Intramolecular Charge Transfer Reaction Research Articles

The effect of the number of conjugated C=C bonds on the ESIPT and ICT reactions of SNCN derivatives

The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics.

Cathode reaction models for Braga-Goodenough Na-ferrocene and Li-MnO2 rechargeable batteries

Braga-Goodenough all-solid-state Na-Fc and Li-MnO2 batteries demonstrate deposition of Na and Li on the cathode during discharge. These reaction mechanisms were investigated in light of the generalized charge neutrality level and the experimental results of Braga et al., and two new types of mechanisms were proposed. The Na-Fc mechanism is represented by a multi-step C[(CE)cC]n mechanism where C is the chemical step, E is the electrochemical step, c is the catalytic (CE) step, and n denotes the number of [(CE)cC] part cycles. The nth cycle corresponds to n moles of Na and Li deposition. For Li-MnO2, two mechanisms were considered. One is the C[(CE)cC]n mechanism which is the same as Na-Fc, and the other is the C[2(CE)cC]n mechanism, which involves two consecutive (CE)c steps. In the C step of (CE)c of both mechanisms, Fc and MnO2 reduce Na+(sf) and Li+(sf) (sf - surface states) to deposit Na and Li, respectively, which are intramolecular charge transfer reactions within the adsorbed molecules. Fc and MnO2 are oxidized to inter­mediates immediately reduced to Fc and MnO2 by their anodes in the subsequent E step. Based on these mechanisms, these batteries' discharge capacity and cathode alkali metal deposition were examined in detail.

Electrochemically Tunable Mixed Valence Conduction in Ruthenium Hexacyanoruthenate

Reversible electrochemical doping of ruthenium hexacyanoruthenate, a type of Prussian blue analogue (PBA), enables the on-demand tuning of electronic conductivity by more than four orders of magnitude. Inkjet-printed electrochemical random access memory (ECRAM) devices based on Ru-PBA and lithium- or proton-conducting ionogel electrolytes exhibit excellent switching efficiency and long-term memory retention, important characteristics for analog artificial synapses in neuromorphic circuits. We also demonstrate excellent biocompatibility with live neurons and the use of Ru-PBA ECRAM devices to detect dopamine, promising first steps toward connecting artificial and biological neural networks. In-situ probing of metal-metal charge transfer by UV/Vis/NIR absorption spectroscopy reveals a switching mechanism whereby electrochemically tunable valence mixing between N-coordinated Ru sites controls the carrier concentration and mobility, as independently supported by both Marcus-Hush electron transfer theory and more conventional band structure predictions from DFT. The experimental agreement achieved by both theoretical approaches supports a general mechanistic picture that intramolecular charge transfer reactions, more commonly studied in polynuclear mixed valence small molecules, are central to electronic conductivity in extended coordination frameworks.

The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine

AbstractThe excited‐state dynamics of the excited‐state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time‐dependent density functional theory. The excited‐state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen‐bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited‐state double proton transfer (ESDPT) along the hydrogen‐bonded bridge. Importantly, the previous proposed ESDPT‐triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.

Solvent Effect on Excited-State Intramolecular Proton-Coupled Charge Transfer Reaction in Two Seven-Membered Ring Pyrrole-Indole Hydrogen Bond Systems

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g-PPDBI and e-PPDBI, by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.

Construction and Study of the Application of Novel Norfloxacin Fluorescent Probes

In this experiment, a fluorescent probe of tricyanohydrofuran-norfloxacin was successfully constructed. Fluorescence spectrophotometer was used to detect the fluorescence intensity of the system under the condition of the excitation wavelength was 362nm and the emission wavelength was 448nm. The optimal experimental conditions were optimized (the concentration of the complex, the amount of norfloxacin, pH, temperature and reaction time). The optimal concentration of the complex in the system is in the range of 1.0×10−3-3.5×10−4 mol·L−1, the dosage was 0.6mL, and optimal dosage of norfloxacin was 3.0 ml, the optimum pH of the system was 5.5, under this condition the fluorescence intensity of the system was the strongest. At the same time, the intramolecular charge transfer reaction mechanism of fluorescent probe was studied. And testing the content of norfloxacin capsules, the results were satisfactory. A new testing method of norfloxacin content successfully created.

Deciphering the photobehaviour of ensemble and single crystals of Zr-based ITQ MOF composites

Understanding the interactions that govern the photophysical behaviour of Metal-organic frameworks (MOFs) and their composites is paramount for their photochemical and photonic applications. In this work, we report on the photobehaviour of Nile Red (NR) adsorbed on the surface of 2D Zr-ITQ-HB (NR@Zr-ITQ-HB) MOF using ultrafast (fs-ns) and single-molecule fluorescence techniques. The results show the occurrence of energy transfer processes happening in 0.34 ns between neighbouring NR molecules, intramolecular charge transfer reaction (∼0.6 ps) and a vibrational cooling (10 ps) of the adsorbed dye. Time-resolved single-composite fluorescence microscopy experiments on different NR-loaded composites allowed to resolve the emission from adsorbed aggregates (0.9 ns) and from the charge separated state (2.5 ns) of excited NR. The different photobehaviour of NR adsorbed on ZrO2 nanoparticles indicates the relevance of the Zr-ITQ-HB MOFs surface to the observed photoinduced reactions in the composites. We discuss how the nature of both, the organic linkers and metal clusters in this kind of MOFs, affects the photodynamics of the composites. These results may help in designing MOFs "on demand" for their application in photons-based science and technology, like photocatalysis and lighting.

Femto- to Millisecond Time-Resolved Photodynamics of a Double-Functionalized Push-Pull Organic Linker: Potential Candidate for Optoelectronically Active MOFs.

The design of improved organic linkers for the further engineering of smarter metal–organic framework (MOF) materials has become a paramount task for a wide number of material scientists. In this report, a luminescent double-functionalized push–pull (electron donor–acceptor) archetype organic molecule, dimethyl 4-amino-8-cyanonaphthalene-2,6-dicarboxylate (Me2CANADC), has been synthesized and characterized. The optical steady-state properties of Me2CANADC are strongly influenced by the surrounding environment as a direct consequence of its strong charge transfer (CT) character. The relaxation from its first electronically excited singlet state follows a double pathway: (1) on one side deactivating from its local excited (LE) state in the sub-picosecond or picosecond time domain, and (2) on the other side undergoing an ultrafast intramolecular charge transfer (ICT) reaction that is slowing down in viscous solvents. The deactivation to the ground state of these species with CT character is the origin of the Me2CANADC luminescence, and they present solvent-dependent lifetime values ranging from 8 to 18 ns. The slow photodynamics of Me2CANADC unveils the coexistence of a non-emissive triplet excited state and the formation of a long-lived charge separated state (2 µs). These observations highlight the promising optical properties of Me2CANADC linker, opening a window for the design of new functional MOFs with huge potential to be applied in the fields of luminescent sensing and optoelectronics.

Solvent effect on excited state potential energy surfaces of Thioflavin T. Qualitatively different results by TDDFT and SA-2-CASSCF methods.

Thioflavin T (ThT) is a viscosity-sensitive fluorescent dye and its emission intensity undergoes a significant enhancement upon binding to DNA or amyloid fibrils. This fluorescence light-up feature has been attributed earlier to restriction of structural rearrangements in the excited state that are coupled to an intramolecular charge transfer (ICT) reaction. In this work TDDFT (using B3LYP and CAM-B3LYP functionals) and SA-2-CASSCF calculations were carried out to obtain relaxed excited-state potential energy surfaces (PES) along twisting φ and wagging δ angles that describe mutual orientation of benzothiazole (BTZ) and dimethylaniline (DMA) fragments in ThT. For isolated ThT molecule both methods predict that during structural rearrangements of the initially excited Franck-Condon state, besides twisting along CC bond which connects BTZ and DMA fragments, a considerable wagging motion is expected to occur. Account for solvent effect using polarized continuum model showed qualitative differences in the excited state PES features calculated by SA-2-CASSCF and TDDFT methods. Single-reference TDDFT calculations failed to describe solvation of TICT state and predicted increase of its energy in more polar media.

Excitation wavelength dependent ICT character and ISC efficiency in a photocleavage agent of 1-aminoanthraquinone.

Anthraquinone derivatives have been widely used as photocleavage agents and dyes. Here, photoinduced excited state intramolecular charge transfer (ICT) dynamics of 1-aminoanthraquinone in ethanol are studied by femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical (QC) calculations. Four decay associated difference spectra and corresponding lifetime components were obtained by singular value decomposition and global fitting analysis from the fs-TA spectra. The QC calculations prove that the S1 state has obvious ICT character. Planar ICT (PICT) and twisted ICT (TICT) reaction coordinates are observed. On the PICT coordinate, vibrational relaxation (VR) and intersystem crossing (ISC) processes are observed. The ISC efficiency is dependent on excitation wavelength, which elucidates that T'2 state participates in the ISC process on a higher-level than the S'1 state. On the TICT coordinate, the TICT process is dependent on excitation wavelength, which elucidates a direct experimental evidence of an energy barrier in excited state TICT potential energy.

Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(i)-viologen dyad assemblies.

Two covalently linked donor-acceptor copper phenanthroline complexes (C-A dyads) of interest for solar energy conversion/storage schemes, [Cu(i)(Rphen(OMV)2 4+)2]9+ = RC+A4 8+ with RC+ = [Cu(i)Rphen2]+ involving 2,9-methyl (R = Me) or 2,9-phenyl (R = Ph)-phenanthroline ligands that are 5,6-disubstituted by 4-(n-butoxy) linked methylviologen electron acceptor groups (A2+ = OMV2+), have been synthesized and investigated via quantum chemical calculations and nanosecond laser flash spectroscopy in 1,2-difluorobenzene/methanol (dfb/MeOH) mixtures. Upon photoexcitation, charge transfer (CT) states RC2+A+A3 6+ are formed in less than one ns and decay by charge recombination on a time scale of 6-45 ns. The CT lifetime of RC2+A+A3 6+ has a strong dependence on MeOH solvent fraction when R = Me, but is unaffected if R = Ph. This solvent effect is due to coordination of MeOH solvent in MeC+A4 8+ (i.e. exciplex formation) allowed by conformational flattening of the ligand sphere, which cannot occur in PhC+A4 8+ having bulkier Phphen ligand framework. Interestingly, the decay time of the CT state increases for both species at low magnetic fields with a maximum increase of ca. 30% at ca. 150 mT, then decreases as the field is increased up to 1500 mT, the highest field investigated. This magnetic field effect (MFE) is due to magnetic modulation of the spin dynamics interconverting 3CT and 1CT states. A quantitative modeling according to the radical pair mechanism involving ab initio multireference calculations of the complexes revealed that the spin process is dominated by the effect of Cu hyperfine coupling. The external magnetic field suppresses the hyperfine coupling induced spin state mixing thereby lengthening the CT decay time. This effect is counteracted by the field dependent processes of T0-S mixing through the Δg-mechanism and by a local mode spin-orbit mechanism. Further, the maximum MFE is limited by a finite rate of direct recombination of 3CT states and the spin-rotational mechanism of spin relaxation. This study provides a first comprehensive characterization of Cu(ii)-complex spin chemistry and highlights how spin chemistry can be used to manipulate solar energy harvesting and storage materials.

Unravelling Why and to What Extent the Topology of Similar Ce-Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics.

Metal–organic frameworks (MOFs) are emerging materials for luminescent and photochemical applications. Armed with femto to millisecond spectroscopies, and fluorescence microscopy, the photobehaviors of two Ce‐based MOFs are unravelled: Ce‐NU‐1000 and Ce‐CAU‐24‐TBAPy. It is observed that both MOFs show ligand‐to‐cluster charge transfer reactions in ≈100 and ≈70 fs for Ce‐NU‐1000 and Ce‐CAU‐24‐TBAPy, respectively. The formed charge separated states, resulting in electron and hole generation, recombine in different times for each MOF, being longer in Ce‐CAU‐24‐TBAPy: 1.59 and 13.43 µs than in Ce‐NU‐1000: 0.64 and 4.91 µs. The linkers in both MOFs also undergo a very fast intramolecular charge transfer reaction in ≈160 fs. Furthermore, the Ce‐NU‐1000 MOF reveals excimer formation in 50 ps, and lifetime of ≈14 ns. The lack of this interlinkers event in Ce‐CAU‐24‐TBAPy arises from topological restriction and demonstrates the structural differences between the two frameworks. Single‐crystal fluorescence microscopy of Ce‐CAU‐24‐TBAPy shows the presence of a random distribution of defects along the whole crystal, and their impact on the observed photobehavior. These findings reflect the effect of linkers topology and metal clusters orientations on the outcome of electronic excitation of reticular structure, key to their applicability in different fields of science and technology, such as photocatalysis and photonics.

Cover Feature: Molecular Dynamics of Excited State Intramolecular Charge Transfer in Solution by Coherent Nuclear Wave Packets (ChemPhysChem 11/2019)

The Cover Feature illustrates the trajectory of the wave packets on the excited state following photoexcitation and coherent intramolecular charge transfer reaction. Amplitude of the wave packet of the ring deformation mode is modified by the reaction rate, whereas the ring breathing mode is orthogonal to the reaction coordinate and therefore impervious to the reaction rate. More information can be found in the Article by W. Heo and T. Joo on page 1448 in Issue 11, 2019 (DOI: 10.1002/cphc.201801103).

Unraveling Competitive Electron and Energy-Transfer Events at the Interfaces of a 2D MOF and Nile Red Composites: Effect of the Length and Structure of the Linker.

The distribution and interactions of organic molecules adsorbed on the surface of materials play important roles in many catalytic and photonic processes. Here, we show that the length and chemical structure of the linker in new Al-ITQ metal-organic frameworks (MOFs) are fundamental for the dynamics of the dye Nile Red (NR) adsorbed on its surface. For the studied composites using Al-ITQ-4-ethylbenzoic acid (EB), Al-ITQ-4-aminobenzoic acid (AB), and Al-ITQ-EB exposed to the aniline (AN) or N, N-dimethylaniline (DMA) atmospheres, we observed a very fast (∼1.2 ps) intramolecular charge-transfer reaction in adsorbed NR molecules. For NR@Al-ITQ-EB, where the linker has a shorter aliphatic chain (two carbons), the dye molecules present a homoenergy-transfer (ET) process, which is faster (∼90 ps) than in the previously reported NR@Al-ITQ-4-heptylbenzoic acid composite with longer aliphatic chain (seven carbons, ∼220 ps). The more polar environment created by the Al-oxide nodes in Al-ITQ-EB surface around the NR populations strongly favors the ET event. When the linker structure contains phenyl amine moieties, the resulting NR@Al-ITQ-AB composites show different and rich photodynamics, in which a fast electron transfer reaction from the MOF aniline moiety to the adsorbed NR occurs in ∼17 ps, inhibiting the ET process between the dye molecules near the MOF surface. This process also was confirmed in Al-ITQ-EB MOF exposed to AN and DMA gas atmospheres, as well as NR in pure aniline. The obtained results demonstrate how modifications in the length and structure of the organic linker in this MOF change the interface interactions and outcome of the photoinduced processes in the composites. Our findings on dye-MOF interface photobehavior are relevant to the design of new materials in which the interface plays a key role in their performance in the fields of catalysis and photonics.

Fluorescence of 4-(Diisopropylamino)benzonitrile (DIABN) Single Crystals from 300 K down to 5 K. Intramolecular Charge Transfer Disappears below 60 K.

Single crystals of 4-(diisopropylamino)benzonitrile (DIABN) undergo an intramolecular charge transfer (ICT) reaction in the excited singlet state. At 300 K, the fluorescence consists of emissions from the locally excited (LE) and from the ICT state. Upon cooling to 5 K, the ICT fluorescence intensity gradually decreases relative to that of the LE emission and is absent below 60 K. With crystalline 4-(dimethylamino)benzonitrile (DMABN), in contrast, only LE emission is found over the entire range from 300 to 5 K. The phosphorescence spectra of the DIABN and the DMABN crystals do not present any evidence for an additional ICT emission, showing that ICT does not occur in the triplet state. An activation energy Ea of ∼4 kJ/mol is determined for the LE → ICT reaction of DIABN crystals, from the temperature dependence of the fluorescence decay times τ2 and τ1. Ea is attributed to changes in the molecular conformation of DIABN other than a full rotation of the large diisopropylamino group with respect to the benzonitrile moiety. In a comparison with crystal and solution data, literature results from transient vibrational and absorption spectra are discussed and it is concluded that they cannot be employed to favor the TICT (perpendicular twist) over the PICT (planar) model for DIABN and DMABN.

Tuning ‘Stokes Shift’ and ICT Character by Varying the Donor Group in Imidazo[1,5 a]pyridines: A Combined Optical, DFT, TD‐DFT and NLO Approach

AbstractImidazo[1, 5a]pyridine based novel donor‐acceptor dyes (2a‐e) with the large Stokes shift (ca. 140–200 nm) were synthesized and characterized. We report on the steady‐state and nanosecond time‐resolved emission studies of 2a‐e in different microenvironments which reveal that the dyes 2a‐c (average lifetime of 0.65, 1.36 and 1.63 ns respectively) perform reversible proton motion in chloroform due to variable donor group present at the para position of substituted phenyl group attached to imidazopyridine core as an acceptor group whereas 2d and 2e (average lifetime of 3.50 and 4.06 ns respectively) compete with 2a‐c in agreement with considerable intramolecular charge transfer (ICT) characteristics. Solvent polarity plots viz. Lippert‐Mataga, McRae, and Bakhshiev equation furnish the confirmation of ICT character. Density functional theory (DFT) calculations [(B3LYP/6‐311++G(d, p)] provide the information of structural as well as the electronic properties of dyes 2a‐e. Natural bond orbital (NBO) analyses enlighten the nature of the charge or proton transfer within the selected donor‐acceptor orbital interaction with the large energy of stabilization (the donor‐acceptor orbital interactions increases in the order 2a > 2b > 2c > 2d > 2e). The large difference in dipole moment (ca. 1.57‐21.55 D) results in a strong non‐linear optical (NLO) properties. The NLO properties were evaluated theoretically as well as spectroscopically in solvents of different polarities and the resulting hyperpolarizabilities fall within the expected limiting values. These results bring new findings in coupled ICT and proton transfer reactions which will be useful for further research and promising applications in sensing polarity or H‐bonding of microenvironments similar to that of the biological ones.

Excited-State Dynamics of Thioflavin T: Planar Stable Intermediate Revealed by Nuclear Wave Packet Spectroscopies.

Time-domain spectroscopies with time resolution shorter than the vibrational periods of interest were employed to reveal the reaction kinetics and molecular dynamics of the intramolecular charge transfer (ICT) reaction of thioflavin T in liquids. Time-resolved fluorescence spectra provided detailed reaction kinetics, and vibrational wave packets observed in the time-resolved fluorescence and transient absorption provided structural information on the reaction intermediate. Upon photoexcitation, the Franck-Condon state undergoes vibrational relaxation and minor conformational change to form a stable planar intermediate followed by the twisting of the central C-C single bond to form the twisted ICT state. The ICT reaction rate is determined by the solvent fluctuation excluding the inertial component in the solvation function.

Photoprotection or photodamage: a direct observation of nonradiative dynamics from 2-ethylhexyl 4-dimethylaminobenzoate sunscreen agent.

Apart from being an analogue of the prototype for photoinduced intramolecular charge transfer (ICT), 2-ethylhexyl 4-dimethylaminobenzoate (EHDMABA) is also one of the earliest patented and most commonly used sunscreen components. There is, however, little documented information about the photophysics and factors affecting the photophysics of this molecule. Such information is of importance for both the understanding of the ICT reaction and assessing the underlying process of photoprotection, especially in view of the "sunscreen controversy" that has arisen from the contrasting in vivo vs. in vitro photobiological results on this and related UV filters. We report herein a femtosecond broadband time-resolved fluorescence (fs-TRF), complemented by transient absorption (fs-TA) to allow a full probe of the excited state cascades for EHDMABA and two of its derivatives in solvents of varied properties. The results provide direct evidence for a nearly solvent independent inner sphere ICT reaction occurring on the sub-picosecond time scale, and an ensuing solvent dictated deactivation of the ICT state. The ICT state in the aprotic solvent acetonitrile decayed solely through the intrinsic intersystem crossing (ISC) to produce a potentially harmful triplet excited state. In the protic solvent, the solvation and formation of ICT-induced solute-solvent hydrogen (H)-bonding opened the originally inaccessible internal conversion (IC) channel of the ICT state, leading to the rapid reformation of the ground state molecule with a unitary efficiency in the aqueous solution. This H-bonding-mediated IC restrained or eliminated the intrinsic ISC, providing a mechanism at the molecular level for the benign dissipation of the electronic excitation. The precise rate of IC was observed to vary with the alkoxy substituent and its efficiency was affected by the H-bonding capacity of the solvent. The findings of this work demonstrate the pivotal role of the microenvironment and the direct participation of solvent molecules through H-bonding in drastically altering the nonradiative dynamics and promoting or inhibiting photostability and photoprotection. This may assist in developing next-generation UV filters and help in improving formulation design for the optimal efficacy of sunscreen products. The pronounced H-bonding-induced fluorescence quenching and variation in the fluorescence wavelength imply that these molecules may also serve as a sensitive fluorescence probe for the H-bonding properties of the microenvironment.

Nonradiative dynamics determined by charge transfer induced hydrogen bonding: a combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water.

As a case study of the interplay and the consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a combined femtosecond time-resolved fluorescence (fs-TRF) and density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) largely in a water solvent. Direct observation of the broadband spectra, anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with a time constant of ∼0.7 ps from an initial locally excited (LE) state identified to have the Laππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting a decay time of ∼49.7 ps, unusually faster than the nanosecond range lifetime in a polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to the study of the solvent kinetic isotope effect, is identified to originate from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows elimination of the ISC, and enables the nonradiative decay to proceed almost entirely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for the extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain an important insight for the in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the "sunscreen controversy" of the DMABA type of UVB sunscreen molecule.

Computational Spectroscopic Characterization of a Bistable Binuclear Complex [(CO)2(benzoate)FeII/III(terephthalate)CoIII/II(benzoate)(CO)2

Electric dipole moments, polarizabilities, and IR, Raman, optical rotatory dispersion, and electronic and vibrational circular dichroism spectra of the four cis–trans isomers of the proposed [(CO)2(benzoate)FeII/III(terephthalate)CoIII/II(benzoate)(CO)2]+ binuclear complex, having bistablity due to intramolecular charge transfer (IMCT), is investigated using the time-dependent density functional theory ((TD)DFT) B3LYP/6–31G(d,p)[LanL2DZ] method. Results show that the two FeII–CoIII and FeIII–CoII IMCT states of this binuclear complex have distinctly different spectroscopic, optical, and electric response properties, and are sensitive to the cis–trans arrangement of the ligands around the two metallic centres. Furthermore, intrinsic reaction coordinates inter-connecting the two IMCT states are identified using the Duschinsky matrix method. Only one or two of the normal coordinates remain almost (above 80 %) intact during the IMCT reaction which denotes global changes in the bonding strengths and potential energy hypersurface of this bistable binuclear complex. Analysis of the calculated spin densities characterizes the IMCT transition state structures of the trans–trans, cis–cis, and trans–cis isomers as early, early, and late transition states, respectively.