Photoinduced processes in riboflavin: superposition of pi pi*-n pi* states by vibronic coupling, transfer of vibrational coherence, and population dynamics under solvent control

Femtosecond dynamics of riboflavin, the parent chromophore of biological blue-light receptors, was measured by broadband transient absorption and stationary optical spectroscopy in polar solution. Rich photochemistry is behind the small spectral changes observed: (i) loss of oscillator strength around time zero, (ii) sub-picosecond (ps) spectral relaxation of stimulated emission (SE), and (iii) coherent vibrational motion along a' (in-) and a' (out-of-plane) modes. Loss of oscillator strength is deduced from the differences in the time-zero spectra obtained in water and DMSO, with stationary spectroscopy and fluorescence decay measurements providing additional support. The spectral difference develops faster than the time resolution (20 fs) and is explained by formation of a superposition state between the optically active (1pi pi*) S1 and closely lying dark (1n pi*) states via vibronic coupling. Subsequent spectral relaxation involves decay of weak SE in the blue, 490 nm, together with rise and red shift of SE at 550 nm. The process is controlled by solvation (characteristic times 0.6 and 0.8 ps in water and DMSO, respectively). Coherent oscillations for a' and a' modes show up in different regions of the SE band. a' modes emerge in the blue edge of the SE and dephase faster than solvation. In turn, a' oscillations are found in the SE maximum and dephase on the solvation timescale. The spectral distribution of coherent oscillations according to mode symmetry is used to assign the blue edge of the SE band to a 1n pi*-like state (A'), whereas the optically active 1pi pi* (A') state emits around the SE maximum. The following model comes out: optical excitation occurs to the Franck-Condon pi pi* state, a pi pi*-n pi* superposition state is formed on an ultrafast timescale, vibrational coherence is transferred from a' to a' modes by pi pi*-n pi* vibronic coupling, and subsequent solvation dynamics alters the pi pi*/n pi* population ratio.     

The lowest singlet (n,pi*) and (pi,pi*) excited states of the hydrogen-bonded complex between water and pyrazine

The hydrogen bonding between water and pyrazine in its ground, lowest (n,pi*), and lowest (pi,pi*) states is investigated using density-functional theory (DFT), time-dependent density function theory (TD-DFT), coupled-cluster singles and doubles (CCSD) theory and equation-of-motion coupled cluster (EOM-CCSD) theory. For all states, the minimum-energy configuration is found to be an orthodox linear hydrogen-bonded species, with the bond strength increasing by 0.4 kcal mol-1 upon formation of the (pi,pi*) state and decreasing by 1.0 kcal mol-1 upon formation of the (n,pi*) state. The calculated solvent shifts for the complexes match experimental data and provide a basis for the understanding of the aqueous solvation of pyrazine, and the excited-state complexes are predicted to be only short-lived, explaining the failure of molecular beam experiments to observe them. Quite a different scenario for hydrogen bonding to the (n,pi*) excited state is found compared to those of H2O:pyridine and H2O:pyrimidine: for pyridine linear hydrogen bonds are unstable and hydrogen bonds to the electron-enriched pi cloud are strong, whereas for pyrimidine the excitation localizes on the nonbonded nitrogen leaving the hydrogen-bonding unaffected. For H2O:pyrazine, the (n,pi*) excitation remains largely delocalized, providing a distinct intermediary scenario.     

The different photoisomerization efficiency of azobenzene in the lowest n pi* and pi pi* singlets: the role of a phantom state

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.     

Enhanced pi ... pi interactions in alpha,beta-unsaturated carbonyls

High level ab initio calculations on complexes of benzene with acrolein and ethene reveal that pi ... pi interactions to electron deficient acrolein are remarkably similar to those found in the benzene dimer.     

Noncovalent interactions between cytosine and SWCNT: curvature dependence of complexes via pi...pi stacking and cooperative CH...pi/NH...pi

Fragments of C24H12, adapted from a variety of armchair [(n,n), (n = 5, 7, and 8)] and zigzag [(m,0) (m = 8, 10, and 12)] single-walled carbon nanotube (SWCNT), are used to model corresponding SWCNTs with different diameters and electronic structures. The parallel binding mainly through pi...pi stacking interaction, as well as the perpendicular binding via cooperative NH...pi and CH...pi between cytosine and the fragments of SWCNT have been extensively investigated with a GGA type of DFT, PW91LYP/6-311++G(d,p). The eclipsed tangential (ET) conformation with respect to the six-membered ring of cytosine and the central ring of SWCNT fragments is less stable than the slipped tangential (ST) conformation for the given fragment; perpendicular conformations with NH2 and CH ends have higher negative binding energy than those with NH and CH ends. At PW91LYP/6-311++G(d,p) level, two tangential complexes are less bound than perpendicular complexes. However, as electron correlation is treated with MP2/6-311G(d,p) for PW91LYP/6-311++G(d,p) optimized complexes, it turns out there is an opposite trend that two tangential complexes become more stable than three perpendicular complexes. This result implies that electron correlation, a primary source to dispersion energy, has more significant contributions to the pi...pi stacking complexes than to the complexes via cooperative NH...pi and CH...pi interactions. In addition, it was found for the first time that binding energies for two tangential complexes become more negative with increasing nanotube diameter, while those for three perpendicular complexes have a weaker dependence on the curvature; i.e., binding energies are slightly less and less negative. The performance of a novel hybrid DFT, MPWB1K, was also discussed.     

Sigma/pi versus pi/pi conjugation: DFT studies on oligocyclopropenones and related systems

DFT studies on oligocyclopropenones and related systems 5(n)-9(n) are reported. A strong sigma/pi interaction between the pi system and the sigma framework of the three-membered rings is observed in almost all cases, leading to a perpendicular arrangement of the pi systems in the most favored conformation. Also the shape of cyclic congeners is strongly influenced by sigma/pi interactions.     

Site-selective and stereoselective intramolecular (2pi + 2pi) photocycloaddition of arylalkenes to pyrene and its photocycloreversion

Irradiation of a benzene solution containing methyl p-(1-pyrenylmethoxymethyl)cinnamate (1a) with a high-pressure Hg lamp through Pyrex filter stereoselectively gave an intramolecular (2pi + 2pi) photocycloadduct (2a) in an 83% yield in a site-selective manner at the 4,5-position of the pyrene ring. Similar irradiation of an ortho-substituted derivative (3) afforded the corresponding (2pi + 2pi) cycloadduct (4) as a sole product at the 9,10-position of pyrene. The site-selective photocycloaddition can be reasonably explained by the intramolecular sandwich-type singlet exciplexes between the pyrene and phenyl rings.     

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.     

Quenching of tryptophan (1)(pi,pi*) fluorescence induced by intramolecular hydrogen abstraction via an aborted decarboxylation mechanism

CASSCF computations show that the hydrogen-transfer-induced fluorescence quenching of the (1)(pi,pi*) excited state of zwitterionic tryptophan occurs in three steps: (1) formation of an intramolecular excited-state complex, (2) hydrogen transfer from the amino acid side chain to the indole chromophore, and (3) radiationless decay through a conical intersection, where the reaction path bifurcates to a photodecarboxylation and a phototautomerization route. We present a general model for fluorescence quenching by hydrogen donors, where the radiationless decay occurs at a conical intersection (real state crossing). At the intersection, the reaction responsible for the quenching is aborted, because the reaction path bifurcates and can proceed forward to the products or backward to the reactants. The position of the intersection along the quenching coordinate depends on the nature of the states and, in turn, affects the formation of photoproducts during the quenching. For a (1)(n,pi*) model system reported earlier (Sinicropi, A.; Pogni, R.; Basosi, R.; Robb, M. A.; Gramlich, G.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed. 2001, 40, 4185-4189), the ground and the excited state of the chromophore are hydrogen acceptors, and the excited-state hydrogen transfer is nonadiabatic and leads directly to the intersection point. There, the hydrogen transfer is aborted, and the reaction can return to the reactant pair or proceed further to the hydrogen-transfer products. In the tryptophan case, the ground state is not a hydrogen acceptor, and the excited-state hydrogen transfer is an adiabatic, sequential proton and electron transfer. The decay to the ground state occurs along a second reaction coordinate associated with decarboxylation of the amino acid side chain and the corresponding aborted conical intersection. The results show that, for (1)(pi,pi*) states, the hydrogen transfer alone is not sufficient to induce the quenching, and explain why fluorescence quenching induced by hydrogen donors is less general for (1)(pi,pi*) than for (1)(n,pi*) states.     

The lowest excited triplet (T1) energies of p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors estimated from the temperature dependence of the T2(n,pi*) phosphorescence and the S1(n,pi*) fluorescence spectra

Invisible energy levels of the T1(pi, pi*) state of p-methoxybenzaldehyde (anisaldehyde) and p-cyanobenzaldehyde vapors have been estimated through the temperature dependence of the T2(n, pi*) --> S0 phosphorescence and the S1(n, pi*) --> S0 delayed fluorescence spectra. It is shown that the T1(pi, pi*) levels are located at 900 +/- 100 and 300 +/- 100 cm(-1) below the T2(n, pi*) levels, respectively, for p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors. The estimated T1 energy levels are in good agreement with the phosphorescence origins in rigid glass at 77 K.     

Ultrafast decay of electronically excited singlet cytosine via a pi,pi* to n(O),pi* state switch

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.     

Stabilizing the elusive ortho-quinone/copper(I) oxidation state combination through pi/pi interaction in an isolated complex

The heterodinuclear compound [(PhenQ)Cu(dppf)](BF4), PhenQ = 9,10-phenanthrenequinone and dppf = 1,1'-bis(diphenylphosphino)ferrocene, was identified structurally and spectroscopically (NMR, IR, UV-vis) as a copper(I) complex of a completely unreduced ortho-quinone. Crystallographic and DFT calculation results suggest that this stabilization of a hitherto elusive arrangement is partially owed to intramolecular pi/pi interactions phenyl/PhenQ. Intermolecular PhenQ/PhenQ pi stacking is also observed in the crystal. According to DFT calculations, the pi interactions are responsible for the considerably distorted coordination geometry at CuI with one short and one longer Cu-O and Cu-P bond, respectively, and with bond angles at copper ranging from 99 degrees to 133 degrees. Electrochemical reduction proceeds reversibly at low temperatures to yield an EPR spectroscopically characterized semiquinone-copper(I) species.     

Time-dependent quantum wave-packet description of the 1pi sigma* photochemistry of phenol

The photoinduced hydrogen elimination reaction in phenol via the conical intersections of the dissociative 1pi sigma* state with the 1pi pi* state and the electronic ground state has been investigated by time-dependent quantum wave-packet calculations. A model including three intersecting electronic potential-energy surfaces (S0, 1pi sigma*, and 1pi pi*) and two nuclear degrees of freedom (OH stretching and OH torsion) has been constructed on the basis of accurate ab initio multireference electronic-structure data. The electronic population transfer processes at the conical intersections, the branching ratio between the two dissociation channels, and their dependence on the initial vibrational levels have been investigated by photoexciting phenol from different vibrational levels of its ground electronic state. The nonadiabatic transitions between the excited states and the ground state occur on a time scale of a few tens of femtoseconds if the 1pi pi*-1pi sigma* conical intersection is directly accessible, which requires the excitation of at least one quantum of the OH stretching mode in the 1pi pi* state. It is shown that the node structure, which is imposed on the nuclear wave packet by the initial preparation as well as by the transition through the first conical intersection (1pi pi*-1pi sigma*), has a profound effect on the nonadiabatic dynamics at the second conical intersection (1pi sigma*-S0). These findings suggest that laser control of the photodissociation of phenol via IR mode-specific excitation of vibrational levels in the electronic ground state should be possible.     

Cleavable chiral auxiliaries in 8pi (8pi, 6pi) electrocyclizations

[reaction: see text] Low to moderate diastereoselectivity was observed in the 8pi electrocyclization of a series of chiral auxiliary-bearing tetraenic esters. In the 8-arylmenthyl series, diastereomeric products were separated by chromatography.     

The diatropic sigma ring currents of [pi2s + pi2s + pi2s] pericyclic transition states

Ab initio current-density maps for the (4n + 2)-electron transition states of two thermally allowed [pi2s + pi2s + pi2s] trimerisations, of ethyne to benzene (1), and of ethene to cyclohexane (2), show that both support sigma diatropic ring currents, dominated by four-electron sigma --> sigma* virtual excitation of HOMO electrons.     

Halogen bonding and pi...pi stacking control reactivity in the solid state

The halogen bonding and the pi...pi stacking interactions induce the noncovalent self-assembly of modules into photoreactive supramolecular architecture. The pi...pi interaction pre-organizes the template, and the halogen bonding aligns the olefins to conform to the topochemical principle for photoreaction. The UV irradiation of the crystal resulted in a cyclization product with quantitative yield and stereospecificity.     

The benzophenone S1(n,pi*) --> T1(n,pi*) states intersystem crossing reinvestigated by ultrafast absorption spectroscopy and multivariate curve resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.     

Preparation of a resin-based chromium catalyst for effecting [6pi + 2pi] cycloaddition reactions

[reaction: see text] A resin-based chromium catalyst for performing [6pi + 2pi] cycloaddition reactions has been prepared from chloromethylated polystyrene. The catalyst provides cycloadducts in yields comparable to the photochemical and thermal versions of these transformations, and the process is effective with a wide range of 6pi and 2pi reaction partners.     

Photophysics of aromatic molecules with low-lying pi sigma* states: fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S(1)) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S(1) state of pi pi(*) character for fluorinated benzenes with four or less F atoms, and S(1) state of pi sigma(*) character for PFB and HFB. The pi sigma(*) character of the S(1) state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the (1)pi pi(*) (L(b))<--S(0) transition, which can be identified with the predicted low-energy (1)pi sigma(*)<--S(0) absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the (1)pi sigma(*) state and efficient S(1) (pi sigma(*))-->S(0) internal conversion between two electronic states of very different geometries.     

Geometrical effects on the intramolecular quenching of pi,pi* aromatic ketones by phenols and indoles

Laser flash photolysis of a series of bichromophoric compounds 1-12 containing the 2-benzoylthiophene (BT) and phenol (PhOH) or indole (InH) moieties has been used to determine the possible geometrical effects in the intramolecular quenching of triplet excited ketones, resulting in formal hydrogen abstraction. The results are compared with those obtained in the intermolecular process. In both cases, substitution either at the thienyl or the phenyl moiety has a marked influence on the photoreactivity. Time-resolved experiments showed that the rate constants for bimolecular quenching by phenol and indole of 2-benzoylthiophene substituted at the thienyl 5-position were lower than those for BT substituted at the phenyl p-position, which agrees with the higher energy found for the excited triplet state of the latter compounds. However, the rate constant for hydrogen abstraction in the bichromophoric compounds by the pi,pi* triplet state of the derivatives with the spacer linked to the thienyl 5-position are higher than those of their regioisomers. These results indicate a possible geometry-dependence in the intramolecular quenching process. Theoretical DFT studies have been carried out in order to estimate the optimum conformation for hydrogen abstraction in two pairs of phenolic and indolic bichromophoric regioisomers. The energy profile for photoactivation/deactivation of the aromatic ketone and the structures of the triplet states and biradicals involved in the process have been determined. The observed regiodifferentiation in the experimental studies is consistent with a dependence of the rate constant on orbital overlap between the carbonyl oxygen and the X-H bonds.