with an increased adiabatic energy gap between the interacting states. Blue shift of the S(1) potential energy surface by about 0.2 eV in this polar, aprotic environment places the intersection between the S(1) and T(1) potentials close to the S(1) minimum, thus increasing the overlap of the vibrational wavefunctions and consequently speeding up the spin-forbidden nonradiative transition.

Intersystem crossing rates of S1 state keto-amino cytosine at low excess energy

The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S1((1)ππ(∗)) state. We investigate these ISC rates experimentally and theoretically as a function of S1 state vibrational excess energy Eexc. The S1 vibronic levels are pumped with a ∼5 ns UV laser, the S1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S1 and T1 ionization cross sections, we obtain energy dependent ISC quantum yields QISC (corr)=1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates kISC = 0.4-1.5 ⋅ 10(9) s(-1), the corresponding S1⇝S0 internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S1; v = 0 state with kISC = 3 ⋅ 10(9) s(-1) to the T1((3)ππ(∗)) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of (1)nOπ(∗) character into the S1((1)ππ(∗)) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below Eexc = 550 cm(-1) in the S1 state, S1⇝S0 internal conversion dominates the nonradiative decay with kIC ≥ 2 ⋅ 10(10) s(-1), (2) the calculated S1⇝T1 ((1)ππ(∗)⇝(3)ππ(∗)) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S1⇝T1 ISC is moderately fast (kISC = 3 ⋅ 10(9) s(-1)), and not ultrafast, as claimed by other calculations, and (4) at Eexc ∼ 550 cm(-1) the IC rate increases by ∼50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels.

Theoretical Investigation of the Excited States of 2‐Nitrobenzyl and 4,5‐Methylendioxy‐2‐nitrobenzyl Caging Groups†

The photochemistry of caged compounds of the o‐nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol.84 162–171] that we found long‐lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5‐methylendioxy‐2‐nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o‐nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5‐methylendioxy‐2‐nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro‐group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o‐nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.

Reverse Intersystem Crossing in Rhodamines by Near-Infrared Laser Excitation

The population of the long-lived first excited triplet state (T1) of a fluorescence dye represents a major limitation in single-molecule spectroscopy. Reverse intersystem crossing (ReISC) is one of the processes that may prevent considerable loss of luminescence. In the present quantum chemical study we have analyzed rhodamine A in aqueous environment. The T2 ⇝ S1 and T3 ⇝ S2 ReISC channels are predicted to be viable. The rate constant computed for the former channel is ≈2 × 10(6) s(-1). Hence, an excitation with suitable wavelength to one of the triplets should help repopulate the optically bright singlet state S1.