Intramolecular Charge Transfer Model Research Articles

Intramolecular photo‐driven electron transfer in the series of DMABN related compounds with para‐substituted acceptors. Study of the rate constants by Marcus theory

AbstractA series of DMABN‐related compounds with two‐band fluorescence was studied by steady‐state absorption and fluorescence spectroscopy, time‐resolved absorption spectroscopy upon excitation with a 30‐fs laser pulse, and by TDDFT and xMCQDPT2 quantum chemical methods. The efficiency of the intramolecular electron transfer was found to depend on the excitation wavelength in MeCN. The reaction is described by a two‐state scheme (LE↔CT); the Stevens‐Ban method gives underestimated values for the reaction enthalpy ΔH (SB). The spectral luminescence and kinetic parameters, rate constants, and barriers for the forward (k1, Ea) and reverse (k−1, Ed) electron transfer were calculated. The Marcus plot for k1 versus the driving force (−ΔG) and the total reorganization energy (λ) were calculated for six compounds. It was shown that without a barrier, the 1/k1 value (267 fs) is close to the mean solvation time in MeCN (260 fs), ie, the reaction rate is completely determined by the solvent. The results of conformational analysis for all studied compounds are consistent with the twisted intramolecular charge transfer model of structural relaxation.

Intramolecular charge transfer model in fluorescence processes

The absorption–emission processes that characterize the fluorescent phenomena are analyzed by assuming that the changes in the ground state electronic density, when the system is promoted to the first excited state, and that the changes in the first excited state electronic density, when the system decays to a ground state with a distorted geometry, can be interpreted as intramolecular charge transfer processes. Thus, it is shown that in this context the hardness is the property that describes the absorption and the emission energies. Additionally, it is indicated that the substituent effects on the oscillator strengths may be described through the frontier molecular orbital condensed Fukui functions for the charge donating process.

Reinvestigation of photoinduced intramolecular charge transfer reaction in p-Dimethylaminobenzaldehyde by spectroscopic method and Density Functional Theory (DFT) calculation

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 6-311++G ⁎⁎ 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.

Ultrafast excited-state dynamics of ferrocene-bridge-acceptor system

We employed both femtosecond fluorescence up-conversion and transient absorption techniques with ∼150 fs time resolution to study the excited-state deactivation process of an intra-molecular charge transfer model compound, 4-(ferrocen-1-yl)benzylidene-malononitrile (Fc-ph-DCV), which consists of ferrocene (Fc) unit as an electron donor, dicyanovinly (DCV) as an electron acceptor and phenyl (ph) ring as the central bridge. The results showed that after photoexcitation into the higher excited S 2 state, ultrafast internal conversion into S 1 takes place. The rate of S 2 → S 1 internal conversion is markedly faster (with a typical time of 120 fs ± 20 fs) than the diffusive solvation process. On the other hand, the lifetime of the relaxed S 1 state was strongly dependent on the solvent polarity, changing from 40 to 50 ps in acetonitrile to ∼20 ps in cyclohexane. Time-resolved fluorescence data also showed subpicosecond transient component that is attributable to the spectral relaxation caused by solvation and/or vibrational relaxation in the S 1 state.

Evidence of acid mediated enhancement of photoinduced charge transfer reaction in 2-methoxy-4-(N,N-dimethylamino)benzaldehyde: Spectroscopic and quantum chemical study

The photophysical behavior of 2 -methoxy-4-(N,N-dimethylamino)benzaldehyde (2-MDMABA) has been investigated by steady state absorption and emission spectroscopy, time resolved emission spectroscopy and quantum chemical calculations. The molecule 2-MDMABA having a weak acceptor aldehyde group shows strong local emission in all solvents and a weak solvent polarity dependent red shifted emission in polar aprotic solvents for the charge transfer (CT) state which follows well the Lippert–Mataga equation. Instead of usual protonation at the donor site with inhibition of CT process, protonation at the acceptor aldehyde site enhances acceptor properties and hence increases excited state charge transfer reaction. The ground state structural calculation at Density Functional Theory (DFT) level with B3LYP functional and 6-31++ G** basis set and potential energy surfaces (PESs) following twisted intramolecular charge transfer model by using DFT method in the gas phase and in acetonitrile solvent have been performed for the neutral and cation to correlate with experimental findings.

Optically detected magnetic resonance in bismuth-doped silica glass

We report on the observation of optically detected magnetic resonance (ODMR) in bismuth-doped silica glass. To explain the results of the experiment we adopted the intramolecular charge transfer model for Bi(5+)O(n)(2-) molecules in the frame of a semiempirical molecular orbitals approach. The results of our calculations are in good agreement with observed features of luminescence and ODMR experiments.

Ultrafast nonlinear optical properties of dye-doped PMMA discs irradiated by 40 fs laser pulses

The two-photon absorption (TPA) characteristics of PMMA discs doped with three different dyes were studied using an fs-pulsed Ti-Sapphire laser as the pump source, and employing the open-aperture Z-scan technique. TPA cross-sections obtained for PMMA doped with the dyes PM597, DCM and rhodamine 6G–rhodamine B (co-doped) were found to be equal to 24.7, 33.3 and 32.3 GM, respectively (1 GM=10 −50 cm 4 s phot −1 mol −1). Furthermore, two-photon fluorescence was measured for the samples containing DCM and rhodamine 6G–rhodamine B (co-doped). Compared to the one-photon fluorescence spectrum, the peaks in the two-photon fluorescence spectrum were red shifted and the extent of red shift increased with increasing doping concentration. We have also observed that the red shift in the two-photon fluorescence peak of the samples in the solid form is much larger than that in the solution state. This phenomenon could be explained by a twisted intra-molecular charge transfer model.

RESULTS OF ONE THOUSAND CONSECUTIVE BIPOLAR TRANSURETHRAL RESECTION IN SALINE SYSTEM(TURIS)

The two-photon absorption (TPA) characteristics of PMMA discs doped with three different dyes were studied using an fs-pulsed Ti-Sapphire laser as the pump source, and employing the open-aperture Z-scan technique. TPA cross-sections obtained for PMMA doped with the dyes PM597, DCM and rhodamine 6G–rhodamine B (co-doped) were found to be equal to 24.7, 33.3 and 32.3 GM, respectively (1 GM=10−50 cm4 s phot−1 mol−1). Furthermore, two-photon fluorescence was measured for the samples containing DCM and rhodamine 6G–rhodamine B (co-doped). Compared to the one-photon fluorescence spectrum, the peaks in the two-photon fluorescence spectrum were red shifted and the extent of red shift increased with increasing doping concentration. We have also observed that the red shift in the two-photon fluorescence peak of the samples in the solid form is much larger than that in the solution state. This phenomenon could be explained by a twisted intra-molecular charge transfer model.

Photo-induced charge transfer emission in (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid and its use as metal ions fluorosensor

In this study, spectroscopic properties of (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid (DMANAA) have been explored using absorption and emission spectroscopy in combination with quantum chemical calculations. In addition to the local emission, the molecule shows a red-shifted emission from the charge transfer state. The charge transfer emission band is found to be dependent on polarity and hydrogen-bonding ability of the solvents. This photo-induced charge transfer process has been explored theoretically at Density Functional Theory level using twisted intramolecular charge transfer model. Structural calculations and potential energy surfaces along the donor twist coordinate done by Density Functional Theory correlate well with the spectroscopic observations. Addition of metal ions to DMANAA solutions in acetonitrile solvent shows progressive quenching of the charge transfer band and growth of a blue-shifted band, making way for its use as a good fluorosensor for various metal ions.

A THEORETICAL STUDY ON DUAL FLUORESCENCE OF 4-DIMETHYLAMINOPYRIDINE BY POLARIZABLE CONTINUUM MODEL

Dual fluorescence spectra of 4-dimethylaminopyridine (DMAP) is investigated using time-dependent density functional theory and complete active space self-consistent field methods. Electronic absorption and emission spectra of DMAP have been investigated in three solvents, that is, cyclohexane, chloroform, and acetonitrile. The present study reveals that the dual fluorescence phenomena of DMAP appear in the cases of acetonitrile and chloroform, but not in cyclohexane. The electronic structures of the ground state and the intramolecular charge transfer states are, therefore, studied in order to reveal the insight of dual fluorescence. Our theoretical results suggest that the twisting of dimethylamino moiety in DMAP is necessary for the intramolecular charge transfer. The mechanism of the dual fluorescence of DMAP is discussed based on the twisted intramolecular charge transfer model and the dual fluorescence phenomenon is explained theoretically.

Spectroscopic study of excited state intramolecular charge transfer in ethyl ester of N, N′-Dimethylaminonaphthyl-(acrylic)-acid

In this work, we have studied the twisted intramolecular charge transfer process in ethyl ester of N, N′-Dimethylaminonaphthyl-(acrylic)-acid (EDMANA) using absorption and emission spectroscopy. Single conformer of EDMANA present in the ground state gives rise to local and charge transfer emission in different solvents. Solvatochromic effect on the Stokes shifted emission band clearly indicates the charge transfer character of the emitting singlet excited state. Considering twisted intramolecular charge transfer model, the ground and excited state properties of EDMANA have been evaluated along the donor and acceptor twisting coordinates separately using density functional theory (DFT) and time dependent density functional theory (TDDFT), respectively, with B3LYP functional and 6-31G basis set. The effect of solvent on the spectral properties has been explored theoretically at the same level with time dependent density functional theory-polarized continuum model (TDDFT-PCM) and the theoretical results support the solvent polarity dependent Stokes shifted emission of EDMANA molecule.

Two-photon fluorescence properties of a styrylpyridinium derivative organic chromophore in different solvents

The fluorescence properties of a new two-photon absorption chromophore in dimethyl formamide, methanol, acetone, benzyl alcohol, methylene chloride and chloroform are reported. The lifetime, intensity and central wavelength of the fluorescence signal vary significantly in different solvents. The fluorescence properties are explained by using the twisted intra-molecular charge-transfer model, the viscosity of the solvents and the formation of the hydrogen bond. For the dye in all solvents, the longer the fluorescence lifetime, the higher the fluorescence intensity. Generally, the higher the dipole moments, the longer the central wavelengths of the dye.

Rational classification of a series of aromatic donor–acceptor systems within the twisting intramolecular charge transfer model, a time-dependent density-functional theory investigation

The goal of this paper is to rationalize the fluorescence activity, experimentally observed for 21 molecules of the same family as the 4-(N,N-dimethyl)anilines, based on their potential energy surfaces calculated within the twisting intramolecular charge transfer model. A classification in four groups is proposed according to the sign of two parameters, ΔEgap, characterizing the energy difference between the vertical locally and charge transfer excited states, and ΔE1S (or ΔẼ1S for pretwisted systems), representing the energy gain of the charge transfer excited state with a perpendicular conformation compared to the first vertical excited state. In this study, the time-dependent density-functional theory has been used to calculate the potential energy surfaces of the ground and excited states along the twisting angle. Computed excitation energies and optimized ground state geometries have been obtained with both B3LYP and MPW1PW91 functionals using a 6-311+G(2d,p), and a 6-31G(d) basis set, respectively. From this study, it follows that ΔEgap and ΔE1S are the main parameters necessary to understand the fluorescence activity of these molecules. The fact that the same fluorescence activity is observed for the members of each group (or subcategory for the particular case of group II), reveals the underlying twisting mechanism as a common process for all the investigated molecules, which explains their experimental dual and nondual emission.

Supersonic jet and solution studies of intramolecular complexes with TICT formation mimicking solute–solvent interaction

Three ester derivatives of N, N-dimethylaniline have been studied, which possess a flexibly linked intramolecular hydroxyl group amenable to mimic 1:1 complexation of donor–acceptor molecules with solvent molecules. The supersonic jet fluorescence excitation spectra indicate a small redshift for these intramolecularly complexed compounds with respect to the noncomplexed model compounds giving evidence for the intramolecular complexation by hydrogen bond formation at the ester group. The dispersed fluorescence spectra broaden upon an increase in the excitation energy. Semiempirical simulated annealing calculations using AMPAC/AM1 show that there exist many possible conformations of similar energy for the molecule, with H bonding at the carbonyl or alkoxy oxygen in the most stable conformer. A shift in the carbonyl vibrational frequencies is consistent with a carbonyl self-complexed species. The dual fluorescence observed in acetonitrile as solvent is very similar for all three compounds, with a slight redshift of the long wavelength charge transfer (CT) band for the complexed species with respect to the uncomplexed one consistent with the twisted intramolecular charge transfer model. The absence of a clear CT band in spite of complete intramolecular hydrogen bond complexation in alkanes is interpreted as evidence that formation of a specific solute–solvent complex alone is not sufficient to create the large anomalous CT band observed for N, N-dimethylaminobenzonitrile in polar and mixed polar/nonpolar solvents.

A semiempirical study of the fluorescence properties of large charge transfer compounds: N,N‐dimethylanilino‐bis‐pyrazolopyridine

Abstract4‐(4′‐N,N‐dimethylaminophenyl)‐3,5‐dimethyl‐1,7‐diphenyl‐bis‐pyrazolo‐[3,4‐b;4′,3′e]‐pyridine (DMA‐DMPP), was recently synthesized as an example of a bulky electron donor‐acceptor compound and was examined by spectroscopic methods. The molecule consists of several subunits of different redox properties and the observed behavior was thus interpreted by an intramolecular charge transfer model. This hypothesis is validated by semiempirical excited state calculations. They are focused on the coupling of intramolecular charge transfer to various relaxation modes, e.g. rotation about single bonds leading to decoupling π systems. Rotation about the single bond connecting the dimethylanilino and heteroaromatic subunit results as the most important relaxation coordinate. The two moieties non orthogonal in the ground state become perpendicularly oriented in the optimum excited state. In this conformation the dipole moment also becomes maximal. These results agree satisfactorily with the observation of a marked red shift of its fluorescence in polar solvents and dual fluorescence found in polar, protic environment.

Picosecond Transient Absorption Spectra and the Twisted Intramolecular Charge Transfer Phenomenon

An improved model of a ckputer-controlled picosecond absorption spectrometer was designed and constructed. Its operatioii is described, and applied to the measurement of transient spectra of p-N, N-dioctylamino-acetophenone. The spectroscopy and photophysics of this ckpound are ckpared to the other ckpounds exhibiting the dual fluorescence, assigned usually to the adiabatic formation of the twisted intramolecular charge transfer excited state. The data are discussed in relatioii to alternative suggestion of an inversioii of the pyramidal amino nitrogen atk. The whole evidence supports the decoupling of the π-electronic systems of D+ and Ain the D+-A polar excited state, in full accord with the twisted intramolecular charge transfer model. The peculiar behaviour of the long alkyl -NR2 derivatives (favouring the twisted intramolecular charge transfer state emission) seems to be primarily due to the decreasing oxidation potential of such electron donor groups. PACS numbers: 36.20.Kd, 06.60.1n

The Electronic Spectra of Aniline and N-Methylaniline Adsorbed on Aluminum Chloride. II. The Theoretical Treatment

Abstract The electronic spectra of aniline and N-methylaniline adsorbed on aluminum chloride have been examined by the LCMO method and have been compared with those of gaseous aniline, N-methylaniline, toluene and ethylbenzene. All of these molecules can be treated by the intramolecular charge-transfer model. In order for us to be able to interpret the spectra from the LCMO point of view, the ionization potential of the electron donating group in aniline and N-methylaniline molecules should rise upon adsorption on aluminum chloride.