Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission

To design ultrabright fluorescent solid dyes, a crystal engineering strategy that enables monomeric emission by blocking intermolecular electronic interactions is required. We introduced propylene moieties to distyrylbenzene (DSB) as bridges between the phenyl rings either side of its C=C bonds. The bridged DSB derivatives formed compact crystals that emit colors similar to those of the same molecules in dilute solution, with high quantum yields. The introduction of flexible seven-membered rings to the DSB core produced moderate distortion and steric hindrance in the DSB π-plane. However, owing to this strategy, it was possible to control the molecular arrangement with almost no decrease in the crystal density, and intermolecular electronic interactions were suppressed. The bridged DSB crystal structure differs from other DSB derivative structures; thus, bridging affords access to novel crystalline systems. This design strategy has important implications in many fields and is more effective than the conventional photofunctional molecular crystal design strategies.     

Jahn-Teller and related effects in the silver trimer. II: vibrational analysis of the A 2E"-X 2E' electronic transition

The laser-excited, jet-cooled A 2E"-X 2E' electronic spectrum of the silver trimer yields detailed information about its A- and X-state vibronic structure. Following extensive parameter fitting, the absorption and emission spectra are simulated and the bands are assigned. The Jahn-Teller analysis includes both linear and quadratic coupling terms, considered simultaneously with spin-orbit coupling. The spin-orbit splitting is shown to be largely quenched in both the A and X electronic states. The Jahn-Teller analysis of the A and X vibronic structures reveals the distortion of their corresponding potential energy surfaces.     

Theoretical study of the effects of solvent environment on photophysical properties and electronic structure of paracyclophane chromophores

We use first-principles quantum-chemical approaches to study absorption and emission properties of recently synthesized distyrylbenzene (DSB) derivative chromophores and their dimers (two DSB molecules linked through a [2.2]paracyclophane moiety). Several solvent models are applied to model experimentally observed shifts and radiative lifetimes in Stokes nonpolar organic solvents (toluene) and water. The molecular environment is simulated using the implicit solvation models, as well as explicit water molecules and counterions. Calculations show that neither implicit nor explicit solvent models are sufficient to reproduce experimental observations. The contact pair between the chromophore and counterion, on the other hand, is able to reproduce the experimental data when a partial screening effect of the solvent is taken into account. Based on our simulations we suggest two mechanisms for the excited-state lifetime increase in aqueous solutions. These findings may have a number of implications for organic light-emitting devices, electronic functionalities of soluble polymers and molecular fluorescent labels, and their possible applications as biosensors and charge/energy conduits in nanoassemblies.     

Solute-solvent intermolecular vibronic coupling as manifested by the molecular near-field effect in resonance hyper-Raman scattering

Vibronic coupling within the excited electronic manifold of the solute all-trans-β-carotene through the vibrational motions of the solvent cyclohexane is shown to manifest as the "molecular near-field effect," in which the solvent hyper-Raman bands are subject to marked intensity enhancements under the presence of all-trans-β-carotene. The resonance hyper-Raman excitation profiles of the enhanced solvent bands exhibit similar peaks to those of the solute bands in the wavenumber region of 21,700-25,000 cm(-1) (10,850-12,500 cm(-1) in the hyper-Raman exciting wavenumber), where the solute all-trans-β-carotene shows a strong absorption assigned to the 1A(g) → 1B(u) transition. This fact indicates that the solvent hyper-Raman bands gain their intensities through resonances with the electronic states of the solute. The observed excitation profiles are quantitatively analyzed and are successfully accounted for by an extended vibronic theory of resonance hyper-Raman scattering that incorporates the vibronic coupling within the excited electronic manifold of all-trans-β-carotene through the vibrational motions of cyclohexane. It is shown that the major resonance arises from the B-term (vibronic) coupling between the first excited vibrational level (v = 1) of the 1B(u) state and the ground vibrational level (v = 0) of a nearby A(g) state through ungerade vibrational modes of both the solute and the solvent molecules. The inversion symmetry of the solute all-trans-β-carotene is preserved, suggesting the weak perturbative nature of the solute-solvent interaction in the molecular near-field effect. The present study introduces a new concept, "intermolecular vibronic coupling," which may provide an experimentally accessible∕theoretically tractable model for understanding weak solute-solvent interactions in liquid.     

Electronic Substitution Effect on the Ground and Excited State Properties of Indole Chromophore: A Computational Study**

Indole, being the main chromophore of amino acid tryptophan and several other biologically relevant molecules like serotonin, melatonin, has prompted considerable theoretical and experimental interest. The current work focuses on the investigation of substitution effect on the ground and excited electronic states of indole using computational quantum chemistry. Having three close-lying excited electronic states, the vibronic coupling effect becomes extremely important yet challenging for the photophysics and photochemistry of indole. Here, we have evaluated the performance of time-dependent density functional theory against available experimental and ab initio results from the literature. The electronic effects on the excited states of indole and indole derivatives e. g. tryptophan, serotonin and melatonin are reported. A bathochromic shift has been observed in the absorption spectrum for the L_a state. The absorption wavelength increases in the order of indole<tryptophan <serotonin <melatonin. While the contribution of the in-plane small adjacent groups increases the electron density of the indole ring, the out-of-plane long substituent groups have minor effect. The absorption spectra calculated including the vibronic coupling are in good agreement with experiments. These results can be used to estimate the error in photophysical observables of indole derivatives calculated considering indole as a prototypical system.     

Probing intermolecular communication with surface-attached pyrene

We report on the covalent attachment of pyrene derivatives to solid substrates and their spectroscopic and electrochemical characterization. We have constructed several molecular assemblies attached to silica and indium-doped tin oxide surfaces where pyrene molecules are co-immobilized with other functionalities. It was shown that the addition of hydrophobic molecules to the pyrene-containing interface results in a significant decrease in the pyrene I1/I3 vibronic emission band ratio and an increase in the water drop contact angle due to increased hydrophobicity of the interface. The co-attachment of perylenedodecanoic acid, for which the absorption band overlaps with the emission spectrum of pyrene, shows significant intermolecular communication between these species. The co-immobilization of ferrocene serves as an effective fluorescence quencher for tethered pyrene. In all cases, our data point to significant intermolecular communication between adsorbate species, and the combination of spectroscopic and electrochemical interrogation provides insight into the loading density and local environment(s) characteristic of these interfaces.     

Intramolecular excimer formation for covalently linked boron dipyrromethene dyes

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.     

The HOMO Nodal Arrangement in Polychromophoric Molecules and Assemblies Controls the Interchromophoric Electronic Coupling

Triptycenes spontaneously assemble into two‐dimensional networks in which long‐range charge transport is facilitated by the extensive electronic coupling through the triptycene framework (intramolecularly) and by cofacial π‐stacking (intermolecularly). While designing and synthesizing next‐generation triptycenes containing polyaromatic chromophores, the electronic coupling amongst the chromophores was observed to be highly dependent on the nature and position of the substituents. Herein, we demonstrate using hexaalkoxytriptycenes that the electronic coupling amongst the chromophores is switched on and off by a simple repositioning of the substituents, which alters the nodal arrangement of the HOMOs of the individual chromophores. A visual inspection of the HOMOs can thus provide a ready evaluation of the electronic coupling in polychromophoric molecules/assemblies, and will serve as an important tool for the rational design of modern charge‐transport materials.     

Efficient intramolecular energy transfer in single endcapped conjugated polymer molecules in the absence of appreciable spectral overlap

Intramolecular energy transfer is investigated in an endcapped conjugated polymer on the single molecule level at low temperature. While light harvesting in one dimension is on average inefficient in the ensemble, the efficiency scatters widely on the single molecule level, with some molecules exhibiting near-unity transfer probability from the polymer backbone donor to the acceptor endcap. This transfer occurs in the absence of spectral overlap between donor and acceptor, as the electronic and vibronic transitions narrow substantially at low temperatures once inhomogeneous disorder broadening is overcome. The results illustrate how far-field absorption and emission characteristics of molecular transitions are insufficient to describe resonant energy transfer processes following F?rster theory in multichromophoric aggregates. Rather, exciton trapping due to efficient multiphonon emission has to be invoked with a possible contribution of strong polaronic coupling.     

Identifiability analysis of models for reversible intermolecular two-state excited-state processes coupled with species-dependent rotational diffusion monitored by time-resolved fluorescence depolarization

A deterministic identifiability analysis of the kinetic model for a reversible intermolecular two-state excited-state process with species-dependent rotational diffusion described by Brownian reorientation is presented. The cases of both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation on interconversion in the latter case, are specifically considered. The identifiability analysis is carried out in terms of compartmental modeling based on the S(t) identical with I( parallel)(t)+2I( perpendicular)(t) and D(t) identical with I( parallel)(t)-I( perpendicular)(t) functions, where I( parallel)(t) and I( perpendicular)(t) are the delta-response functions for fluorescence, polarized, respectively, parallel and perpendicular to the electric vector of linearly polarized excitation. It is shown that, from polarized time-resolved fluorescence data collected at two concentrations of coreactant and three appropriately chosen emission wavelengths, (a) a unique set of rate constants for the overall excited-state process is always obtained by making use of polarized measurements and (b) the rotational diffusion constants and geometrical factors associated with the different anisotropy decay components can be uniquely determined and assigned to each species. The geometrical factors are determined by the absorption and emission transitions in the two rotating species. For spherical rotors, these factors depend directly on the relative orientations of the transition moments, while for cylindrically symmetric rotors they depend on the orientations with respect to each other and to the symmetry axis.     

Electron-vibrational dynamics of photoexcited polyfluorenes

The highly polarizable pi-electron system of conjugated molecules forms the basis for their unique electronic and photophysical properties, which play an important role in numerous biological phenomena and make them important materials for technological applications. We present a theoretical investigation of the dynamics and relaxation of photoexcited states in conjugated polyfluorenes, which are promising materials for display applications. Our analysis shows that both fast (approximately 20 fs) and slow (approximately 1 ps) nuclear motions couple to the electronic degrees of freedom during the excited-state dynamics. Delocalized excitations dominate the absorption, whereas emission comes from localized (self-trapped) excitons. This localization is attributed to an inherent nonlinear coupling among vibronic degrees of freedom which leads to lattice and torsional distortions and results in specific signatures in spectroscopic observables. Computed vertical absorption and fluorescence frequencies as well as photoluminescence band shapes show good agreement with experiment. Finally, we demonstrate that dimerization such as spiro-linking does not affect the emission properties of molecules because the excitation becomes confined on a single chain of the composite molecule.     

On Theoretical Study of a Molecular Dimer System

In this paper, the vibronic structure of a dimer system is studied both theoretically and numerically. To construct adiabatic potential surfaces and electronic and vibrational wave functions for a dimer system, the adiabatic approximation is applied to two identical molecules, each of which has two electronic states with one vibrational mode. In this scheme, the excitonic splitting results not only from the electronic coupling of two molecules, but also from the vibronic coupling in each molecule. By using the resulting wavefunctions and the corresponding energies, the absorption and fluorescence spectra are studied. The effect of temperature on these spectra is also studied.     

What is the best DFT functional for vibronic calculations? A comparison of the calculated vibronic structure of the S1-S0 transition of phenylacetylene with cavity ringdown band intensities

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.     

Vibronic coupling of electronic states. IV. Harmonic CBO potentials with equal minima positions—the influence of different vibronic coupling strengths on the electronic absorption and emission spectra containing a longest-wavelength electronically forbidden transition

The influence of vibronic coupling between two harmonic CBO potentials with equal minima positions on the electronic absorption and emission spectra is investigated in the framework of our model using the variational procedure. The numerical results, being identical with those obtained through the vibronic coupling model of Fulton, Gouterman, and Brickmann, are discussed with regard to the longest-wavelength electronically forbidden transition, its vibrational structure, and the Stokes loss.     

Functionally relevant electric-field induced perturbations of the prosthetic group of yeast ferrocytochrome c mutants obtained from a vibronic analysis of low-temperature absorption spectra

We have measured the low temperature (T = 20 K) absorption spectra of the N52A, N52V, N52I, Y67F, and N52AY67F mutants of ferrous Saccharomyces cerevisiae (baker's yeast) cytochrome c. All the bands in the Q0- and Q(v)-band region are split, and the intensity distributions among the split bands are highly asymmetric. The spectra were analyzed by a decomposition into Voigtian profiles. The spectral parameters thus obtained were further analyzed in terms of the vibronic coupling model of Schweitzer-Stenner and Bigman (Schweitzer-Stenner, R.; Bigman, D. J. Phys. Chem. B 2001, 7064-7073) to identify parameters related to electronic and vibronic perturbations of the heme macrocycle. We report that the electronic perturbation is of B(1g) symmetry and reflects the heterogeneity of the electric field at the heme, that is, the difference between the gradients along the perpendicular N-Fe-N axis of the heme core. We found that all the investigated mutations substantially increase this electronic perturbation, so that the spectral properties become similar to those of horse heart cytochrome c. Moreover, the electronic perturbation was found to correlate nonlinearly with the enthalpy changes associated with the reduction of the heme iron. Group theoretical arguments are invoked to propose a simple model which explains how a perturbation of the obtained symmetry can stabilize the reduced state of the heme iron. Finally, vibronic coupling parameters obtained from the analysis of the Q(v)-band region suggest that the investigated mutations decrease the nonplanar deformations of the heme group. This finding was reproduced by a normal mode structural decomposition (NSD) analysis of the N52V and N52VY67F heme conformations obtained from a 1 ns molecular dynamics simulation. We argue that the reduced nonplanarity contributes to the stabilization of the reduced state.     

Photodynamics of polyene-polymethine transformations and spectral fluorescent properties of merocyanine dyes

Absorption and fluorescence spectrum band moments (center of gravity, width, asymmetry, excess, and fine structure) have been determined in a wide range of solvents with different polarities for inverse solvatochromic di-, tetra-, and hexamethinemerocyanines derived from 1,3-diphenyl-2,3-dihydro-1H-benzimidazole. Juxtaposition of the quantum-chemically calculated (by the semiempirical AM1 method) charges, bond orders, and dipole moments of the merocyanine molecules in the ground and excited singlet states with the experimentally observed spectral fluorescent characteristics suggests that the molecular electronic structure in the two states can vary from a nonpolar polyene via a polymethine to a charge-separated polyene, depending on the length of the polymethine chain and the medium polarity. As shown, solvatofluorochromism gives rise to smaller spectral band shifts than those of solvatochromism. This effect is attributable to weaker intermolecular solute-solvent interactions in the fluorescent excited state due to the more equalized charges as compared to those of the ground state. A lack of mirror symmetry of the absorption and fluorescence spectra has been revealed for di- and tetramethinemerocyanines (broadened fluorescence bands) as well as for hexamethinemerocyanines (narrowed fluorescence bands); the two cases are accounted for by the different behavior of vibronic and intermolecular interactions in the course of absorption and emission. As found for merocyanines, the electronic structure of their fluorescent state approaches the cyanine limit and the ground state becomes increasingly polyene-like with lengthening of the polymethine chain. A close vicinity of the excited state to the cyanine limit causes a dramatic increase in fluorescence quantum yields and a decrease in Stokes shifts observed for higher merocyanine vinylogues.     

Dimeres composes de molecules dont l'excitation electronique entraine un deplacement de la configuration nucleaire d'equilibre accompagne d'un changement de constante de force

We have studied the validity of the traditional model of a dimer that has been treated exactly by Merrifield and Fulton and Gouterman, solving the vibronic coupled equations by a numerical method. This model takes into account the modification of the nuclear equilibrium configuration, but it neglects the variation of the force constant when the monomer is electronically excited from the fundamental to a given excited state (the corresponding electronic potentials are both considered as harmonic). We have shown by inspection of the absorption and fluorescence spectra calculated by solving the vibronic equation exactly that the variation force constant cannot be neglected, even if it is weak, particularly in the weak coupling region. The weak, intermediate and strong coupling criteria have been deduced, for the model studied, by examination of the dimeric electronic potential surfaces for different cases of intermolecular interactions.     

Analysis of the Vibronic Structure in the Emission and Absorption Spectra of (&mgr;-1,1-Dicyanoethylene-2,2-dithiolato-S,S')bis(triphenylphosphine)digold(I) and Assignment of the Emissive State

Both the 77 K single crystal absorption and 20 K emission spectrum of (&mgr;-1,1-dicyanoethylene-2,2-dithiolato-S,S')bis(triphenylphosphine)digold(I), (AuPPh(3))(2)[i-MNT], show resolved vibronic structure. Progressions in the 1410 cm(-)(1) C=C stretching mode of the dithiolate ligand, and in the 480 cm(-)(1) mode, which involves gold-dithiolate stretching, are observed in the emission spectrum. The resonance Raman spectra of the title compound and related compounds were used to identify the modes that give rise to the vibronic structure observed in the emission spectrum. The emission spectrum is fit using the time dependent theory of electronic spectroscopy. The theoretical fit to the spectrum requires distortions in vibrations involving both the metal-sulfur and dithiolate centered modes. These distortions show that the transition is a charge transfer involving the gold and the dithiolate ligand. The emission spectrum of an analogous complex, (AuAsPh(3))(2)[i-MNT], is red shifted relative to the title complex. This red shift allows the direction of the charge transfer emission in the title complex to be assigned as a dithiolate to gold, ligand to metal charge transfer.     

On the excited state dynamics of vibronic transitions. High-resolution electronic spectra of acenaphthene and its argon van der Waals complex in the gas phase

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state. Additionally, some other vibronic bands exhibit anomalous intensity patterns in several of their rotational transitions. A Fermi resonance involving two near degenerate vibrations has been proposed to explain this behavior. The high-resolution electronic spectrum of the ACN-Ar vdW complex has also been obtained and fully analyzed. The results indicate that the weakly attached argon atom is located on top of the plane of the bare molecule at ~3.48 ? away from its center of mass in the S(0) electronic state.     

Azide-water intermolecular coupling measured by two-color two-dimensional infrared spectroscopy

We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D(2)O. The azide-bound D(2)O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ~500 fs. The waters around the ion are present in an electrostatically better defined environment. Afterwards, ~1 ps, the sample thermalizes and selectivity is lost. On the other hand, the excited state absorption signal of the azide-bound D(2)O is much broader. The asymmetry in spectral width between bleach/stimulated emission versus excited absorption has been observed in very much the same way for isotope-diluted ice Ih, where it has been attributed to the anharmonicity of the OD potential.