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Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

Aryl‐maleimides undergo a novel [2+4]‐photodimerization instead of the expected [2+2]‐photodimerization under both direct irradiation with visible light and under sensitized energy transfer conditions. This new excited state reactivity in aryl‐maleimides is deciphered through photochemical, photophysical, and spectroscopic studies. The stereochemistry of the photodimer depends on the type of non‐bonding interactions prevalent during photodimerization which is in turn dictated by the substituents on the maleimide ring. More importantly, the stereochemistry of the photodimer formed is complementary to the product observed under thermal conditions. [ABSTRACT FROM AUTHOR]

A photochemical synthetic method to access sulfones from aryl halides and sodium alkylsulfinates in the presence of CuCl is reported. Regio‐ and chemoselectivity for the sulfonylation is observed for polyhalogenated arenes. Mechanistic studies indicate that a copper sulfinate reagent is formed in situ from CuCl and sodium sulfinate based on NMR experiments. Potential reaction intermediates, including excited state aryl halides are studied using density functional theory calculations and reveal energetically accessible non‐planar triplet geometries. [ABSTRACT FROM AUTHOR]

The front cover of the journal "Advanced Synthesis & Catalysis" features an illustration of a copper-enabled, regioselective photo-sulfonylation method for polyhalogenated arenes using alkylsulfinate salts. This method is compared to thermal conditions and offers advantages in terms of safety and the use of starting materials. The details of this research can be found in the Research Article by Lehnherr, Lam, and co-workers. [Extracted from the article]

Graphical abstract Highlights • Methyl anthranilate (MA), an ortho sunscreen, is a non-ideal chemical filter. • Two structural isomers, para- and meta -MA, have very different photochemistry. • para -MA shows a long-lived excited state, also not an ideal sunscreen. • meta -MA decays much more rapidly, has non-unity luminescence. • Theory suggests prefulvenic conical intersections drive faster dynamics. • These conical intersections present new avenues for novel sunscreen design. Abstract Towards the development of a bottom-up rationale for sunscreen design, the effects of substituent position on the ultrafast photodynamics of the sunscreen precursor methyl anthranilate (MA, an ortho compound) were evaluated by studying para- and meta- MA in vacuum. Time-resolved ion yield (TR-IY) measurements reveal a long-lived S 1 excited state (≫1.2 ns) for para- MA, proposed to be the result of a weakly fluorescent, bound excited state. In the case of meta- MA, TR-IY transients reveal a much faster (∼2 ns) excited state relaxation, possibly due to multiple low-lying S 1 /S 0 conical intersections of prefulvenic character. While meta- MA may not be an ideal sunscreen ingredient due to a low ultraviolet absorbance, its comparatively efficient relaxation mechanism may constitute an alternative to common sunscreen relaxation pathways. Thus, our results should prompt further studies of prefulvenic relaxation pathways in potential sunscreen agents. [ABSTRACT FROM AUTHOR]

The performance of the extended multi-state (XMS)-complete active space second-order perturbation theory (CASPT2) method has been assessed for the benchmark of a truncated retinal model, the penta-2,4-dieniminium cation (PSB3). This benchmark presents a challenge for multireference electronic structure methods because the wave function character is changing considerably. The assessment comprises ground and excited state pathways of the isomerisation, including transition states and conical intersection (CI) points. It also includes circular paths centred around different CIs, and 2D potential energy scans located in the branching planes. In this work, we compare the performance of the previous formulations of CASPT2, the single-state and the multi-state, with the recently developed XMS-CASPT2. Besides, we have also tested two variants of internal contraction in XMS-CASPT2, namely, the single-state single reference (SS-SR) and multi-state multireference (MS-MR) schemes. In our study, we find that XMS-CASPT2 corrects the artefacts and discontinuities present in other CASPT2 variants. The investigation of a circular loop and 2D potential energy surfaces around the surface crossing point shows that XMS-CASPT2 exhibits a smooth topology at the CI with the correct degeneracy. It also agrees better with the reference method MRCISD+Q in regions of the potential energy surfaces further away from CIs. Another observation is the close agreement between the results from the SS-SR contraction scheme and the more demanding MS-MR scheme. [ABSTRACT FROM AUTHOR]

The electronic ground and excited states of the coordinatively unsaturated complex Ni(η¹-N2)2, isolated in an Ar matrix, are analyzed in detail by vibrational and electronic absorption and emission spectroscopies allied with quantum chemical calculations. The bond force constants are determined from a normal coordinate analysis and compared with those of the isoelectronic carbonyl complex. The consequences for the bond properties are discussed, and the trend in the force constants is compared with the standard formation enthalpies. The linear complex Ni(η¹-N2)2 with two terminal dinitrogen ligands can be photoisomerized to two isomeric, metastable forms Ni(η¹-N2)(η²-N2) and Ni(η²-N2)2, with one and two side-on coordinated dinitrogen ligands, respectively. [ABSTRACT FROM AUTHOR]

A quantitative treatment of quenching processes under restricted geometry conditions in the Slow Exchange Limit is presented. The expressions for KSV have been obtained for this limit in some common situations that can arise in studies under restricted geometry conditions, such as an inhomogeneous quencher distribution in the solution, the presence of oxygen in solution, or the case of solutions containing a mixture of receptors. These situations have been considered and incorporated into the treatment. [ABSTRACT FROM AUTHOR]

Biochemical reactions are subject to the particular environmental conditions of planet earth, including solar irradiation. How DNA responds to radiation is relevant to human health because radiation damage can affect genetic propagation and lead to cancer and is also important for our understanding of how life on earth developed. A reductionist approach to unravelling the detailed photochemistry seeks to establish intrinsic properties of individual DNA building blocks, followed by extrapolation to larger systems, to incorporate interactions between the building blocks and the role of the biomolecular environment. Advances in both experimental and computational techniques have lead to increasingly detailed insights in the excited state dynamics of DNA bases in isolation as well as the role of the solvent and intermolecular interactions. This review seeks to summarise current findings and understanding. [ABSTRACT FROM AUTHOR]

The extended S1/S0 conical intersection seam for the photochemical 2s + 2s, 2s + 2a and 2a + 2a cycloadditions of two ethylene molecules has been documented using the methodology described by Sicilia et al. (J. Chem. Theory Comput. 4, 257 (2008)). Two additional critical points on this prototype crossing seam were found as a result, although both lie more than 100 kcal mol−1 above the rhomboidal crossing located previously, and are not minima in the intersection space. Using VB theory, the branching space conditions for the conical intersection can be derived analytically, in fair agreement with calculated CASSCF results. Woodward–Hoffmann symmetry forbidden and allowed reactions are associated with the same extended conical intersection seam in this case. [ABSTRACT FROM AUTHOR]

The photochemistry of pivaloyl, benzoyl, 4-phenylbenzoyl, and 2-anthroyl azides has been studied using femtosecond (fs) time-resolved infrared (TRIR) and UV-vis spectroscopy and interpreted with the aid of computational chemistry. Density functional theory calculations revealed a significant difference in the nature of the lowest singlet excited state for these carbonyl azides. The lowest singlet excited states ( S1) of p-phenylbenzoyl and 2-anthroyl azides are (π,π*) in nature, while the pivaloyl and benzoyl azides S1 states involve ( n,π*) excitations. Nevertheless, for all acyl azides studied here, a similar, and intense, IR band at about 2100 cm−1 has been detected in the ultrafast TRIR experiments following 270 nm excitation. These bands were shifted to lower energy by about 100 cm−1 relative to the N3 stretching mode for the ground states of these azides. These 2100 cm−1 vibrational bands were assigned to the S1 states of acyl azides in agreement with density functional theory calculations. The decay of the acyl azide S1 states was described by bi-exponential functions. The fast component was attributed to the decay of the hot S1 state and the longer component to the decay of the thermally relaxed S1 state. A strong and broad transient absorption in the 350-650 nm spectral range was observed in the fs UV-vis experiments for p-phenylbenzoyl and 2-anthroyl azides. The carrier of this absorption also decayed bi-exponentially, and the time constants were in excellent agreement with those found in the fs TRIR experiments. The slow component of the S1 state decay was found to be dependent on the solvent polarity. When the lifetime of the acyl azide S1 state is substantially longer than the time constant for vibrational cooling of nascent (hot) isocyanate, the correlation between the S1 decay and isocyanate formation was clear. The 270 nm excitation populates the Sn ( n ≥ 2) states of these acyl azides. It was established that a hot nitrene is produced more efficiently from both the Sn and hot S1 states than from the relaxed S1 state of these acyl azides. Thus, time-resolved study provides direct experimental evidence that the S1 state is the precursor of nitrene only when the S1 state is pumped directly and when the S1 state lifetime is longer than the time constant of vibrational cooling of the newborn nitrene. All of these results are consistent with the data obtained recently for 2-napththoyl azide. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: This study presents new insight into the photochemical degradation of polybrominated diphenyl ethers (PBDEs), and it provides details about the structures and properties of 27 PBDE congeners in the electronically excited state using the time-dependent density functional theory method. Each PBDE congener exhibited remarkably different geometries in the ground state and the excited state. The significant lengthening of C–Br bond in each PBDE congener was observed in the excited state for the first time by theoretical calculation, which is directly involved in the photochemistry reductive debromination of n-BDE to (n−1)-BDE. Generally, the lengthening of C–Br bonds cannot occur at the para position. Furthermore, the calculated results demonstrated that the photoreactivity of PBDEs increased with an increase of bromination degree. It was also found that the pattern of Br substituents had an effect upon the photoreactivity of PBDEs. These findings suggest that the information obtained in the excited state is crucial to the mechanism explanation of the photochemical degradation of PBDEs. [Copyright &y& Elsevier]

Peroxides are ubiquitous in the atmosphere and their photochemistry at ice surfaces is important. Here the primary steps following photoexcitation of methyl hydroperoxide (MHP) on ice particles are investigated using the MNDO method that describes semiempirically multiple electronic states and treats non-adiabatic dynamical transitions between them by surface hopping. Results are compared with the isolated MHP. Important findings are as follows. (1) Ice catalyzes the deactivation of MHP from the excited state to the ground state. (2) The deactivation process takes place on a femtosecond timescale and is followed by dissociation into fragments. (3) Recombination of fragments occurs to a small extent on ice, but not for the isolated peroxide. [ABSTRACT FROM AUTHOR]

In the present work, in order to investigate the electronic excited-state intermolecular hydrogen bonding between the chromophore coumarin 153 (C153) and the room-temperature ionic liquid N, N-dimethylethanolammonium formate (DAF), both the geometric structures and the infrared spectra of the hydrogen-bonded complex C153-DAF in the excited state were studied by a time-dependent density functional theory (TDDFT) method. We theoretically demonstrated that the intermolecular hydrogen bond C = O···H-O in the hydrogen-bonded C153-DAF complex is significantly strengthened in the S state by monitoring the spectral shifts of the C=O group and O-H group involved in the hydrogen bond C = O···H-O. Moreover, the length of the hydrogen bond C = O···H-O between the oxygen atom and hydrogen atom decreased from 1.693 Å to 1.633 Å upon photoexcitation. This was also confirmed by the increase in the hydrogen-bond binding energy from 69.92 kJ mol in the ground state to 90.17 kJ mol in the excited state. Thus, the excited-state hydrogen-bond strengthening of the coumarin chromophore in an ionic liquid has been demonstrated theoretically for the first time. [ABSTRACT FROM AUTHOR]

Abstract: Direct photolysis of many PCBs has a non quenchable component which survives oxygen quenching. In this study, the nature of the excited states of PCB 138 and PCB 77 was analyzed to understand the origins of reaction from singlet as well as triplet excited states. Data on luminescence quenching, luminescence profiles, reaction quantum yields, and product distributions as functions of excitation wavelength were collected. The most striking and unambiguous observation was the excitation wavelength dependence of the vibronic structure of a fluorescence. Dependence of the apparent quantum yields on the reaction conditions (concentration, intensity) was also recognized during this study. Upon correlating all collected data, sufficient evidence for slow vibrational relaxation in a 2,2′-dichloro congener emerged. This could be seen to open opportunities for reaction from the singlet in competition with intersystem crossing. The slow vibrational relaxation can be understood through analogy to quantitatively modeled biphenyl spectra, to arise from steric hindrance and solvent resistance to rotation about the central C–C bond. [Copyright &y& Elsevier]

Abstract: A theoretical study was conducted on the effects of three substituents in the Ru(II) polypyridyl complex, [Ru(phen)2(6-R-dppz)]2+, (R=H for complex 1, OH for complex 2, and NO2 for complex 3), on the photocleavage of DNA. The geometric and electronic structures of these complexes in the ground (S0), first singlet (S1), and triplet (T1) excited states, were calculated using the density functional theory (DFT), Hartree–Fock (HF), and configuration interaction singles (CIS) methods. The excited reduction potentials of the Ru(II) complexes in aqueous solution, which appear to be primarily responsible for the DNA-photocleavage behavior, were calculated by thermodynamic cycle, and determined to be higher than the oxidation potentials of some DNA-bases. The order of the reduction potentials changes from 3 > 2 > 1 in the ground state to 2 > 1 > 3 in the excited state, suggesting that the substituent on the main ligand has a significant effect on the electrochemical properties of the excited Ru(II) polypyridyl complexes. In addition, these theoretical results can explain two experimentally-observed phenomena: (a) the cleavage of DNA by complex 3 in the absence of light irradiation; (b) the inability of complex 3 to emit luminescence in its excited triplet state in aqueous solution. [ABSTRACT FROM AUTHOR]

Abstract: The isomerization pathways of 4,4′-azobenzene disulfonate in the S0 and T1 states have been studied by using density functional theory (DFT) with B3LYP method at the levels of 6-31G(d,p) and 6-311++G(d,p), respectively. There are two isomerization pathways in the S0 state. One is the inversion of one CNN angle combined with certain degree of rotation around the CN bond, and it is worthy to notice that the potential energy profile includes three sequential transition states. The other pathway is the rotation of CNNC dihedral angle involved inversion of one CNN angle, while the two isomers are connected through the only one transition state. Calculation indicates that the molecular structures at the highest points on the potential energy profiles of two pathways are identical, and the energy barriers are the same, 20.52kcal/mol. In the T1 state, there exists the rotation pathway (rotation of CNNC dihedral angle) and its energy barrier is 4.11kcal/mol. In the excited states (T1, S1, T2, and S2), the potential energy profiles of the vertical excitation are obtained by time dependent density functional theory (TD-DFT) at the B3LYP/6-311++G(d,p) level. The photoexcitation at 342nm results in the reactant molecule populated in the S2 state, but isomerization does not occur directly on the S2 state due to the high energy barrier. It could undergo a rapid relaxation to the minimum of S1 state, and then the isomerization occurs using the inversion or rotation pathway. The results show that the rapid energy redistribution among the various vibrations renders the concentration of such amounts of energy in the inversion coordinate very improbable, while there are two possible photoisomerization pathways by the rotation pathway. The isomerization can easily occur through the S0/S1 conical intersection and the S0–T1–S0 crossing to reach the product. The primary isomerization pathways for 4,4′-azobenzene disulfonate can go through the inversion and rotation forms in the S0 state and the rotation mechanism in the excited state. [Copyright &y& Elsevier]

Abstract: Photoinduced charge transfer (CT) and excited state properties in the mixed coaggregates of 1,3,5-triphenyl-2-pyrazoline (TPP) and 1,4-dicyanonaphthalene (DCN) are investigated theoretically, using time-dependent density functional theory (TD-DFT) as well as the two-dimensional (2D) site (transition density matrix) and three-dimensional (3D) cube (transition density and charge difference density) representations. The calculated results indicate that a strong absorption band stems from the S0 →S4 transition. There are electron–hole coherences between TPP and DCN monomers, which are shown by 2D site representation. Direct visual evidence revealed by 3D cube representations indicates that photoinduced CT mechanism for the mixed coaggregates of TPP and DCN is the mixture of intermolecular and intramolecular CT in the vertical absorption. Some phenyl group of TPP monomer not only serves as the electronic donor in the intramolecular CT, but also in the intermolecular CT process. [Copyright &y& Elsevier]

Abstract: This review article summarizes the research progress made since the year 2000 in the field of long-range electron transfer with synthetic systems that use d6 or d8 metal complexes as photosensitizers. This includes charge transfer over distances greater than 10Å in both covalent and noncovalent donor–bridge–acceptor systems. Metal complexes of Ru(II), Os(II), Re(I), Ir(III), Ir(I), and Pt(II) are considered. [Copyright &y& Elsevier]

Structures and properties of the low-lying states in 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated theoretically. Calculations show that the dimethylamino and dimethylanilino twisted conformations of DCM on the potential energy surface of the first excited state (S1) have relatively high stabilities and remarkable intramolecular charge transfers (ICT). Both structures can serve as candidates for the red-shifted emissive state in polar solvent. In particular, the dimethylanilino twisted ICT state has been predicted to have a dipole moment increment of 20 Debye with respect to the ground state by CASSCF calculations, in good agreement with the suggested experimental values. The optimized geometry of the S1 state exhibits a long central CC bond of 1.458 Å, which makes the trans–cis isomerization quite facile through intramolecular rotation around the central CC bond on the S1 potential surface. The S1 state is a precursor to the formation of the ICT emissive state and photoinduced trans–cis isomerization. The S1/S0 crossing in polar solvent and avoid-crossing in the gas phase as well as in non-polar solvent are involved in the trans–cis isomerization process. The presence of an early S1/S0 crossing in the strong polar solvent reduces the isomerization efficiency. [ABSTRACT FROM AUTHOR]

Photo-induced β-bond dissociation of phenacyl phenyl sulfide (PPS) has been investigated in acetonitrile by laser photolysis techniques. Direct excitation of PPS at 295 K provided the acetylmethyl and phenylthiyl radicals with a quantum yield (Φrad) of 0.18, whereas triplet sensitization using xanthone revealed an efficiency for β-cleavage of triplet PPS (αrad) of ≥0.64. From disagreement between the Φrad and αrad values, it was concluded that both the lowest excited single and triplet states are reactive for β-bond dissociation in PPS. The photochemical processes of excited PPS, including β-cleavage, are discussed in detail. [ABSTRACT FROM AUTHOR]

Depending on the ligands L and N,N (=polypyridine, α-diimine), the complexes [Re(L)(CO)3(N,N)]n+ undergo different photophysical and photochemical processes. Herein, we compare the behavior of complexes with an electron-accepting ligand L = N-methyl-4,4′-bipyridinium (MQ+), isomerizable ligand L = trans-4-styrylpyridine (t-stpy), and L = trans-N-methyl-4,4′-dipyridiniumethylene (Medpe+), which can undergo either reduction, or isomerization, or both. In particular, we discuss the dynamics and mechanisms of (i) NN− → MQ+ interligand electron transfer (ILET) from a 3MLCT state of [ReI(MQ+)(CO)3(dmb)]2+ (dmb = 4,4′-dimethyl-2,2′-bipyridine), (ii) intramolecular energy transfer from a 3MLCT state of the Re(CO)3(bpy) unit of [Re(t-stpy)(CO)3(bpy)]+ to the intraligand 3IL ππ* state of the axial ligand t-stpy and its subsequent isomerization, and (iii) competition between charge and energy transfer/isomerization in [ReI(Medpe+)(CO)3(dmb)]2+.Optical excitation of [ReI(MQ+)(CO)3(dmb)]2+ populates a Re → dmb 3MLCT excited state [ReII(MQ+)(CO)3(dmb−)]2+. Ultrafast dmb− → MQ+ ILET follows, producing a Re → MQ+ MLCT excited state [ReII(MQ)(CO)3(dmb)]2+. The ILET rate (8–18 ps, depending on the solvent) is much faster than predicted by conventional electron-transfer theories, being accelerated by a combination of large (ca. 130 cm−1) electronic coupling through the ReII atom and vibrational excitation of the 3MLCT(dmb) precursor state. Irradiation of [ReI(t-stpy)(CO)3(bpy)]+ populates a Re → bpy 3MLCT excited state, which converts into a 3IL state of the t-stpy ligand with a 3.5 ps time constant. This 3MLCT → 3IL conversion amounts to an intramolecular energy transfer from the electronically excited chromophore Re(CO)3(bpy)+ to the t-stpy ligand. The t-stpy ligand in the 3IL state undergoes a 12 ps twist around the CC bond to the perpendicular geometry, followed by further 18 ns twist to the ground state and the cis isomer. The same isomerization mechanism operates for [ReI(Cl)(CO)3(t-stpy)2]+ whose 3IL state is populated directly. Excitation of [ReI(Medpe+)(CO)3(bpy)]2+ leads to both Re → bpy and Re → Medpe+ MLCT states. A ps energy transfer to a 3IL state follows, triggering rotation around the CC bond. In contrast, the electron-acceptor character of the Medpe+ ligand prevails in [ReI(Cl)(CO)3(Medpe+)2]2+, where only a Re → Medpe+ 3MLCT state is populated, which decays to the ground state with ca. 100 ps lifetime by a Medpe → ReII electron transfer. [Copyright &y& Elsevier]

Abstract: The electronic-absorption and -emission spectroscopy and photophysical properties of metal–alkylidyne (–carbyne) complexes are reviewed. Emission has been observed in fluid solution at room temperature from compounds of a variety of different metals (Mo, W, Re, Os) and electron configurations (d0, d1, d2). The emissive excited states are of the types d xy →π*, π→π*, π→d xy , and MLCT. This compositional and electronic diversity enables the luminescence properties of metal–alkylidyne complexes to be broadly tuned. [Copyright &y& Elsevier]

Nanosecond and picosecond laser flash photolysis (LFP) of 2,2'- and 4,4'-biphenyldiols has been investigated in aqueous acidic and alkaline solutions to understand the details of the photodissociation (PD) and photoionization (PI) mechanisms of the two diols. For 2,2'-biphenyldiol, the photodissociation and photoionization following the excitation using UV light (248 or 266 nm) is seen to be a bi-photonic process, whereas that of the 4,4'-biphenyldiol is seen to follow a monophotonic mechanism. A detailed analysis of the nanosecond and picosecond LFP results indicate that the PD/PI of 2,2'-biphenyldiol involves the participation of the triplet (T1) state of the molecule. For 4,4'-biphenyldiol, however, the PD/PI occurs from the excited singlet (S1) state. It is inferred that the difference in the PD/PI behavior of 2,2'- and 4,4'-biphenyldiols arises due to the presence and absence of intramolecular hydrogen bonding in the two respective molecules. Qualitative potential energy diagrams have been presented to rationalize how the presence and absence of intramolecular hydrogen bonding causes a difference in the stabilization of the electronic states in the two diols and that ultimately determines the possibility of the bi-photonic mono-photonic PD/PI mechanisms for 2,2'- and 4,4'-biphenyldiols, respectively. [ABSTRACT FROM AUTHOR]

Seventeen singlet excited states of ethylene have been calculated via time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional and the geometries of 11 excited states were optimized successfully. The local vibrational mode theory was employed to examine the intrinsic C=C/C–H bond strengths and their change upon excitation. The natural transition orbital (NTO) analysis was used to further analyze the C=C/C–H bond strength change in excited states versus the ground state. For the first time, three excited states including π y ′ → 3s, π y ′ → 3p y and π y ′ → 3p z were identified with stronger C=C ethylene double bonds than in the ground state. [ABSTRACT FROM AUTHOR]