Formation and stabilization of twisted intramolecular charge transfer states in binary mixed solvents (original) (raw)
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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.
Excited-State Charge Transfer Dynamics of p-Dimethylaminobenzonitrile in Quadrupolar Solvents
The Journal of Physical Chemistry A, 2002
Excited-state intramolecular charge transfer of p-dimethylaminobenzonitrile (DMABN) in benzene, toluene, and dioxane is studied. By combining the recent continuum quadrupolar solvent theory of Jeon and Kim [J. Phys. Chem. A 2000, 104, 9812] and the two-dimensional formulation of the DMABN photoreaction in dipolar solvents by Fonseca et al. [J. Mol. Liq. 1994, 60, 161], the influence of both solvent quadrupole reorganization and solute twist on the reaction free energetics and dynamics is accounted for. The solutionphase reaction paths are investigated with the aid of experimental information on the frequencies associated with solute torsional and collective solvent quadrupole dynamics. The rate constants and transmission coefficients are analyzed using transition state theory with the neglect of dissipative dynamics. Our results are in reasonable agreement with the experimental findings in quadrupolar solvents.
Organic molecules substituted with the nitro group show efficient nonlinear optical (NLO) properties, which are a consequence of the strong intramolecular charge transfer (ICT) character of the molecules because of the strong electron withdrawing nature of the nitro group and rapid responsiveness because of highly movable p-electrons. Dynamics of the ICT process in the excited states of a push–pull biphenyl derivative, namely, 4-N,N-dimethylamino-4 0-nitrobiphenyl (DNBP), an efficient NLO material, has been investigated using ultrafast transient absorption spectroscopy. The experimental results have been corroborated with DFT and TDDFT calculations. In solvents of large polarity, e.g. acetonitrile, the ultrafast ICT process of DNBP is associated with the barrierless twisting of the N,N-dimethylaniline (DMA) group with respect to the nitrobenzene moiety to populate the twisted ICT (or TICT) state, and the rate of this process is solely governed by the viscosity of the medium. In solvents of moderate polarity, e.g. ethyl acetate, the rate of the twisting process is significantly slowed down and the LE and TICT states remain in equilibrium because of a low energy barrier for interconversion between these two states. By further lowering the polarity of the solvent, e.g. in dioxane, the twisting process is completely retarded. In nonpolar solvents, e.g. cyclohexane, a reverse twisting motion towards the planar geometry (i.e. the PICT process) has been evident in the excited state dynamics. In this solvent, the S 1 state undergoes an ultrafast intersystem crossing to the triplet state because of its close proximity with the T 2 state.
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.
The Journal of Physical Chemistry, 1991
The twisted intramolecular charge-transfer (TICT) phenomenon of p-(N~~imethylamino)ben~nitrile (DMABN) is studied by using the CNDO-S/CI method for calculating transition energies. The ground state geometry is fully optimized by the MNDO method. The solvent is treated as a continuous dielectric. The TICT state is found to correspond to a state of perpendicular geometry and large dipole moment (18.656 D). The "nonpolar" emission is ascribed to either of two nearly degenerate states at nearly planar geometry having relatively small dipole moments. The calculations indicate that the TICT emission should be observed only if the dielectric constant of the medium exceeds 2.2. The model also provides reasonably good estimates for the activation barrier for the TICT process. Finally, the energy difference between the TICT emission and the nonpolar emission turns out to be predominantly due to the energy difference between the excited states, the energies of the two ground-state conformations (planar and perpendicular) being very nearly the same.
Journal of Physical Chemistry A, 2008
The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE f ICT reaction time τ 2 at 22°C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE a ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(k a + k d). In the photostationary S 0 f S n absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm-1 lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25°C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE f ICT reaction just starts to occur under these conditions. In din -pentyl ether (DPeE) and din -butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25°C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S 1) of the lowest excited singlet state the condition E(ICT) e E(S 1) holds. E(ICT) is calculated from the difference E(D/D +)-E(A-/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment µ e (ICT)) 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius F. The carbazole/phenyl twist angle θ) 45°of NP4CN in the S 0 ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature.
Chemical Physics, 2006
A donor acceptor substituted aromatic system trans-3-(4-monomethylamino-phenyl)-acrylonitrile (MMAPA) has been synthesized and its photophysical behavior has been investigated in the solvent of different polarity by steady state absorption and emission, time-resolved emission and quantum chemical calculations. The observed dual fluorescence of MMAPA in polar aprotic solvents has been assigned to emission from the locally excited and twisted intramolecular charge transfer states. The low-energy emission in protic solvent is attributed to the hydrogen-bonded complex. Potential energy surfaces for the ground and excited states along the donor (-NHMe group) and acceptor (acrylonitrile group) twist coordinates have been calculated by time-dependent density functional theory (TDDFT) and time-dependent density functional theory-polarized continuum model (TDDFT-PCM) in the gas phase and in acetonitrile solvent, respectively. Calculations predict that the stabilized excited state along the twist coordinate is responsible for the solvent dependent red shifted charge transfer emission. It is found that the twisting along the donor site is energetically favorable compared to that of the acceptor site. The canonical crossing of the excited states for the twisting of the donor group and localized nitrogen lone pair orbital of-NHMe group at the perpendicular configuration with respect to p-orbitals of benzene ring support TICT model for photo-induced charge transfer reaction in MMAPA molecule.
Intramolecular charge transfer (ICT) promoted fluorescence transfer has been investigated in two polar solvents, acetonitrile and water. The ICT species of 4-N,N-dimethylaminobenzonitrile (DMABN) produced exclusively in the photoexcited state serves as the donor while phenosafranin (PSF), a cationic phenazinium dye, acts as the acceptor. A transfer of the fluorescence occurs from the ICT state of DMABN to PSF in both the media. Time-resolved fluorescence decay analysis in combination with the steady state fluorometric observations conclusively rule out the involvement of the fluorescence resonance energy transfer (FRET) process and establish the fluorescence transfer as a consequence of re-absorption of the fluorescence of the donor by the acceptor. Thus, apart from citing an example of a coupled interaction of ICT and fluorescence transfer, importance of the present work lies in providing a demonstration in differentiating between the Förster resonance energy transfer and the trivial energy transfer through re-absorption.
The Journal of Physical Chemistry A, 2015
The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S 2 (L a , CT) and S 1 (L b , LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S 2 to S 1. Polar solvent effects are not found to have a significant influence on this situation. Applying C s symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.