Intramolecular charge transfer effects on 4-hydroxy-3-methoxybenzaldehyde

The absorption and fluorescence spectral characteristics of 4-hydroxy-3-methoxybenzaldehyde (HMB) have been studied in different solvents, pH and beta-cyclodextrin (beta-CD) and compared with 4-hydroxy-3,5-dimethoxybenzaldehyde (HDMB). The inclusion complex of HMB with beta-CD is analysed by UV-vis, fluorimetry, FT-IR, (1)H NMR, SEM and AM1 methods. In HMB, the normal emission (B band) is originates from a locally excited state and the longer emission (A band) is due to intramolecular charge transfer state (ICT). The OH group of HMB is present in the interior part of the beta-CD cavity and aldehyde group present in the upper part of the beta-CD cavity.     

Intramolecular photoinduced charge transfer in rotaxanes

We report the synthesis and photophysical investigation of a series of rotaxanes in which the physical confinement of the donor and acceptor (DA) pair leads, in some cases, to emissive exciplexes. As a comparison, we examined the photoinduced charge-transfer processes in the same DA mixtures under intermolecular conditions. The interlocked configuration of the rotaxane facilitates pi orbital overlap of the excited state DA pair by keeping their center-to-center distance extremely small. This increased interaction between the DA pair significantly lowers the activation energy for exciplex formation (E(a)) and helps stabilize the highly polar charge-transfer complex. We find that the stabilizing effect of the rotaxane architecture compensates for the modest thermodynamic driving force for some charge-transfer interactions. In addition, we examined the temperature dependence on the rotaxanes' optical properties. Metal coordination to the tetrahedral cavity disrupts the cofacial conformation of the DA pair and quenches the fluorescence. Binding of alkali metal ions to the 3,4-ethylenedioxythiophene (EDOT)-based rotaxane, however, gives rise to the emergence of a new weak emission band at even lower energies, indicative of a new emissive exciplex.     

Intramolecular charge transfer in a star-shaped oligoarylamine

The intervalence (IV) states of the monocationic states of the star-shaped nonaamine (3) and the triamine (2) as the branched unit in 3 have been examined by electrochemical, spectroelectrochemical, and temperature-dependent ESR spectroscopy. The oligoarylamines 2 and 3 were synthesized by using the successive palladium-catalyzed amination reaction. The redox property of 3 was basically the same as that of 2. However, there exist small potential differences between the first three one-electron oxidations for 3, indicating electronic coupling among the peripherally substituted triamine moieties via the central 1,3,5-benzenetriyl bridging unit. The observed ESR spectral pattern for 2+ remained unchanged over the measured temperature range. From the spectral simulation analyses, it was concluded that the unpaired electron in 2+ is fully delocalized over the whole molecule on the ESR time scale. This conclusion was corroborated by comparison of its optical absorption spectrum with TD-DFT-calculated results. In contrast, the peak-to-peak ESR line width (DeltaHPP) of 3+ exhibited temperature dependency. This behavior is ascribed to the thermally activated intramolecular charge transfer (ICT) among the branched three triamine moieties via the central 1,3,5-benzenetriyl bridging unit. From the spectral simulations based on the stochastic Liouville method, the first-order rate constant at each temperature and the parameters of the energy barrier for the ICT in 3+ were successfully determined.     

Intramolecular charge transfer dual fluorescence of p-dimethylaminobenzoates

A series of ester, p-N,N-dimethylaminobenzoates ((CH3)2NC6H4COOR, R=n-CnH2n+1, n=1,2,...8), were synthesized and their fluorescence spectra were recorded. Intramolecular charge transfer (CT) dual fluorescence was found in common organic solvent or in water with cetyltrimethylammonium bromide (CTAB) micelle. The fluorescence intensity ratio of CT band to normal band (ICT/ILE) was decreased dramatically with the increasing of carbon number of R group when the carbon number of R group were 1-3, but the dependence was not obvious when the carbon number of R group were 4-8. Oppositely, the ICT/ILE was increased with increasing of carbon number of R group in water with CTAB micelle.     

Excited state intramolecular charge transfer in N,N-heterocyclic-4-aminobenzonitriles: a DFT study

The excited state intramolecular charge transfer (ICT) reaction in a series of N,N-heterocyclic 4-aminobenzonitriles is investigated theoretically by a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Experimentally, increasing ICT emission is observed with increasing ring size. Formation of both a planar and twisted ICT (PICT and TICT) state are energetically unfavorable in the small systems due to high inversion barriers. With increasing ring size, the TICT state is more stabilized than the PICT state. A good agreement of the computed TICT state dipole moment is found with experimental values. The red-shifted fluorescence of all systems is explained by the TICT model due to both arguments.     

Intramolecular charge transfer effects on 3-aminobenzoic acid

Effect of solvents, buffer solutions of different pH and β-cyclodextrin on the absorption and fluorescence spectra of 3-aminobenzoic acid (3ABA) have been investigated. The solid inclusion complex of 3ABA with β-CD is discussed by UV–Vis, fluorimetry, semiempirical quantum calculations (AM1), FT-IR, ¹H NMR and Scanning Electron Microscope (SEM). The thermodynamic parameters (ΔH, ΔG and ΔS) of the inclusion process are also determined. The experimental results indicated that the inclusion processes is an exothermic and spontaneous. The large Stokes shift emission in solvents with 3ABA are correlated with different solvent polarity scales suggest that, 3ABA molecule is more polar in the S₁ state. Solvent, β-CD studies and excited state dipole moment values confirms that the presence of intramolecular charge transfer (ICT) in 3ABA. Acidity constants for different prototropic equilibria of 3ABA in the S₀ and S₁ states are calculated. β-Cyclodextrin studies shows that 3ABA forms a 1:1 inclusion complex with β-CD. β-CD studies suggest COOH group present in non-polar part and amino group present in hydrophilic part of the β-CD cavity. A mechanism is proposed to explain the inclusion process.     

Intramolecular charge transfer and dielectric solvent relaxation in n-propyl cyanide. N-phenylpyrrole and 4-dimethylamino-4'-cyanostilbene

Fast intramolecular charge transfer (ICT) accompanied by dual fluorescence from a locally excited (LE) and an ICT state taking place with N-phenylpyrrole (PP) in the solvent n-propyl cyanide (PrCN) is investigated as a function of temperature between 25 and -112 degrees C. The LE and ICT fluorescence decays from -45 to -70 degrees C can be adequately fitted with two exponentials, in accordance with a two state (LE + ICT) reaction mechanism, similar to what has been observed with PP in the more polar and less viscous alkyl cyanides acetonitrile (MeCN) and ethyl cyanide (EtCN). At lower temperatures, triple-exponential fits are required for the LE and ICT decays. The ICT emission band maximum of the time-resolved fluorescence spectra of PP in PrCN at -100 degrees C displays a spectral shift from 29 230 cm-1 at t = 0 to 27 780 cm-1 at infinite time, which equilibration process is attributed to dielectric solvent relaxation. From the time dependence of this shift, in global analysis with that of the band integrals BI(LE) and BI(ICT) of the time-resolved LE and ICT fluorescence bands, the decay times 119 and 456 ps are obtained. Dielectric relaxation times of 20 and 138 ps are determined from the double-exponential spectral solvation response function C(t) of the probe molecule 4-dimethylamino-4'-cyanostilbene in PrCN at -100 degrees C. It is concluded from the similarity of the times 119 ps (PP) and 138 ps (DCS) that the deviation from double-exponential character for the fluorescence decays of PP in PrCN below -70 degrees C is due to the interference of dielectric solvent relaxation with the ICT reaction. This fact complicates the kinetic analysis of the LE and ICT fluorescence decays. The kinetic analysis for PP in PrCN is hence restricted to temperatures between -70 and -45 degrees C. From this analysis, the forward and backward ICT activation energies Ea (12 kJ/mol) and Ed (17 kJ/mol) are obtained, giving an ICT stabilization enthalpy -DeltaH of 5 kJ/mol. A comparison of the reaction barriers for PP in the three alkyl cyanides PrCN, EtCN, and MeCN (J. Phys. Chem. A 2005, 109, 1497) shows that Ea becomes smaller with increasing solvent polarity (from 12 to 6 kJ/mol), whereas Ed remains effectively constant. Both observations are indicative of a late transition state for the LE --> ICT reaction. The significance of the Leffler-Hammond postulate in this connection is discussed: not primarily the energy of the LE, ICT, and transition states but rather the extent of charge transfer in these states determines whether an early or a late transition state is present.     

Intramolecular energy transfer through charge transfer state in lanthanide compounds: a theoretical approach

A theoretical approach for the intramolecular energy transfer process involving the ligand-to-metal charge transfer (LMCT) state in lanthanide compounds is developed. Considering a two-electron interaction, both the direct Coulomb and exchange interactions are taken into account, leading to expressions from which selection rules may be derived and transfer rates may be calculated. These selection rules show that the direct Coulomb and exchange mechanisms are complementary, in the same way as obtained in previous works for the case of ligand-lanthanide ion energy transfer processes. An important result from numerical estimates is that the channel ligand-LMCT state is by far the dominant case, leading to transfer rates higher than for the channel lanthanide ion-LMCT state by several orders of magnitude. The analysis of the emission quantum yield as a function of the relative energy position of the LMCT state in a typical Eu(3+) compound allows the identification of two quenching regions, the most pronounced one occurring close to the lower ligand triplet level.     

Intramolecular charge transfer and biomimetic reaction kinetics in galactose oxidase model complexes

One-electron oxidation of two structurally similar CuII-diphenolate complexes, 1 and 2, creates EPR-silent CuII-phenoxyl complexes [1]+ and [2]+ that mimic the oxidized form of the enzyme galactose oxidase (GOase). Both model complexes display novel NIR absorptions assigned to phenolate-phenoxyl charge transfer that resemble a tyrosinate-tyrosyl charge-transfer band observed in the enzymatic system. [1]+ and [2]+ react with benzyl alcohol to form 0.5 equivs of benzaldehyde per complex; biomimetic reduction to CuI-phenol complexes is not observed, but such species may exist transiently. Initial kinetic studies show that [2]+ reacts faster with benzyl alcohol than does [1]+, despite being a significantly weaker oxidant (DeltaE degrees = 370 mV). This acceleration is ascribed to mechanistic differences: [2]+ appears to bind substrate prior to the rate-determining step. Large, nonclassical kinetic isotope effects confirm C-H bond cleavage as the rate-determining step in the reactions of both [1]+ and [2]+ with benzyl alcohol, as is the case for GOase.     

Intramolecular charge transfer with the planarized 4-aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap DeltaE(S(1),S(2)), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to micro(e)(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 degrees C, the forward ICT rate constant (1.3 x 10(11) s(-1)) is much smaller than that of the back reaction (5.9 x 10(9) s(-1)), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when DeltaE(S(1),S(2)) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing DeltaE(S(1),S(2)) relative to that of DMABN.     

Intramolecular charge transfer and electronic absorption and luminescence spectra of fluoroquinolinones

TD-DFT study on the effect of donor and acceptor substituents on molecular orbital localization and charge distribution in fluoroquinolinone molecules showed that their photoexcitation is accompanied by electron density redistribution over particular fragments. Depending on the protolytic form, frontier molecular orbitals are localized on different fragments, whereas variation of substituents weakly affects localization of these orbitals.     

Intramolecular charge transfer with the planarized 4-cyanofluorazene and its flexible counterpart 4-cyano-N-phenylpyrrole. Picosecond fluorescence decays and femtosecond excited-state absorption

The fluorescence spectrum of the rigidified 4-cyanofluorazene (FPP4C) in n-hexane consists of a dual emission from a locally excited (LE) and an intramolecular charge-transfer (ICT) state, with an ICT/LE fluorescence quantum yield ratio of Phi'(ICT)/Phi(LE) = 3.3 at 25 degrees C. With the flexible 4-cyano- N-phenylpyrrole (PP4C) in n-hexane, such an ICT reaction also takes place, with Phi'(ICT)/Phi(LE) = 1.5, indicating that for this reaction, a perpendicular twist of the pyrrole and benzonitrile moieties is not required. The ICT emission band of FPP4C and PP4C in n-hexane has vibrational structure, but a structureless band is observed in all other solvents more polar than the alkanes. The enthalpy difference Delta H of the LE --> ICT reaction in n-hexane, -11 kJ/mol for FPP4C and -7 kJ/mol for PP4C, is determined by analyzing the temperature dependence of Phi'(ICT)/Phi(LE). Using these data, the energy E(FC,ICT) of the Franck-Condon ground state populated by the ICT emission is calculated, 41 (FPP4C) and 40 kJ/mol (PP4C). These large values for E(FC,ICT) lead to the conclusion that with FPP4C and PP4C, direct ICT excitation, bypassing LE, does not take place. FPP4C has an ICT dipole moment of 15 D, similar to that of PP4C (16 D). Picosecond fluorescence decays allow the determination of the ICT lifetime, from which the radiative rate constant k'(f)(ICT) is derived, with comparable values for FPP4C and PP4C. This shows that an argument for a twisted ICT state of PP4C cannot come from k'(f)(ICT). After correction for the solvent refractive index and the energy of the emission maximum nu(max)(ICT), it appears that k'(f)(ICT) is solvent-polarity-independent. Femtosecond transient absorption with FPP4C and PP4C in n-hexane reveals that the ICT state is already nearly fully present at 100 fs after excitation, in rapid equilibrium with LE. In MeCN, the ICT state of FPP4C and PP4C is likewise largely developed at this delay time, and the reaction is limited by dielectric solvent relaxation, which shows that the ICT reaction is ultrafast, at the experimental time limit of 50 fs. PP4C and FPP4C have a similar planar ICT structure, without an appreciable twist of the pyrrole and benzonitrile subgroups. Their crystal structure is compared with calculations for the S0 ground state.     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.     

Intramolecular charge transfer in 1:1 Cu(II)/pyrenylcyclam dendrimer complexes

Two newly synthesized pyrenylcyclam dendrons (1 and 2) exhibit a new emission band centered at 450 nm when coordinated with copper triflate. Observed fluorescence shifts induced by coordinative metalation indicate an unusual intramolecular charge transfer from a pyrenyl excited state to the coordinated metal ion that competes with pyrene excimer formation. This interaction likely proceeds by photoexcitation of pi-complex of the appended arene, followed by intramolecular charge transfer within the dendritic 1:1 cyclam/metal complex, effecting reduction of the bound Cu(II) metal ion. The appended dendritic groups not only decrease the equilibrium binding constant with Cu(II) but also participate in a new excited-state pathway as an alternative to energy-dissipative excimer formation.     

Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers

The strong solvatochromism observed for two fluorene-dibenzothiophene-S,S-dioxide oligomers in polar solvents has been investigated using steady-state and time-resolved fluorescence techniques. A low-energy absorption band, attributed to a charge-transfer (CT) state, is identified by its red shift with increasing solvent polarity. In nonpolar solvents, the emission of these conjugated luminescent oligomers shows narrow and well-resolved features, suggesting that the emission comes from a local excited state (LE), by analogy to their conjugated fluorene-based polymer counterparts. However, in polar solvents, only a featureless broad emission is observed at longer wavelengths (CT emission). A linear correlation between the energy maximum of the fluorescence emission and the solvent orientation polarizability factor Deltaf (Lippert-Mataga equation) is observed through a large range of solvents. In ethanol, below 230 K, the emission spectra of both oligomers show dual fluorescence (LE-like and CT) with the observation of a red-edge excitation effect. The stabilization of the CT emissive state by solvent polarity is accompanied/followed by structural changes to adapt the molecular structure to the new electronic density distribution. In ethanol, above 220 K, the solvent reorganization occurs on a faster time scale (less than 10 ps at 290 K), and the structural relaxation of the molecule (CT(unrelaxed) --> CT(Relaxed)) can be followed independently. The magnitude of the forward rate constant, k(1)(20 degrees C) approximately 20 x 10(9) s(-1), and the reaction energy barrier, E(a) approximately 3.9 kcal mol(-1), close to the energy barrier for viscous flow in ethanol (3.54 kcal mol(-1)), show that large-amplitude molecular motions are present in the stabilization of the CT state.     

Electrophoretic mobility does not always reflect the charge on an oil droplet

Electrophoresis is widely used to determine the electrostatic potential of colloidal particles. Oil droplets in pure water show negative or positive electrophoretic mobilities depending on the pH. This is commonly attributed to the adsorption of hydroxyl or hydronium ions, resulting in a negative or positive surface charge, respectively. This explanation, however, is not in agreement with the difference in isoelectric point and point of zero charge observed in experiment. Here we present molecular dynamics simulations of oil droplets in water in the presence of an external electric field but in the absence of any ions. The simulations reproduce the negative sign and the order of magnitude of the oil droplet mobilities at the point of zero charge in experiment. The electrostatic potential in the oil with respect to the water phase, induced by anisotropic dipole orientation in the interface, is positive. Our results suggest that electrophoretic mobility does not always reflect the net charge or electrostatic potential of a suspended liquid droplet and, thus, the interpretation of electrophoresis in terms of purely continuum effects may need to be reevaluated.     

Intramolecular charge transfer processes in a series of styryl derivatives of pyridine and quinoline N-oxides

Intramolecular charge transfer processes in a series of thetrans-isomers of styryl derivatives of pyridines and quinolines, their hydrochlorides and molecular complexes with BF₃ have been studied by electronic and IR spectroscopy. Electron donor groups involved in direct resonance conjugation with the N-oxide group intensify these processes, while electron acceptor nitro groups somewhat weaken them. Protons and BF₃ coordinate to the oxygen of the NO group to form strong 11 complexes, except in the cases of 4-(4-dimethylaminostyryl)quinoline and pyridine N-oxides, in which a second acceptor molecule adds to the amino group.Petrozavodsk State University, Petrozavodsk 185640, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, 1093–1102, August, 1998.