trans/cis‐Isomerization of Fluorene‐Bridged Azo Chromophore with Significant Two‐Photon Absorbability at Near‐Infrared Wavelength

Azo‐containing materials have been proven to possess second‐order nonlinear optical (NLO) properties, but their third‐order NLO properties, which involves two‐photon absorption (2PA), has rarely been reported. In this study, we demonstrate a significant 2PA behavior of the novel azo chromophore incorporated with bilateral diphenylaminofluorenes (DPAFs) as a π framework. The electron‐donating DPAF moieties cause a redshifted π–π* absorption band centered at 470 nm, thus allowing efficient blue‐light‐induced trans‐to‐cis photoisomerization with a rate constant of 2.04×10^?1 min^?1 at the photostationary state (PSS). The open‐aperture Z‐scan technique that adopted a femtosecond (fs) pulse laser as excitation source shows an appreciably higher 2PA cross‐section for the fluorene‐derived azo chromophore than that for common azobenzene dyes at near‐infrared wavelength (λ_ex=800 nm). Furthermore, the fs 2PA response is quite uniform regardless of the molecular geometry. On the basis of the computational modeling, the intramolecular charge‐transfer (ICT) process from peripheral diphenylamines to the central azo group through a fluorene π bridge is crucial to this remarkable 2PA behavior.     

Thermodynamics and Conformations in the Formation of Excited States and Their Interconversions for Twisted Donor‐Substituted Tridurylboranes

We synthesized a series of donor‐substituted tridurylboranes containing different types and number of chromophores including 1‐pyrene (PB1–3), 3‐carbazole (CBC1–3), or substituted p‐carbazol‐N‐phenyl (CBN3a–c) as various donor–acceptor (D–A) molecules. The photophysical and electrochemical properties of these twisted D–A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge‐separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge‐transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited‐state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited‐state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes.     

Versatile Photophysical Properties of meso‐Aryl‐Substituted Subporphyrins: Dipolar and Octupolar Charge‐Transfer Interactions

Donor–acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso‐phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types of charge‐transfer interactions, that is, from center to periphery and periphery to center depending on the peripheral substitution, in which the subporphyrin moiety plays a dual role as both donor and acceptor. Based on the solvent‐polarity‐dependent photophysical properties, we have shown that the fluorescence emission of para isomers originates from the solvatochromic, dipolar, symmetry‐broken, and relaxed excited states, whereas the non‐solvatochromic fluorescence of meta isomers is of the octupolar type with false symmetry breaking. The restricted meso‐(4‐aminophenyl) rotation at low temperature prevents the intramolecular charge‐transfer (ICT)‐forming process. The two‐photon absorption (TPA) cross‐section values were determined by photoexcitation at 800 nm in nonpolar toluene and polar acetonitrile solvents to see the effect of ICT on the TPA processes. The large enhancement in the TPA cross‐section value of approximately 3200 GM (1 GM=10^?50 cm⁴ s photon^?1) with donor–acceptor substitution has been attributed to the octupolar effect and ICT interactions. A correlation was found between the electron‐donating/‐withdrawing abilities of the peripheral groups and the TPA cross‐section values, that is, p‐aminophenyl>m‐aminophenyl>nitrophenyl. The increased stability of octupolar ICT interactions in highly polar solvents enhances the TPA cross‐section value by a factor of approximately 2 and 4, respectively, for p‐amino‐ and m‐nitrophenyl‐substituted subporphyrins. On the other hand, the stabilization of the symmetry‐broken, dipolar ICT state gives rise to a negligible impact on the TPA processes.     

Aromatic/proaromatic donors in 2-dicyanomethylenethiazole merocyanines: from neutral to strongly zwitterionic nonlinear optical chromophores

Push–pull compounds, in which a proaromatic electron donor is conjugated to a 2‐dicyanomethylenethiazole acceptor, have been prepared, and their properties compared to those of model compounds featuring an aromatic donor. A combined experimental (X‐ray diffraction, ¹H NMR, IR, Raman, UV/Vis, nonlinear optical (NLO) measurements) and theoretical study reveals that structural and solvent effects determine the ground‐state polarisation of these merocyanines: whereas 4H‐pyran‐4‐ylidene‐ and 4‐pyridylidene‐containing compounds are zwitterionic and 1,3‐dithiol‐2‐ylidene derivatives are close to the cyanine limit, anilino‐derived merocyanines are essentially neutral. This very large range of intramolecular charge transfer (ICT) gives rise to efficient second‐order NLO chromophores with μβ values ranging from strongly negative to strongly positive. In particular, pyranylidene derivatives are unusual in that they show an increase in the degree of ICT on lengthening the π‐spacer, a feature that lies behind the very large negative μβ values they display. The linking of the formally quinoidal 2‐dicyanomethylenethiazole moiety to proaromatic donors seems a promising approach towards the optimisation of zwitterionic NLO chromophores.     

Tailoring the donor moieties in TPA-based organic dyes for efficient photovoltaic,optical and nonlinear optical response properties

The current study reports tailoring the electronic donor structures of organic dyes to modify their optical and nonlinear optical (NLO) response properties. Five (5) tri-phenyl amine (TPA) based Donor-π-Acceptor (D-π-A) organic dyes with the codes ICAA1 , ICAA2 , ICAA3 , ICAA4 , and ICAA5 were designed and investigated for their optical and NLO properties using quantum chemical methods. Optical and NLO properties of these dyes were studied by CAM-B3LYP method and 6-311G* basis set. The focus has been on the impact of adding secondary donors and shifting their substitutions at ortho (o), meta (m) and para (p) positions. Among all designed compounds, ICAA4 showed the highest amplitude of average third-order NLO polarizability <γ>, which is calculated to be 1316 × 10⁻³⁶ esu. Time-dependent Density Functional Theory (TD-DFT) method was used to determine how a change in the position of the donor affected the excitation energy (E_g) and NLO response properties. The findings showed that changing the position of the secondary donor results in a red shift among absorption spectra as well as the increase in their NLO responses. Complete process of intramolecular charge transfer (ICT) has been investigated in terms of different optical parameters such as frontier molecular orbitals (FMOs), molecular electrostatic potentials (MEPs), transition density matrix (TDMs), density of states (DOS), electron density difference (EDD), and natural bond orbital (NBO) analysis. Our calculations for study of ICT process indicate that p-position of methoxy group performs better among all other positions and even it has better NLO response properties than the compound with three collective methoxy groups. The calculated V_oc values of all designed molecules range from 1.09 to 1.30, all of them are positive while their ΔG_inject is found to be in the range of −0.87 to −1.79 eV indicating their decent potential for photovoltaic applications. The studied optical, NLO and photovoltaic parameters illustrated that ICAA1 to ICAA5 are appropriate molecules not only for NLO applications but also for efficient photovoltaic purposes.     

Photophysical Properties of Intramolecular Charge Transfer in a Tribranched Donor–π–Acceptor Chromophore

The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor–π–acceptor chromophore, triphenoxazine‐2,4,6‐triphenyl‐1,3,5‐triazine (tri‐PXZ‐TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady‐state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri‐PXZ‐TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum‐chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri‐PXZ‐TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited‐state dynamics of this push–pull molecule were strongly influenced by solvent polarity through the formation of a solvent‐stabilized ICT state. The excited‐state relaxation mechanism of tri‐PXZ‐TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri‐PXZ‐TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.     

吩噻嗪给体受体衍生物激发态的电荷转移

A series of N-bonded donor-acceptor derivatives of phenothiazine containing phenyl （PHPZ）, anisyl （ANPZ）, pyridyl （PYPZ）, naphthyl （NAPZ）, acetylphenyl （APPZ）, and cyanophenyl （CPPZ） as an electron acceptor have been synthesized. Their photophysical properties were investigated in solvents of different polarities by absorption and emission techniques. These studies clearly revealed the existence of an intramolecular charge transfer （ICT） excited state in the latter four compounds. The solvent dependent Stokes shift values were analyzed by the modified Lippert-Mataga equation to obtain the excited state dipole moment values. The large excited state dipole moment suggests that the full （or nearly full） electron transfer take place in the A-D systems. In the system of A-D phenothiazine derivatives, the transition dipole moments Mflu were determined mainly by direct interactions between the solvent-equilibrated fluorescence ^1CT state and ground state because of their lack of significant change with increase of the solvent polarity. The electron structure and molecular conformation of phenothiazine derivatives will be significantly changed with the increase of the electron affinity of the N-10 substituent.     

A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

The D‐π‐A type phosphonium salts in which electron acceptor (A=‐⁺PR₃) and donor (D=‐NPh₂) groups are linked by polarizable π‐conjugated spacers show intense fluorescence that is classically ascribed to excited‐state intramolecular charge transfer (ICT). Unexpectedly, salts with π=‐(C₆H₄)_n‐ and ‐(C₁₀H₆C₆H₄)‐ exhibit an unusual dual emission (F₁ and F₂ bands) in weakly polar or nonpolar solvents. Time‐resolved fluorescence studies show a successive temporal evolution from the F₁ to F₂ emission, which can be rationalized by an ICT‐driven counterion migration. Upon optically induced ICT, the counterions move from ‐⁺PR₃ to ‐NPh₂ and back in the ground state, thus achieving an ion‐transfer cycle. Increasing the solvent polarity makes the solvent stabilization dominant, and virtually stops the ion migration. Providing that either D or A has ionic character (by static ion‐pair stabilization), the ICT‐induced counterion migration should not be uncommon in weakly polar to nonpolar media, thereby providing a facile avenue for mimicking a photoinduced molecular machine‐like motion.     

Quadratic and cubic nonlinear optical properties of salts of diquat-based chromophores with diphenylamino substituents

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.     

基于对二烷基氨基苯甲酰苯胺分子内电荷转移的单糖比值法荧光受体

Two dual fluorescent receptors (1 and 2) for monosaccharides based on 4-dialky(alkyl=methyl and n-butyl) containing boronic acid group at the amido aniline were synthesized and their spectral properties were investigated. These receptors exhibited dual fluorescence with the long-wavelength band displaying strong solvent-polarity dependence, indicating the occurrence of the excited-state intramolecular charge transfer （ICT).With increasing pH value in aqueous solutions, the hybridization of the boron atom changed from sp^2 to sp^3, inducing a decrease in the total fluorescence quantum yield. The experimental results indicated that the anionic form of the boronate group acted as an electron donor and the benzanilide-like charge transfer was promoted upon hybridization change. In the presence of monosaccharides, the boronic acid in 1 and 2 changed from neutral to anionic form. The intensity of the locally excited (LE) state emission decreased in the presence of sugars while a slight increase in the intensity at the charge transfer (CT) emission occurred. Based on the change in the CT to LE intensity ratios of 1 and 2 due to sugar binding, ratiometric fluorescent assays for monosaccharide sensing were established.     

Excitation‐Wavelength‐Dependent,Ultrafast Photoinduced Electron Transfer in Bisferrocene/BF2‐Chelated‐Azadipyrromethene/Fullerene Tetrads

Donor–acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron‐transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor–acceptor dyads, revealing excitation‐wavelength‐dependent photochemical properties demands multimodular, photosynthetic‐reaction‐center model compounds. Here, we successfully demonstrate donor– acceptor excitation‐wavelength‐dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF₂‐chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured “closely” and “distantly” positioned donor–acceptor systems. The near‐IR‐emitting BF₂‐chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation‐wavelength‐dependent, photoinduced, electron‐transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc⁺–ADP–C₆₀^.? charge‐separated state upon C₆₀ excitation, and Fc⁺–ADP^.?–C₆₀ formation upon ADP excitation is demonstrated.     

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.     

Large Two‐Photon Absorption of Terpyridine‐Based Quadrupolar Derivatives: Towards their Applications in Optical Limiting and Biological Imaging

Developing organic chromophores with large two‐photon absorption (TPA) in both organic solvents and aqueous media is crucial owing to their applications in solid‐state photonic devices and biological imaging. Herein, a series of novel terpyridine‐based quadrupolar derivatives have been synthesized. The influences of electron‐donating group, type of conjugated bridge, as well as solvent polarity on the molecular TPA properties have been investigated in detail. In contrast to the case in organic solvents, bis(thienyl)‐benzothiadiazole as a rigid conjugated bridge will completely quench molecular two‐photon emission in aqueous media. However, the combination of alkylcarbazole as the donor and bis(styryl)benzene as a conjugation bridge can enlarge molecular TPA cross‐sections in both organic solvent and aqueous media. The reasonable two‐photon emission brightness for the organic nanoparticles of chromophores 3 – 5 in the aqueous media, prepared by the reprecipitation method, enables them to be used as probes for in vivo biological imaging.     

BODIPY‐Bridged Push–Pull Chromophores for Nonlinear Optical Applications

A set of linear and dissymmetric BODIPY‐bridged push–pull dyes are synthesized. The electron‐donating substituents are anisole and dialkylanilino groups. The strongly electron‐accepting moiety, a 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) group, is obtained by insertion of an electron‐rich ethyne into tetracyanoethylene. A nonlinear push–pull system is developed with a donor at the 5‐position of the BODIPY core and the acceptor at the 2‐position. All dyes are fully characterized and their electrochemical, linear and nonlinear optical properties are discussed. The linear optical properties of dialkylamino compounds show strong solvatochromic behavior and undergo drastic changes upon protonation. The strong push–pull systems are non‐fluorescent and the TCBD‐BODIPY dyes show diverse photochemistry and electrochemistry, with several reversible reduction waves for the tetracyanobutadiene moiety. The hyperpolarizability μβ of selected compounds is evaluated using the electric‐field‐induced second‐harmonic generation technique. Two of the TCBD‐BODIPY dyes show particularly high μβ (1.907 μm) values of 2050×10^?48 and 5900×10^?48 esu. In addition, one of these dyes shows a high NLO contrast upon protonation–deprotonation of the donor residue.     

Extensive Reduction in Back Electron Transfer in Twisted Intramolecular Charge‐Transfer (TICT) Coumarin‐Dye‐Sensitized TiO2 Nanoparticles/Film: A Femtosecond Transient Absorption Study

We report the synthesis, characterization, and optical and electrochemical properties of two structurally similar coumarin dyes ( C1 and C2 ). These dyes have been deployed as sensitizers in TiO₂ nanoparticles and thin films, and the effect of molecular structure on interfacial electron‐transfer dynamics has been studied. Steady‐state optical absorption, emission, and time‐resolved emission studies on both C1 and C2 , varying the polarity of the solvent and the solution pH, suggest that both photoexcited dyes exist in a locally excited (LE) state in solvents of low polarity. In highly polar solvents, however, C1 exists in an intramolecular charge‐transfer (ICT) state, whereas C2 exists in both ICT and twisted intramolecular charge‐transfer (TICT) states, their populations depending on the degree of polarity of the solvent and the pH of the solution. We have employed femtosecond transient absorption spectroscopy to monitor the charge‐transfer dynamics in C1 ‐ and C2 ‐sensitized TiO₂ nanoparticles and thin films. Electron injection has been confirmed by direct detection of electrons in the conduction band of TiO₂ nanoparticles and of radical cations of the dyes in the visible and near‐IR regions of the transient absorption spectra. Electron injection in both the C1 /TiO₂ and C2 /TiO₂ systems has been found to be pulse‐width limited (<100 fs); however, back‐electron‐transfer (BET) dynamics has been found to be slower in the C2 /TiO₂ system than in the C1 /TiO₂ system. The involvement of TICT states in C2 is solely responsible for the higher electron injection yield as well as the slower BET process compared to those in the C1 /TiO₂ system. Further pH‐dependent experiments on C1 ‐ and C2 ‐sensitized TiO₂ thin films have corroborated the participation of the TICT state in the slower BET process in the C2 /TiO₂ system.     

Donor/Acceptor Effects on the Linear and Nonlinear Optical Properties of Geminal Diethynylethenes (g‐DEEs)

A series of geminal diethynylethenes (g‐DEEs) with electron‐donating and/or electron‐accepting (D/A) groups were synthesized via a Pd‐catalyzed cross‐coupling sequence. The UV/VIS spectra for donor–acceptor (D–A) functionalized g‐DEEs 5, 8 , and 11 show distinctive absorption trends attributable to intramolecular charge‐transfer (ICT). The bond‐length‐alternation (BLA) index for the cross‐conjugated enediyne framework varies slightly with different terminal substituents as determined by density‐functional theory (DFT) calculations and single‐crystal X‐ray analysis. Ultrafast third‐order optical nonlinearities for the g‐DEEs were measured by the differential optical Kerr effect (DOKE) technique and show that terminal donor–acceptor substitution of g‐DEEs enhances molecular second hyperpolarizabilities (γ) in comparison to donor or acceptor g‐DEEs. A small increase in the two‐photon‐absorption cross‐section (σ⁽²⁾) is observed in the series 9 – 11 as a result of increased functionalization. The effects of donor/acceptor substitution on electron delocalization along the cross‐conjugated enediyne structure are evaluated on the basis of natural‐bond‐orbital (NBO) analysis. Solid‐state structures of the four derivatives 3b, 4b, 7 and 8 were characterized by single‐crystal X‐ray structural analysis and show an asymmetric unit cell for one derivative, D–A g‐DEE 8 .     

Luminescent Quadrupolar Borazine Oligomers: Synthesis,Photophysics, and Two‐Photon Absorption Properties

A set of monodisperse bent donor–acceptor–donor‐type conjugated borazine oligomers, BnNn+1 (n=1–4), incorporating electron‐rich triarylamine donor and electron‐deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon–boron and tin–boron exchange reactions. The effect of chain elongation on the electrochemical, one‐ and two‐photon properties and excited‐state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl‐centered acceptor sites results in emissions with high quantum yields (Φ_fl>0.5) in the range of 400–500 nm. Solvatochromic effects lead to solvent shifts as large as ～70 nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two‐photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410 GM (n=4), and broadband excited‐state absorption (ESA) attributed to long‐lived singlet–singlet and radical cation/anion absorption. The excited‐state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6‐31G*) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.     

A Supramolecular Tetrad Featuring Covalently Linked Ferrocene–Zinc Porphyrin–BODIPY Coordinated to Fullerene: A Charge Stabilizing,Photosynthetic Antenna–Reaction Center Mimic

A novel photosynthetic‐antenna–reaction‐center model compound, comprised of BF₂‐chelated dipyrromethene (BODIPY) as an energy‐harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole‐shifting agent, and phenylimidazole‐functionalized fulleropyrrolidine (C₆₀Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy‐level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the ¹ZnP* (formed either as a result of singlet–singlet energy transfer or direct excitation) or ¹C₆₀* of the coordinated fullerene resulting into the formation of the Fc–(C₆₀ ^{. ?}Im:ZnP ^{. +})–BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc⁺–(C₆₀ ^{. +}Im:ZnP)–BODIPY radical ion pair that persisted for 7–15 μs, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC₆₀, as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long‐lived charge‐separated state with the help of the present “antenna–reaction‐center” model system has been successfully demonstrated.     

Density Functional Theory Investigation on the Second‐Order Nonlinear Optical Properties of Chlorobenzyl‐o‐Carborane Derivatives

The structures and second‐order nonlinear optical (NLO) properties of a series of chlorobenzyl‐o‐carboranes derivatives ( 1 – 12 ) containing different push‐pull groups have been studied by density functional theory (DFT) calculation. Our theoretical calculations show that the static first hyperpolarizability (β_tot) values gradually increase with increasing the π‐conjugation length and the strength of electron donor group. Especially, compound 12 exhibits the largest β_tot (62.404×10^?30 esu) by introducing tetrathiafulvalene (TTF), which is about 76 times larger than that of compound 1 containing aryl. This means that the appropriate structural modification can substantially increase the first hyperpolarizabilities of the studied compounds. For the sake of understanding the origin of these large NLO responses, the frontier molecular orbitals (FMOs), electron density difference maps (EDDMs), orbital energy and electronic transition energy of the studied compounds are analyzed. According to the two‐state model, the lower transition energy plays an important role in increasing the first hyperpolarizability values. This study may evoke possible ways to design preferable NLO materials.     

Vibrational and quantum-chemical study of nonlinear optical chromophores containing dithienothiophene as the electron relay

A series of nonlinear optical (NLO) donor-acceptor (D-A) chromophores containing a fused terthiophene, namely dithienothiophene (DTT), as the electron relay, the same donor group, and acceptors of various strengths, has been investigated by means of infrared and Raman spectroscopies, both in the solid state as well as in a variety of solvents, to evaluate the effectiveness of the intramolecular charge transfer from the electron-donor to the electron-acceptor end groups. The Raman spectral profiles of these NLO-phores measured from their dilute solutions have been found to be rather similar to those of the corresponding solids, and thus their intramolecular charge transfer (ICT) shows very little dependence on the solvent polarity. The experimental results obtained for the DTT-containing NLO-phore with a 4-(N,N-dibutylamino)styryl end group as the donor and a 2,2-dicyanoethen-1-yl end group as the acceptor differ from those previously obtained for two parent "push-pull" chromophores with the same D-A pair but built-up around either a bis(3,4-ethylenedioxythienyl) (BEDOT) or a bithienyl (BT) electron relay. Vibrational spectroscopy shows that DTT is significantly more efficient as an electron relay than BT (which has the same number of conjugated C=C bonds) or BEDOT (which can be viewed as a rigidified version of BT on account of noncovalent intramolecular interactions between heteroatoms of adjacent rings). Density functional theory (DFT) calculations have also been performed on these NLO-phores to assign their main electronic and vibrational features and to obtain information about their structures. An additional merit of these molecular materials was revealed by the infrared spectra of the DTT-based NLO-phores recorded at different temperatures. Thus, spectra recorded between -170 and 150 degrees C did not show any substantial change, indicating that the materials have a high thermal stability, which is of significance for their use as active components in optoelectronic devices.