Photoinduced Intramolecular Charge Transfer Research Articles

Probing the interaction of a globular protein with a small fluorescent probe in the presence of silver nanoparticles: spectroscopic characterization of its domain specific association and dissociation

Photoinduced intramolecular charge transfer produces a polar excited state in the small fluorescent probe 1-anilino-8-naphthalene-sulfonate (ANS) rendering the resulting emission sensitive to the medium polarity. The binding of ANS to proteins has been well characterized in the literature. The probe binds to the less polar hydrophobic, restricted proteinous environment of bovine serum albumin (BSA) resulting in a blue shift of the emission maximum with an increase in emission intensity. However, upon addition of silver nanoparticles to the probe–BSA system, there was found to be an interaction between the nanoparticle and protein resulting in a weakening of the probe–BSA binding and an associated conformational change of the protein in order to accommodate the silver nanoparticle. This has been hitherto undeciphered. Studies using site markers of flufenamic acid and phenylbutazone coupled with other spectroscopic techniques, REES, time resolved studies, FTIR, and SEM imaging studies have shed light on the interaction of silver nanoparticles with the probe bound protein. ANS was observed to move from its binding site in the protein to accommodate the silver nanoparticle surface which involved conformational changes of the protein near the probe binding site. The association of the probe to the protein and its dissociation from the sites on introduction of silver nanoparticles has thus been investigated using spectroscopic studies.

Fluorine–thiophene-substituted organic dyes for dye sensitized solar cells

Novel triphenylamine (TPA)-based organic dyes were synthesized and assessed for their performance in dye-sensitized solar cells (DSSCs). In the dyes considered the TPA group and the cyanoacetic acid have the role of electron-donor and -acceptor, respectively, whereas a thienyl–fluoro-phenyl-substituted was introduced as π-linker to improve the dye performance in DSSCs. Experimental characterizations empasize that the presence of electron withdrawing substituents in the linker close to the electron-acceptor moiety leads to a more efficient intramolecular photoinduced charge transfer. In fact, photovoltaic experiments reveal that the DSSCs based on the thienyl–o-fluoro-phenyl substituted dyes yield a better solar-energy-to-electricity conversion efficiency.

THEORETICAL STUDY OF PHOTO-PHYSICAL PROCESSES IN 2-ARYL SUBSTITUTED INDOLES

Ab initio method is used to figure out the structures and photo-physical processes of 2-aryl substituted indoles, 2-phenylindole (2PI), 2-naphthylindole (2NI), and 2-anthracenylindole (2AI), synthesized experimentally with strong fluorescence. The results show that the photoabsorption and fluorescence measured experimentally correspond to the monovalent anions deprotonated in the indole NH end, other than the neutral compounds. During the photochemical reaction, the angles between the planes of the indole and 2-aryl substituted moieties keep unchanged, but the photo-induced intramolecular charge transfer can immediately occur in the anions. Furthermore, the photo-physical processes after photoexcitation are analyzed by using a linear reaction coordinate.

New fluorescent chemosensor based on quinoline and coumarine for Cu2+

A new fluorescent chemosensor based a coumarin fluorophore and an 8-hydroxyquinoline ionophore was synthesized and characterized by 1H NMR, IR and ESI-HRMS. Its selectivity toward metal ions in acetonitrile (1% water, v/v) was investigated by UV absorption and fluorescence spectroscopy. The results showed that this chemosensor exhibited good selectivity toward Cu2+ over other metal ions by enhancement of fluorescence intensity more than 13-fold, and the fluorescent color change could be observed by naked eyes under the light of 365nm. The absorption spectra of the sensor (L) toward the concentration of Cu2+ indicated the formation of L–Cu2+ complex, and the complex was 1:1 ratio according to Job plot experiment. Both of photo-induced electron transfer (PET) and intramolecular charge transfer (ICT) mechanisms were considered to be operational for fluorescence enhancement and spectral shift. The detection limit of Cu2+ was determined to be 1.16μM, a satisfying level to detect Cu2+ in micromolar scale.

Solvent effects on the absorption and fluorescence spectra of rhaponticin: Experimental and theoretical studies

Rhaponticin (RH) possesses a variety of pharmacological activities including potent antitumor, antitumor-promoting, antithrombotic, antioxidant and vasorelaxant effects. The fundamental photophysics of RH is not well understood. In this work, solvent effect on the photoluminescence behavior of RH was studied by fluorescence and absorption spectra. The bathchromic shift was observed in absorption and fluorescence spectra with the increase of solvents polarity, which implied that transition involved was π→π*. A quantitative estimation of the contribution from different solvatochromic parameters, like normalized transition energy value (ETN), was made using the linear stokes shift (Δν) relationship based on the Lippert–Suppan equation. The ground state and excited state dipole moments were calculated by quantum–mechanical second-order perturbation method as a function of the dielectric constant (ε) and refractive index (n). The result was found to be 2.23 and 3.67D in ground state and excited state respectively. The density functional theory (DFT) was used to obtain the most stable structure, electronic excitation energy, dipole moments and charge distribution. The analysis revealed that the RH exhibited strong photoinduced intramolecular charge transfer (ICT), and the intermolecular hydrogen bonding ability of the solvent was the most important parameter to characterize the photophysics behavior of RH. The hydrogen bonding effect occurred at the localized electron-acceptor oxygen at the glycoside bond. The experimental and theoretical results would help us better understand the photophysical properties of RH.

Large Stokes Shift Induced by Intramolcular Charge Transfer in N,O-Chelated Naphthyridine–BF2 Complexes

Novel N,O-chelated naphthyridine-BF(2) complexes with push-pull structures have been synthesized and characterized. Spectral investigations on these complexes reveal that photoinduced intramolecular charge transfer occurs and results in a large Stokes shift, which is further supported by density functional theory based theoretical calculations.

Direct observation and control of ultrafast photoinduced twisted intramolecular charge transfer (TICT) in triphenyl-methane dyes.

Femtosecond time-resolved infrared spectroscopy was employed to study intramolecular charge transfer in triphenylmethane dyes, including malachite green (MG), malachite green carbinol base (MGCB), and leucomalachite green (LMG). A local excited state (LE) and a twisted intramolecular charge-transfer (TICT) state have been observed directly in MG. Furthermore, solvent-controlled TICT measurements in a series of linear alcohols indicate that the transition time (4-11 ps) from LE to TICT is strongly dependent on alcohol viscosity, which is due to rotational hindrance of dimethylaniline in high-viscosity solvents. For LMG, no TICT is observed due to steric hindrance caused by the sp(3)-hybridized central carbon atom. However, for MGCB, TICT is rescued by the addition of the electron-donating hydroxyl group to the bridge. These results for MG and its analogues provide new insight regarding the dynamics and mechanism of twisted intramolecular charge transfer (TICT) in triphenylmethane dyes.

Resolving Photo-Induced Twisted Intramolecular Charge Transfer with Vibrational Anisotropy and TDDFT

The interplay between reaction environment and photochemical outcome has wide ranging implications for designing and directing light driven chemical conversions. We present a detailed mechanistic description of photoisomerization in julolidine malononitrile (JDMN) as the first step to characterizing this interplay between reaction pathways and reaction environment. We have used polarization resolved UV pump-mid-IR probe spectroscopy and time dependent DFT calculations to investigate the dynamics of charge transfer induced intramolecular rotation in JDMN. We have probed the mechanism and dynamics of photoisomerization with the symmetric and antisymmetric CN-stretch of the malononitrile group. These measurements show the S1 electronic excited state relaxes with a 12.3 ps time constant by isomerizing around both the C-C single and C-C double bond of the malononitrile group with a branching ratio of 1:5. Isomerization around the single bond leads to the formation of a metastable twisted excited state, while isomerization around the double bond leads to excited state quenching via a conical intersection between the S1 and S0 electronic states. We have characterized the electronic and nuclear structure of the long-lived excited state with pump-probe anisotropy measurements and time dependent DFT calculations using the CAM-B3LYP functional and the 6-31G(d,p) basis set. These calculations further confirm that isomerization around the malononitrile single bond forms a twisted intermolecular charge transfer excited state.

Intramolecular charge transfer, solvatochromism and hyperpolarizability of compounds bearing ethenylene or ethynylene bridges

Emission or both absorption/emission solvatochromic effects induced by intramolecular charge transfer (ICT) have been used to determine the excited-state dipole moments of the locally excited μe(LE) and the polar-solvent induced μe(ICT) states and the first hyperpolarizability parameter βCT of six asymmetric distyrylbenzene analogues, two 9,10-(diarylethynyl)anthracene derivatives and two 2,7-(diarylethynyl)fluorene analogues. The change in dipole moment under excitation (Δμeg) was also evaluated by theoretical calculations and compared with experimental data. All compounds display more or less remarkable positive solvatochromism which leads to relatively high hyperpolarizabilities and makes them interesting candidates for applications associated with photoinduced ICT. The structural effect on the transfer is also briefly discussed and related to the relaxation properties (previously investigated) of these compounds.

More than Restriction of Twisted Intramolecular Charge Transfer: Three-Dimensional Expanded #-Shaped Cross-Molecular Packing for Emission Enhancement in Aggregates

This study developed a new solid-state, highly emissive phenylbenzoxazole-based organic compound, N-(4-(benzo[d]oxazol-2-yl)phenyl)-4-tert-butylbenzamide, which exhibited a distinct aggregation-induced enhanced emission. The solid fluorescence efficiency of the newly developed compound was 50.3%, whereas that in THF solution was only 0.22%. The single-crystal analyses revealed that a specific three-dimensional #-shaped cross stacking between molecules was observed in the solid/aggregated state, driven by specific C–H···π interaction and various hydrogen bonds. The expansion of the cross-dipole stacking into the three-dimensional network was believed to be the dominant factor for the emission enhancement in the solids/aggregates, with respect to the assistant effect of the photoinduced twisted intramolecular charge transfer restriction.

Intramolecular charge transfer reaction in 3-amino-4-methyl benzoic acid: a study from condensed phase to jet-cooled molecular beam

The present work reports the spectral signatures of intramolecular charge transfer (ICT) reaction in 3-amino-4-methyl benzoic acid (AMBA). AMBA exhibits large red-shifted emission in various solvents with the emission maxima being highly sensitive to the solvent polarity. Solvatochromic measurements of AMBA on the basis of its steady state absorption and emission properties appears to confirm the operation of photoinduced ICT reaction in it with the ICT state being characterized by a higher excited state dipole moment than that of the ground state. Laser induced fluorescence excitation spectra and dispersed fluorescence spectra of AMBA and its solvated clusters with polar solvent methanol in the cooled jet yield quite distinct evidence for the occurrence of ICT reaction in polar solvent.

Domain Specific Association of Small Fluorescent Probe trans-3-(4-Monomethylaminophenyl)-Acrylonitrile (MMAPA) with Bovine Serum Albumin (BSA) and Its Dissociation from Protein Binding Sites by Ag Nanoparticles: Spectroscopic and Molecular Docking Study

Photoinduced intramolecular charge transfer produced a polar excited state in trans-3-(4-monomethylaminophenyl)acrylonitrile (MMAPA), rendering the resulting emission sensitive to the medium polarity. Strong binding interaction of silver nanoparticles with the probe was observed, causing fluorescence quenching through the static quenching process. The probe MMAPA was found to bind to the less polar hydrophobic, restricted proteinous environment of bovine serum albumin (BSA) resulting in the blue shift of the emission maximum with an increase in emission intensity and fluorescence anisotropy. Studies using site markers of flufenamic acid and phenylbutazone coupled with molecular docking results predicted that the binding site of the probe is in between subdomains IIIA and IB of BSA and is different from the conventional Sudlow sites. The denaturation of the probe-bound BSA by urea or heat released the probe from this proteinous environment to water marked by exactly reverse spectral changes. On the interaction of silver nanoparticles with the probe bound protein, the probe was observed to move from its binding site in the protein to the Ag(0) nanoparticle surface involving conformational changes of the protein near the probe binding site.

Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution

Abstract The photophysics of a red-emitting push–pull triarylamine compound (fvin) comprising a triphenylamino electron donor core and a dicyanovinylene electron acceptor group is investigated by steady-state absorption and emission and femtosecond time-resolved absorption spectroscopy in room-temperature n -hexane, toluene, ethanol, and acetonitrile solvents. Fvin is strongly fluorescent in apolar solvents upon excitation of the S 0 → S 1 intramolecular charge transfer (ICT) transition, but hardly emissive in polar solvents. Time-resolved spectra reveal a strong dependence of the excited-state dynamics on both the solvent polarity and viscosity. Unlike n -hexane solutions where the fluorescent ICT excited state remains weakly solvated and keeps a structure close to that of the Franck–Condon level, significant stabilization of the ICT state operates in toluene, leading to a relaxed fluorescent ICT′ form with stronger charge transfer character. This was featured by a notable dynamic Stokes shift of the stimulated emission (SE) band occurring in 15 ps. However the similarly high fluorescence quantum yield and lifetime in n -hexane and toluene excludes any large structural distortion on going from ICT to ICT′. In acetonitrile and ethanol, the SE shift dynamics is enhanced and clearly biphasic. The first step can be ascribed to an ICT → ICT′ conformational relaxation as in toluene. The second step, accompanied by a drastic drop of the SE intensity and a strong evolution of the excited-state absorption spectrum (time-constant 1.5 ps in acetonitrile, 9 ps in ethanol), corresponds to the appearance of a new ICT″ state with enhanced charge localization and separation, most probably associated with a rotation of the phenyldicyanovinylene group. This large-amplitude distortion is followed by fast internal conversion to the ground state (9 ps in acetonitrile, 21 ps in ethanol), accounting for the almost nonemissive character of fvin in polar solvents.

π-Conjugated molecular heterojunctions with multi[60]fullerene: photophysical, electrochemical, and photovoltaic properties

A series of well-defined D–π-B–A C60 adducts containing three chromophores and their corresponding precursors were facilely developed as organic photovoltaic materials. The photophysical and electrochemical properties indicated that effective photo-induced intramolecular and/or intermolecular energy transfer and charge transfer were formed from the donor chromophores to the C60 moiety in thin films. Solution processed molecular heterojunction solar cells using 12 as active layer were fabricated with 2.0% monochromatic efficiency. Our results showed that further exploration might lead the way toward high-efficient solution-processed single molecular solar cell materials based on conjugated oligomer covalent to fullerene units.

The impact of bromine substitution on the photophysical properties of a homodinuclear Ru–tpphz–Ru complex

Ruthenium–polypyridine complexes play an important role as photosensitizers in supramolecular photocatalysis. Using multiple Ru-centers within a single supramolecular catalyst might be a promising path for improving its efficiency. The connection of several chromophores may, however, lead to direct interaction amongst individual photoactive centers, which is at the core of the work at hand. The work focuses on the photophysics of [(tbbpy) 2Ru(3,16-Br 2-tpphz)Ru(tbbpy) 2](PF 6) 4 ( 1, tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2″′,3″′-j]phenazine, tbbpy = 4,4′-di- tert.-butyl-2,2′-bipyridine) and aims at detailing the impact of the bromine substituents on bridging ligand photoinduced intramolecular charge-transfer dynamics. It is shown that the introduction of the bromine reduces the driving force for intra-ligand charge-transfer steps and impacts exciton–exciton annihilation at high pump intensities.

Photoinduced intramolecular charge transfer process of betaine pyridinium: A theoretical spectroscopic study

Using Time-Dependent Density Functional Theory and taking into account bulk solvent effects, we investigate the absorption and emission spectra of a betaine pyridinium molecule, the 2-(1-pyridinio) benzimidazolate (SBPa). This molecule exhibits strong photoinduced intramolecular charge transfer (ICT). We have identified two different electronic states involved, respectively, in the strong bathochromic ICT absorption band (S 2) and in the moderate emission band (S 1). The ICT process is analyzed in terms of charge distribution and dipole moment evolutions upon photoexcitation. These results are compared with steady-state spectroscopic measurements.

Theoretical study of charge transfer mechanism in fullerene-phenyl-phenothiazine compound: A real-space analysis

Quantum chemistry methods as well as two-dimensional (2D) and three-dimensional (3D) real-space analysis have been conducted to study the photo-induced intramolecular charge-transfer (ICT) and excited state properties of fullerene-phenylphenothiazine, which has recently been developed for solar cells. Firstly, we obtained the energy levels and spatial distributions of HOMO/LUMO, energy gap ( Δ E H – L ) and excitation energies on the basis of quantum chemistry study. Secondly, two-dimensional (2D) and three-dimensional (3D) real-space analysis were used to visualize the CT process and to reveal the nature of the excited states. In the above analyses, the 2D real-space analysis of the transition density matrix provided information about the electron-hole coherence, and the 3D real-space analysis of charge difference density enabled the visualization of the orientation and result of the ICT. The results of real-space analysis directly indicate that some states are ICT states, and others belong to locally excited states. Moreover, according to the generalized Mulliken Hush theory, we calculated the electronic coupling matrix elements and predict that electron transfer for some ICT states more easily takes place than that for some locally excited states.

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.

Highly Twisted Triarylamines for Photoinduced Intramolecular Charge Transfer

9-(N,N-Dianisylamino)anthracene (9DAAA), 9-(N,N-dianisylamino)dinaphth([1,2-a:2'-1'-j]-anthracene (9DAAH), and 9,10-bis(N,N-dianisylamino)anthracene (910BAA) were synthesized as highly twisted triarylamines with potential for photoexcited internal charge transfer. Crystallography of 9DAAA shows its dianisylamino group to be twisted nearly perpendicular to its anthracene unit, similar to a report for 910BAA. The solution fluorescence spectra show strong bathochromic shifts for each of the three molecular systems with strongly decreased quantum efficiency in higher polarity solvents. Solution-phase (ensemble) time-resolved photoluminescence measurements show up to 4-fold decreases in fluorescence lifetime in acetonitrile compared to hexane. The combined results are consistent with photoinduced, transient intramolecular charge-transfer from the bis-anisylamine unit to the polycyclic aromatic unit. Computational modeling is in accord with intramolecular transfer of electron density from the bis-anisylamino unit to the anthracene, based on in comparisons of HOMO and LUMO.

Photo-induced intramolecular charge transfer from antenna to anchor groups in phenoxazine dyes

Time-resolved transient absorption spectroscopy in the visible region and density functional theory methods are used to investigate the excited-state dynamics of two metal-free organic sensitizers used in dye-sensitized solar cells (DSSCs). The triphenylamine (TPA) group in TH305 acts as an energy antenna to absorb light at approximately 381 nm. The energy absorbed by TPA can be efficiently used by DSSCs through intramolecular charge transfers from the TPA group to the cyanoacrylic acid group.