Excited S1 Research Articles

GW-BSE approach on S1 vertical transition energy of large charge transfer compounds: A performance assessment.

In this work, we apply many-body perturbation theory (MBPT) on large critical charge transfer (CT) complexes to assess its performance on the S1 excitation energy. Since the S1 energy of CT compounds is heavily dependent on the Hartree-Fock (HF) exchange fraction in the reference density functional, MBPT opens a new way for reliable prediction of CT S1 energy without explicit knowledge of suitable amount of HF-exchange, in contrary to the time-dependent density functional theory (TD-DFT), where depending on various functionals, large errors can arise. Thus, simply by starting from a (semi-)local reference functional and performing update of Kohn-Sham (KS) energies in the Green's function G while keeping dynamical screened interaction (W(ω)) frozen to the mean-field level, we obtain impressingly highly accurate S1 energy at slightly higher computational cost in comparison to TD-DFT. However, this energy-only updating mechanism in G fails to work if the initial guess contains a fraction or 100% HF-exchange, and hence considerably inaccurate S1 energy is predicted. Furthermore, eigenvalue updating both in G and W(ω) overshoots the S1 energy due to enhanced underscreening of W(ω), independent of the (hybrid-)DFT starting orbitals. A full energy-update on top of HF orbitals even further overestimates the S1 energy. An additional update of KS wave functions within the Quasi-Particle Self-Consistent GW (QSGW) deteriorates results, in stark contrast to the good results obtained from QSGW for periodic systems. For the sake of transferability, we further present data of small critical non-charge transfer systems, confirming the outcomes of the CT-systems.

TD-M06-2X insights into the absorption and emission spectra of dichlorvos and its molecularly imprinted recognition by methacrylic acid.

The absorption and emission spectra of dichlorvos and the dichlorvos-MAA complex in methanol, water, and chloroform in the molecularly imprinted recognition were investigated systematically. The M06-2X results revealed that: 1) the hydroxyl groups in polar solvents such as methanol and water may markedly influence the weak interactions, and then alter the adsorption and emission spectra; 2) the electronic excitation in absorption spectra of dichlorvos is dominated by the configuration HOMO → LUMO, but in the most stable dichlorvos-MAA it becomes the ππ* excitation of HOMO → LUMO + 1; 3) Mulliken charges reveal that dichlorvos almost dissociates to Cl- and a cation in its S1 excitation state; 4) the phosphorescence spectra of dichlorvos-MAA are relatively weak. Graphical Abstract The absorption and emission spectra of dichlorvos and the dichlorvos-MAA complex in the molecularly imprinted recognition of dichlorvos were investigated systematically in methanol, water, and chloroform as solvents.

Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling.

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Hitherto, it has not been established if it involves a higher lying "dark" state or vibrational relaxation in the excited S2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic S2 potential.

Intersystem crossing-branched excited-state intramolecular proton transfer for o-nitrophenol: An ab initio on-the-fly nonadiabatic molecular dynamic simulation.

The 6SA-CASSCF(10, 10)/6-31G (d, p) quantum chemistry method has been applied to perform on-the-fly trajectory surface hopping simulation with global switching algorithm and to explore excited-state intramolecular proton transfer reactions for the o-nitrophenol molecule within low-lying electronic singlet states (S0 and S1) and triplet states (T1 and T2). The decisive photoisomerization mechanisms of o-nitrophenol upon S1 excitation are found by three intersystem crossings and one conical intersection between two triplet states, in which T1 state plays an essential role. The present simulation shows branch ratios and timescales of three key processes via T1 state, non-hydrogen transfer with ratio 48% and timescale 300 fs, the tunneling hydrogen transfer with ratios 36% and timescale 10 ps, and the direct hydrogen transfer with ratios 13% and timescale 40 fs. The present simulated timescales might be close to low limit of the recent experiment results.

According theory and experiment in CaH: Laser-induced fluorescence study of new B/B′–X bands in the UV region

Despite the astrophysical importance of calcium monohydride (CaH), a long-standing discrepancy exists between the experimental and theoretical analysis of its first two excited 2Σ+ states. In a bid to resolve this discrepancy, we observed the rotationally-resolved laser-induced fluorescence spectrum of CaH in the 23,300–27,800cm−1 region. We assigned all newly observed vibrational levels, and five levels previously assigned to the D state, to the B/B′ state. The level properties alternate strongly with vibrational excitation and this new assignment brings the experimental vibronic structure into remarkably good agreement with the predictions of Carlsund-Levin et al. (2002).

Control of Nonadiabatic Passage through a Conical Intersection by a Dynamic Resonance.

Nonadiabatic processes, dominated by dynamic passage of reactive fluxes through conical intersections (CIs), are considered to be appealing means for manipulating reaction paths, particularly via initial vibrational preparation. Nevertheless, obtaining direct experimental evidence of whether specific-mode excitation affects the passage at the CI is challenging, requiring well-resolved time- or frequency-domain experiments. Here promotion of methylamine-d2 (CH3ND2) molecules to spectral-resolved rovibronic states on the excited S1 potential energy surface, coupled to sensitive D photofragment probing, allowed us to follow the N-D bond fission dynamics. The branching ratios between slow and fast D photofragments and the internal energies of the CH3ND(X̃) photofragments confirm correlated anomalies for predissociation initiated from specific rovibronic states. These anomalies reflect the existence of a dynamic resonance that strongly depends on the energy of the initially excited rovibronic states, the evolving vibrational mode on the repulsive S1 part during N-D bond elongation, and the manipulated passage through the CI that leads to CH3ND radicals excited with C-N-D bending. This resonance plays an important role in the bifurcation dynamics at the CI and can be foreseen to exist in other photoinitiated processes and to control their outcome.

Packing of Large Two- and Three-Photon Activity Into Smallest Possible Unsymmetrical Fluorene Chromophores.

The quantum chemical study of one-, two-, and three-photon absorption (1PA, 2PA, and 3PA) properties for a set of compact fluorene derivatives (FD) with combination of different donor and acceptor moieties on both sides of fluorene ring system is presented. The main goal of the study is to pack large two-photon (2P) and three-photon (3P) activity into smallest possible chromophore. Linear, quadratic, and cubic response time-dependent density functional theory was used to calculate 1PA, 2PA, and 3PA properties, respectively. We used CAMB3LYP/cc-pVDZ level of theory for all the property calculations. The 2P and 3P transition probabilities were recalculated using two-state model approach and found to be in good agreement with the response theory results for first excited state. To include the contributions from higher states, the three-state model was also employed to recalculate the 2P transition probabilities and found to be in excellent agreement with response theory. The 2P/3P tensor elements were also analyzed to find reasons behind large 2P/3P activities. All the orbitals involved in transition processes were studied in detail by both molecular orbital pictures (qualitatively) and overlap diagnostic Λ-values (quantitatively). The study reveals that the novel fluorene derivatives FD-12 and FD-13 have shown large 2PA cross-section values of 1100 G.M. and 1030 G.M.; and 3PA transition probabilities of 6.10 × 10(10) a.u. and 4.85 × 10(10) a.u., respectively, for transition S0 → S1. The largest 3PA transition probability of 4.04 × 10(11) a.u. was found with FD-12 for S0 → S2 excitation. The linear relationship between Λ-values and 2PA cross-section values was also studied.

Ultrafast excited-state deactivation of 9-methylhypoxanthine in aqueous solution: A QM/MM MD study.

Photoinduced ultrafast non-adiabatic decay of 9-methylhypoxanthine (9MHPX) in aqueous solution was investigated by ab initio surface-hopping dynamics calculations using a combined quantum mechanical/molecular mechanical approach. The absorption spectra of 9MHPX in aqueous solution were also explored by the hybrid cluster-continuum model at the level of time-dependent density functional theory along with the polarizable continuum model (PCM). The static electronic-structure calculations indicate that the absorption spectra of 9MHPX simulated by TD-B3LYP/PCM and TD-X3LYP/PCM can reproduce very well the experimental findings, with the accuracy of about 0.20 eV. According to dynamics simulations, irradiation of 9MHPX populates the bright excited singlet S1 state, which may undergo an ultrafast non-radiative deactivation to the S0 state. The lifetime of the S1 state of 9MHPX in aqueous solution is predicted to be 115.6 fs, slightly longer than that in the gas phase (88.8 fs), suggesting that the solventwater has no significant influence on the excited-state lifetime of 9MHPX. Such a behavior in 9MHPX is distinctly different from its parent hypoxanthine keto-N9H tautomer in which the excited-state lifetime of the latter in watersolution was remarkably enhanced as compared to the gas phase. The significant difference of the photodynamical behaviors between 9MHPX and keto-N9H can be ascribed to their different hydrogen bond environment in aqueous solution.

Femtosecond Heterodyne Transient Grating Studies of Nonradiative Deactivation of the S2 (1(1)Bu(+)) State of Peridinin: Detection and Spectroscopic Assignment of an Intermediate in the Decay Pathway.

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state of peridinin in methanol solution. Just as previously observed by this laboratory for β-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S2 state decays in 12 fs to produce an intermediate state, Sx. The excited state absorption spectrum from the Sx state of peridinin, however, is found to be markedly blue-shifted from that of β-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S2 state rather than to a vibrationally excited (or hot) S1 state. The Sx state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation-energy barrier between planar and twisted structures on the S2 potential surface. The lengthened lifetime of the Sx state of peridinin in methanol, 900 ± 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, ∼150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system-bath coupling is significantly enhanced over that in β-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the Sx state, and the distorted structures reached subsequently along the torsional gradient on the S2 potential surface, may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin-chlorophyll a protein from dinoflagellates.

Proton Quantization and Vibrational Relaxation in Nonadiabatic Dynamics of Photoinduced Proton-Coupled Electron Transfer in a Solvated Phenol-Amine Complex.

Nonadiabatic dynamics simulations of photoinduced proton-coupled electron transfer (PCET) in a phenol-amine complex in solution were performed. The electronic potential energy surfaces were generated on-the-fly with a hybrid quantum mechanical/molecular mechanical approach that described the solute with a multiconfigurational method in a bath of explicit solvent molecules. The transferring hydrogen nucleus was represented as a quantum mechanical wave function calculated with grid-based methods, and surface hopping trajectories were propagated on the adiabatic electron-proton vibronic surfaces. Following photoexcitation to the excited S1 electronic state, the overall decay to the ground vibronic state was found to be comprised of relatively fast decay from a lower proton vibrational state of S1 to a highly excited proton vibrational state of the ground S0 electronic state, followed by vibrational relaxation within the S0 state. Proton transfer can occur either on the highly excited proton vibrational states of S0 due to small environmental fluctuations that shift the delocalized vibrational wave functions or on the low-energy proton vibrational states of S1 due to solvent reorganization that alters the asymmetry of the proton potential and reduces the proton transfer barrier. The isotope effect arising from replacing the transferring hydrogen with deuterium is predicted to be negligible because hydrogen and deuterium behave similarly in both types of proton transfer processes. Although an isotope effect could be observed for other systems, in general the absence of an isotope effect does not imply the absence of proton transfer in photoinduced PCET systems. This computational approach is applicable to a wide range of other photoinduced PCET processes.

Theoretical study of the effect of π-conjugated transmitter of D–π–A ruthenium systems on the quadratic NLO properties

In this work, we present the theoretical quantum chemical calculation of effect of π-conjugated transmitter variation of two types of acceptor based on barbituric acid (with R=H or R=Me) of the organometallic molecules. The one-photon absorption OPA, dipole moments (μ), first hyperpolarizabilities (β), frontier molecular orbital energies based on the semi-empirical ZINDO/1 method within a framework of the restricted Hartree–Fock approach have been calculated. This method was employed to study relationship between first hyperpolarizabilities and the a kind of transmitter of π-conjugated ligand in the investigated organometallic compounds. Using this method, we found excellent agreement with experimental S1 → S0 excitation energies. The investigated complexes show large optical second order nonlinearities in comparison with metal alkynyl and related complexes.

Structural and photophysical properties of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone dyes

A series of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone compounds were synthesized. Spectroscopic and photophysical properties of these compounds have been measured in a variety of solvents. Absorption and fluorescence maxima have been correlated with the Empirical Scale of Solvent Polarity (ET(30)). Theoretical TD-DFT spectral calculations and Lippert–Mataga analysis support the internal charge transfer (ICT) nature of the S0 → S1 excitation for these compounds, with higher degrees of ICT depicted for the alkylamino substituted 2,5-diarylidene cyclopentanones. Photophysical properties consisted of measuring the fluorescence quantum yields (Φf) and lifetimes (τf) in a variety of solvents. Radiative and nonradiative decay constants have been determined from the Φf and τf data. Variation with solvent in the nonradiative rate of decay is interpreted in terms of a competition between internal conversion and intersystem crossing. Lastly, two compounds presented have been shown to undergo excited state protonation in glacial acetic acid.

Chiral conversion and periodical decay in bridged-azobenzene photoisomerization: an ab initio on-the-fly nonadiabatic dynamics simulation

On-the-fly trajectory surface hopping dynamics simulations on the cis ↔ trans photoisomerization mechanisms of bridged-azobenzene upon S1 excitation at the CASSCF level.

Application of spectroscopic and theoretical methods in the studies of photoisomerization and photophysical properties of the push-pull styryl-benzimidazole dyes.

The synthesis and spectroscopic properties of a series of substituted 1,3-dimethyl-2-aminostyrylbenzimidazolium iodides are described and discussed. The products were identified by NMR, IR and UV-Vis spectroscopy and elemental analysis. Their electronic absorption and fluorescence band positions are affected by the character of the substituent and by the solvent polarity. The fluorescence decay of the dyes shows two lifetimes interpreted in terms of emission from two forms of the dye in the excited state. Moreover, the photochemical trans→cis isomerization is reported for these compounds. It occurs from the first excited singlet state of the trans isomer to the cis isomer following a trans-S0→ S1 excitation. The electron-donating character of the substituent in a styrene moiety is one of the crucial factors influencing the photoisomerization process. The structure of the cis isomer was established by (1)H and (15)N NMR. Experimental studies are supported by the results of quantum chemical calculations.

The mechanism of excited state proton dissociation in microhydrated hydroxylamine clusters.

The dynamics and mechanism of excited-state proton dissociation and transfer in microhydrated hydroxylamine clusters are studied using NH2OH(H2O)n (n = 1-4) as model systems and the DFT/B3LYP/aug-cc-pVDZ and TD-DFT/B3LYP/aug-cc-pVDZ methods as model calculations. This investigation is based on the Förster acidity scheme and emphasizes the photoacid dissociation in the ground (S0) and lowest singlet-excited states (S1) and the interplay between the photo and thermal excitations. The quantum chemical results suggest that the intermediate complexes are formed only in the S1 state in a low local-dielectric environment (e.g., ε = 1) and that upon the S0→ S1 transition, the photon energy excites mostly NH2OH, which leads to a homolytic cleavage of the O-H bond and to dynamically stable charge-separated Rydberg-like H-bond complexes (e.g., NH2O˙-H3O(+)˙). The potential energy surfaces for proton displacement in the smallest Rydberg-like H-bond complex support the intersection of the S0 and S1 states in low local-dielectric environments, whereas in a high local-dielectric environment (e.g., ε = 78), these two states are completely separated. Based on the static results, a photoacid-dissociation mechanism that involves Rydberg-like H-bond complex formation, an H-bond chain extension and fluctuations in the local-dielectric environment is proposed. NVT-BOMD simulations confirm the static results and show that the dynamic behavior of the dissociating proton in the S1 state is not different from that of the protonated H-bond systems in the ground state, which consists of the oscillatory shuttling and structural diffusion motions. These findings allow our theoretical methods, which have been used successfully in protonated H-bond systems in the ground state, to be applied in the study of the photoacid-dissociation processes. The current theoretical study suggests effective steps as well as guidelines for the investigation of the dynamics of the photoacid-dissociation and transfer processes in the Förster acidity scheme, provided that the exciting photon does not lead to a significant change in the structure of the intermediate complex in the excited state.

Metal–organic green dye: chemical and physical insight into a modified Zn-benzoporphyrin for dye-sensitized solar cells

A novel green benzoporphyrin has been synthesized, characterized, studied by theoretical methods and tested in DSSC devices. Ab initio simulations predict the actual charge displacement during S0 → S1 excitation.

Direct photoisomerization of CH2I2vs. CHBr3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study.

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S1 excitation of both CH2I2 and CHBr3 in the gas phase. The photochemical conversion process is direct and intramolecular, i.e., it proceeds without caging media that have long been implicated in the photo-induced isomerization of polyhalogenated alkanes in condensed phases. Using multistate complete active space second order perturbation theory (MS-CASPT2) calculations, we investigate the structure of the photochemical reaction paths connecting the photoexcited species to their corresponding isomeric forms. Unconstrained minimum energy paths computed starting from the S1 Franck-Condon points lead to S1/S0 conical intersections, which directly connect the parent CHBr3 and CH2I2 molecules to their isomeric forms. Changes in the chemical bonding picture along the S1/S0 isomerization reaction path are described using multireference average coupled pair functional (MRACPF) calculations in conjunction with natural resonance theory (NRT) analysis. These calculations reveal a complex interplay between covalent, radical, ylidic, and ion-pair dominant resonance structures throughout the nonadiabatic photochemical isomerization processes described in this work.

Unexpected formation of π-expanded isoquinoline from anthracene possessing four electron-donating groups via the Duff reaction.

New synthetic methods leading towards π-expanded heterocycles are sought after mainly due to their promising opto-electronic properties. Subjecting 1,5,9,10-tetramethoxyanthracene to the modern Duff reaction conditions led to the formation of a compound possessing the 2-azabenzoanthrone (dibenzo[de,h]isoquinolin-7-on) skeleton instead of the expected dialdehyde. This non-typical course of reaction can be rationalized by the double electrophilic aromatic substitution at two neighboring electron-rich positions of anthracene followed by oxidation of the resulting intermediate to form a pyridine ring. Optical studies supported by the quantum chemistry calculations indicated the lack of excited-state intramolecular proton transfer (ESIPT); for energy reasons, only one tautomeric form, with a hydrogen atom bonded to one of the two nearby oxygen atoms, was populated in the electronic ground S0 and in the excited S1 states. Nonradiative depopulation of the S1 state proceeded via internal conversion stimulated by the presence of the low frequency vibrational modes. Our serendipitous discovery represents the most complex case of rearrangement of aromatic compounds under Duff reaction conditions and could help to design analogous processes. At the same time this is the simplest method for the synthesis of derivatives of 2-azabenzoanthrone.

Infrared and Visible Photodissociation Spectra of Rhodamine Ions at 3 K in the Gas Phase.

Helium-tagging predissociation spectroscopy in the visible spectral range (He@VisPD) is shown for Rhodamine 123, Rhodamine 110, and Rhodamine 110's silver salt (silver carboxylate). It is shown that the spectra reflect single-photon absorption. The helium-tagged ions are in the ground vibrational state, and the He@VisPD spectra feature the Franck-Condon envelopes for the excitation to the first excited singlet state that agree very well with theoretical simulations. The S0 → S1 excitation energies are 2.712 ± 0.006 eV for Rhodamine 123, 2.700 ± 0.006 eV for Rhodamine 110, and 2.751 ± 0.006 eV for the silver salt of Rhodamine 110. The determined energies can be slightly blue-shifted due to the binding energy of helium. The Rhodamine ions were also characterized by helium-tagging infrared photodissociation spectroscopy. The distinctive spectral features of the individual derivatives are described and the spectra are compared to the classical solid-state IR spectra.

Radiation damage yields across the carbon 1s excitation edge

X-rays are used as a probe in many materials analysis techniques, but they may induce changes in the sample that can invalidate the measurement. Strategies that overcome “radiation damage” are therefore desirable. Tuning the incident photon energy below a core electron excitation edge of a material (where only valence excitation takes place) rather than above it (where primarily core excitation occurs) has been theorized as a way to reduce radiation damage. To investigate this strategy, we used a scanning transmission X-ray microscope to expose thin films of organic polymers to X-rays with photon energies below and above the C 1s excitation edge. The extent of radiation damage was quantified by X-ray absorption spectroscopy and lithography. We found that the amount of radiation damage per unit absorbed dose was the same using incident photons below and above the C 1s excitation edge. We conclude that the turn on of Auger processes when C 1s holes are created does not affect the overall radiation damage outcome.