Solvent Dependence of the Spectra and Kinetics of Excited-State Charge Transfer in Three (Alkylamino)benzonitriles (original) (raw)
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Excited state dynamics of 4(1H-pyrrole 1-yl) benzoic acid and different environmental effects
Journal of Photochemistry and Photobiology A: Chemistry, 1998
This paper describes the dual fluorescence of 4-N,N-dimethylamino cinnamaldehyde (DMACA) in various solvents. Based on major findings the first fluorescence band has been assigned to be arising out of delocalized excited state (DE) and the anomalous fluorescence band in polar aprotic solvents has been assigned to twisted intramolecular charge transfer (TICT) state whereas, in protic solvent it is arising out due to hydrogen bonding interaction between hydrogen donor part of the solvent and carbonyl group of the probe molecule. This hydrogen bonding being an efficient fluorescence quenching channel trammels the TICT formation and thus an apparent competition exists between TICT and hydrogen bonding formation in protic solvent. The calculated absorption spectrum generated from CNDO calculations follows nicely the experimental absorption band. The AM1 calculations corroborate the large dipole moment change and the charge transfer in orthogonal position as found from experimental observations.
The Journal of Physical Chemistry A, 2007
Time-resolved studies with three intramolecular charge-transfer molecules 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in ethyl acetate in presence of LiClO 4 indicate that the average reaction time for LE f CT conversion increases at low electrolyte concentration and then decreases to become comparable and even lower than that in the pure solvent. Except for Mg 2+ , the average reaction time decreases linearly with the ion size in perchlorate solutions of ethyl acetate. The observed electrolyte-concentration dependence of the average reaction time for these molecules agrees well with the predictions from the theory of Zwan and Hynes [Chem. Phys. 1991, 152, 169] in the broad barrier overdamped limit with barrier frequency, ω b ≈ 2 × 10 12 s-1 .
The Journal of Chemical Physics, 2009
For the double exponential fluorescence decays of the locally excited ͑LE͒ and intramolecular charge transfer ͑ICT͒ states of 4-͑dimethylamino͒benzonitrile ͑DMABN͒ in acetonitrile ͑MeCN͒ the same times 1 and 2 are observed. This means that the reversible LE ICT reaction, starting from the initially excited LE state, can be adequately described by a two state mechanism. The most important factor responsible for the sometimes experimentally observed differences in the nanosecond decay time, with 1 ͑LE͒ Ͻ 1 ͑ICT͒, is photoproduct formation. By employing a global analysis of the LE and ICT fluorescence response functions with a time resolution of 0.5 ps/channel in 1200 channels reliable kinetic and thermodynamic data can be obtained. The arguments presented in the literature in favor of a ء state with a bent CN group as an intermediate in the ICT reaction of DMABN are discussed. From the appearance of an excited state absorption ͑ESA͒ band in the spectral region between 700 and 800 nm in MeCN for N , N-dimethylanilines with CN, Br, F, CF 3 , and C͑vO͒OC 2 H 2 p-substituents, it is concluded that this ESA band cannot be attributed to a ء state, as only the CC w N group can undergo the required 120°bending.
Photochemical & Photobiological Sciences, 2010
We report herein the solvent and temperature effects on the emission of the intermolecular exciplexes 1-cyanonaphthalene/triethylamine and 1-methylnaphthalene/triethylamine and the intramolecular exciplexes formed by the bichromophoric compounds diethyl-(3-naphthalen-1-yl-propyl)-amine (I), diethyl-(2-naphthalen-1-yl-ethyl)-amine (II), 3-[ethyl-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (III) and 3-[(2-cyano-ethyl)-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (IV). The results are interpreted within the formalism of the semiclassical Marcus theory for radiative back electron transfer (BET) reactions. We show that, following a few simple assumptions, reliable values of the Gibbs free energy changes (DG e -et ) and the solvent reorganization energies (l e s ) associated to the BET reactions (and their corresponding enthalpic and entropic contributions) can be estimated directly from the emission of the CT states. We also show that for the 1-cyanonaphthalene/triethylamine exciplex, which exhibits emission in the vapour phase, the experimental calculation of the absolute energy of solvation of the CT state (DG e s ) is also possible. The calculated DG e -et correlate quite satisfactorily with the corresponding values obtained from independent electrochemical and kinetics experiments. The temperature effects on DG e -et , l e s and DG e s are discussed qualitatively using different solvation models. The limitations of the present approach for the estimation of DG e -et and l e s and its possible application to more complex A/D systems are also examined. † Electronic supplementary information (ESI) available: Additional electrochemical and spectroscopic data of the compounds studied, a broader discussion of some theoretical aspects of the results obtained and the implementation of the proposed method to other systems previously studied by other authors. See
Journal of Photochemistry and Photobiology A-chemistry, 2011
Photophysical characterization of a molecule, trans-4- [4 -(N,N -dimethylamino)styryl]pyridine (4-DMASP) containing donor and acceptor moieties has been done experimentally as well as theoretically. Upon single excitation a charge-transfer state with high dipole moment is formed through rapid relaxation of a locally excited (LE) state in polar medium. A complete charge transfer process occurs as a result of twisting of donor group with respect to the acceptor part of the molecule resulting in the highly stabilized twisted state in polar medium giving fluorescence from LE state as well as from twisted state. However, in a nonpolar solvent emission occurs explicitly from a LE state. Hydrogen bond donor ability of solvents rather than dipolar interactions contributes more to the stability of ground state. However, dipolar interactions have greater contribution towards the stability of an excited state. All such interactions have higher contribution towards the stability of excited state than that of ground state. Very low fluorescence quantum yield in water is because of high rate of nonradiative processes as a result of high degree of stabilization of charge transfer state thereby making closer proximity of this state to triplet as well as ground charge transfer states. Monocation of 4-DMASP formed due to the protonation of pyridine nitrogen atom is more stable than neutral and dication species at ground as well as excited states because of greater extent of flow of charge from donor to acceptor part in monocation. Basicity of pyridine nitrogen atom being greater at excited state than that in ground state results in higher extent of pulling of charge from donor to acceptor part of monocation at excited state. Theoretical calculations suggest donor twisting as a possible path for creation of a charge transfer state rather than acceptor twisting. Excited state dipole moment value obtained from theoretical calculation corroborates well with the value determined experimentally. Theoretical calculation suggests no cis-trans photoisomerization in the ground state as well as excited state at room temperature.
Journal of the American Chemical Society, 1996
Intramolecular photoinduced electron transfer in 9-(p-N,N-dimethylanilino)phenanthrene (9DPhen) has been studied in solution. The solvent dependence of the fluorescence spectra of 9DPhen indicates that the emission occurs from a highly polar excited state. The quantum yield of fluorescence (Φ f ) of 9DPhen is quite high and increases with increasing solvent polarity. The radiative rate constant (k f ), however, shows a maximum for solvents of intermediate polarity, e.g., in butyl acetate a value of 2.3 × 10 8 s -1 is attained. These results are difficult to explain within the "TICT" (twisted intramolecular charge transfer) model, which predicts a strongly forbidden fluorescence caused by a minimum overlap of the orbitals involved in the transition. The above-mentioned trend as a function of the solvent polarity is observed in particular donor-acceptor substituted arenes where the L b state of the corresponding arenes is lower in energy than the L a state. The quantum chemical calculations actually could explain this behavior on the basis of an ICT state which interacts with the lower lying 1 L a and 1 L b states of the acceptor. The quantum mechanical mixing of states can occur by two pathways, namely orbital mixing and mixing of configurations, and is modified by geometrical changes and by solvent polarity. The single exponential fluorescence decay, obtained with time-correlated single-photon-timing, suggests emission from an excited charge-transfer state, resulting from a solvent-induced rapid relaxation of the initial delocalized excited state of 9DPhen, obtained immediately after picosecond pulsed excitation. Picosecond transient absorption spectra in acetonitrile show a rapid decay within a few picoseconds from a less polar but delocalized excited state toward a more polar ICT state. Even the triplet state of 9DPhen in isopentane at 77 K shows a significant polar character. As a reference compound, 9-phenylphenanthrene (9PhPhen) was also examined by means of stationary and time-resolved fluorescence measurements as well as transient absorption experiments.
The Journal of Physical Chemistry A, 1998
The photophysics of 9,9′-bianthryl (BA) were investigated by means of fluorescence spectroscopy, nanosecond transient-absorption spectroscopy, and semiempirical calculations. Fluorescence spectra and lifetimes were measured in more than 50 solvents in order to get a detailed picture of the solvent dependence. The results show that the fluorescence lifetime is constant in solvents of low polarity (D < 5) and increases with solvent polarity in more polar solvents. Departures from this trend can be traced to specific solute-solvent interactions. Excited-state singlet-singlet absorption spectra were measured in the ultraviolet range and show a marked solvent dependence. In polar solvents, the spectrum (λ max) 315 nm) is closely related to those of the radical ions of both BA and anthracene. The decay rate constant of this band is identical with that of the fluorescence emission in a range of solvents of varying polarity (D > 5), thus providing direct proof of the chargeseparated character of the fluorescent state in polar solvents. The 315 nm band is absent in isooctane, indicating that the fluorescent state is not of charge-separated character in this case. Semiempirical calculations were carried out in order to rationalize the experimental data. Careful consideration of the symmetry character of the electronic states involved and of the solvent effect on these states indicates that two distinct transitions are responsible for the observed fluorescence emission; in nonpolar solvents, a nonpolar state with D 2 symmetry and a torsion angle that is markedly smaller than 90°is the fluorescent state, whereas in polar solvents fluorescence originates from a charge-separated perpendicular state of D 2d symmetry. This latter state is responsible for the large solvent effects on fluorescence and singlet-singlet absorption. Triplet-triplet absorption and intersystem-crossing efficiency data were also measured in several solvents. They too are solvent-dependent but do not show characteristics of charge separation; they rather are influenced by specific solute-solvent interactions.
2020
Steady-state spectroscopic studies have been performed with three intramolecular charge-transfer molecules, 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C), in ethyl acetate and acetonitrile in presence of lithium perchlorate (LiClO 4 ) at room temperature to investigate the effects of electrolytes on excited-state intramolecular charge-transfer reaction. Electrolyte-concentration and ion-size dependences of several spectroscopic properties such as quantum yield, absorption and emission transition moments, radiative and nonradiative rates, and changes in reaction free energies associated with LE f CT conversion have been determined for these molecules and reported. For P4C, quantum yield decreases by a factor of ∼7 at the highest electrolyte concentration relative to that in pure ethyl acetate whereas it is a factor of ∼4 for both P5C and P6C. However, in acetonitrile with 1.0 M LiClO 4 , quantum yield reduces to almo...
Chemical Physics, 2007
The fluorescence spectrum of 5-cyano-2-(1-pyrrolyl)-pyridine (CPP) was measured in several solvents as well as in an argon matrix. Based on comparison with other compounds and on ab initio calculations it is proposed that the fluorescence in the argon matrix and in non-polar solvents is due to two electronic excited states: one is of locally excited nature, the other a charge transfer (CT) state. In polar solvents the spectrum is dominated by the CT emission. The photo-physical behavior of CPP is discussed by comparison with that of other molecules exhibiting dual fluorescence and in view of a recent model developed for the benzene analog.