Excited State Relaxation Processes Research Articles

Charge dynamics in the 2D/3D semiconductor heterostructure WSe2/GaAs

Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices, which integrate van der Waals 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe2 on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe2/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in GaAs and holes in WSe2. The type-II band alignment and fast photoexcited carrier dissociation shown here indicate that WSe2/GaAs is a promising junction for advanced photovoltaic and other optoelectronic devices, making use of the best properties of van der Waals (2D) and conventional (3D) semiconductors.

Effect of confinement on photoluminescence of MeLPPP/SBA-15 nanocomposite

Photoluminescence spectra and kinetics of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP)/SBA-15 mesoporous silica (SBA-15) nanocomposite have been studied and compared with photoluminescence properties of the MELPPP film and solution. Relaxation processes of excited singlet states in the closed geometry of the nanocomposite and in the diluted solution compared to the film based on the temperature dependence of photoluminescence (5–220 K) were studied. It was shown that the nanocomposite has twice the photoluminescence lifetime and three times weaker temperature dependence of photoluminescence intensity in comparison with the film. These peculiarities are due to a decrease in the number of polymer chains in a low-dimensional structure and a reduced role of exciton diffusion.

Sulfur oxidation states manipulate excited state electronic configurations for constructing highly efficient organic type I photosensitizers.

The multiple relaxation processes of excited states are a bridge connecting molecular structures and properties, providing enormous application potential for organic luminogens. However, a systematic understanding and manipulation of the relationship between the molecular structure, excited state relaxation processes, and properties of organic luminogens is still lacking. Herein, we report a strategy for manipulating excited state electronic configurations through the regulation of the sulfur oxidation state to construct eminent organic type I PSs. Combined with the experimental results and theoretical calculations, we have successfully revealed the decisive role of high sulfur oxidation states in promoting ROS production capacity. Impressively, a higher sulfur oxidation state can reduce the singlet-triplet energy gap (ΔE ST), increase the matching degree of transition configurations, promote the changes of the excited state electronic configurations, and boost the effective ISC proportion by enhancing intramolecular interactions. Therefore, DBTS2O with the highest sulfur oxidation state exhibits the strongest type I ROS generation ability. Additionally, guided by our strategy, a water-soluble PS (2OA) is designed and synthesized, showing selective imaging capacity and photokilling ability against Gram-positive bacteria. This study broadens the horizons for both molecular design and mechanism study of high-performance organic type I PSs.

Nonadiabatic molecular dynamics under adiabatic representation

<sec>In this paper, we develop a nonadiabatic molecular dynamics method based on Su-Schriffer-Heeger (SSH) Hamiltonian, and this method is widely used to study the photoexcitation dynamics and polaron motion in conjugated polymers. However, in this method, the time-dependent Schrödinger equation has so far been solved in a diabatic representation, also known as site representation. In order to provide a deeper insight into the nonadiabatic molecular dynamics method, we solve the time-dependent Schrödinger equation in an adiabatic representation. The new method can directly provide the important information about the strength of nonadiabatic couplings between different molecular orbitals in the excited-state relaxation process, helping us to predict the electron and energy transfer within or between polymer chains.</sec><sec>Solving the time-dependent Schrödinger equation in an adiabatic representation is much more complicated, it is mainly because we need to calculate the nonadiabatic couplings between different molecular orbitals. In this paper, the detailed formula derivation and actual calculation process of the nonadiabatic molecular dynamics method in an adiabatic representation are given. Using this new method, we simulate three photoexcitation processes in a conjugated polymer chain, HOMO→LUMO, HOMO–1→LUMO+1 and HOMO–2→LUMO+2. We analyze in detail the time evolutions of lattice configuration for these three photoexcitation processes, and compare these results with those obtained by diabatic representation (site representation) showing that the results obtained from these two representations are consistent with each other.</sec>

Synthesis technique and electron beam damage study of nanometer-thin single-crystalline thymine.

Samples suitable for electron diffraction studies must satisfy certain characteristics such as having a thickness in the range of 10-100 nm. We report, to our knowledge, the first successful synthesis technique of nanometer-thin sheets of single-crystalline thymine suitable for electron diffraction and spectroscopy studies. This development provides a well-defined system to explore issues related to UV photochemistry of DNA and high intrinsic stability essential to maintaining integrity of genetic information. The crystals are grown using the evaporation technique, and the nanometer-thin sheets are obtained via microtoming. The sample is characterized via x-ray diffraction and is subsequently studied using electron diffraction via a transmission electron microscope. Thymine is found to be more radiation resistant than similar molecular moieties (e.g., carbamazepine) by a factor of 5. This raises interesting questions about the role of the fast relaxation processes of electron scattering-induced excited states, extending the concept of radiation hardening beyond photoexcited states. The high stability of thymine in particular opens the door for further studies of these ultrafast relaxation processes giving rise to the high stability of DNA to UV radiation.

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains.

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal, and electronic properties. Despite recent advances in their synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds) as the simplest model of finite carbon wire and as a prototype of sp-carbon-based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H─(C≡C)6─H synthesized by laser ablation in liquid. With the support of computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200 fs time constant, followed by thermalization on the picosecond time scale and decay of the low-energy singlet state with hundreds of picoseconds time constant. We also show that the time scale of these processes does not depend on the end groups capping the sp-carbon chain. The understanding of the primary photoinduced events in polyynes is of key importance both for fundamental knowledge and for potential optoelectronic and light-harvesting applications of low-dimensional nanostructured carbon-based materials.

Understanding of Intramolecular Charge Transfer Dynamics of a Push–Pull Dimethylamino-phenylethynylphenyl-dicyanoimidazole by Steady-State and Ultrafast Spectroscopic Studies

Photophysical behaviors of D−π–A compound 2-{4-[4-(N,N-dimethylamino)phenylethynyl]phenyl-1-methyl-1H-imidazole-4,5-dicarbonitrile (DMAPPIDCN) were explored using steady-state absorption, fluorescence emission, and femtosecond time-resolved absorption and emission spectroscopic techniques at room temperature along with computational time-dependent density functional theory (TD-DFT) calculation. The spectroscopic studies were carried out in different solvents of varying polarities including binary solvent mixtures. The role of the solvent polarity, viscosity, and temperature on the relaxation mechanism of DMAPPIDCN is disclosed. The observed steady-state and time-resolved spectroscopic features were attributed to intramolecular charge transfer (ICT) dynamics. The ICT in DMAPPIDCN is rationalized to a sequential twisted motion of both N(CH3)2 and whole N,N-dimethylaminophenyl moieties around the molecular axis interconnecting the adjacent imidazolephenyl moiety leading to the TICT1 and TICT2 (σ*) states. The increased solvent polarity affected mostly the fluorescence emission spectra pointing to a significant increase in the excited state dipole moment. This result clearly reveals formation of the TICT2 (σ*) involving efficient charge transfer from the (N,N-dimethylamino)phenyl (DMAP) donor to the phenyl-1-methyl-1H-imidazole-dicarbonitrile (PIDCN) acceptor in the excited state in a polar environment. In the TICT2 (σ*) state, the planes of electron-withdrawing and electron-donating moieties are perpendicular with the angle (DMAP)C–C≡C being 141.1°. This nonplanar arrangement accounts for the observed large Stokes shift. Time-resolved fluorescence spectroscopic studies unveil the excited state relaxation processes confirming the increase in the nonradiative decay rate in aprotic medium with increase in the solvent dielectric constants. Femtosecond transient spectroscopic studies unambiguously confirmed the existence of well separated LE and TICT states and their ensuing kinetics in polar medium. In nonpolar solvents, DMAPPIDCN shows strong fluorescence which emits from the LE (ππ*) state, whereas in polar solvents, formation of two consecutive TICT states occurs from the LE (ππ*) in a sub picosecond to few picosecond time domain depending on polarity of the solvents and the non-radiative decay from the TICT states.

(Invited) Excited States and Reaction Mechanisms of Catalysts with Redox Active Ligands Investigated with X-Ray Spectroscopy

Metal dithiolenes and related molecular catalysts known for their redox-active ligands and their efficient and robust electro- and photo-catalysis of the hydrogen evolution reaction are investigated. To understand how the redox-active ligands of these catalysts influence their electronic excited states and participate in their chemical reactions, we need element-specific probes of charge and bonding at both metal and ligand atomic sites. X-ray absorption spectroscopy (XAS) affords us this opportunity and is used to study the electro- and photo-catalytic reaction mechanisms of these complexes, with a specific focus on the role of metal versus ligand atomic sites. Steady-state and time-resolved XAS studies at the Ni and S K-edges will be presented to identify the redox activity of metal vs. ligand sites on timescales spanning from the ultrafast excited state relaxation and photochemical activation processes to the steady-state creation of important intermediates in the multi-step hydrogen evolution reaction.

Electronic structure calculations and nonadiabatic dynamics simulations on reversible photochromic mechanism of spirooxazine

In this work, the combined electronic structure calculations (OM2/MRCI and MS-CASPT2//CASSCF) and Tully's fewest-switches surface hopping dynamics simulations (OM2/MRCI) have been conducted to investigate the photoinduced ring-opening, relaxation, and subsequent isomerization of spirooxazine. The OM2/MRCI and MS-CASPT2//CASSCF methods share very similar structures of S0 and S1 minima and S1/S0 conical intersections, and also provide comparable relative energies at the same time. The relevant reaction paths indicate that two bonded conical intersections and two unbonded ones will play very important roles in the nonadiabatic transitions. In addition, the S1 state nonadiabatic dynamics simulations reveal two effective and competitive S1 → S0 decay pathways via the bonded and unbonded conical intersections. And the C–O bond dissociation could take place either in the S1 state before hopping or in the S0 state after nonadiabatic transitions, which will finally generate three types of ground-state products, i.e. the closed-loop spirooxazine, a variety of merocyanine intermediates, and a small amount of hydrogen-transferred merocyanine isomers. Besides, the out-of-plane hydrogen (HOOP) oscillations could be an efficient and ultrafast inactivation channel for the excited-state relaxation processes. The currently simulated results provide important mechanistic insights into spirooxazine-based photochromic materials.

Dopamine Photochemical Behaviour under UV Irradiation.

To understand the photochemical behaviour of the polydopamine polymer in detail, one would also need to know the behaviour of its building blocks. The electronic absorption, as well as the fluorescence emission and excitation spectra of the dopamine were experimentally and theoretically investigated considering time-resolved fluorescence spectroscopy and first-principles quantum theory methods. The shape of the experimental absorption spectra obtained for different dopamine species with standard, zwitterionic, protonated, and deprotonated geometries was interpreted by considering the advanced equation-of-motion coupled-cluster theory of DLPNO-STEOM. Dynamical properties such as fluorescence lifetimes or quantum yield were also experimentally investigated and compared with theoretically predicted transition rates based on Fermi’s Golden Rule-like equation. The results show that the photochemical behaviour of dopamine is strongly dependent on the concentration of dopamine, whereas in the case of a high concentration, the zwitterionic form significantly affects the shape of the spectrum. On the other hand, the solvent pH is also a determining factor for the absorption, but especially for the fluorescence spectrum, where at lower pH (5.5), the protonated and, at higher pH (8.0), the deprotonated forms influence the shape of the spectra. Quantum yield measurements showed that, besides the radiative deactivation mechanism characterized by a relatively small QY value, non-radiative deactivation channels are very important in the relaxation process of the electronic excited states of different dopamine species.

String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy.

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers (PSs) for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in phase II human clinical trials for treating bladder cancer by PDT. The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper, we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge-transfer excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge-transfer excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited-state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these PSs.

GW/BSE nonadiabatic dynamics simulations on excited-state relaxation processes of zinc phthalocyanine-fullerene dyads: Roles of bridging chemical bonds

In this work, we employ electronic structure calculations and nonadiabatic dynamics simulations based on many-body Green function and Bethe-Salpeter equation (GW/BSE) methods to study excited-state properties of a zinc phthalocyanine-fullerene (ZnPc-C60) dyad with 6-6 and 5-6 configurations. In the former, the initially populated locally excited (LE) state of ZnPc is the lowest S1 state and thus, its subsequent charge separation is relatively slow. In contrast, in the latter, the S1 state is the LE state of C60 while the LE state of ZnPc is much higher in energy. There also exist several charge-transfer (CT) states between the LE states of ZnPc and C60. Thus, one can see apparent charge separation dynamics during excited-state relaxation dynamics from the LE state of ZnPc to that of C60. These points are verified in dynamics simulations. In the first 200 fs, there is a rapid excitation energy transfer from ZnPc to C60, followed by an ultrafast charge separation to form a CT intermediate state. This process is mainly driven by hole transfer from C60 to ZnPc. The present work demonstrates that different bonding patterns (i.e. 5-6 and 6-6) of the C−N linker can be used to tune excited-state properties and thereto optoelectronic properties of covalently bonded ZnPc-C60 dyads. Methodologically, it is proven that combined GW/BSE nonadiabatic dynamics method is a practical and reliable tool for exploring photoinduced dynamics of nonperiodic dyads, organometallic molecules, quantum dots, nanoclusters, etc.

Vibronic resonance is inadequately described by one-particle basis sets

Vibrational-electronic (vibronic) resonance and its possible role in energy and charge transfer have been experimentally and theoretically investigated in several photosynthetic proteins. Using a dimer modeled on a typical photosynthetic protein, we contrast the description of such excitons provided by an exact basis set description, as opposed to a basis set with reduced vibrational dimensionality. Using a reduced analytical description of the full Hamiltonian, we show that in the presence of vibrational excitation both on electronically excited as well as unexcited sites, constructive interference between such basis states causes vibronic coupling between excitons to become progressively stronger with increasing quanta of vibrational excitation. This effect leads to three distinguishing features of excitons coupled through a vibronic resonance, which are not captured in basis sets that restrict ground state vibrations: (1) the vibronic resonance criterion itself, (2) vibronically assisted perfect delocalization between sites even though purely electronic mixing between the sites is imperfect due to energetic disorder, and (3) the nuclear distortion accompanying vibronic excitons becoming increasingly larger for resonant vibronic coupling involving higher vibrational quanta. In terms of spectroscopically observable limitations of reduced basis set descriptions of vibronic resonance, several differences are seen in absorption and emission spectra but may be obscured on account of overwhelming line broadening. However, we show that several features such as vibronic exciton delocalization and vibrational distortions associated with electronic excitations, which ultimately dictate the excited state wavepacket motions and relaxation processes, are fundamentally not described by basis sets that restrict ground state vibrations.

Multireference Description of Nickel-Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis.

Multireference electronic structure calculations consistent with known experimental data have elucidated a novel mechanism for photo-triggered Ni(II)-C homolytic bond dissociation in Ni 2,2'-bipyridine (bpy) photoredox catalysts. Previously, a thermally assisted dissociation from the lowest energy triplet ligand field excited state was proposed and supported by density functional theory (DFT) calculations that reveal a barrier of ∼30 kcal mol-1. In contrast, multireference ab initio calculations suggest that this process is disfavored, with barrier heights of ∼70 kcal mol-1, and highlight important ligand noninnocent and multiconfigurational contributions to excited state relaxation and bond dissociation processes that are not captured with DFT. In the multireference description, photo-triggered Ni(II)-C homolytic bond dissociation occurs via initial population of a singlet Ni(II)-to-bpy metal-to-ligand charge transfer (1MLCT) excited state, followed by intersystem crossing and aryl-to-Ni(III) charge transfer, overall a formal two-electron transfer process driven by a single photon. This results in repulsive triplet excited states from which spontaneous homolytic bond dissociation can occur, effectively competing with relaxation to the lowest energy nondissociative triplet Ni(II) ligand field excited state. These findings guide important electronic structure considerations for the experimental and computational elucidation of the mechanisms of ground and excited state cross-coupling catalysis mediated by Ni heteroaromatic complexes.

Simulation of time-resolved x-ray absorption spectroscopy of ultrafast dynamics in particle-hole-excited 4-(2-thienyl)-2,1,3-benzothiadiazole.

To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of π-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen K edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our ab initio molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum.

Relaxation Dynamics of the Triazene Compound Berenil in DNA-Minor-Groove Confinement after Photoexcitation.

The effects of biomolecular embedding on the photoinduced relaxation process of the DNA-minor-groove binder berenil, diminazene aceturate, are studied with quantum mechanics/molecular mechanics, QM/MM, calculations that employ the algebraic diagrammatic construction through second-order, ADC(2), for the quantum mechanical part and an atomistic polarizable embedding for the classical part. The lowest singlet excitation to the S1 state is a bright transition with a ππ* character and a perichromatic red shift, due to the interactions with the solvent and DNA. The excited-state relaxation pathway is a two-step mechanism, an N═N azo-bond stretch followed by a volume-conserving bicycle-pedal twist. The DNA confinement and the coupling to solvent molecules via hydrogen bonds lead, for the excited-state relaxation process, only to small deviations from the ideal bicycle-pedal relaxation. Because of its volume-conserving character, the S1 excited-state relaxation proceeds almost unhindered, even in a fully rigid minor-groove confinement. With a fully frozen DNA minor groove and solvent, the energy gap for deexcitation from S1 to the ground state increased to 2.0 eV compared to 0.16 eV in aqueous solution. When the relaxation of the first solvation shell is included, the relaxation process on the S1 potential energy surface proceeds to a region on the potential energy surface, where only a small gap to the ground-state potential energy surface remains, 0.43 eV. These results show that the solvent relaxation has a significant effect in controlling the energy gap between the ground and S1 electronically excited states, which explains the experimental observations of the fluorescence characteristics of berenil in DNA confinement.

Simulating the Nonadiabatic Relaxation Dynamics of 4-(N,N-Dimethylamino)benzonitrile (DMABN) in Polar Solution.

The compound 4-(N,N-dimethylamino)benzonitrile (DMABN) represents the archetypal system for dual fluorescence, a rare photophysical phenomenon in which a given fluorophore shows two distinct emission bands. Despite extensive studies, the underlying mechanism remains the subject of debate. In the present contribution, we address this issue by simulating the excited-state relaxation process of DMABN as it occurs in polar solution. The potential energy surfaces for the system are constructed with the use of the additive quantum mechanics/molecular mechanics (QM/MM) method, and the coupled dynamics of the electronic wave function and the nuclei is propagated with the semiclassical fewest switches surface hopping method. The DMABN molecule, which comprises the QM subsystem, is treated with the use of the second-order algebraic diagrammatic construction (ADC(2)) method with the imposition of spin-opposite scaling (SOS). It is verified that this level of theory achieves a realistic description of the excited-state potential energy surfaces of DMABN. The simulation results qualitatively reproduce the main features of the experimentally observed fluorescence spectrum, thus allowing the unambiguous assignment of the two fluorescence bands: the normal band is due to the near-planar locally excited (LE) structure of DMABN, while the so-called "anomalous" second band arises from the twisted intramolecular charge transfer (TICT) structure. The transformation of the LE structure into the TICT structure takes place directly via intramolecular rotation, and is not mediated by another excited-state structure. In particular, the oft-discussed rehybridized intramolecular charge transfer (RICT) structure, which is characterized by a bent nitrile group, does not play a role in the relaxation process.

Ultrafast Intermolecular Vibrational Energy Transfer in Hexahydro-1,3,5-trinitro-1,3,5-triazine in Molecular Crystal by 2D IR Spectroscopy

Cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine (HHTT) is an important energetic molecule with a variety of applications. In this work, structural dynamics and vibrational-energy transfer dynamics of HHTT were investigated using steady-state infrared (IR), femtosecond time-resolved IR, and two-dimensional (2D) IR spectroscopic methods. Two different structural forms of HHTT (isolated molecules dissolved in DMSO and microcrystal dispersed in paraffin oil) were examined using the NO2 asymmetric stretching mode as a structural marker. Both intra- and intermolecular vibrational energy transfers were observed in HHTT microcrystal, where the well-known α-conformation was retained. However, less perfect α-HHTT species was found in DMSO. An intermolecular energy dissipation pathway was found to be related to Davydov splitting in the microcrystal form of HHTT, which originates from intermolecular interactions (e.g., electrostatic and hydrogen bonds). Experimental results were supported by quantum-chemistry computations. In addition, both vibrational excited-state relaxation and vibrational energy-transfer processes were found to be more efficient in HHTT microcrystal than in the solvated form. Our work provides a chemical-bond level of insight into the nitro group-containing energetic materials.

Experimental and DFT/TD-DFT approach on photo-physical and NLO properties of 2, 6-bis (4-Chlorobenzylidene) cyclohexanone

The crystalline nature and lattice parameters of synthesized compound 2, 6-bis (4-Chlorobenzylidene) cyclohexanone (B4CBC) were confirmed by powder X-ray diffraction technique. The splitting of the carbonyl mode is attributed to the intramolecular association on the basis of CO ….H type hydrogen bonding in the molecule. The doublet of the CO mode band originates from Fermi resonance. The solvatochromic behaviour of B4CBC in acetone solvent was investigated by UV–Vis, fluorescence spectroscopy and supported by TD-DFT calculations. Fluorescence lifetime measurement of the title compound exhibits a lifetime of the order of 2–3 ns and blue emission observed from CIE chromatic diagram. Decay curve also shows that excited state life time of BBC is longer, indicating that the chlorine substituent atom has a significant impact on the excited state relaxation process. Our result shows that NLO response and photo-physical parameters of cyclohexanone derivatives are tunable via peripheral substituent chlorine atom.

Excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines

The excited-state relaxation processes of three newly synthesized multi-branched alkyl-triphenylamine end-capped triazines ATT-(1–3) are characterized in different solvents by steady-state and time-resolved spectroscopy. In toluene, a weakly polar solvent, the emission originates from the intramolecular charge transfer (ICT) state; in tetrahydrofuran (THF), a strongly polar solvent, the existence of a nonradiative channel from ICT to twisted intramolecular charge transfer (TICT) accelerates the relaxation rate of the ICT state. The rate of the evolution process of ATT-(1–3) increases with increasing number of donor branches, which could ascribed to enhancements in the electron donor and acceptor abilities of the triazines.