Franck-Condon Excited State Research Articles

A fluorescent temperature probe based on the association between the excited states of 4-(N,N-dimethylamino)benzonitrile and beta-cyclodextrin.

A temperature probe based on the fluorescence properties of the two excited states of 4-(N,N-dimethylamino)benzonitrile (DMABN) in equilibrium with beta-cyclodextrin (CD) in aqueous solution is presented. The fluorescence intensity of the Franck-Condon excited state (FB) as a function of temperature shows a straight line with a correlation better than 0.99 in the 283-308 K temperature interval. On the other hand, the fluorescence intensity of the twisted internal charge-transfer state (FA) remains constant in the same temperature interval because the binding of DMABN in the A* state to CD is isoenthalpic and entropy driven. It is found that the FA/FB ratio is independent of the excitation intensity at a specified temperature, shows a linear relationship with temperature, and allows temperature measurements with a resolution of +/- 2.5 K.

Excited state dynamics of 4(1 H-pyrrole 1-yl) benzoic acid and different environmental effects

The absorption and fluorescence characteristics of 4(1 H-pyrrole 1-yl) benzoic acid (PBA) in solvents of different polarities, acidities, glass matrices and also in β-cyclodextrin cavity were studied with regard to the influence of flexibility of the molecule in both the ground and excited sates. The study of dynamics of excited state reveals the presence of two competitive fluorescence emission from a common excited state of PBA. Of these two fluorescence bands one originates from delocalized excited (DE) state [i.e., Franck-Condon (F-C) excited state] and the other from twisted intramolecular charge transfer (TICT) state. The excitation of ground state of PBA leads to the immediate formation of F-C excited state and the TICT state is formed within 220 ps from the parent F-C excited state. The 220 ps time has also been interpreted as the time required for the molecule to relax by rotational motion along the C-N bond of PBA to achieve the TICT state. The steady state photophysical measurements show that the molecule is very weakly fluorescent and nonradiative rate parameters get prominence compared to radiative rate parameters. The fluorescence and phosphorescence studies at 77 K have also been reported. Quantum chemical calculations of the energies and dipole moments were performed for planar and different twisted conformations to confirm experimental findings qualitatively.

Solvent effects on the nπ* transition of pyrimidine in aqueous solution

A hybrid quantum mechanical and molecular mechanical potential is used in Monte Carlo simulations to examine the solvent effects on the electronic excitation energy for the n→π* transition of pyrimidine in aqueous solution. In the present study, the pyrimidine molecule is described by the semi-empirical AM1 model, while the solvent molecules are treated classically. Two sets of calculations are performed: the first involves the use of the pairwise three-point charge TIP3P model for water, and the second computation employs a polarizable many-body potential for the solvent. The latter calculation takes into account the effect of solvent polarization following the solute electronic excitation, and makes a correction to the energies determined using pairwise potentials, which neglects such fast polarization effects and overestimates the solute-solvent interactions on the Franck-Condon excited states. Our simulation studies of pyrimidine in water indicate that the solvent charge redistribution following the solute electronic excitation makes modest corrections (about −130␣cm−1) to the energy predicted by using pairwise potentials. Specific hydrogen bonding interactions between pyrimidine and water are important for the prediction of solvatochromic shifts for pyrimidine. The computed n→π* blue shift is 2275±110 cm−1, which may be compared with the experimental value (2700 cm−1) from isooctane to water.

Microsecond atomic force sensing of protein conformational dynamics: implications for the primary light-induced events in bacteriorhodopsin.

In this paper a new atomic force sensing technique is presented for dynamically probing conformational changes in proteins. The method is applied to the light-induced changes in the membrane-bound proton pump bacteriorhodopsin (bR). The microsecond time-resolution of the method, as presently implemented, covers many of the intermediates of the bR photocycle which is well characterized by spectroscopical methods. In addition to the native pigment, we have studied bR proteins substituted with chemically modified retinal chromophores. These synthetic chromophores were designed to restrict their ability to isomerize, while maintaining the basic characteristic of a large light-induced charge redistribution in the vertically excited Franck-Condon state. An analysis of the atomic force sensing signals lead us to conclude that protein conformational changes in bR can be initiated as a result of a light-triggered redistribution of electronic charge in the retinal chromophore, even when isomerization cannot take place. Although the coupling mechanism of such changes to the light-induced proton pump is still not established, our data question the current working hypothesis which attributes all primary events in retinal proteins to an initial trans<==>cis isomerization.

Photophysics of 4-dimethylamino-4′-cyanostilbene and 4-azetidinyl-4′-cyanostilbene. Time-resolved fluorescence and trans-cis photoisomerisation

Abstract The fluorescence decays of trans-4-dimethylamino-4′-cyanostilbene (DCS) and trans-4-azetidinyl-4′-cyanostilbene (ACS) in an unpolar (n-heptane) and a polar (acetonitrile) solvent are single exponential down to a time resolution of around 5 ps and dual fluorescence is not observed. The dipole moment increases in two steps: from 7 D in the ground state via 13 D (DCS) or 10 D (ACS) for the Franck-Condon excited state to 21 D (DCS) or 22 D (ACS) for the relaxed fluorescing CT state. This means that the intramolecular charge transfer (ICT) in the excited state at most involves an intermediate with a subpicosecond lifetime. The presence of the azetidinyl group in ACS does not slow down the ICT reaction, contrary to what has been found for the 4-aminobenzonitriles, showing that inversion of the amino group is not an important reaction coordinate here. The activation energy E tc for trans-cis photoisomerisation is determined from the fluorescence decays in n-heptane (14.0 kJ/mol, DCS) and in acetonitrile (22.7 kJ/mol, DCS and 22.5 kJ/mol, ACS). This increase of E tc with solvent polarity, opposite to what has been found for trans-stilbene, shows that the photoisomerisation transition state is less polar than the CT state.

Ultrafast electron transfer in arene - Br atom charge transfer complexes

Ultrafast pump-probe transient absorption spectroscopy was applied to study, return electron transfer in the areneBr atom charge transfer (CT) complexes. Ultrafast optical excitation of the charge transfer band, of the complex, yields an excited Franck-Condon state of the CT complex, which relaxes to form an ion pair. The rate of charge recombination between the bromide and arene cation strongly depends on arene concentration, indicating on efficient quenching of the ion pair by another donor molecule. The return electron transfer has been studied in various solvents. In general, the observed kinetics is nonexponential, with distribution of relaxation times from 1 ps to ∼1000 ps. The slowest component in the relaxation is attributed to charge recombination of the ion pair in an equilibrium configuration. The rate of charge recombination of the excited areneBr complex does not depend significantly on the donor strength.

Electronic structure and polarizabilities of some heterocycles: I. Hydroxypyrazoles and related systems

The (hyper)polarizabilities of different tautomer forms of hydroxypyrazoles and pyrazolones have been calculated by the finite-field procedure in the MNDO approximation and the sum of states formalism in the PPP approximation, with all singly- and doubly-excited electronic configurations in the CI method. It was shown that while in the ground electronic state the values of the (hyper) polarizabilities are not essentially different, in the first excited singlet Franck-Condon state an increase of the molecular polarizabilities of some tautomers is observed. This increase is attributed to a specific change in the electronic structure of the excited state, demonstrated by the localization of the electronic transition in the different pyrazolone tautomers. The electron-donor capabilities of phenyl-substituted hydroxypyrazoles and pyrazolones are discussed.

Band shape analysis of electronic spectra of polar dye solutions

A semiclassical theory of electronic spectra of polar dye solutions is presented and analytical expressions of the spectra are given. A quasi-molecular approach is applied and the quasi-molecule model of the spectra previously published is reanalyzed. It is assumed that a large-amplitude motion plays a key role in the broadening of the spectra of polar dye solutions. An energy-level diagram of a quasi-molecule, considered as a dye molecule with its nearest neighborhood, is presented. In addition, the energy of reorientation in going from a Franck-Condon to an equilibrated state is determined. The orientation energy is equal to that part of the excitation energy given by the difference of the excited Franck-Condon state of a quasi-molecule from that of its equilibrated excited state. It is shown that after excitation, not only is excess vibrational, but also part of the electrostatic interaction energy is transferred to the surroundings. This energy may be obtained directly from absorption and fluorescence spectra of polar dye solutions. Experimental verification is performed on several coumarin solutions. The mean value for the reorientation energy in this case is found to be 645 cm-1.

Sub-picosecond pump-probe spectroscopy of ESIPT in 3-hydroxyflavone

Sub-picosecond pump-probe spectroscopy of 3-hydroxyflavone in a range of solvents reveals the presence of two transient species which are assigned to the initial Franck–Condon excited state and the proton-transferred tautomer.

Photochemistry of an unsymmetrical polymethine-cyanine dye; solute—solvent interactions and relaxation dynamics of LDS 751

The photophysical and photochemical properties of the LDS 751 or styryl 8 molecule have been investigated in various solvents. Our results indicate that the photostability of this laser dye is mostly due to an efficient S1 → S0 non-radiative internal conversion process which is faster than the radiative process. This radiationless relaxation is governed by a viscosity-dependent process, tentatively assigned to the free rotor effect between the polymethinic chain and the end groups due to the single bond character of the dominant benzenoid structure. Moreover, intersystem crossing to the triplet state and trans-cis photoisomerization are totally inefficient. The shape (relatively small molar extinction coefficient and large bandwidth) and the blue shift of the absorption spectrum in polar solvents show that this unsymmetrical polymethine-cyanine dye is highly polar in the ground state with the cationic positive charge mainly localized on the benzothiazole nitrogen and inducing a large orientation polarization of the surrounding solvent molecules. The absorption transition involves a much less polar Franck—Condon excited state and a charge transfer from the dimethylanilino nucleus to the benzothiazole nucleus. The shape of the fluorescence spectrum, similar to that of symmetrical polymethine-cyanines and almost independent of solvent polarity, confirms the unpolar character of the first singlet excited state. The sub-picosecond analysis of the time-dependent fluorescence Stokes shift thus probes the relaxation of solvent molecules freed from solute—solvent interactions after the Franck—Condon absorption transition.

A study of the inclusion complexes of β-cyclodextrin with three electronic states of 4-(N,N-dimethylamino)benzonitrile

Abstract A marked difference among the inclusion formation constants of β-cyclodextrin (β-CD) and three electronic states of 4-( N , N -dimethylamino)benzonitrile (DMABN), namely the ground state S O , the Franck-Condon excited state B* and the change transfer state A*, is found. The complexation reaction involving the B* state is exothermic, while the complexation of the A* state is entropy driven, indicating different mechanisms. Evidence is presented to suggest that B* and A* occupy different microenvironments within the CD. A 1:1 stoichiometry is confirmed for the three reactions. Values of Δ H O and Δ S O are derived from the temperature dependence of the equilibrium constants.

Comparative study of primary photochemical events of two retinal proteins, bacteriorhodopsin and halorhodopsin, by use of subpicosecond time-resolved spectroscopy

The primary photochemical events of bacteriorhodopsin (bR) were compared with those of halorhodopsin (hR) by subpicosecond time-resolved spectroscopy. The absorption and stimulated emission spectra display similar features. It is proposed that the isomerization reaction and relaxation to the fluorescent state occur simultaneously from the Franck—Condon excited state. The quantum yield of cis—trans isomerization in bR is about twice that in hR.

Photoinduced charge separation and broken symmetry in Franck-Condon excited ππ'* states of tetraphenylpentatetraenes

Abstract

Femtosecond-picosecond laser photolysis studies on the dynamics of excited charge-transfer complexes: aromatic hydrocarbon-acid anhydride, -tetracyanoethylene, and -tetracyanoquinodimethane systems in acetonitrile solutions

Formation processes of contact ion pairs (CIP) from the excited Franck-Condon (FC) state of charge-transfer (CT) complexes of aromatic hydrocarbons with acid anhydride as well as cyano compound acceptors in acetonitrile solution and charge recombination (CR) rates (k{sub CR}{sup CIP}) of produced CIP states have been investigated by femtosecond and picosecond laser phototlysis and time-resolved absorption spectral measurements covering a wide range of free energy gap-{Delta}G{degree}{sub ip} between the ion pair and the ground state. It has been confirmed that the CIP formation becomes faster and k{sub CR}{sup CIP} of the produced CIP increases with increase of the strengths of the electron donor (D) and acceptor (A) in the complex, i.e., with decrease of the {minus}{Delta}G{degree}{sub ip} value. This peculiar energy gap dependence of k{sub CR}{sup CIP}, quite different from the bell-shaped one observed in the case of the solvent-separated ion pairs (SSIP) or loose ion pairs (LIP) formed by encounter between fluorescer and quencher in the fluoresence quenching reaction, has been interpreted by assuming the change of electronic and geometrical structures of CIP depending on the strengths of D and A.

Franck-Condon States of Coumarins in Alcoholic Solutions

Abstract The temperature dependence of the fluorescence spectra of coumarin 2, coumarin 10 and coumarin 120 in ethyl alcohol is investigated. A blue-shift of the spectra with increase of temperature is observed. The blue-shift is explained by means of excited and ground Franck-Condon state parameters obtained by the adjustment of analytical shape factors to experimental spectra.

Femtosecond-picosecond laser photolysis studies on the dynamics of excited charge-transfer complexes in solution. 1. Charge separation processes in the course of the relaxation from the excited Franck-Condon state of 1,2,4,5-tetracyanobenzene in benzene and methyl-substituted benzene solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFemtosecond-picosecond laser photolysis studies on the dynamics of excited charge-transfer complexes in solution. 1. Charge separation processes in the course of the relaxation from the excited Franck-Condon state of 1,2,4,5-tetracyanobenzene in benzene and methyl-substituted benzene solutionsSeishi. Ojima, Hiroshi. Miyasaka, and Noboru. MatagaCite this: J. Phys. Chem. 1990, 94, 10, 4147–4152Publication Date (Print):May 1, 1990Publication History Published online1 May 2002Published inissue 1 May 1990https://doi.org/10.1021/j100373a050RIGHTS & PERMISSIONSArticle Views178Altmetric-Citations57LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (751 KB) Get e-Alerts

N,π* Fluorescence from selected vibronic levels of pyrimidine- d4 vapour: franck-condon factors, ground and excited state anharmonic coupling

Some “single vibronic level” fluorescence spectra of pyrimidine- d 4 are reported. The progression forming modes are ν″ 6a (659 cm −1) and ν″ 12 (1048 cm −1) and in contrast to pyrimidine- d 0 excitation of up to 7 quanta may be clearly identified. The ground-state Fermi resonance between ν 6a 1 and 2ν 16b 2 , so significant in the fluorescence spectra of pyrimidine- d 0, is absent for pyrimidine- d 4. Analyses of the spectra have enabled comparable studies of Franck-Condon factors and Fermi resonances to be carried out. They have provided confirmation of vibrational assignments and have provided the underlying reason for the much sharper and more discrete nature of the pyrimidine- d 4 spectra.

Bichromophores symétriques: torsion intramoléculaire sans transfert de charge à l'état électronique excité S 1 dans le dicarbazyle- N,N′

N,N′-dicarbazyl (NNDC) was investigated in the gas phase and in fluid and rigid solutions of non-polar and polar protic and aprotic solvents over a wide temperature range (77 – 300 K). A comparison of the spectral and photophysical parameters in a series of N-substituted carbazole derivatives allows us to conclude that the symmetry of the relaxed excited singlet S 1 state in NNDC has been lowered compared with that in the electronic ground state. We have shown that the 90° geometry in the excited Franck—Condon state is transformed to an oblique geometry through intramolecular interaction processes only. This is in contrast with the only other well-studied symmetrical bichromophoric system, 9,9′-bianthryl, where a polar-solvent-induced charge transfer operates in the Franck—Condon excited state relaxation process.

Intramolecular charge transfer state and unusual fluorescence from an upper excited singlet of a nonplanar derivative of p-cyano-N, N-dimethylaniline

The TICT (twisted internal charge transfer state) fluorescence of 4-cyano-2,6,N,N-tetramethylaniline (CTMA) was found in the vapour phase. The dipole moments of excited Franck-Condon and Franck-Condon twisted ground states were estimated from the solvent shifts of absorption and fluorescence. The near-equality of excited Franck-Condon and relaxed TICT state dipole moments is discussed. The fluorescence from an upper excited singlet state is found and its mechanism is explained.

Adiabatic photoreactions in dilute solutions of p-substituted N, N-dialkylanilines and related donor—acceptor compounds

Twisted intramolecular charge transfer (TICT) stable excited states can occur in molecules in which two chromophores are separated by one (twisted) single bond. The TICT state is achieved by an adiabatic photoreaction after absorption to the lowest-lying Franck—Condon excited state of that molecule. This lower-lying TICT state potential minimum in the hypersurface of the lowest-lying excited state gives rise to the effect of dual fluorescence which allows us to study the kinetics of the twist mechanism. If the para substituent to the N, N-dialkylaniline is a large π electonic system (9-anthryl or sulphone) the aniline twists around that substituent, but if the substituent is small (nitrile or ester) the dialkylamino group twists around its phenyl ring. The population of the TICT state is favoured by (1) the dialkylamino group ionization potential, (2) the substituent electron affinity, (3) the lifetime of the excited state, (4) its dipole—dipole interaction with the solvent and (5) a low viscosity of the solution.