Theoretical study of the two-photon absorption properties of several asymmetrically substituted stilbenoid molecules

Two-photon absorption (TPA) properties of noncentrosymmetric pi-conjugated stilbenoid molecules with D-pi-A structures, TPA spectra of which have been reported [L. Antonov et al., Phys. Chem. Chem. Phys. 5, 1193 (2003)], have been investigated theoretically by ab initio molecular orbital methods. The difference in the observed one-photon absorption and TPA spectra among compounds with the same donor (D) and acceptor (A) units is well reproduced by the present calculations, although the calculated excitation energies are overestimated by the configuration interaction with single excitation method used. It was found that the spectral differences among the compounds were mainly due to the deviation from the planar structure by intramolecular rotation around the N[Single Bond]C (phenyl) bond of the N-benzilideneanilines having the C[Double Bond]N linkage as the central pi bridge. Substitution of the end donor or acceptor groups with weaker ones leads to a decrease in the TPA intensity of the lowest pi-pi(*) TPA states, resulting mainly from the decrease in the dipole moment of the excited states. The total TPA cross section spectra have been separated into contributions of the dipolar term, which appear only in noncentrosymmetric systems, and the three-state term, which appear in any systems irrespective of symmetry. The dipolar term predominates only for the lowest pi-pi(*) state, while for the higher excited states the three-state terms become predominant. An analysis employing the index R(f) defined with the transition polarizability shows that the TPA properties of the higher excited states are well described by the three-state approximation mediated by the lowest pi-pi(*) state. The differences found between the centrosymmetric and dipolar molecules in the enhancement mechanism of the TPA intensity by substituting the end groups with strong donors are discussed by comparison with the TPA properties of azobenzenes symmetrically substituted with the same donors.     

One- and two-photon spectra of some selected molecules: A comparative study

The complementarity of one- and two-photon spectroscopy has been utilised for throwing light on the following problems of chemical interest: (1) Weak interaction between identical chromophores separated by insulating bridges gives rise to split states of different symmetries. Two-photon spectroscopy (TPA), in conjunction with one-photon absorption (OPA), has been used to identify the states and hence to estimate the magnitude of interaction in bimolecules and trimolecules. From the shifts between the one- and the two-photon spectra, the splittings have been estimated. Calculations confirm that the dominant interaction is the through-bond one. (2) The second type of problem is the identification ofg andu vibrations in molecules. We have initiated studies on three molecules in jet-cooled conditions: 9,10-dihydro-anthracene (DHA). 9,10-dihydro-phenanthrene (DHP) and octa-fluoronaph-thalene (OFN). Only the one-photon fluorescence excitation spectra have so far been obtained by us and the TPA spectra are under investigation. (3) The third class of molecules discussed here are the Ln³⁺ complexes wheref ⁿ⇒ fⁿ transitions are intrinsically two-photon allowed. We have studied two GD³⁺ single crystals. The CF-splittings, observed clearly in TPA, have been fitted with a parametric model. Some of our observations on the variations of TPA intensity patterns from crystal to crystal, such as circular:linear polarisation ratios, relative intensities of transitions to differentJ-states, do not quite fit in with the Axe-Judd-Downer model. The discrepancies call for a reappraisal of the role of ligand in the TPA process.     

Theoretical investigation of two-photon absorption properties for a series of bifluorene molecules

The two-photon absorption (TPA) cross-sections of a series of bifluorene molecules with different substituents (constructing three types of structures D–π–D, A–π–A and D–π–A) were calculated using ZINDO/SOS program. The results showed that the A–π–A structure with strong substituent-nitro had the largest TPA cross-section for these molecules. To verify the results, we analyzed the charge quantity of the ground and the main excited states as well as the frontier orbitals of the investigated molecules. The equilibrium geometries were obtained with AM1 method, and using ZINDO-SOS to calculate the third-order nonlinearities of the molecules, then gained the TPA cross-sections. We found that for the compounds with bifluorene as π center, the donor and acceptor strength are the important factor for the enhancement of the TPA properties, and compared with molecules with fluorene as π center, the large TPA cross-sections of bifluorene are caused by coupling effects between the two monomers.     

A selective colorimetric chemosensor for thiols based on intramolecular charge transfer mechanism

Compound 1 as an electron donor-acceptor compound with N,N-dimethylaniline and quinone units was designed for a highly selective colorimetric determination of thiol-containing amino acids and peptides, by making use of the unique reactivity of thiol towards quinone. Compound 1 shows a strong intramolecular charge transfer (ICT) band around 582 nm; but, it decreased after addition of either cysteine (Cys) or glutathione (GSH). Moreover, the ICT band intensity at 582 nm decreased linearly with the increasing concentrations of Cys or GSH. The interference from other amino acids can be neglected. Therefore, compound 1 can be employed as a selective colorimetric visual chemosensor for thiol-containing amino acids and peptides.     

A novel fluoride selective optical chemosensor based on internal charge transfer signaling

A new internal charge transfer probe, NAPH-1, synthesized by incorporating photoemitive naphthalimide core with an acidic imidazolium ring, offers highly selective colorimetric and ratiometric ‘off-on’ signaling for targeting F⁻, while Cl⁻, Br⁻, I⁻, , SCN⁻, AcO⁻, and do not appreciably perturb the photophysical properties of the probe even at relatively higher concentrations than the F⁻. Deprotonation of the imidazolium ring, supported by the ¹H NMR and theoritical studies, seems to cause the spectral modulations.     

A theoretical study of solvent effects on the 13C chemical shifts of some polar molecules

Calculations of the dependence of ¹³C chemical shifts on the dielectric constant, ?, of the medium are reported using the INDO/S parameterized version of Pople's method together with the solvaton model. Satisfactory agreement with the available experimental data is obtained. The rather sensitive dependence of chemical shift upon ? suggests that polar solute—solvent interactions in aprotic media should be taken into account when considering ¹³C chemical shifts.     

A theoretical study of the chiroptical properties of molecules with isotopically engendered chirality

There has been a considerable interest in the chiroptical properties of molecules whose chirality is exclusively due to an isotopic substitution and numerous examples for the electronic circular dichroism (CD) spectra of isotopically chiral systems have been reported in literature. Four different explanations have been proposed for the mechanism as to how the isotopic substitution induces a chiral perturbation of the otherwise achiral electronic wave function; however, up to now no conclusive answer has been given about the dominating effect responsible for the experimental observations. In this study we will present, for the first time, fully quantum-mechanical calculations of the CD spectra of three different molecular systems with isotopically engendered chirality. As examples, we consider the spectra of organic molecules with ketone and alpha-diketone carbonyl and diene chromophores. The effect of vibronic couplings for the reorientation of the electric and magnetic transition dipole moments is taken into account within the Herzberg-Teller approximation. The ground and excited state geometries and vibrational normal modes are obtained with (time-dependent) density functional theory [(TD)DFT], while the vibronic coupling effects are calculated at the TDDFT and density functional theory/multireference configuration interaction (DFT/MRCI) levels of theory. Generally, the band shapes of the experimental CD spectra are reproduced very well, and also the absolute CD intensities from the simulations are of the right order of magnitude. The sign and the intensity of the CD band are determined by a delicate balance of the contributions of a large number of individual vibronic transitions, and it is found that the vibrational normal modes with a large displacement are dominant. The separation of the calculated CD spectrum into the different contributions due to the overlap of the in-plane and out-of-plane components (regarding the symmetry plane of the unsubstituted molecule) of the electric and magnetic transition dipole moments yields information about the influence of the vibronic coupling effects for the reorientation of the corresponding transition dipole moments. In conclusion, the calculations clearly show that vibronic effects are responsible or at least dominant for the chiroptical properties of isotopically chiral organic molecules.     

A theoretical study of the heats of formation of some small molecules using non-empirical wavefunctions

Total energies, obtained from non-empirical LCAO-MO-SCF calculations on a series of reactions involving only closed-shell molecules and ions, have been used to calculate the heats of formation H ₂₉₈ ⁰ of a large number of small molecules. The Double- basis set calculations, after empirical corrections for inadequacies in the basis set and systematic errors found in all calculations involving oxygen and carbon atoms, usually predict the heats of formation within 10 kcal/mole of the experimental value. A series of similar calculations predicts the heats of formation of some negative ions for which experimental values are either not available or are unreliable.     

Effect of one ligand substitution on charge transfer and optical properties in mer-Alq3: a theoretical study

In tris(8-hydroxyquinolinato)aluminum (mer-Alq3) position for substitution plays an important role. We explain the push–pull effect on the charge transfer and optical properties, if only one of the ligand among three ligands of meridianal isomer of mer-Alq3 has been substituted. To check this effect, we substituted A-ligand with electron-donating group at position 4 and electron-withdrawing group at position 6. We designed 4-methyl-6-chloro-(8-hydroxyquino)bis(8-hydroxyquinolinato)aluminum (1), 4-methyl-6-cyano-(8-hydroxyquino)bis(8-hydroxyquinolinato)aluminum (2), 4-amino-6-chloro-(8-hydroxyquino)bis(8-hydroxyquinolinato)aluminum (3), and 4-amino-6-cyano-(8-hydroxyquino)bis(8-hydroxyquinolinato)aluminum (4) derivatives of mer-Alq3. All the studied derivatives in the ground (S₀) and first excited (S₁) states have been optimized at the B3LYP/6-31G* and CIS/6-31G* level of theory, respectively. We have designed green light-emitting materials like mer-Alq3 and blue light-emitting materials. These derivatives are good candidates for comparable/better charge carrier mobility as mer-Alq3.     

Theoretical study of one- and two-photon absorption properties of pyrene derivatives

The geometrical structure, electronic structure, one-photon absorption (OPA) properties of pyrene and its derivatives have been theoretically studied by using density functional theory (DFT) method and Zerner’s intermediate neglect of differential overlap (ZINDO) methods, and their two-photon absorption (TPA) properties are calculated by the ZINDO/sum-over-states method. The results show that introducing donor groups to pyrene molecule, increasing the number of donor groups, extending the conjugated length, or forming circular conjugated dimer can increase the oscillator strength (f) in the TPA process and ultimately result in extremely large TPA cross-sections and strong OPA around 400 nm of pyrene derivatives. All these results give us some basic principles to design pyrene derivatives with large TPA cross-sections. This shed light into the significance of the pyrene derivatives as promising fluorescent probes in biochemistry when they were linked to some special recognizing groups.     

A thermodynamic study of the charge transfer complexes of MV with some macrocyclic hosts

Formation of charge transfer complexes of methyl viologen, MV, and hosts (1?C3) in ethanol were studied using UV?Cvisible spectrophotometry. The stability constants and the thermodynamic parameters of the resulting 1:2 (MV:host ratio) complexes were determined. All charge transfer complexes formed were enthalpy destabilized, but entropy stabilized. The effect of donor atoms, their orientation, the substituents, flexibility and the cavity size of the crown ethers on the formation constants and thermodynamic parameters will be discussed.     

Spectrophotometric study of the charge transfer complexes of some pharmaceutical butyrophenones

The molecular interactions between haloperidol and droperidol as electron donors and each of iodine; 7,7,8,8-tetracyanoquinodimethane (TCNQ); 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); tetracyanoethylene (TCNE); 2,4,7-trinitro-9-fluorenon (TNF); and 2-3-5-6-tetrabromo-1,4-benzoquinone (Bromanil) as acceptors have been investigated spectrophotometrically. Different variables affecting the reaction were studies and optimized. Beer's law was obeyed in a concentration limit of 2.5-2500 mug ml(-1) for the studied drugs with various acceptors used. Electron affinities (E(A)) of the acceptors were found to correlate with both the time required for maximum colour formation and the molar absorptivities of haloperidol and droperidol. A Job's plot of the absorbance versus the molar ratio of the drugs to iodine indicated 1:1 ratio. The proposed methods were found to be rapid and sensitive and may be applied for estimation of named drugs in pharmaceutical dosage forms without interferences from the common additives encountered. Percentage recoveries ranged from 99.1-102.2%.     

Theoretical study of two-photon absorption properties of a series of ferrocene-based chromophores

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.     

Experimental and theoretical study of two-photon absorption in nitrofuran derivatives: Promising compounds for photochemotherapy

We report experimental and theoretical studies of the two-photon absorption spectrum of two nitrofuran derivatives: nitrofurantoine, (1-(5-nitro-2-furfurilideneamine)-hidantoine) and quinifuryl, 2-(5(')-nitro-2(')-furanyl)ethenyl-4-{N-[4(')-(N,N-diethylamino)-1(')-methylbutyl]carbamoyl} quinoline. Both molecules are representative of a family of 5-nitrofuran-ethenyl-quinoline drugs that have been demonstrated to display high toxicity to various species of transformed cells in the dark. We determine the two-photon absorption cross-section for both compounds, from 560 to 880 nm, which present peak values of 64 GM for quinifuryl and 20 GM for nitrofurantoine (1 GM = 1×10(-50)cm(4).s.photon(-1)). Besides, theoretical calculations employing the linear and quadratic response functions were carried out at the density functional theory level to aid the interpretations of the experimental results. The theoretical results yielded oscillator strengths, two-photon transition probabilities, and transition energies, which are in good agreement with the experimental data. A higher number of allowed electronic transitions was identified for quinifuryl in comparison to nitrofurantoine by the theoretical calculations. Due to the planar structure of both compounds, the differences in the two-photon absorption cross-section values are a consequence of their distinct conjugation lengths.     

A ratiometric and exclusively selective Cu fluorescent probe based on internal charge transfer (ICT)

A new ratiometric and exclusively selective fluorescent probe N-butyl-4,5-di[N-(phenyl)-2-(amino)-acetamino]-1,8-naphthalimide (1) was designed and synthesized on the basis of the mechanism of internal charge transfer (ICT). The probe 1 showed exclusively selectivity for Cu^II in the presence of a variety of other metal ions in aqueous ethanol solutions and the binding mode of probe 1 with Cu^II was 1:1 metal-ligand complex. Fluorescent emission spectra of probe 1 in the presence of Cu^II showed a 50 nm blue shift, which is from 521 nm to 471 nm. Furthermore, probe 1 shows the same fluorescent change with the Cu^II in living cells.     

Electrochemical study of thymine dimer based on DNA charge transfer

Differential pulse voltammetry was used to study the formation and level of thymine dimer in DNA duplex modified on a gold electrode. The electrochemical signal of methylene blue coupled with ferricyanide can be obtained via DNA mediated electron transfer, which would be blocked during the formation of thymine dimer. DNA duplexes with different sequences differ in the level of thymine dimer under the same UV irradiation. Futhermore, the presence of guanine base directly preceding -TT- can effectively decrease the level of thymine dimer, possibly due to the self-repair process in which guanine participates. The proposed method can be further applied to DNA self-repair analysis.     

A theoretical study of the charge transfer behavior of the highly regioregular poly-3-hexylthiophene in the ordered state

We use quantum mechanical (QM) methods to interpret the charge transport properties of the self-assembled poly-3-hexylthiophene (P3HT) molecules along the intrachain and interchain directions. Our approach is illustrated by a hopping transport model, in which we examine the variation of the electron-coupling strength (transfer integral) with the torsional angle and the intermolecular distance between two adjacent thiophene segments. We also simulate the packed P3HT structures at various values of temperature and regioregularity via the molecular dynamics (MD) simulations. The MD results indicate that with decreasing the molecular regioregularity and/or increasing temperature, the P3HT backbone chains experience a larger distortion of the thiophene rings out of coplanarity, and thus the charge mobility along the main chains is reduced. However, as long as the P3HT molecules remain in the ordered lamellar state due to the presence of the pi-pi interaction, the resultant mobility along the pi-pi interchain direction is still significantly less than that along the intrachain direction. Accordingly, the main charge transfer route within the P3HT ordered domains is along the intrachains instead of the interchains.     

A theoretical study on the detection of proton transfer pathways in some mutants of human carbonic anhydrase II

Structural and kinetic studies of mutants can give much insight into the function of an enzyme. We report the detection of possible proton transfer pathways into the active site of a number of mutants of the enzyme human carbonic anhydrase II (HCA II). Using a recently developed method of path search in the protein conformational space, we identify hydrogen-bonded networks (or proton paths) that can dynamically connect the protein surface to the active site through fluctuations in protein structure and hydration. The feasibility of establishing such dynamical connectivities is assessed by computing the change in free energy of conformational fluctuations and compared to those identified earlier in the wild type enzyme. It is found that the point mutation facilitates or suppresses one or more of the alternative pathways. Our results allow the use of a generic set of pathways to correlate qualitatively the residual activity in the mutants to the molecular mechanism of proton transfer in the absence of His at position 64. We also demonstrate how the detected pathways may be used to compare the efficiencies of the mutants His-64-Ala/Asn-62-His and His-64-Ala/Asn-67-His using the empirical valence bond theory.