1B2u State Research Articles

Ab Initio Quantum Study of Nonadiabatic S1–S2 Photodynamics of s-trans-Butadiene

The nonadiabatic photodynamics of s-trans-butadiene in its lowest singlet excited states is studied theoretically, using a fully quantal approach. The coupled 1Bu and 2Ag states are considered in the calculation, representing the lowest dipole-allowed electronic transition, and the dipole-forbidden state with substantial double-excitation character, respectively. Up to six nuclear degrees of freedom, including out-of-plane dihedral angles, are included. The calculation of the underlying potential energy surfaces relies on the CASPT2 method, where widely different CAS spaces have been compared. The ultrafast electronic population decay is confirmed, proceeding on a time scale of 30-40 fs. Pronounced out-of-plane distortions are obtained for the first time from fully quantal calculations. The complexity of the electronic absorption spectrum increases substantially upon including additional vibrational modes in the calculation. Further computations were performed to facilitate inclusion of the coupling to the ground state in subsequent work.

Energy Transfer Pathways in Light-Harvesting Complexes of Purple Bacteria as Revealed by Global Kinetic Analysis of Two-Dimensional Transient Spectra

Excited state dynamics in LH2 complexes of two purple bacterial species were studied by broad-band two-dimensional electronic spectroscopy. The optical response was measured in the 500-600 nm spectral region on the 0-400 fs time scale. Global target analysis of two-dimensional (2D) transient spectra revealed the main energy transfer pathways between carotenoid S2, 1Bu(-) and S1 states and bacteriochlorophyll Qx state. Global analysis ascertained the evolutionary and vibration-associated spectra, which also indicated the presence of a higher-lying vibrational level in the carotenoid S1 state. The estimation of the spectral overlap between the 1Bu(-) state and the Qx state indicated a significant contribution of the 1Bu(-) state to the overall S2-to-Qx excitation energy transfer.

TD-DFT and DFT/MRCI study of electronic excitations in Violaxanthin and Zeaxanthin

We report vibrationally broadened Franck–Condon (FC) spectra of Violaxanthin (Vx) and Zeaxanthin (Zx) for the lowest-energy 1Ag→1Bu band that arises from the bright HOMO→LUMO single-electron excitation. Geometries were optimized using standard (1Ag) and time-dependent (1Bu) density functional theory (DFT) at the (TD-)CAM-B3LYP/6-31G(d) level, both in the gas phase and in acetone using a polarizable continuum model (PCM). DFT/MRCI multireference calculations were performed at the optimized (TD)-CAM-B3LYP structures to evaluate the energies of doubly excited states that are not accessible to linear response TD-DFT theory. The FC spectra were calculated using the time-independent (TI) scheme. The calculated spectra of Vx and Zx are very similar, with a red shift of about 0.1eV for Zx relative to Vx, which is in agreement with the experimental data. The predicted spectral peaks of Vx and Zx deviate from experiment by less than 0.1eV when performing the calculations in the gas phase. In the presence of acetone (PCM model), there are larger deviations so that a state specific correction scheme needs to be applied, which accounts for non-equilibrium solvent relaxation. The 1Ag→1Bu vertical absorption energies and the corresponding vertical fluorescence energies from TD-CAM-B3LYP and DFT/MRCI agree reasonably well. The DFT/MRCI absorption and fluorescence energies for the doubly excited 2Ag and 2Bu states are found to be rather sensitive to the underlying geometry, in particular to the bond length alternation in the polyene chain. In acetone (PCM), Vx and Zx show little bond alternation, and thus the doubly excited Bu state becomes the lowest excited Bu state. (TD)-CAM-B3LYP appears to be suitable for generating realistic geometries for higher-level calculations in such molecules.

Photophysics and photostability of 9,10-bis(phenylethynyl)anthracene revealed by single-molecule spectroscopy

The photophysics and photostability of 9,10-bis(phenylethynyl)anthracene (BPEA) diluted in a 40-nm-thick Zeonex polymer film have been investigated by single-molecule spectroscopy (SMS). The single-molecule detection of BPEA was verified by recording fluorescence intensity trajectories, fluorescence lifetimes, and fluorescence spectra. The intensity trajectories showed frequent on/off blinking and one-step photobleaching behaviors. The observed blinking was attributed to the temporary occupation of the excited triplet state T1via intersystem crossing (ISC). Assuming a three-state model (e.g., S0, S1, and T1), the distributions of triplet lifetime and S1→T1 ISC quantum yield of BPEA were both derived from the analyses of the blinking statistics and the intensity autocorrelation. We found extremely low ISC yields (on the order of 10−5–10−4), which were theoretically rationalized by the large energy gap between 3B2u and S1(1B1u)/T1(3B1u) states. SMS measurements were also conducted under both air and Ar atmospheres in order to gain insight into the influence of oxygen on photobleaching. The results reveal that, although the presence of oxygen considerably degraded the photostability of BPEA, under deoxygenated conditions, BPEA delivers more than 107 photons before photobleaching and possesses an appreciably low photobleaching yield of 10−9–10−8. This study shows that BPEA has a relatively high degree of photostability at room temperature and can serve as a useful green fluorescent probe for SMS studies.

Absorption spectra of α,ω-diphenylhexadecaoctaene and shorter diphenylpolyenes

Absorption spectrum of α,ω-diphenylhexadecaoctaene has been measured along with those of α,ω-diphenylpolyenes with one to seven double bonds in the polyene chain in carbon tetrachloride at room temperature. Extrapolation of the observed vibrational frequencies in the 11Bu state measured as a function of polyene double bond number provides the CC and CC stretching frequencies of 1520±20 and 1080±20cm−1, respectively, for diphenylpolyenes with infinite polyene chain length.

Solute–solvent intermolecular vibronic coupling as manifested by the molecular near-field effect in resonance hyper-Raman scattering

Vibronic coupling within the excited electronic manifold of the solute all-trans-β-carotene through the vibrational motions of the solvent cyclohexane is shown to manifest as the "molecular near-field effect," in which the solvent hyper-Raman bands are subject to marked intensity enhancements under the presence of all-trans-β-carotene. The resonance hyper-Raman excitation profiles of the enhanced solvent bands exhibit similar peaks to those of the solute bands in the wavenumber region of 21,700-25,000 cm(-1) (10,850-12,500 cm(-1) in the hyper-Raman exciting wavenumber), where the solute all-trans-β-carotene shows a strong absorption assigned to the 1A(g) → 1B(u) transition. This fact indicates that the solvent hyper-Raman bands gain their intensities through resonances with the electronic states of the solute. The observed excitation profiles are quantitatively analyzed and are successfully accounted for by an extended vibronic theory of resonance hyper-Raman scattering that incorporates the vibronic coupling within the excited electronic manifold of all-trans-β-carotene through the vibrational motions of cyclohexane. It is shown that the major resonance arises from the B-term (vibronic) coupling between the first excited vibrational level (v = 1) of the 1B(u) state and the ground vibrational level (v = 0) of a nearby A(g) state through ungerade vibrational modes of both the solute and the solvent molecules. The inversion symmetry of the solute all-trans-β-carotene is preserved, suggesting the weak perturbative nature of the solute-solvent interaction in the molecular near-field effect. The present study introduces a new concept, "intermolecular vibronic coupling," which may provide an experimentally accessible∕theoretically tractable model for understanding weak solute-solvent interactions in liquid.

ChemInform Abstract: Resonance Raman Scattering of Butadiene: Vibronic Activity of a bu Mode Demonstrates the Presence of a 1Ag Symmetry Excited Electronic State at Low Energy

Resonance Raman spectra of buta‐1,3‐diene‐d0 and buta‐1,3‐diene‐1,1,4,4‐d4 have been obtained with ultraviolet excitation from 239.5 to 199.9 nm. Activity of the first overtone of mode 24, the bu symmetry CCC chain deformation mode, is observed with excitation energy below the origin of the 1 1Bu state. This vibronic activity of a nontotally symmetric mode is shown to be evidence of resonance with the 2 1Ag state of butadiene. A quantitative analysis of the ratio of intensities of 2ν24 to ν9, the ag symmetry CCC chain deformation mode, demonstrates that enhancement of 2ν24 cannot be due to resonance with the 1 1Bu state. The resonance enhancement behavior of this overtone band also shows that it is of vibronic origin rather than Franck–Condon allowed. The intensity pattern seen for the modes of bu symmetry is fully consistent with the results of a quantitative calculation of vibronic activity for the eight bu symmetry modes. The 2 1Ag electronic state is estimated to be ca. 0.25 eV below the 1 1Bu electro...

ChemInform Abstract: Resonant Third-Harmonic Generation in all-trans β-Carotene: The Vibronic Origins of the Third-Order Nonlinear Susceptibility in the Visible Region.

The dispersion of the modulus of the third‐order nonlinear susceptibility, ‖χ(3)zzzz(−3ω;ω,ω,ω)‖, of an all‐trans β‐carotene doped polystyrene thin film has been measured by third‐harmonic generation throughout the 1 1Bu absorption band of the β‐carotene. The observed dispersion is interpreted in terms of a three‐electronic state model including explicit regard to six Franck–Condon active normal modes in the resonant electronic state. Both the sum‐over‐states method and, equivalently, the time domain approach are used. A fit of the three‐state model to the data shows that the large nonlinear susceptibility of β‐carotene in this wavelength range can be explained by strong dipole coupling between the strong one‐photon allowed 1 1Bu state and a nearby two‐photon allowed n 1Ag state. The electric dipole transition moment between the 1 1Bu excited state and the n 1Ag excited state, located at 22 200 cm−1, was determined to be 31 D.

ChemInform Abstract: Hydrogen Bonding in the Symmetry-Equivalent C2h Dimer of 2-Pyridone in Its S0 and S2 Electronic States. Effect of Deuterium Substitution.

The properties of the two intermolecular N–H⋅⋅⋅O bonds that are responsible for the formation of the cis‐peptide‐like dimer of 2‐pyridone (2PY) have been examined using deuterium substitution of the bridging hydrogen atoms as a probe. Studies of the fully resolved S2←S0 electronic spectrum of (2PY)2 in a molecular beam show that the protonated dimer has a symmetry‐equivalent planar C2h structure in its S0(1Ag) and S2(1Bu) states. Analogous studies of (2PY)2–d1 and (2PY)2–d2 show that (2PY)2 and (2PY)2–d2 are energy delocalized dimers in their S2 states, with an exciton splitting of less than 20 cm−1. Effective structures of the symmetric dimers in both states are derived from the measured rotational constants. A comparison of these structures shows that there is a distortion of the hydrogen‐bonding geometry when hydrogen is replaced by deuterium, along both in‐plane and out‐of‐plane coordinates. ΔR(N–H⋅⋅⋅O)=0.008 A, Δθ[C=O⋅⋅⋅(H)–N]=0°, and Δφ (the dihedral angle)=0.96° in S0 (2PY)2–d2 and ΔR=0.003 A, Δθ=0...

Excited states of all-trans and 11-cis retinal: All valence-electron SCF-MO-CI calculations

All valence-electron (cndo/s) scf-mo-ci calculations are carried out on the excited states of a number of conformers of all-trans and 11-cis retinal. Singlet state transition energies, oscillator strengths, polarization directions, and ground and excited state dipole moments are examined, and the relationship of the retinal 1ππ* states to polyene 1ππ* states is analyzed. Particular emphasis is given to the nature and location of the cis-band, and to the relative location of the 1ππ* and 1Bu states. Several aspects of the electronic structure of triplet states are also examined.

Ab initio studies of excited electronic state S2 of pyrazine and Franck–Condon simulation of its absorption spectrum

The equilibrium geometry and 24 vibrational-normal-mode frequencies of the excited state S2(1B2u) of pyrazine are calculated and characterized using the complete active space self-consistent field method in the adiabatic representation. The displaced harmonic oscillator approximation is used to simulate the absorption spectrum of the S2(1B2u) state along with the Franck–Condon approximation. It is found that the totally symmetric mode ν1 plays the most important role and this exactly agrees with the experimental observations. The simulated absorption spectrum agrees well with those experimentally observed. This indicates that the present S2(1B2u) state calculated in the adiabatic representation effectively includes contribution from the diabatic vibronic coupling through the conical intersection.

On the controversial nature of the 1 B1u and 2 B1u states of trans-stilbene: The n-electron valence state perturbation theory approach

The nature of two lowest-energy states of B(u) symmetry of trans-stilbene and the accurate calculation of their vertical excitation energy have been the subject of a controversy because time dependent density functional theory (TD-DFT) calculations, in agreement with experimental observations, have questioned the results obtained with multireference perturbation theory (MRPT) in the CASPT2 implementation. This paper aims to solve this controversy. By using a different version of MRPT, the n-electron valence state perturbation theory method, the description provided by TD-DFT is confirmed: the lowest B(u) singlet state has a HOMO-->LUMO nature and a large oscillator strength (HOMO refers to highest occupied molecular orbital and LUMO refers to lowest unoccupied molecular orbital), while the second B(u) singlet state has a mixed HOMO-1-->LUMO and HOMO-->LUMO+1 nature, has an oscillator strength almost vanishing, and is located at 0.6-0.7 eV higher than the first excited state. The computed vertical excitation energy to the first state (3.8-4.0 eV) is in good agreement with the experimental value.

The vibronic structures of absorption and magnetic circular dichroism (MCD) in the low energy 1 1B 2u and 1 1B 3u states of 1,4,5,8-naphthalenetetracarboxy dianhydride. The analysis in terms of DFT and CASSCF methods

The magneto-optical properties in low energy 1 1B 2u and 1 1B 3u states of 1,4,5,8-naphthalenetetracarboxy dianhydride (NTCDA) are examined in terms of time-dependent density functional theory at TD-B3LYP/aug-cc-pVDZ and TD-SVWN/aug-cc-pVDZ levels. The Franck–Condon (FC) analysis performed for two 1 1A g → 1 1B 2u and 1 1A g → 1 1B 3u overlapping transitions led to excellent agreement between the theory and experiment providing that the calculated absorption and MCD spectra are confronted with those measured in chloroform. We argue that absorption and MCD spectra measured in ethanol solution and already known in literature do not characterize NTCDA molecule. Rather, these spectra diagnose a product of chemical reaction between NTCDA and ethanol solvent.

Theoretical study on S1(1B3u) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S0(1Ag)) and the first (S1(1B3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S1(1B3u) state of pyrazine. The assignment of main vibronic transitions is made for S1(1B3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S1(1B3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S1(1B3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν10a. The calculated S1(1B3u) absorption spectrum is in reasonable agreement with the experimental spectra.

Electronic relaxation and vibrational dynamics in a thiophene oligomer studied under the same experimental condition with a sub-5 fs laser

Using a sub-5 fs pulse laser, the pump–probe experiment was performed on a quinoid thiophene derivative and both electronic relaxation and vibrational dynamics were clarified with the same experimental data. From the data, two population decay times of the lowest electronic excited state were determined to be 200 fs and 1.8 ps, which can be explained by the assumption that the 21Ag state lies below the exciton 11Bu state together with the very low fluorescence quantum efficiency. The relaxation process after excitation by the pump pulse was studied to be 11Bu→11B*u→21Ag→11Ag. According to the experimental data, the location of the energy level was also discussed. The electronic dephasing time was determined to be 64 ± 4 fs by utilizing the data in the ‘negative time’ range. The vibrational dephasing time constant of the most strongly coupled mode with frequency of 1466 cm-1 was determined to be 520 ± 20 fs from the widths of the corresponding Fourier power spectra.

11Bu (S2) and 21Ag (S1) Fluorescence and the 21Ag State of α,ω-Dithienylbutadiene and α,ω-Dithienylethylene

Fluorescence and absorption spectra and fluorescence lifetimes have been measured for alpha,omega-dithenylbutadiene (DTB) and alpha,omega-dithenylethylene (DTE) in different solvents and at different temperatures as well as in the vapor phase. It is shown that the emission of DTB consists of the S(2) (1(1)B(u)) and S(1) (2(1)A(g)) fluorescence, while the emission of DTE consists of solely the S(1) (1(1)B(u)) fluorescence. The 2(1)A(g) and 1(1)Bu state levels of DTB are found to invert in solvents with the polarizability of near 0.32. All the available data for DTB on temperature and solvent dependence of the absorption and fluorescence spectra and the fluorescence lifetime support the presence of the lowest excited singlet state, 2(1)A(g), located below the strongly allowed 1(1)B(u) state. Quantitative analyses of the spectral data provide the physical parameters that characterize the excited state dynamical behavior of DTB.

Fluorescence spectra of all trans 1,4-diphenylbutadiene obtained by the two-photon excitation into the 2 1Ag state

The fluorescence spectra obtained by the two-photon excitation into the 2 1Ag state of diphenylbutadiene (DPB) have been measured in room temperature solutions, along with the one- and two-photon absorption spectra. It is shown that DPB molecule excited into the two-photon allowed 2 1Ag state through the two-photon process exhibits the fluorescence from the two-photon forbidden 1 1Bu state irrespective of the solvent polarizability. Relaxation processes followed by the two-photon excitation into the 2 1Ag state, and the shape of the two-photon absorption spectra measured when the 2 1Ag origin is located below the 1 1Bu origin are discussed.

Investigation of the Electronic Spectra and Excited-State Geometries of Poly(para-phenylene vinylene) (PPV) and Poly(para-phenylene) (PP) by the Symmetry-Adapted Cluster Configuration Interaction (SAC-CI) Method

The symmetry-adapted cluster-configuration interaction (SAC-CI) method has been used to investigate the optical and geometric properties of the oligomers of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PP). Vertical singlet and triplet absorption spectra and emission spectra have been calculated accurately; the mean average deviation from available experimental results lies within 0.2 eV. The chain length dependence of the transition energies has been improved in comparison to earlier TDDFT and MRSDCI calculations. The present analysis suggests that conventional TDDFT with the B3LYP functional should be used carefully, as it can provide inaccurate estimates of the chain length dependence of the excitation energies of these molecules with long pi conjugation. The T1 state was predicted to be at a lower energy, by 1.0-1.5 eV for PPV and by 0.9-1.7 eV for PP, than the S1 state, which indicates a localized T1 state with large exchange energy. By calculating the SAC-CI electron density difference between the ground and excited states, the geometry relaxations due to excitations can be analyzed in detail using electrostatic force theory. For trans-stilbene, the doubly excited 21Ag state was studied, and the calculated transition energy of 4.99 eV agrees very well with the experimental value of 4.84 eV. In contrast to previous ab initio calculations, we predict this doubly excited 21Ag state to lie above the 11Bu state.

Fluorescence Studies of Terrylene in a Supersonic Jet: Indication of A Dark Electronic State Below the Allowed Transition

Jet-cooled terrylene has been studied in helium buffer gas using a pulsed nozzle by means of laser-induced fluorescence. Fluorescence excitation and two-color depletion experiments (resulting in hole burning spectra) are presented. Analysis of the spectra leads to the conclusion that another excited electronic state is present in the vicinity of the allowed 1B1u state. Assuming (according to previous literature suggestions Karabunarliev, S.; Baumgarten, M.; Müllen, K. J. Phys. Chem. A 1998, 102, 7029) that this dark state is the 21Ag state, we discuss the vibrational structure of the fluorescence excitation spectrum in terms of two manifolds of vibronic states belonging to Sd(21Ag) and S1(1B1u) states. The anomalous shift between excitation and dispersed fluorescence spectra observed earlier for terrylene in a neon matrix is discussed as a consequence of terrylene electronic relaxation to the low-energy dark state.

Photophysics of α,ω-Diphenyloctatetraene in the Vapor Phase

Fluorescence and fluorescence excitation spectra of diphenyloctatetraene vapor have been measured at different temperatures from 98 to 136 degrees C and at different buffer gas pressures from 0 to 300 Torr. The fluorescence quantum yields were determined as functions of the excitation energy and buffer gas pressure. It is shown that diphenyloctatetraene vapor exhibits weak fluorescence from the S2 (1(1)Bu) state in addition to the fluorescence from the S1 (2(1)Ag) state. The quantum yield of the S1 fluorescence is shown to decrease with decreasing pressure and with increasing excitation energy. The electronic relaxation processes of diphenyloctatetraene vapor are discussed based on the pressure and excitation-energy dependence of the fluorescence quantum yield.