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Recently, two contradicting experimental investigations have been published on the fluorescence behavior of 4-fluoro-N,N-dimethylaniline (FDMA). Motivated by the discrepancies between these studies, we have examined the deactivation pathways of photoexcited FDMA using high-level quantum chemical methods such as time-dependent density functional theory (TDDFT), coupled cluster (CC2) theory, and the algebraic diagrammatic construction scheme (ADC(2), ADC(3)) simulating solvation using continuum solvation models. Our results show the initial population of the bright S2 (ππ*) state of FDMA relax to a linear, almost planar S1 state minimum from which emission occurs. Our study further reveals that TDDFT/B3LYP erroneously predicts a planar twisted intramolecular charge transfer (TICT) S1 state minimum that vanishes when the amount of nonlocal Hartree-Fock exchange in the functional is increased. [ABSTRACT FROM AUTHOR]

Conspectus: The use of mechanical force to initiate a chemical reaction is an efficient alternative to the conventional sources of activation energy, i.e., heat, light, and electricity. Applications of mechanochemistry in academic and industrial laboratories are diverse, ranging from chemical syntheses in ball mills and ultrasound baths to direct activation of covalent bonds using an atomic force microscope. The vectorial nature of force is advantageous because specific covalent bonds can be preconditioned for rupture by selective stretching. However, the influence of mechanical force on single molecules is still not understood at a fundamental level, which limits the applicability of mechanochemistry. As a result, many chemists still resort to rules of thumb when it comes to conducting mechanochemical syntheses. In this Account, we show that comprehension of mechanochemistry at the molecular level can be tremendously advanced by quantum chemistry, in particular by using quantum chemical force analysis tools. One such tool is the JEDI (Judgement of Energy DIstribution) analysis, which provides a convenient approach to analyze the distribution of strain energy in a mechanically deformed molecule. Based on the harmonic approximation, the strain energy contribution is calculated for each bond length, bond angle and dihedral angle, thus providing a comprehensive picture of how force affects molecules. This Account examines the theoretical foundations of quantum chemical force analysis and provides a critical overview of the performance of the JEDI analysis in various mechanochemical applications. We explain in detail how this analysis tool is to be used to identify the "force-bearing scaffold" of a distorted molecule, which allows both the rationalization and the optimization of diverse mechanochemical processes. More precisely, we show that the inclusion of every bond, bending and torsion of a molecule allows a particularly insightful discussion of the distribution of mechanical strain in deformed molecules. We illustrate the usefulness of the JEDI analysis by rationalizing the finding that a knot tremendously weakens a polymer strand via a "choking" motion of the torsions in the curved part of the knot, thus leading to facilitated bond rupture in the immediate vicinity of the knot. Moreover, we demonstrate that the JEDI analysis can be exploited to devise methods for the stabilization of inherently strained molecules. In addition to applications in the electronic ground state, the JEDI analysis can also be used in the electronically excited state to determine the mechanical energy that a molecular photoswitch can release into its environment during photoisomerization. This approach allows the quantification of the mechanical efficiency of a photoswitch, i.e., the part of the energy that becomes available for the motion into a specific direction, which enables us to judge whether a photoswitch is capable of performing a desired switching function. [ABSTRACT FROM AUTHOR]

In quantum mechanochemistry, quantum chemical methods are used to describe molecules under the influence of an external force. The calculation of geometries, energies, transition states, reaction rates, and spectroscopic properties of molecules on the force-modified potential energy surfaces is the key to gain an in-depth understanding of mechanochemical processes at the molecular level. In this review, we present recent advances in the field of quantum mechanochemistry and introduce the quantum chemical methods used to calculate the properties of molecules under an external force. We place special emphasis on quantum chemical force analysis tools, which can be used to identify the mechanochemically relevant degrees of freedom in a deformed molecule, and spotlight selected applications of quantum mechanochemical methods to point out their synergistic relationship with experiments. [ABSTRACT FROM AUTHOR]

The excited states of fac-tris(phenylpyridinato)iridium[Ir(ppy)3] and the smaller model complex Ir(C3H4N)3are computed using a number of high-levelab initio methods, including the recently implemented algebraic diagrammaticconstruction method to third-order ADC(3). A detailed descriptionof the states is provided through advanced analysis methods, whichallow a quantification of different charge transfer and orbital relaxationeffects and give extended insight into the many-body wave functions.Compared to the ADC(3) benchmark an unexpected striking differenceof ADC(2) is found for Ir(C3H4N)3, which derives from an overstabilization of charge transfer effects.Time-dependent density functional theory (TDDFT) using the B3LYP functionalshows an analogous but less severe error for charge transfer states,whereas the ωB97 results are in good agreement with ADC(3).Multireference configuration interaction computations, which are inreasonable agreement with ADC(3), reveal that static correlation doesnot play a significant role. In the case of the larger Ir(ppy)3complex, results at the TDDFT/B3LYP and TDDFT/ωB97levels of theory are presented. Strong discrepancies between the twofunctionals, which are found with respect to the energies, characters,as well as the density of the low lying states, are discussed in detailand compared to experiment. [ABSTRACT FROM AUTHOR]

The article presents a survey of single-reference ab initio excited state methods, which can be used for large molecules and to not explicitly include correlation through the ground-state wave function. Time-dependent density functional theory is currently the most popular technique for the computation of excited states of medium-sized and large molecules. With the advances in computer technology, systems up to 300 second-row atoms became possible.

It is well-known that time-dependent density functional theory (TDDFT) yields substantial errors for the excitation energies of charge-transfer (CT) excited states, when approximate standard exchange-correlation (xc) functionals are used, for example, SVWN, BLYP, or B3LYP. Also, the correct 1/R asymptotic behavior of CT states with respect to a distance coordinate R between the separated charges of the CT state is not reproduced by TDDFT employing these xc-functionals. Here, we demonstrate by analysis of the TDDFT equations that the first failure is due to the self-interaction error in the orbital energies from the ground-state DFT calculation, while the latter is a similar self-interaction error in TDDFT arising through the electron transfer in the CT state. Possible correction schemes, such as inclusion of exact Hartree-Fock or exact Kohn-Sham exchange, as well as aspects of the exact xc-functional are discussed in this context. Furthermore, a practical approach is proposed which combines the benefits of TDDFT and configuration interaction singles (CIS) and which does not suffer from electron-transfer self-interaction. The latter approach is applied to a (1,4)-phenylene-linked zincbacteriochlorin-bacteriochlorin complex and to a bacteriochlorophyll-spheroidene complex, in which CT states may play important roles in energy and electron-transfer processes. The errors of TDDFT alone for the CT states are demonstrated, and reasonable estimates for the true excitation energies of these states are given. [ABSTRACT FROM AUTHOR]

To date, B[sub 6]H[sub 6][sup 2-] and some of its derivatives are the smallest members of the closo-borates that have been synthesized and analyzed in condensed phases. In contrast, no stable diaeionic tetraborate has yet been observed, either in solution or solids or in the gas phase. In this work, the gas-phase stability of dianionic tetraborates B[sub 4]X[sub 4][sup 2-] (X = H, CN, NC, or BO) is investigated with ab initio methods. For this objective, the geometries of the dianions are optimized, the electronic stability is tested, and various fragmentation channels are studied. In agreement with previous examinations, tetrahedral isomers of all examined tetraborates have been found to represent geometrically stable isomers and to exhibit a triplet electronic ground state. However, these isomers are electronically unstable, i.e., their additional electrons are not bound. Furthermore, new D[sub 2σ]-symmetric isomers of B[sub 4]X[sub 4][sup 2-] (X = H, CN, NC, or BO) have been identified that have a closed-shell singlet ground state and are lower in energy than their tetrahedral counterparts. Moreover, B[sub 4](CN)[sub 4][sup 2-] and B[sub 4](BO)[sub 4][sup 2-] represent stable gas-phase dianions and are predicted to be observable in suitable experiments. The electronic properties and geometries of these dianions are discussed in detail and explained in terms of the electrostatic repulsion of the excess electrons and the aromaticity of the dianions. [ABSTRACT FROM AUTHOR]

Examines the cyclic carbon cluster dianions using ab initio methods. Components on the structures of the dianions; Representation of stable gas-phase dianions by the examines dianions; Geometries and aromaticity of the cyclic clusters.

Presents a study which focused on stable and long-lived multiply charged anions in the gas phase. Developments in alkaline earth tetrahalides; Mixed silicon-carbon dianions; Mixed beryllium-carbon dianions.

Discusses the characterization of the relevant excited states in the photodissociation of carbon monoxide-ligated hemoglobin and myoglobin. Role of the iron motion in the decay of the excited singlet into the final high-spin state; Optimization of the ground-state structure along the iron-carbon bond stretch; Calculation of the vertical electronic excitation stage.

Examines the characterization of the relevant excited states in the photodissociation of carbon oxide-ligated hemoglobin and myoglobin. Structure of the myoglobin model complex; Potential energy curves of the lowest-excited singlet states of the complex.

Photoinduced biochemical reactions are ubiquitously governed by derivatives of flavin, which is a key player in a manifold of cellular redox reactions. The photoreactivity of flavins is also one of their greatest disadvantages as the molecules are sensitive to photodegradation. To prevent this unfavorable reaction, UV-light-exposed archaea bacteria, such as Halobacterium salinarum, manage the task of protecting flavin derivatives by dodecin, a protein which stores flavins and efficiently photoprotects them. In this study, we shed light on the photoprotection mechanism, i.e., the excited state quenching mechanism by dodecin using computational methodology. Molecular dynamics (MD) simulations unraveled the hydrogen bond network in the flavin binding pocket as a starting point for proton transfer upon preceding electron transfer. Using high-level ab initio quantum chemical methods, different proton transfer channels have been investigated and an energetically feasible Mg2+-promoted channel has been identified fully explaining previous experimental observations. This is the first extensive theoretical study of archaeal dodecin, furthering the understanding of its photocycle and manipulation. [ABSTRACT FROM AUTHOR]

Photoinduced biochemical reactions are ubiquitously governed by derivatives of flavin, which is a key player in a manifold of cellular redox reactions. The photoreactivity of flavins is also one of their greatest disadvantages as the molecules are sensitive to photodegradation. To prevent this unfavorable reaction, UV-light-exposed archaea bacteria, such as Halobacterium salinarum, manage the task of protecting flavin derivatives by dodecin, a protein which stores flavins and efficiently photoprotects them. In this study, we shed light on the photoprotection mechanism, i.e., the excited state quenching mechanism by dodecin using computational methodology. Molecular dynamics (MD) simulations unraveled the hydrogen bond network in the flavin binding pocket as a starting point for proton transfer upon preceding electron transfer. Using high-level ab initio quantum chemical methods, different proton transfer channels have been investigated and an energetically feasible Mg2+-promoted channel has been identified fully explaining previous experimental observations. This is the first extensive theoretical study of archaeal dodecin, furthering the understanding of its photocycle and manipulation. [ABSTRACT FROM AUTHOR]

Diphenylacetylene (tolane) exhibits a rich photochemistry that depends on the relative orientation of the phenyl rings and the external conditions. Here, state-of-the-art quantum chemical methods based on the algebraic diagrammatic construction scheme for a polarization propagator of second order are employed to investigate the luminescence properties of tolane and its derivatives. It is explained why different derivatives of tolane exhibit different absorption but practically identical fluorescence spectra, why the fluorescence quantum yield and the population of a "dark" state are temperature-dependent, and why initially twisted tolanophanes phosphoresce in glassy media at 77 K while planar ones do not. [ABSTRACT FROM AUTHOR]

Many strained hydrocarbons are prone to isomerization, dimerization, and trimerization under normal laboratory conditions. Here we investigate a method to stabilize angle-strained cycloalkynes by applying a mechanical pulling force to the carbon atoms adjacent to the triple bond, which partially linearizes the CC-C bond angles. We discuss various methods of applying such pulling forces, including photoswitches and incorporation into additional strained macrocycles. We use the computational JEDI (Judgement of Energy DIstribution) analysis to quantify the distribution of energy in strained cycloheptyne and judge the change in stability upon application of an external force via isodesmic and homodesmotic reactions. We find that cycloheptyne can indeed be stabilized by external forces. However, the force generated by photoswitches during isomerization is too low to lead to a significant stabilization of the molecule. Hence, stronger forces are needed, which can be achieved by incorporating cycloheptyne into a second strained macrocycle. [ABSTRACT FROM AUTHOR]

4-(1H-pyrrol-1-yl)benzonitrile (PBN) is a typical intramolecular donor-acceptor (D/A) molecule that shows dual fluorescence in weakly polar environments. In this work the underlying photochemical reaction mechanism is investigated theoretically by using high-level ab initio methods including the recently implemented third order algebraic diagrammatic construction of the polarization propagator (ADC(3)). Solvation effects have been considered by using different sophisticated continuum model approaches. Our results conclusively explain all available experimental findings including the effects of excitation wavelength, temperature and solvent polarity. After photoexcitation in gas phase to the bright 2A (S2, ππ*) state, PBN relaxes on the 2A state surface until a conical intersection with the energetically close-lying dark 1B (S1, LE) state is reached. After passing this conical intersection PBN further relaxes on the LE state surface toward a minimum from which emission can occur. In polar environments this picture changes. Then the polar 2B (S3, CT) state is stabilized and an energy barrier along the twisting coordinate vanishes. As a consequence population of the twisted 2B state minimum becomes the dominating decay channel and red-shifted fluorescence occurs. [ABSTRACT FROM AUTHOR]

Themechanism of the photochemical conversion of spiropyran tomerocyanine is investigated theoretically. Calculations were performedat TD-DFT/ωB97XD/cc-pVDZ level of theory, which shows good agreementwith the reference RI-CC2 method. A two-dimensional scan of the potentialenergy surface has been performed along the C–O distance andthe central torsion angle in the ground state and in the first excitedstate, where the reaction takes place. Starting at the Franck–Condongeometry, the energy of the first excited state decreases in the directionof the C–O dissociation while the ground-state energy increases.This leads to a barrierless C–O bond dissociation in the firstexcited state. While relaxing on the S1PES toward longerC–O distances, the torsion angle hardly changes, but othercoordinates start to vary, leading to a conical intersection of theground state and the first excited state at a C–O distanceof about 3.4 Å. Passing the conical intersection, the reactioncontinues on the ground-state PES. At these large C–O distances,either barrierless Cspiro–O rebinding occurs thatquenches spiropyran isomerization or rotation around the central torsionangle occurs that leads to merocyanine. For the latter an energy barrierof 0.1 eV must be overcome explaining the low quantum yield of spiropyranto merocyanine photoswitching. [ABSTRACT FROM AUTHOR]

The (6–4) photolyases areblue-light-activated enzymes thatselectively bind to DNA and initiate splitting of mutagenic thymine(6–4) thymine photoproducts (T(6–4)T-PP) via photoinducedelectron transfer from flavin adenine dinucleotide anion (FADH–) to the lesion triggering repair. In the present work,the repair mechanism after the initial electron transfer and the effectof the protein/DNA environment are investigated theoretically by meansof hybrid quantum mechanical/molecular mechanical (QM/MM) simulationsusing X-ray structure of the enzyme–DNA complex. By comparisonof three previously proposed repair mechanisms, we found that thelowest activation free energy is required for the pathway in whichthe key step governing the repair photocycle is electron transfercoupled with the proton transfer from the protonated histidine, His365,to the N3′ nitrogen of the pyrimidone thymine. The transfersimultaneously occurs with concerted intramolecular OH transfer withoutformation of an oxetane or isolated water molecule intermediate. Incontrast to previously suggested mechanisms, this newly identifiedpathway requires neither a subsequent two-photon process nor electronicexcitation of the photolesion. [ABSTRACT FROM AUTHOR]

Vinylogous Nazarov-type cyclizations yield seven-membered rings from butadienyl vinyl ketones via a photochemical [6p] photocyclization followed by subsequent isomerization steps. The mechanism of this recently developed method was investigated using unrestricted DFT, SF-TDDFT, and CASSCF/NEVPT2 calculations, suggesting three different pathways that lead either to pure trans, pure cis, or mixed cis/trans configured products. Singlet biradicals or zwitterions occur as intermediates. The computational results are supported by deuterium-labeling experiments. [ABSTRACT FROM AUTHOR]

A series of diazapentacenes (5,14-diethynyldibenzo[b,i]phenazine, 6,13-diethynylnaphtho[2,3-b]phenazine) and tetraazapentacenes (7,12-diethynylbenzo[g]quinoxalino [2,3-b]quinoxaline, 6,13-diethynylquinoxalino [2,3-b]phenazine) were reduced to their radical anions and dianions, employing either potassium anthracenide or lithium naphthalenide in THF. The anionic species formed were investigated by UV-vis-NIR, fluorescence and EPR spectroscopy, spectroelectrochemistry, and quantum chemical calculations. Single crystal X-ray structures of three of their radical anions and of three of their dianions were obtained. In contrast to the acenes, the anions of the azapentacenes are persistent and, in some cases, even moderately stable toward air, and were characterized. [ABSTRACT FROM AUTHOR]

Wereport the implementation and evaluation of a perturbative,density-based correction scheme for vertical excitation energies calculatedin the framework of a polarizable continuum model (PCM). Because theproposed first-order correction terms depend solely on the zeroth-orderexcited-state density, a transfer of the approach to any configurationinteraction-type excited-state method is straightforward. Employingthe algebraic-diagrammatic construction (ADC) scheme of up to thirdorder as well as time-dependent density-functional theory (TD-DFT),we demonstrate and evaluate the approach. For this purpose, we assembleda set of experimental benchmark data for solvatochromism in molecules(xBDSM) containing 44 gas-phase to solvent shifts for 17 molecules.These data are compared to solvent shifts calculated at the ADC(1),ADC(2), ADC(3/2), and TD-DFT/LRC-ωPBE levels of theory in combinationwith state-specific as well as linear-response type PCM-based correctionschemes. Some unexpected trends and differences between TD-DFT, thelevels of ADC, and variants of the PCM are observed and discussed.The most accurate combinations reproduce experimental solvent shiftsresulting from the bulk electrostatic interaction with maximum errorsin the order of 50 meV and a mean absolute deviation of 20–30meV for the xBDSM set. [ABSTRACT FROM AUTHOR]

Chiral helical-based phosphanes are challenging and promising ligands, with a great potential for the generation of both organic and organometallic catalysts. We report here the preparation of novel chiral thiahelicene-based alkyl phosphanes, isolated and characterized as air-stable borane adducts, and the investigation of their experimental and theoretical (chir)optical properties. X-ray characterization of a mono- and a disubstituted derivative as a racemic mixture has been performed, which confirms the influence of the number and nature of substituents on the flexibility of the helix. In addition, the absolute configuration inferred from CD spectra of the two enantiomers of a diborane complex has been established from X-ray analysis. State-of-the-art quantum chemical calculations of vibrationally resolved spectra allow, for the first time, for an unambiguous assignment of the experimentally observed peaks in linear absorption and circular dicliroism spectra to excited electronic states of this class of thiahelicene phosphorus derivatives. [ABSTRACT FROM AUTHOR]

We herein describe the synthesis and property evaluation of three novel aldehyde-substituted pentameric phenylenevinylenes carrying branched oligo(ethylene glycol) (swallowtail, Sw) substituents. The targets were synthesized by a combination of Heck coupling and Wittig or Horner reactions of suitable precursor modules. If the pentameric phenylenevinylene carries only two of these Sw substituents, it is no longer water-soluble. When six of the Sw substituents are attached, regardless of their position, the pentameric phenylenevinylenes are well water-soluble. The dialdehydes were investigated with respect to their amine-sensing capabilities both in water as well as in the solid state, sprayed onto thin layer chromatography (TLC) plates (alox, silica gel, reversed phase silica gel). The recognition of amine vapors using the sprayed-on phenylenevinylene dialdehydes is superb and allows the identification of different amines on regular silica TLC plates via color changes, analyzed by a statistical tool, the multivariate analysis of variance (MANOVA) protocol. [ABSTRACT FROM AUTHOR]

The rotational motion of tolanes along their acetylene axis is not fully understood. What happens to the optical and electronic properties if the tolane backbone is forced into a twisted conformation? Several tethers were investigated to obtain tolanophanes, fixing the torsion angle of the two phenyl rings. X-ray crystal structures revealed tether-specific torsion angles in the solid state. The absorption, emission, and excitation spectra were recorded. Twisted tethered tolane conformers showed blue-shifted absorption; emission spectra were all torsionally independent and identical. The tethered tolanes were embedded in a rigid matrix by freezing to 77 K; well-resolved emission spectra were recorded for planar tolanes, but for twisted systems unexpectedly long-lived phosphorescence was observed. How is this triplet emission explained? Quantum chemical calculations (TDDFT/cam-B3LYP/6-31G*) of the unsubstituted tolane showed that intersystem crossing (ISC) is favored with large spin-orbit coupling, which occurs when the molecular orbitals are orthogonal to each other; this is the case at the crossing of S1/T7. Also, a small energy difference between singlet and triplet states is required; we found that ISC can favorably take place at four crossings: S1/T6, S1/T7, S1/T8,9, S1/T10. [ABSTRACT FROM AUTHOR]

The cleavage of a photolabile nitroveratryloxycarbonyl protecting group, which is widely used as caging group, was studied by femtosecond transient absorption spectroscopy in the visible and infrared spectral range and by flash-photolysis experiments on the longer time scale. On the basis of quantum-chemical calculations it is shown that directly after excitation, triplet absorption that is not part of the reactive pathway dominates the transient spectrum and that the molecules following the triplet pathway are trapped in a nonreactive triplet state. By contrast, photolysis proceeds from the singlet manifold. Therefore, trapping in the triplet state lowers the quantum yield of the process for this compound compared with other o-nitrobenzyl protecting groups. With our integrated approach of time-resolved UV and IR measurements and calculations, we can characterize the entire uncaging mechanism and identify the most relevant intermediate states along the reaction pathway. The final uncaging is accomplished within 32 μs. [ABSTRACT FROM AUTHOR]

New tetrathia[7]helicene-based (7-TH-based) gold(I) complexes 6 and 7 have been readily prepared by reaction of the respective phosphine ligands 2 and 3 with Au(tht)Cl in a 1:1 and 1:2 molar ratio, respectively. These complexes have been fully characterized by analytical and spectroscopic techniques as well as quantum chemical calculations. The molecular structure of dinuclear complex 7 has been determined by single-crystal X-ray diffraction, showing a gold--gold interaction of 3.1825(3) Å and a significant contraction of the 7-TH total dihedral angle. Au(I) complex 7 displays luminescence emission at room and low temperature in diluted solution and in the solid state. Quantum chemical calculations show that the luminescence emission at room temperature is primarily due to slightly perturbed fluorescence emission from purely ππ* excited states of the conjugated helicene scaffold. At 77 K phosphorescence emission is displayed as well. Preliminary studies on the use of 6 and 7 as catalysts in typical Au(I)-catalyzed cycloisomerizations have demonstrated the reactivity of these systems in the intramolecular allene hydroarylations and the hydroxycarboxylation of allene-carboxylates. [ABSTRACT FROM AUTHOR]

Three water-soluble fluorescent aldehyde-substituted distyrylbenzene derivatives were prepared using Heck or Horner methodologies. Water solubility was achieved through the addition of branched oligoethylene glycol side chains; these are attached via an ether bridge to the aromatic nucleus. The aldehydes are almost nonfluorescent in water, but addition of primary amines turns the fluorescence on; formation of imines results. Control of the basicity of the media allows further discrimination of the analytes employed. 1,3-Diaminopropane reacts with these aldehydes. Instead of an imine, a brightly fluorescent aminal forms. Amino acids are almost always nonreactive toward these aldehydes. Exceptions are lysine and cysteine, which form an imine and a thioaminal, respectively, discreating the aldehyde unit under fluorescence turn-on in water. The detection limit and time of completion of the sensing event were evaluated. Dialdehydes 3 and 16 were comparable on both counts. The cross-shaped 16 did react approximately twice as quickly with 1,3-diaminopropane. [ABSTRACT FROM AUTHOR]

The photodecarboxylation mechanismof different structural isomersof nitrophenylacetate (NPA) has been investigated using quantum chemicalcalculations as well as time-resolved UV-pump VIS-probe spectroscopy.It is shown that for a proper theoretical description of the excitedstates of anionic NPA in aqueous solution a careful considerationof the influence of the solvent is indispensable. In this sense, NPAis an example that demonstrates how character and lifetime of theinvolved excited states affect the results of equilibrium and nonequilibriumsolvation approaches. An ultrafast decay channel via a repulsive singletstate has been found to be responsible for observed ultrafast CO2release, while another very efficient but slower CO2release channel is found to proceed via intersystem crossing andsubsequent decay via a repulsive triplet state. After all, differencesand similarities in the observed excited state dynamics of the isomersare conclusively explained. Most notably, the much smaller quantumyield of CO2release from the ortho-isomer is due to analternative excited-state hydrogen-transfer channel, which occursalong a triplet and singlet pathway. On the basis of theoretical andexperimental evidence suggesting that the multiplicity of the routetaken determines the photoproduct yield, we provide guidelines forthe design of ortho-nitrobenzylic caging groups with improved uncagingyield. [ABSTRACT FROM AUTHOR]

Steady-state and time-resolved polarized spectroscopy studies reveal that electronic excitation to the third excited state of 1,4-distyryl-2,5-bis(arylethynyl)benzene cruciforms results in fluorescence emission that is shifted an angle of ca. 60°. This result is consistent with quantum chemical calculations of the lowest electronic excited states and their transition dipole moments. The shift originates from the disjointed nature of the occupied molecular orbitals being localized on the different branches of the cruciforms. [ABSTRACT FROM AUTHOR]

Ion-depletion IA spectroscopy has revealed that at least two water molecules are required in complexes with 4-(dimethylamino)benzoic acid methyl ester (DMABME) for anomalous red-shifted fluorescence to occur in the gas phase. Through the use of high-level quantum-chemical calculations, two experimentally observed isoenergetic isomers are assigned to complexes in which a water dimer is hydrogen-bonded either to the carbonyl oxygen of the ester function or to the amino nitrogen. Surprisingly, computed IA spectra reveal that the N-bonded isomer is responsible for the observed red-shifted fluorescence. For an explanation, the mechanism of twisted intramolecular charge-transfer (TICT) formation and energy dissipation is investigated in detail. In general, for red-shifted fluorescence to occur, the N-bonded complexes must be able to dissipate energy, which in the gas phase can only happen nonradiatively via fragmentation. Arguments are given that only the N-bonded isomer photodissociates rapidly enough into free DMABME and a water dimer as a result of the immediate repulsion between the amino nitrogen and the water dimer in the TICT state. The 0-bonded isomer, on the other hand, stays intact because the hydrogen bond is strengthened by additional electrostatic attraction in the ICT state. Furthermore, an experiment to further corroborate that mechanism is suggested. [ABSTRACT FROM AUTHOR]