UV Excited State Research Articles

Intrinsic Nature of the Ultrafast Deexcitation Pathway of Mycosporine-like Amino Acid Porphyra-334.

Porphyra-334 is a member of natural UV-screening compounds named mycosporine-like amino acids found in several marine organisms. The UV excited porphyra-334 has been identified to deexcite quickly by puckering the intramolecular cyclohexenimine ring; however, the reason for such a ring-puckering occurrence is yet unclear. In this study, we show the ring-puckering to be the relaxation pathway of the UV excited π electron which shifts from the in-ring bond to the out-of-ring bond. The ring-puckering is characterized by the torsion among the in-ring and out-of-ring bonds. Since the π electron shift is possible in two different directions at the Franck-Condon UV excited state, it enables two ring-puckering pathways: the previously reported pathway and another one newly identified at present. We also examine the ring-unpuckering pathways which are an analogy of cis/trans photoisomerization, and we find them to be not suited for the π electron shift character of the UV excited state and thus not related to the deexcitation pathway. The present study provides insight into how porphyra-334 exerts the UV-screening ability based on its cyclohexenimine ring structure.

Excited-State Evolution of Keto-Carotenoids after Excess Energy Excitation in the UV Region.

Carotenoids are molecules with rich photophysics that are in many biological systems involved in photoprotection. Yet, their response to excess energy excitation is only scarcely studied. Here we have explored excited state properties of three keto-carotenoids, echinenone, canthaxanthin and rhodoxanthin after excess energy excitation to a singlet state absorbing in UV. Though the basic spectral features and kinetics of S2 , hot S1 , relaxed S1 states remain unchanged upon UV excitation, the clear increase of the S* signal is observed after excess energy excitation, associated with increased S* lifetime. A multiple origin of the S* signal, originating either from specific conformations in the S1 state or from a non-equilibrated ground state, is confirmed in this work. We propose that the increased amount of energy stored in molecular vibrations, induced by the UV excitation, is the reason for the enhanced S* signal observed after UV excitation. Our data also suggest that a fraction of the UV excited state population may proceed through a non-sequential pathway, bypassing the S2 state.

A molecular crystal with an unprecedentedly long-lived photoexcited state.

The Au(iii)-complex anions in a newly synthesised compound BPY[Au(dmit)2]2 (BPY = N,N'-ethylene-2,2'-bipyridinium, dmit = 1,3-dithiole-2-thione-4,5-dithiolate) reversibly exhibit a molecular distortion in the solid state under UV-radiation. The photoexcited state is maintained for a week at 298 K, during which time molecules relax to their original structures and energy is gradually released as heat without decomposition or light emission. Most Au atoms adopt square planar (SP) coordination geometries, but some anions have unusual non-planar (NP) coordination geometries that produce disorder at the Au sites. The total (Gibbs) energy of the system depends on the proportion of Au atoms of NP geometry, which is directly determined from the occupancy (Occ (%)) by X-ray diffractometry. Due to phase transition, Occ substantially changes at a critical temperature (TC) of ∼280 K without other structural changes; however it remains almost constant in each phase. In addition, due to UV-promoted charge-transfer transitions between BPY and Au(dmit)2, Occ can be controlled by UV irradiation (∼250-450 nm). The UV-excited states have unprecedentedly long relaxation times (t1/2 > 36 h at 298 K), which is attributed to the close connection between the degrees of freedom on charge, spin, and molecular structures.

Electronic Excitations in Complex Molecular Environments: Many-Body Green's Functions Theory in VOTCA-XTP.

Many-body Green’s functions theory within the GW approximation and the Bethe-Salpeter Equation (BSE) is implemented in the open-source VOTCA-XTP software, aiming at the calculation of electronically excited states in complex molecular environments. Based on Gaussian-type atomic orbitals and making use of resolution of identity techniques, the code is designed specifically for nonperiodic systems. Application to a small molecule reference set successfully validates the methodology and its implementation for a variety of excitation types covering an energy range from 2 to 8 eV in single molecules. Further, embedding each GW-BSE calculation into an atomistically resolved surrounding, typically obtained from Molecular Dynamics, accounts for effects originating from local fields and polarization. Using aqueous DNA as a prototypical system, different levels of electrostatic coupling between the regions in this GW-BSE/MM setup are demonstrated. Particular attention is paid to charge-transfer (CT) excitations in adenine base pairs. It is found that their energy is extremely sensitive to the specific environment and to polarization effects. The calculated redshift of the CT excitation energy compared to a nucelobase dimer treated in vacuum is of the order of 1 eV, which matches expectations from experimental data. Predicted lowest CT energies are below that of a single nucleobase excitation, indicating the possibility of an initial (fast) decay of such an UV excited state into a binucleobase CT exciton. The results show that VOTCA-XTP’s GW-BSE/MM is a powerful tool to study a wide range of types of electronic excitations in complex molecular environments.

Capturing transiently charged states at the C60/TiO2(110) interface by time-resolved soft X-ray photoelectron spectroscopy

Time-resolved soft X-ray photoelectron spectroscopy is utilized to determine an energy level alignment and the photoexcited carrier dynamics at a C60/TiO2(110) interface. The interface electronic structure is characterized by a type II junction, which favors an injection of photoexcited electrons from C60 to TiO2. Ultraviolet (UV) laser pulse irradiation induces transient shifts of both C 1s and Ti 2p core levels towards the higher binding energies. These energy shifts are caused by a laser-induced charge transfer between the C60 layer and the TiO2(110) surface. Upon UV absorption, valence electrons of C60 are promoted to unoccupied levels, followed by a resonant transfer to TiO2, leaving C60 in a cationized state. On the TiO2(110) side, the electrons are injected into the conduction band to raise the carrier density so that downward bending of the TiO2 band is induced. The UV-excited states of C60 and TiO2 have sufficiently longer lifetime than the lifetime of the electron–hole pairs in solid C60. The C60/TiO2(110) interface is, thus, proved to be efficient for separating the electron–hole pairs generated within the C60 layer.

Excited state dynamics of the isolated green fluorescent protein chromophore anion following UV excitation.

A combined frequency-, angle-, and time-resolved photoelectron spectroscopy study is used to unravel the excited state dynamics following UV excitation of the isolated anionic chromophore of the green fluorescent protein (GFP). The optically bright S3 state, which is populated for hv > 3.7 eV, is shown to decay predominantly by internal conversion to the S2 state that in turn autodetaches to the neutral ground state. For hv > 4.1 eV, a new and favorable autodetachment channel from the S2 state becomes available, which leads to the formation of the neutral in an excited state. The results indicate that the UV excited state dynamics of the GFP chromophore involve a number of strongly coupled excited states.

Base stacking in adenosine dimers revealed by femtosecond transient absorption spectroscopy.

Excitons formed in DNA by UV absorption decay via poorly understood pathways that can culminate in mutagenic photoproducts. In order to gain insight into how base stacking influences UV excited states in DNA, five dinucleosides composed of adenosine or 2'-deoxyadenosine units joined by flexible linkers were studied by femtosecond transient absorption spectroscopy. In aqueous solution, transient absorption signals recorded at pump and probe wavelengths of 267 and 250 nm, respectively, show that UV absorption produces excimer states in all dimers that decay orders of magnitude more slowly than excitations in a single adenine nucleotide. Adding methanol as a cosolvent disrupts π-π stacking of the adenine moieties and causes the excimer states in all five dinucleosides to vanish for a methanol concentration of 80% by volume. These observations confirm that base stacking is an essential requirement for the slow decay channel seen in these and other DNA model compounds. This channel appears to be insensitive to the precise stacking conformation at the instant of photon absorption as long as the bases are cofacially stacked. Notably, circular dichroism (CD) spectra of several of the dinucleosides are weak and monomer-like and lack the exciton coupling that has been emphasized in the past as an indicator of base-stacked structure. For these dimers, the coupled transition dipole moments of the two adenines are proposed to adopt left- and right-handed arrangements upon stacking with roughly equal probability. Although the mechanism behind slow nonradiative decay in DNA is still uncertain, these results show that the signature of these states in transient absorption experiments can be a more reliable diagnostic of base stacking than the occurrence of exciton-coupled CD signals. These observations also draw attention to the important role the backbone plays in producing structures with axial (helical) chirality.

Environmental Effects on Photochemical Properties of Betaine Dyes in Rigid Media

We report luminescence of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (B30)- or other betaine dye-dispersed rigid media at 77 K and at room temperature (r.t.). Their luminescence in acetone at 77 K were remarkably influenced by water addition. UV-excited blue ∼ bluish green luminescence of B30-dispersed polymers at r.t. we reported in our previous reports appeared to resemble that of B30-dispersed acetone at 77 K in the presence of water. It suggests the participation of protonation and/or deprotonation processes in the UV-excited states of B30. A weak emission shoulder in visible emission wavelength range was also observed for B30-dispersed polymeric solids at r.t., similar to those emission bands for betaine dye-dispersed polymeric solids or organic solvents at 77 K.

Theoretical study on the UV excited states of UF 6 using the discrete-variational Dirac—Slater calculation method

We have performed the relativistic molecular orbital calculations for the ground and UV excited states of UF 6, using the discrete-variational Dirac—Slater method. For the ground state of UF 6, the formation of the UF bonding is explained by the relativistic MOs, in contrast to the non-relativistic MOs which fail to give the bonding character between the U and F atoms. The theoretical excitation energies for the charge transfer transitions are in good agreement with the experimental UV absorption spectrum. The changes in the bond overlap population of the excitation can explain the experiments that the excitation in the A—X band leads to fluorescence, while that in the B—X band to the dissociation of UF bonding. This suggests that the difference in the character of the de-excitation processes is mainly determined by the bonding nature for the initial and the final levels on the excitation.