Intramolecular Charge Transfer of p-(Dimethylamino)benzethyne: A Case of Nonfluorescent ICT State (original) (raw)
Related papers
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.
Chemical Physics Letters, 2000
Ž . Ž . Dual fluorescence and fast intramolecular charge transfer ICT is observed with 4-diisopropylamino benzonitrile Ž . Ž . DIABN in alkane solvents. The rate constant k for the reaction from the locally excited LE to the ICT state has a value a of 3.4 = 10 11 s y1 in n-hexane at 258C, with an activation energy E of 6 kJ mol y1 . Efficient intersystem crossing with a a Ž . yield of 0.94 takes place from the ICT state. With 4-dimethylamino benzonitrile, in contrast, dual fluorescence is not Ž . observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap D E S ,S , in 1 2
The Journal of Physical Chemistry B, 2005
Steady-state absorption and emission spectra and emission decay kinetics are reported for 4-aminobenzonitrile (ABN), 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in 24 room temperature solvents. In solvents of modest to high polarity, P4C, P5C, and P6C exhibit dual fluorescence and emission decays characteristic of the transformation from an initially prepared (LE) state to a more polar charge transfer (CT) state, whereas ABN does not undergo this reaction. The frequencies of the steady-state absorption and emission spectra of all of these solutes can be rationalized using a dielectric continuum description of the solvent and considering only the minima on the reactive surfaces, which are assumed to involve both an intramolecular (twisting) and a solvation coordinate. Characteristics of the gas-phase solutes deduced from this analysis are in good agreement with electronic structure calculations and indicate that differences in their spectra mainly reflect differences in the relative energies of the gasphase LE and CT states. The relative yields of LE and CT emission are not described as satisfactorily by this model, and reasons for this failure are discussed. The kinetics of the LE f CT reaction vary considerably with solute and solvent. In many solvents, the emission decays of P4C are reasonably described by a simple two-state kinetic scheme with time-independent rate constants. In P5C and P6C multiexponential decays are observed that reflect time-dependent shifts of the component spectra as well as time-dependent reaction rates. A simplified analysis of these complex dynamics provides estimates for both the free energy change ∆ r G and (average) LE f CT rate constant k f for a wide range of solute and solvent combinations. The driving force for reaction (-∆ r G) follows the order P6C > P5C > P4C and increases with increasing solvent polarity. The reaction rates are correlated to ∆ r G and follow the opposite trend. The relationships observed between k f and ∆ r G suggest that static solvent effects, i.e., barrier height changes, are the primary determinants of the solvent dependence in P4C. Correlations between barrier-corrected rates and solvation times suggest that dynamical solvent effects contribute substantially to the solvent dependence of the rates in P5C, and especially P6C.
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 Physical Chemistry A, 2006
The kinetics of the intramolecular charge-transfer (ICT) reaction of 4-(dimethylamino)benzonitrile (DMABN) in the polar solvent acetonitrile (MeCN) is investigated by fluorescence quantum yield and picosecond timecorrelated single photon counting (SPC) experiments over the temperature range from-45 to +75°C, together with femtosecond S n r S 1 transient absorption measurements at room temperature. For DMABN in MeCN, the fluorescence from the locally excited (LE) state is strongly quenched, with an unquenched to quenched fluorescence quantum yield ratio of 290 at 25°C. Under these conditions, even very small amounts of the photoproduct 4-(methylamino)benzonitrile (MABN) severely interfere, as the LE fluorescence of MABN is in the same spectral range as that of DMABN. The influence of photoproduct formation could be overcome by a simultaneous analysis of the picosecond and photostationary measurements, resulting in data for the activation barriers E a (5 kJ/mol) and E d (32 kJ/mol) of the forward and backward ICT reaction as well as the ICT reaction enthalpy and entropy: ∆H (-27 kJ/mol) and ∆S [-38 J/(mol K)]. The reaction hence takes place over a barrier, with double-exponential fluorescence decays, as to be expected in a two-state reaction. From femtosecond transient absorption down to 200 fs, the LE and ICT excited state absorption (ESA) spectra of DMABN in n-hexane (LE) and in MeCN (LE and ICT) and also of 4-aminobenzonitrile in MeCN (LE) are obtained. For DMABN in MeCN, the quenching of the LE and the rise of the ICT ESA bands occurs with a single characteristic time of 4.1 ps, the same as the ICT reaction time found from the picosecond SPC experiments at 25°C. The sharp ICT peak at 320 nm does not change its spectral position after a pumpprobe delay time of 200 fs, which suggests that large amplitude motions do not take place after this time. The increase with time in signal intensity observed for the LE spectrum of DMABN in n-hexane between 730 and 770 nm, is attributed to solvent cooling of the excess excitation energy and not to an inverse ICT f LE reaction, as reported in the literature.
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.
The Journal of Physical Chemistry A, 2011
Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25°C, for example, Φ 0 (ICT) = 3.2 Â 10-3 and Φ(LE) = 1.6 Â 10-4. The large value of Φ 0 (ICT)/Φ(LE) indicates that the forward LE f ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S 0. For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1-Φ(ISC)-Φ 0 (ICT)-Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S 0), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S 0), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ 0 (ICT)/Φ(LE), a reaction enthalpy-ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε 25 = 1.88), increasing to 17 kJ/mol in the more polar solvent din -butyl ether (ε 25 = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE f ICT reaction (k a). The femtosecond excited state absorption spectra at 22°C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ 2 increases upon cooling from 2.6 ps at-45°C to 6.9 ps at-95°C. The decay involves ICT and IC as the two main pathways: 1/τ 2 = k a þ k IC. As a model compound (no ICT) is not available, its lifetime τ 0 (LE) ∼ 1/k IC is not known, which prevents a separate determination of k a. The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S 0 f LE a ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 Â 10-5) and dominated by impurities, due to enhanced IC from LE to S 0 .
Intramolecular charge transfer in the excited state. Kinetics and configurational changes
Journal of Photochemistry and Photobiology A-chemistry, 1996
The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three-to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9'-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.
Journal of Luminescence, 2012
The excited state charge transfer (CT) properties of p-Dimethylaminobenzaldehyde (DMABA) have been reinvestigated spectroscopically in combination with quantum chemical calculations. The molecule having weak acceptor group (-CHO) shows weak charge transfer emission, which follows linear dependency on solvent polarity parameters and E T (30) values. However, previously reported CT emission of DMABA in ACN and in DMF solvents by Grabowski et al. (Chem. Rev. 103 (2003) 3899) and Kawski et al. (Chem. Phys. Lett. 448 (2007) 208) are ambiguous and are different from the Grabowski's previous studies (Pure Appl. Chem. 55 (1983) 245) and the present results. Theoretical potential energy surfaces along both the donor and acceptor twisting motion using Density Functional Theory (DFT) with B3LYP functional and 6-311þþG nn basis set following Twisted Intramolecular Charge Transfer model support our experimental results. Time Dependent Density Functional Theory Polarization Continuum Model (TDDFT-PCM) has been used to explore the solvent effect on the emission spectra of DMABA.
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.