Microsolvation effects on the pi*<--pi electronic transitions in simple aromatic chromophores: the role of the Slater-type Gaussian orbitals in the complete active space self-consistent field approach

One of the most studied feature of aromatic molecular clusters in the gas phase is the shift of the pi*<--pi electronic transitions between the bare aromatic chromophore and its clusters. In the present ab initio complete active space self-consistent field study the coupling of the basis set superposition error and counterpoise procedure and a combination of Pople- and Slater-type Gaussian basis sets has been proven to reproduce quantitatively the gas-phase experimental shifts. The quantal results are here analyzed with respect to the electrostatic and polarization forces and electron density differences, and connected with the sign of the shifts of the electronic transitions.     

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.     

Origin of the attraction in aliphatic C-H/pi interactions: infrared spectroscopic and theoretical characterization of gas-phase clusters of aromatics with methane

An attractive intermolecular interaction between an aliphatic C-H bond and a pi-electron system (C-H/pi interaction) was characterized on the basis of infrared spectroscopy and high level ab initio calculations. Infrared spectroscopy was applied to several isolated methane clusters with benzene, toluene, p-xylene, mesitylene, and naphthalene in the gas phase, and the spectral changes of the C-H stretch bands in the methane moiety upon the cluster formation were observed. In the theoretical approach, interaction energies of the clusters were evaluated by high-level ab initio calculations. The forbidden symmetric C-H stretch transition weakly appeared in the IR spectra of the clusters, and it confirmed the small deformation of the methane moiety from the T(d)() symmetry, which was predicted by the ab initio calculations. On the other hand, the degenerated asymmetric C-H stretch band showed complicated splitting, which is qualitatively interpreted by a hindered rotor model. Low-frequency shifts upon the cluster formation were seen in the symmetric C-H stretch frequency, though the magnitude of the shifts was extremely small and no clear correlation with the interaction energy was found. On the other hand, the size of the calculated interaction energy well correlates with the polarizability of aromatics. The S(1)-S(0) electronic transition of the aromatic moiety was also observed, and it showed low-frequency shifts upon cluster formation. These results support the dominance of the dispersion interaction over the electrostatic and charge-transfer terms in the aliphatic C-H/pi interaction.     

C 1s -->pi* excitation in variable size benzene clusters

The C 1s -->pi* transition in molecular benzene and benzene clusters is investigated by photoion yields at high energy resolution. The vibrationally resolved band shows the same shape in clusters as in the bare molecule, but it is redshifted by 50 meV in small clusters, i.e. near the threshold of cluster formation. This redshift increases to 70 meV with increasing cluster size. The results are assigned in comparison with ab initio calculations on model structures of dimers, trimers, and tetramers. These indicate that different carbon sites in the molecular moieties give rise to distinct spectral shifts, where carbon sites that are pointing to the pi-system of another molecule show a larger redshift than the other ones. Such structural properties are found in solid benzene, so that the gas-to-solid shift of C 1s -->pi* excited benzene is derived to be a redshift which is of the order of 100-180 meV.     

Solvent-induced electronic spectral shifts: Benzene·Arn revisited

Just what can be learned about cluster dynamics (and, more generally, about solvation dynamics) from spectral studies of small clusters that have been doped with a chromophore is still an open question. In the present work we suggest a novel procedure for calculating the shift in the electronic absorption spectrum of a chromophore deriving from the attachment to or the incorporation in a cluster. The particular system of interest here is benzene·Ar _n , for which experimental results are readily available although their interpretation has been a point of controversy. In addition, since the present formalism is equally applicable to a chromophore isolated in a bulk phase (either liquid or solid), we are able to venture an explanation for the apparent observation that the spectral shift of cluster-isolated benzene does not approach the asymptotic values characteristic of the bulk-isolated species.     

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.     

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.     

Fluorescent perylene diimide rotaxanes: spectroscopic signatures of wheel-chromophore interactions

[2]- and [3]-rotaxanes with a tetraphenoxy perylene diimide core were synthesized. Hydrogen bonding between the wheel and the imide changes the optical properties of the perylene chromophore: the absorption and fluorescence spectra are red-shifted. The decay times of the rotaxanes are shorter in comparison with that of the axle. Single molecule fluorescence measurements reveal relatively narrow distributions of emission maxima and decay times. The averages are in agreement with ensemble measurements. The observed red shifts make the perylene diimide a suitable chromophore for sensing the position of the wheel on the axle.     

Photochemical structuring and fixing of structures in binary polymer brush layers via 2pi+2pi photodimerization

Binary polymer brushes grafted to Si wafers were prepared from hydrophilic and hydrophobic polymer components, which allow switching the surface properties (as revealed by contact angles) by exposure to varying solvents. The hydrophilic component was poly-2-vinylpyridine; the hydrophobic component was a copolymer of styrene and 2-(4-vinylphenyl)indene containing the photodimerizable phenylindene chromophore. The brushes were prepared via thermal and via photochemical grafting-to methods, which led to distinct layer thicknesses. Structural patterns differing in surface properties were produced and fixed via crosslinking the hydrophobic component by 2pi+2pi photodimerization of phenylindene moieties. The patterns were visually observable.     

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 concerted addition of HBr to aryl alkynes; orthogonal pi bond selectivity

In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic pi bond orthogonal to the extended aromatic pi system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the beta carbon. Attack on this pi bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents.     

Towards highly fluorescent and water-soluble perylene dyes

A systematic approach towards highly fluorescent, water-soluble perylene-3,4:9,10-tetracarboxylic acid diimide chromophores is presented. Water solubility was introduced first through the attachment of four hydrophilic substituents onto the bay region of the perylene dye. Positively and negatively charged groups were then applied to the chromophore, and their number and their distance from the aromatic scaffold were systematically varied. To suppress aggregation, the chromophore was further isolated within a dendritic shell. Such variation of structural features and a thorough investigation of the resulting optical properties facilitated the first synthesis of perylene-3,4:9,10-tetracarboxylic acid diimides combining the properties of water solubility and fluorescence quantum yields (FQYs) close to unity, which makes them attractive as high-performance fluorescence probes in aqueous media.     

Perylene-3,4:9,10-tetracarboxylic acid bisimide dye as an artificial DNA base surrogate

A phosphoramidite of the perylene bisimide dye was synthesized as a DNA building block that allows incorporation of this chromophore as an artificial nucleoside surrogate either at the 5'-terminus or at internal positions of duplex DNA. The internally incorporated perylene bisimide chromophore shows strong interactions with the DNA base stack; the 5'-terminally attached perylene bisimide is able to induce dimerization of two whole DNA duplexes.     

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.     

Block modification of rod-shaped pi conjugated carbon frameworks with donor and acceptor groups toward highly fluorescent molecules: synthesis and emission characteristics

To create organic molecules that are highly fluorescent at a longer wavelength region, we investigated the synthesis (using Pd-catalyzed cross-coupling) and photophysical properties (Phi(f), lambda(em), tau, lambda(abs), and epsilon) of the following pi conjugated molecular rods consisting of p-phenyleneethynylene units modified by donor (OMe) and/or acceptor (CN) groups: (1) side-donor modification systems (SD systems), (2) side-acceptor modification systems (SA systems), and (3) systems consisting of a donor block and an acceptor block (BL systems). As a consequence, very high Phi(f) values (>0.95) were obtained for BL systems. Bathochromic shifts of lambda(em) for the same pi conjugation length were largest for BL systems. Thus we succeeded in creating highly efficient light emitters at a longer wavelength region by block modification (e.g., Phi(f) = 0.97, lambda(em) = 464 nm for ). Considerably intense solid emission (Phi(f) approximately 0.5) at a longer wavelength region (500-560 nm) was also found for BL systems. It has been found that and exhibit interesting two photon absorption characteristics.     

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.     

DNA‐Based Oligochromophores as Light‐Harvesting Systems

The chromophores ethynyl pyrene as blue, ethynyl perylene as green and ethynyl Nile red as red emitter were conjugated to the 5‐position of 2′‐deoxyuridine via an acetylene bridge. Using phosphoramidite chemistry on solid phase labelled DNA duplexes were prepared that bear single chromophore modifications, and binary and ternary combinations of these chromophore modifications. The steady‐state and time‐resolved fluorescence spectra of all three chromophores were studied in these modified DNA duplexes. An energy‐transfer cascade occurs from ethynyl pyrene over ethynyl perylene to ethynyl Nile red and subsequently an electron‐transfer cascade in the opposite direction (from ethynyl Nile red to ethynyl perylene or ethynyl pyrene, but not from ethynyl perylene to ethynyl pyrene). The electron‐transfer processes finally provide charge separation. The efficiencies by these energy and electron‐transfer processes can be tuned by the distances between the chromophores and the sequences. Most importantly, excitation at any wavelength between 350 and 700 nm finally leads to charge separated states which make these DNA samples promising candidates for light‐harvesting systems.     

26pi aromatic core-modified hexaphyrins: syntheses, characterization, and structural diversities

Synthesis and characterization of several 26pi core-modified hexaphyrins are reported. The synthetic methodology involved a well-known acid-catalyzed MacDonald-type condensation reaction of the required tripyrrane with electron deficient pentafluorobenzaldehyde. The nature of the product and yield depends on the nature of the acid catalyst and its concentration. Dioxahexaphyrin 9 was isolated only when 0.5 equiv of TFA was used as a catalyst, while dithiahexaphyrin 10 and diselenahexaphyrin 11 were formed with TFA, PTSA, and even in the absence of catalyst. The detailed 1H and 2-D COSY as well as HSQC experiments reveal the solution structure as well as the conformational mobility of hexaphyrins. In the tetracationic state, 10 and 11 exhibit a four heterocyclic ring inverted structure, while only two completely inverted heterocyclic rings were observed for 9. The other four heterocyclic rings are only partially inverted in 9. All the hexaphyrins reported here show aromatic character inferred from large Deltadelta values (difference in chemical shift between the most shielded and the most deshielded protons). Electronic absorption spectral studies also support the conformational changes observed upon protonation.     

Crystal engineering for topochemical polymerization of muconic esters using halogen-halogen and CH/pi interactions as weak intermolecular interactions

We now report the molecular and crystal structure design of muconic ester derivatives on the basis of crystal engineering using halogen-halogen contacts and CH/pi interactions. The solid-state photoreaction pathway of the dibenzyl (Z,Z)-muconates as the 1,3-diene dicarboxylic acid monomers depends on the structure of the ester groups. The substitution of a halogen atom for the aromatic hydrogen of a benzyl group induces topochemical polymerization to produce stereoregular polymers in a crystalline form, whereas the unsubstituted benzyl derivative isomerizes to yield the corresponding E,E isomer under similar conditions. The topochemical polymerization process is directly confirmed by the fact that the single-crystal structures before and after the polymerization are very similar to each other. From the crystal structure analysis for a series of substituted benzyl (Z,Z)- and (E,E)-muconates, it has been revealed that the planar diene moieties are closely packed to form a columnar structure in the crystals. The stacking of the polymerizable monomers is characterized by a stacking distance of 4.9-5.2 A along the columns. This structure is supported by a halogen-halogen interaction between the chlorine or bromine atoms introduced at the p position of the benzyl groups in addition to an aromatic stacking due to the CH/pi interaction between the benzylic methylene hydrogens and aromatic rings. The design of a monomer packing corresponds to the type and position of the introduced halogen atom and also the polymorphs. To make a stacking distance of 5 A using both halogen-halogen and CH/pi interactions as supramolecular synthons is important for the molecular design of muconic ester derivatives appropriate for topochemical polymerization.     

CH...pi interaction for rhenium-based rectangles: an interaction that is rarely designed into a host-guest pair

Alkoxy- and thiolato-bridged Re(I) molecular rectangles [{(CO)3Re(mu-ER)2Re(CO)3}2(mu-bpy)2] (ER = SC4H9, 1a; SC8H17, 1b; OC4H9, 2a; OC12H25, 2b; bpy = 4,4'-bipyridine) exhibit strong interactions with several planar aromatic molecules. The nature of their binding was studied by spectral techniques and verified by X-ray diffraction analysis. Standard absorption and fluorescence titrations showed that a relatively strong 1:1 interaction occurs between aromatic guests such as pyrene and these rectangles. The results of a single-crystal X-ray diffraction analysis show that the recognition of 1 with a pyrene molecule is mainly due to CH...pi interactions and the face of the guest pyrene is located over the edges of the bpy linkers of 1. This is a fairly novel example of an interaction that is rarely designed into a host-guest pair. Furthermore, the interaction of 1 with Ag+ results in the self-organization of supramolecular arrays, as revealed by solid-state data.