First theoretical framework of di-substituted donor moieties of triphenylamine and carbazole for NLO properties: quantum paradigms of interactive molecular computation

The dependence of polarisability (α) and hyperpolarisability (β) on donor strength has been systematically studied by employing density functional theory method on triphenylamine (TPA) and carbazole (CZ) based compounds. The electronic structures, absorption spectra and non-linear optical (NLO) response were calculated by using quantum chemical methods. All the calculations were performed in gas phase in presence of solvent. The results reveal that the polarizability (α) and hyperpolarizability (β) significantly increased by the addition of second donor moiety. Similarly, the oscillator strength and light harvesting efficiency were also increased, while absorption wavelength was red-shifted by the addition of second donor moiety. These results indicate that the di-substituted donor is an effective way to improve NLO response. The TPA dyes possess a large second-order non-linear response and this is primarily because of the strong donor-π-acceptor conjugation that is ascribed to the excited state intramolecular charge transfer. These theoretical frameworks of carbon architecture might be advantageous to design other organic charge-transfer compounds.     

Solvent‐dependent ultrafast optical response of conjugated push–pull chromophores

Two new difluoroboron β‐carbonyl cyclic ketonate complexes C2B and DC2B were investigated using several spectroscopic methods. Relative to the absorption spectra, the fluorescence spectra were more affected by the polarity of the solvent. Also, compound C2B showed a more pronounced Stokes’ shift after solvent polarity increased. Transient absorption measurements then demonstrated the relaxation behaviour of the excited state compound molecule. The kinetic results showed that the excited state C2B in tetrahydrofuran (THF) can return from the intramolecular charge‐transfer (ICT) state and the initial excited state to the ground state. The kinetic relaxation pathway after THF was replaced by dimethyl sulfoxide became single. When the carbazole unit was introduced, DC2B also exhibited an ICT state but there was no significant difference in the excited state relaxation path after solvent polarity was changed. The results indicated that C2B is more susceptible to solvent polarity regulation. The global fit results revealed that an increase in the solvent polarity prolonged the lifetime of the ICT state of compound C2B and had the opposite effect on compound DC2B. These results provide guidance for understanding the relationship between solvent polarity and the designing and synthesizing advanced compound materials.     

Charge transfer and optoelectronic properties in the triarylamine-based donor–π bridge–acceptor dyes for dye-sensitised solar cells

The triarylamine–based donor–π bridge–acceptor dyes (namely, Ds-3, Ds-5 and Ds-6), with the higher conversion efficiency of sunlight to electricity, have been studied with quantum chemistry methods. The geometrical structure, frontier molecular orbital and electronic vertical excitation energies were calculated by using the density functional theory (DFT) and the time-dependent DFT with the Cam-B3LYP and PBE0 functional. From the calculated results, we perform a three-dimensional real-space analysis, which demonstrates that the lowest energy excited state of the triarylamine-based dye is a charge transfer (CT) excited state and electrons shift from triarylamine to cyanoacrylic acid group. The excited-state oxidation potentials and driving force energy are identified as the essential parameters to study the electron injection ability of the excited dyes. The evaluation of photochemical parameter and the visualised study of CT process provide the important information for revealing the relationship between structure and photochemical property of the triarylamine-based dyes.     

Energy transfer in the major intrinsic light-harvesting complex from Amphidinium carterae

Carbonyl carotenoids are important constituents of the antenna complexes of marine organisms. These carotenoids possess an excited state with a charge-transfer character (intramolecular charge transfer state, ICT), but many details of the carotenoid to chlorophyll energy transfer mechanisms are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the intrinsic light-harvesting complex (LHC) of dinoflagellates, which contains the carbonyl carotenoid peridinin. Carotenoid to chlorophyll energy transfer efficiency is about 90% in the 530-550 nm region, where the peridinin S2 state transfers energy with an efficiency of 25-50%. The rest proceeds via the S1/ICT channel, and the major S1/ICT-mediated energy transfer pathway utilizes the relaxed S1/ICT state and occurs with a time constant of 2.6 ps. Below 525 nm, the overall energy transfer efficiency drops because of light absorption by another carotenoid, diadinoxanthin, that contributes only marginally to energy transfer. Instead, its role is likely to be photoprotection. In addition to the peridinin-Chl-a energy transfer, it was shown that energy transfer also occurs between the two chlorophyll species in LHC, Chl-c2, and Chl-a. The time constant characterizing the Chl-c2 to Chl-a energy transfer is 1.4 ps. The results demonstrate that the properties of the S1/ICT state specific for carbonyl carotenoids is the key to ensure the effective harvesting of photons in the 500-600 nm region, which is of vital importance to underwater organisms.     

Tuning dual emission behavior of p-dialkylaminobenzonitriles by supramolecular interactions with cyclodextrin hosts

Ground state absorption and steady-state and time-resolved fluorescence measurements have been carried out to understand the host-guest interactions of p-diethylaminobenzonitrile (DEABN) and p-dimethylaminobenzonitrile (DMABN) dyes with alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin (beta-CD) hosts. DEABN and DMABN dyes show both locally excited (LE) state and intramolecular charge transfer (ICT) state emissions in solution. The LE and ICT emissions of the dyes are seen to get modulated in the presence of alpha-CD and beta-CD hosts. The results indicate that the dyes form 1 : 1 inclusion complexes with both the hosts. Comparing the binding constants and the fluorescence characteristics of different dye x CD systems it is inferred that DEABN adopts a completely different orientation on complexation with alpha-CD than in the other cases of dye.CD systems. It is indicated that while in all other cases of dye x CD systems the N,N-dialkyl group of the dyes enters the host cavity leaving the C[triple bond, length as m-dash]N group projected out into the water phase, the DEABN dye enters the alpha-CD cavity (smallest CD) with its C[triple bond, length as m-dash]N group entering the host cavity. The differences in the orientation of the dye in the host cavities is understood to be determined by the requirement of maximum van der Waals contact of the encapsulated dye with the host cavity for maximum stability of the complex and the relative sizes of the substituents of the dye compared to the host cavities. From the observation that the binding constants for the present dye x CD systems are not that significantly high, it is inferred that the hydrophobic interaction mainly govern the inclusion complex formation in the present systems.     

Time dependent – density functional theory characterization of organic dyes for dye-sensitized solar cells

We aim at providing better insight into the parameters that govern the intramolecular charge transfer (ICT) and photo-injection processes in dyes for dye-sensitised solar cells (DSSC). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are utilized to study the geometry, electronic structure, electrostatic potential (ESP) and absorption spectrum, for a representative donor-π bridge-acceptor (D–π–A) dye for DSSC. The coplanar geometry of the dye (D1) facilitates strong conjugation and considerable delocalization originating the π CT interaction from donor to acceptor orbitals and the hyper-conjugative interactions involving Rydberg states. A model simulating the adsorption of the dye on the TiO₂ surface is utilized to estimate binding energies. The effect of fluorine substituents in the π-spacer on the quantum efficiency of DSSCs was investigated. Gibb’s free energy values, redox potentials, excited state lifetime, non-linear optical properties (NLO) and driving forces for D1 and its fluorinated derivatives were computed.     

Structure-property relationships for three indoline dyes used in dye-sensitized solar cells: TDDFT study of visible absorption and photoinduced charge-transfer processes

The electronic structures of three D-A-π-A indoline dyes (WS-2, WS-6, and WS-11) used in dye-sensitized solar cells (DSSCs) were studied by performing quantum chemistry calculations. The coplanarity of the A-π-A segment and distinct noncoplanarity of the indoline donor part of each dye were confirmed by checking the calculated geometric parameters. The relationships between molecular modifications and the optical properties of the dyes were derived in terms of the partial density of states, absorption spectrum, frontier molecular orbital, and excited-state charge transfer. 3D real-space analysis of the transition density (TD) and charge difference density (CDD) was also performed to further investigate the excited-state features of the molecular systems, as they provide visualized physical pictures of the charge separation and transfer. It was found that modifying the alkyl chain of the bridge unit near the acceptor unit is an efficient way to decrease dye aggregation and improve DSSC efficiency. Inserting a hexylthiophene group next to the donor unit leads to a complicated molecular structure and a decrease in the charge-transfer ability of the system, which has an unfavorable impact on DSSC performance.     

Comparative study on electronic structures and optical properties of indoline and triphenylamine dye sensitizers for solar cells

The computations of the geometries, electronic structures, dipole moments and polarizabilities for indoline and triphenylamine (TPA) based dye sensitizers, including D102, D131, D149, D205, TPAR1, TPAR2, TPAR4, and TPAR5, were performed using density functional theory, and the electronic absorption properties were investigated via time-dependent density functional theory with polarizable continuum model for solvent effects. The population analysis indicates that the donating electron capability of TPA is better than that of indoline group. The reduction driving forces for the oxidized D131 and TPAR1 are slightly larger than that of other dyes because of their lower highest occupied molecular orbital level. The absorption properties and molecular orbital analysis suggest that the TPA and 4-(2,2diphenylethenyl)phenyl substituent indoline groups are effective chromophores in intramolecular charge transfer (IMCT), and they play an important role in sensitization of dye-sensitized solar cells (DSCs). The better performance of D205 in DSCs results from more IMCT excited states with larger oscillator strength and higher light harvesting efficiency. While for TPA dyes, the longer conjugate bridges generate the larger oscillator strength and light harvesting efficiency, and the TPAR1 and TPAR4 have larger free energy change for electron injection and dye regeneration.     

Theoretical study of the spectroscopic and nonlinear optical properties of <Emphasis Type="Italic">trans</Emphasis>- and <Emphasis Type="Italic">cis</Emphasis>-4-hydroxyazobenzene

We investigate the molecular structure, vibrational and electronic absorption spectra, and electronic hyperpolarizabilities of trans and cis isomers of 4-hydroxyazobenzene (HOAB) via density functional theory. Results show that the azo dye exhibits a high third-order nonlinear optical response and good optical transparency. Both the basis set and the functional are important influences on the results obtained when calculating the absorption spectrum and NLO response. We also study the effect of the solvent on the electronic absorption spectrum to assess the ability of the functional to reproduce the experimental spectrum in combination with a suitable solvent model. Our calculations show that the SMD model of Truhlar et al. handles the electrostatic and the non-electrostatic effects of hydrogen-bonding solvents on the absorption spectrum better than the traditional polarizable continuum model does. In addition, our results indicate that the dye trans-HOAB exhibits a high second hyperpolarizability and excellent optical transparency. Also, although the second hyperpolarizability of cis-HOAB is much lower than that of trans-HOAB, it is non-negligible when calculating the optical nonlinearity of HOAB under an optical pump. We also examine the effect of frequency dispersion on second harmonic generation. This study provides the basis for further research on the spectroscopic and nonlinear optical properties of novel azo dyes and other π-conjugated compounds.     

Charge separation and energy transfer in a caroteno-C60 dyad: photoinduced electron transfer from the carotenoid excited states.

We have designed and synthesized a molecular dyad comprising a carotenoid pigment linked to a fullerene derivative (C-C(60)) in which the carotenoid acts both as an antenna for the fullerene and as an electron transfer partner. Ultrafast transient absorption spectroscopy was carried out on the dyad in order to investigate energy transfer and charge separation pathways and efficiencies upon excitation of the carotenoid moiety. When the dyad is dissolved in hexane energy transfer from the carotenoid S(2) state to the fullerene takes place on an ultrafast (sub 100 fs) timescale and no intramolecular electron transfer was detected. When the dyad is dissolved in toluene, the excited carotenoid decays from its excited states both by transferring energy to the fullerene and by forming a charge-separated C.+ -C(60).- . The charge-separated state is also formed from the excited fullerene following energy transfer from the carotenoid. These pathways lead to charge separation on the subpicosecond time scale (possibly from the S(2) state and the vibrationally excited S(1) state of the carotenoid), on the ps time scale (5.5 ps) from the relaxed S(1) state of the carotenoid, and from the excited state of C(60) in 23.5 ps. The charge-separated state lives for 1.3 ns and recombines to populate both the low-lying carotenoid triplet state and the dyad ground state.     

Synthesis, X-ray crystal structures and linear and nonlinear optical characterization of a series of nickel(II) and copper(II) salicylaldiminato complexes

A series of nickel(II) and copper(II) salicylaldiminato complexes containing side arms with either potentially coordinating (OH) or non-coordinating (Cl) functional groups have been prepared and characterized by X-ray crystallography. The Cu(II) complexes are square planar, but the Ni(II) complexes prefer octahedral coordination. Linear absorption spectra depend on the metal and on its coordination geometry, with the octahedral Ni(II) complexes being the most weakly absorbing at 532 nm and the square planar Cu(II) complexes being the most strongly absorbing at 532 nm. The third-order nonlinear optical properties of the complexes have been characterized using degenerate four-wave mixing (DFWM) and Z-scan. Two different Z-scan experimental configurations were used, one of which employs a Gaussian beam in a tightly focused geometry while the other employs a top-hat beam and a more relaxed focus. The observed third-order optical nonlinearity is primarily due to transient thermal (photo-acoustic) effects associated with linear absorption in the samples. The dependence of the third-order nonlinear optical properties on the linear absorption means that the nonlinear optical properties vary substantially between the complexes even though they all contain the same chromophore. The hyperpolarizability of one of the complexes, γ = 1.3 × 10⁻³⁰ esu, rivals the nonlinearities measured at 532 nm in expanded porphyrin and phthalocyanine complexes.     

Switch between charge transfer and local excited states in 4-aminophenyl-substituted Hantzsch 1,4-dihydropyridine induced by pH change and transition metal ions.

The absorption and fluorescence spectra of a Hantzsch 1,4-dihydropyridine derivative bearing a N,N-dimethylaminophenyl group at 4-position (H(2)Py-PhN(CH(3))(2)) in aprotic solvents have been examined and compared to model compounds 4-phenyl- and 4-methyl-substituted Hantzsch 1,4-dihydropyridines (H(2)Py-Ph and H(2)Py-Me). While H(2)Py-Ph and H(2)Py-Me show fluorescence around 420 nm from the local excited state of the dihydropyridine chromophore, H(2)Py-PhN(CH(3))(2) exhibits fluorescence around 520 nm from the intramolecular charge transfer (ICT) state involving the aniline and dihydropyridine groups as donor and acceptor, respectively. Upon addition of an acid to the solution of H(2)Py-PhN(CH(3))(2), the amino group in the aniline is protonated. Thus, the photoinduced intramolecular charge transfer is prevented, and only the fluorescence from the local excited state of the dihydropyridine chromophore can be detected. These changes in the fluorescence behavior are fully reversible: subsequent addition of a base to the acidic solution leads to the recovery of the ICT fluorescence and the quenching of the local fluorescence. Transition metal ions also can switch the fluorescence of H(2)Py-PhN(CH(3))(2). Evidence for the interaction between transition metal ions and the amino group in the dimethylaniline have been provided by absorption and emission spectrum as well as NMR studies.     

Tuning Energy Transfer in the Peridinin–chlorophyll Complex by Reconstitution with Different Chlorophylls

In vitro studies of the carotenoid peridinin, which is the primary pigment from the peridinin chlorophyll-a protein (PCP) light harvesting complex, showed a strong dependence on the lifetime of the peridinin lowest singlet excited state on solvent polarity. This dependence was attributed to the presence of an intramolecular charge transfer (ICT) state in the peridinin excited state manifold. The ICT state was also suggested to be a crucial factor in efficient peridinin to Chl-a energy transfer in the PCP complex. Here we extend our studies of peridinin dynamics to reconstituted PCP complexes, in which Chl-a was replaced by different chlorophyll species (Chl-b, acetyl Chl-a, Chl-d and BChl-a). Reconstitution of PCP with different Chl species maintains the energy transfer pathways within the complex, but the efficiency depends on the chlorophyll species. In the native PCP complex, the peridinin S₁/ICT state has a lifetime of 2.7 ps, whereas in reconstituted PCP complexes it is 5.9 ps (Chl-b) 2.9 ps (Chl-a), 2.2 ps (acetyl Chl-a), 1.9 ps (Chl-d), and 0.45 ps (BChl-a). Calculation of energy transfer rates using the Förster equation explains the differences in energy transfer efficiency in terms of changing spectral overlap between the peridinin emission and the absorption spectrum of the acceptor. It is proposed that the lowest excited state of peridinin is a strongly coupled S₁/ICT state, which is the energy donor for the major energy transfer channel. The significant ICT character of the S₁/ICT state in PCP enhances the transition dipole moment of the S₁/ICT state, facilitating energy transfer to chlorophyll via the Förster mechanism. In addition to energy transfer via the S₁/ICT, there is also energy transfer via the S₂ and hot S₁/ICT states to chlorophyll in all reconstituted PCP complexes.     

Spectroscopic studies of the low-lying singlet excited electronic states and photochemical properties of carotenoids

Investigations of the singlet excited state properties of carotenoids using steady-state fluorescence, transient absorption pump-probe, two-photon excitation, and resonance Raman excitation spectroscopies are described. The application of these experimental techniques to the specific problem of determining the S1 excited energies of carotenoids is discussed in detail, and the recent literature pertaining to the assignment of charge transfer states in carotenoids and states described as having particular pseudoparity elements is reviewed. Hypothetical schemes for how these states may account for some of the dynamic and photochemical behavior of carotenoids are presented.     

DFT study of linear and nonlinear optical properties of donor-acceptor substituted stilbenes, azobenzenes and benzilideneanilines

A theoretical analysis of the linear and nonlinear optical properties of six push–pull π-conjugated molecules with stilbene, azobenzene and benzilideneaniline as a backbone is presented. The photophysical properties of the investigated systems were determined by using response functions combined with density functional theory (DFT). Several different exchange-correlation potentials were applied in order to determine parameters describing the one- and two-photon spectra of the studied molecules. In particular, the recently proposed Coulomb-attenuated model (CAM-B3LYP) was used to describe charge-transfer (CT) excited states. In order to compare theoretical predictions with available experimental data, calculations with inclusion of solvent effects were performed. The BLYP and the CAM-B3LYP functionals were found to yield values of two-photon absorption (TPA) probabilities closer to experimental values than the B3LYP functional or the HF wavefunction. Moreover, molecular static hyperpolarisabilities were determined using both DFT and second-order Møller-Plesset perturbation (MP2) theory. Likewise, the CAM-B3LYP functional was found to outperform other applied exchange-correlation potentials in determining first hyperpolarisability (β). Moreover, it was confirmed on a purely theoretical basis that the presence of a –C=C– bridge between the phenyl rings leads to a much larger nonlinear optical response in comparison with a –N=N– bridge.     

Characterization of the intramolecular transfer state of marine carotenoid fucoxanthin by femtosecond pump–probe spectroscopy

Fucoxanthin, containing a carbonyl group in conjugation with its polyene backbone, is a naturally occurring pigment in marine organisms and is essential to the photosynthetic light-harvesting function in brown alga and diatom. Fucoxanthin exhibits optical characteristics attributed to an intramolecular charge transfer (ICT) state that arises in polar environments due to the presence of the carbonyl group. In this study, we report the spectroscopic properties of fucoxanthin in methanol (polar and protic solvent) observed by femtosecond pump–probe measurements in the near-infrared region, where transient absorption associated with the optically allowed S₂ (1¹B _u ⁺ ) state and stimulated emission from the strongly coupled S₁/ICT state were observed following one-photon excitation to the S₂ state. The results showed that the amplitude of the stimulated emission of the S₁/ICT state increased with decreasing excitation energy, demonstrating that the fucoxanthin form associated with the lower energy of the steady-state absorption exhibits stronger ICT character.     

Fluorimetric studies on the binding of 4-(dimethylamino)cinnamic acid with micelles and bovine serum albumin

The constrained photophysics of intramolecular charge transfer (ICT) probe 4-(dimethylamino)cinnamic acid (DMACA) was studied in different surfactant systems as well as in presence of model water soluble protein bovine serum albumin (BSA). Binding of the probe in ionic micelles like sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) causes an increase in ICT fluorescence intensity, whereas, in non-ionic TritonX-100 (TX-100) the intensity decreases with a concomitant increase in emission from locally excited (LE) state. The observations were explained in terms of the different binding affinity, location of the probe and also the nature of specific hydrogen bonding interaction in the excited state nonradiative relaxation process of DMACA. The ICT fluorescence emission yield decreases in BSA due to the locking in of the probe buried in the hydrophobic pocket of the protein structure. SDS induced uncoiling of protein and massive cooperative binding between BSA and SDS is manifested by the release of probe molecules in relatively free aqueous environment.     

Photophysical processes of an intramolecular charge transfer fluorescent dye with carbazole units

A carbazole‐based compound with intramolecular charge transfer (ICT) characteristics, 3,6‐bis‐((N‐ethylcarbazole‐3‐)‐propene‐1‐keto)‐N‐ethylcarbazole (BCzPCz) was synthesized by N‐alkylation, acetylation and aldol condensation. BCzPCz was further confirmed by IR and ¹ H NMR. The central N‐ethylcarbazole was connected with two N‐ethylcarbazole units through the propenone group in BCzPCz. N‐ethylcarbazole and carbonyl groups were electron donors (D) and acceptors (A), respectively. The UV–vis absorption and fluorescence characteristics of BCzPCz were also investigated in different solvents. Solvatochromism was attributed to ICT complex formation in singlet excited state. Magnitude of the change in the dipole moment was 24.78 D according to Lippert‐Mataga equation. Fluorescence of BCzPCz was significantly affected by pH and was quenched in acidic medium. Fluorescence quantum yield of BCzPCz was 0.516 in ethanol. Experimental results showed its potential use as a fluorescence probe and as two‐photon absorption material. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural modelling of optical and electrochemical properties of 4-aminodiphenylamines - optoelectronic studies on a polyaniline repeating unit.

Amino-diphenylanilines and their planarized and twisted model compounds have been investigated by steady state and time-resolved absorption and emission, as well as by spectroelectrochemistry. These polyaniline model compounds show that the observation of excited states with full charge separation is linked to molecular twisting where the diaminobenzene is the donor and the phenyl group the acceptor. The observable charge transfer fluorescence shows the characteristic features of twisted intramolecular charge transfer (TICT) excited states, i.e. forbidden emissive properties and strong solvatochromic red shift. The transient absorption spectrum of the TICT state matches the ground state absorption spectrum of the electrochemically produced radical cation of the molecule. This is the first example where excited-state properties of the neutral and ground state properties of the radical cation are directly linked.