Intramolecular Excited State Research Articles

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

NIR Emission and Acid-Induced Intramolecular Electron Transfer Derived from a SOMO–HOMO Converted Non-Aufbau Electronic Structure

Some organic radicals violate the Aufbau principle and have unique electronic structures in which the energy level of the singly occupied molecular orbital (SOMO) is formally lower than that of the highest occupied molecular orbital (HOMO). We synthesized a novel organic radical with a SOMO–HOMO converted electronic structure, TPA-R•, a novel donor–acceptor hybrid of triphenylamine (an electron donor), and a stable polychlorinated diphenyl(4-pyridyl)methyl radical (an electron acceptor). TPA-R• exhibited fluorescence in the near-infrared region (λmax = 910 nm) in cyclohexane from a polar intramolecular charge-transfer excited state. Cyclic voltammetry, absorption spectroscopy, and density functional theory calculation revealed the inversion of the SOMO and HOMO levels in the electronic structure of TPA-R•. Addition of trifluoromethanesulfonic acid to TPA-R• caused a two-step change. Protonation initially occurred on the diphenylpyridylmethyl radical moiety to form TPA-[RH]•+. This protonation induced larg...

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

Bright lanthanide‐based probes rely on sensitised lanthanide luminescence. The outcome of the excited state energy transfer cascade – from sensitiser chromophore to lanthanide(III) ion – should be the population of the lanthanide excited state with an efficiency equal to unity. This is rarely the case, and this is a particular problem for europium(III) due to the low energy of the Eu(II)/Eu(III) redox couple. Thus, photoinduced electron transfer quenching can be an efficient drain for the excited state energy, reducing the brightness of a probe. Here, the effect of intramolecular PeT in a lanthanide‐based probe is investigated by contrasting intramolecular processes in coumarin‐appended 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate (DO3A) complexes with intermolecular processes induced through collisional quenching. We find that the excited energy transfer processes involving terbium(III) and europium(III) are slow in the complex when compared to collisional quenching. Further, we show that the quenching event may lead to lanthanide luminescence and suggest that the fate of a PeT quenching event may not be non‐radiative loss of the excitation energy.

An intramolecular charge transfer and excited state intramolecular proton transfer based fluorescent probe for highly selective detection and imaging of formaldehyde in living cells.

Formaldehyde (FA), as a reactive carbonyl species, is endogenously generated in various biological processes. Abnormal levels of FA could lead to various cellular dysfunction and pathological conditions. Here, we develop a new activatable fluorescent probe for highly selective visualization of FA in living cells. Our probe (Naph-1) is designed using a naphthalene derivative as the fluorophore and hydrazone as a recognition site for FA. Naph-1 is essentially nonemissive. After reacting with FA, the amine moiety is converted into a Schiff base with electron-withdrawing ability and the fluorescence is simultaneously turned on due to synergetic intramolecular charge transfer and favoured excited state intramolecular proton transfer effects. Naph-1 exhibits a large Stokes shift upon reaction with FA. Furthermore, it possesses high selectivity and superior sensitivity toward FA with an estimated limit of detection of 0.35 μM. Moreover, Naph-1 is also successfully applied to image both endogenous and exogenous formaldehyde in living cells. These features demonstrate that Naph-1 holds great potential in the detection and imaging of formaldehyde in biological systems.

A ‘‘naked-eye’’ colorimetric and ratiometric fluorescence probe for trace hydrazine

A new colorimetric and ratiometric fluorescence probe was developed for detection of hydrazine (N2H4) based on the mechanism of intramolecular proton transfer excited state (ESIPT).

Density Functional Theory Applied to Excited State Intramolecular Proton Transfer in Imidazole-, Oxazole-, and Thiazole-Based Systems.

Excited state intramolecular proton transfer (ESIPT) is a photoinduced process strongly associated to hydrogen bonding within a molecular framework. In this manuscript, we computed potential energy data using Time Dependent Density Functional Theory (TDDFT) for triphenyl-substituted heterocycles, which evidenced an energetically favorable proton transfer on the excited state (i.e., ESIPT) but not on the ground state. Moreover, we describe how changes on heterocyclic functionalities, based on imidazole, oxazole, and thiazole systems, affect the ESIPT process that converts an enolic species to a ketonic one through photon-induced proton transfer. Structural and photophysical data were obtained theoretically by means of density functional theory (DFT) calculations and contrasted for the three heterocyclics. Different functionals were used, but B3LYP was the one that adequately predicted absorption data. It was observed that the intramolecular hydrogen bond is strengthened in the excited state, supporting the occurrence of ESIPT. Finally, it was observed that, with the formation of the excited state, there is a decrease in electronic density at the oxygen atom that acts as proton donor, while there is a substantial increase in the corresponding density at the nitrogen atom that serves as proton acceptor, thus, indicating that proton transfer is indeed favored after photon absorption.

Synthesis, Characterization, Physico-Chemical and DFT Studies of Potential Organic NLO Materials: Experimental and Theoretical Combined Study

1-(2,4-difluorobenzyl)-2-(2,4-difluorophenyl)-6-methyl-1H-benzo[d]imidazole(1) and 1-(3-(tri fluoromethyl)benzyl)2-(4-(trifluoromethyl)phenyl)-6-methyl-1H-benzo[d]imidazole (2) were synthesized and characterized using FT-IR,1H-NMR,13C-NMR and mass spectral studies. UV-Vis absorption and emission spectral studies were studied in hexane and ethanol medium. In addition to these, density functional theory studies (DFT/B3LYP/6-31G(d,p)) has also been carried out to compute the relevant electronic properties of the molecules and the observed theoretical results of the non-zero dipole moments, polarizability and hyper polarizability values which show that these types of molecules are promising candidates for applications in various non-linear optical (NLO) devices because of their significant values of the molecular hyperpolarizabilities and fine microscopic NLO behaviour. The high NLO responses of the donor-acceptor π-conjugated molecules are related to the intramolecular charge transfer excited state. The results confirmed that the synthesized benzimidazoles can act as a non-linear optical material and changing the substituents in the aldehydic phenyl ring will change the energy gap in the molecule.

Establishing the Critical Role of Oriented Aggregation in Molecular Solid State Fluorescence Enhancement.

The general occurrence of fluorescence emission quenching in molecular aggregates is circumvented in select classes of molecules. This has largely been attributed to the rigidification of the molecule and its environment, which hinders non-radiative excited state energy loss through structural relaxation; since such an effect should in principle apply to most aggregates and crystals, there must clearly be other critical factors that make the select molecules exceptional. Discovery of three crystalline structures of a new push-pull molecule in its enantiomorphic and racemic forms, exhibiting not only very high, but distinctly different solid state fluorescence enhancements, has now allowed a systematic investigation of the role of intramolecular and intermolecular excited state energy loss pathways. Crystallographic, spectroscopic and computational investigations provide a detailed appraisal of the assembly patterns in the crystals, and rigorous establishment of an inverse correlation between intermolecular energy transfer and solid state fluorescence. The study provides a clear visualization of the critical role of oriented molecular aggregation in solid state fluorescence efficiency enhancement.

Using Density Based Indexes and Wave Function Methods for the Description of Excited States: Excited State Proton Transfer Reactions as a Test Case.

To provide tools to interpret photochemical reactions, in this paper we demonstrate how a recently developed density-based index (DCT), up to now used in conjunction with time dependent density functional theory methods, can be extended to multiconfigurational methods. This index can guide chemists in the interpretation of photochemical reactions providing a measure of the spatial extent of a photoinduced charge transfer and, more generally, of charge transfer phenomena. This qualitative and quantitative description can be particularly relevant in the case of multiconfigurational calculations providing a simple tool for the interpretation of their complex outputs. To prove the potentiality of this approach we have considered a simple intramolecular excited state proton transfer reaction as study case and applied both wave function (CASSCF-CASPT2) and density-based methods in conjunction with a DCT analysis. Our results confirm that, also in the case of multiconfigurational methods, the DCT provides very useful information about the structural reorganization of a molecule at the excited state.

Narrow Band Gap Conjugated Polyelectrolytes.

Two essential structural elements define a class of materials called conjugated polyelectrolytes (CPEs). The first is a polymer framework with an electronically delocalized, π-conjugated structure. This component allows one to adjust desirable optical and electronic properties, for example the range of wavelengths absorbed, emission quantum yields, electron affinity, and ionization potential. The second defining feature is the presence of ionic functionalities, which are usually linked via tethers that can modulate the distance of the charged groups relative to the backbone. These ionic groups render CPEs distinct relative to their neutral conjugated polymer counterparts. Solubility in polar solvents, including aqueous media, is an immediately obvious difference. This feature has enabled the development of optically amplified biosensor protocols and the fabrication of multilayer organic semiconductor devices through deposition techniques using solvents with orthogonal properties. Important but less obvious potential advantages must also be considered. For example, CPE layers have been used to introduce interfacial dipoles and thus modify the effective work function of adjacent electrodes. One can thereby modulate the barriers for charge injection into semiconductor layers and improve the device efficiencies of organic light-emitting diodes and solar cells. With a hydrophobic backbone and hydrophilic ionic sites, CPEs can also be used as dispersants for insoluble materials. Narrow band gap CPEs (NBGCPEs) have been studied only recently. They contain backbones that comprise electron-rich and electron-poor fragments, a combination that leads to intramolecular charge transfer excited states and enables facile oxidation and reduction. One particularly interesting combination is NBGCPEs with anionic sulfonate side groups, for which spontaneous self-doping in aqueous media is observed. That no such doping is observed with cationic NBGCPEs indicates that the interplay between electrostatic forces and the redox chemistry of the organic semiconducting chain is essential for stabilizing the polaronic states and increasing the conductivity of the bulk. Capitalizing upon the properties of NBGCPEs has resulted in a range of new applications. When doped, they can be introduced as interlayers in organic and perovskite solar cells. Single-walled carbon nanotubes can be n- or p-doped with NBGCPEs, depending on whether the same backbone contains attached cationic or anionic side groups, respectively. The resulting dispersions can be used to fabricate flexible thermoelectric devices in which the n- and p-semiconductor legs are nearly identical in terms of chemical composition. Electrostatic interactions with negatively charged cell walls, in combination with the long-wavelength absorption and high photothermal efficiencies, have been used to create effective agents for photothermal killing of bacteria. Additionally, recent results have shown that cationic NBGCPEs can effectively n-dope graphene and that this doping is temperature-dependent. The preferential charge carriers can therefore be chosen to be electrons or holes depending on the applied temperature.

Time-resolved spectroscopic and density functional theory investigation of the influence of the leaving group on the generation of a binol quinone methide

Quinone methides are important reactive intermediates in photobiology and photochemistry, with little known regarding the photo-induces generation of reactive quinone methides species from relevant precursors in aqueous solutions. Here, the time-resolved spectroscopy methods including the transient absorption experiments and time-resolved Resonance Raman were employed to directly study the photophysics and photochemical processes of 6,6′-Bis(hydroxymethyl)-1,1′-binaphthyl-2,2′-diol (BQMP-a) in neat MeCN and 1:1 MeCN:H2O aqueous solutions. In aqueous solution, the intramolecular excited state proton transfer (ESIPT) process was observed where the water molecules help deliver the proton on the hydroxyl group to the naphthol ring within 125 ps in the singlet excited state of BQMP-a. This solvent mediated ESIPT will then lead to the leaving of the H2O group in 906 ps followed by the generation of the binol quinone methide (BQM-a) intermediate with a lifetime of 1.7 μs. As a result, the BQMP-a provides a different production rate of quinone methides compared with the 1,1′-(2,2′-Dihydroxy-1,1′-binaphthyl-6,6′-diyl) bis(N,N,N-trimethylmethanaminium) bromide system, which indicates the leaving group is an important factor in the generation of the reactive DNA alkylated intermediates.

Combined TDDFT and AIM Insights into Photoinduced Excited State Intramolecular Proton Transfer (ESIPT) Mechanism in Hydroxyl- and Amino-Anthraquinone Solution

Time-dependent density functional theory (TDDFT) and atoms in molecules (AIM) theory are combined to study the photoinduced excited state intramolecular proton transfer (ESIPT) dynamics for eight anthraquinones (AQs) derivatives in solution. The calculated absorption and emission spectra are consistent with the available experimental data, verifying the suitability of the theory selected. The systems with the excited-state exothermic proton transfer, such as 1-HAQ, 1,5-DHAQ and TFAQ, emit completely from transfer structure (T), while the reactions for those without ESIPT including 1,4-DHAQ and AAAQ appear to be endothermic. Three reaction properties of three systems (1,8-DHAQ, DCAQ and CAAQ) are between the exothermic and endothermic, sensitive to the solvent. Energy scanning shows that 1,4-DHAQ and AAAQ exhibit the higher ESIPT energy barriers compared to 1-HAQ, 1,5-DHAQ and TFAQ with the “barrierless” ESIPT process. The ESIPT process is facilitated by the strengthening of hydrogen bonds in excited state. With AIM theory, it is observed that the change in electrons density ρ(r) and potential energy density V(r) at BCP position between ground state and excited state are crucial factors to quantitatively elucidate the ESIPT.

Star-Shaped Asymmetrically Substituted Blue Emitting Carbazoles: Synthesis, Photophyscial, Electrochemical and Theoretical Investigations

A series of star-shaped blue-emitting 1,3,6,8-tetrasubstituted carbazoles containing different functional groups are synthesized and characterized. Electron-transporting N-phenylbenzimidazole is attached to C3 and C6 positions while the chromophore at C1 & C8-positions varied from fluorene to triphenylamine. The effect of the nature of the C1 and C8 chromophores on optical, electrochemical and thermal properties is unravelled. Absorption spectra revealed that the choice of meta− mode of connectivity between the chromophores on carbazole nucleus interrupted the interchromophoric electronic interactions in the ground state. However, the molar extinction coefficients of the dyes increased significantly on the addition of chromophores at C1 and C8 attributable to the increment in the chromophore density. Interestingly, triphenylamine and pyrene containing compounds displayed positive solvatochromism in emission attributable to intramolecular charge transfer and excimer excited states, respectively. Fluorescence and thermal properties are strongly influenced by the nature of chromophores present on C1 & C8. Electrochemical studies revealed low oxidation potential for arylamine containing derivatives and led to high-lying highest occupied molecular orbital in the series. However, the lowest unoccupied molecular orbital is stabilized by pyrene substituent due to its π-acceptor character.

A tridentate CNO‐donor palladium(II) complex as efficient catalyst for direct C―H arylation: Application in preparation of imidazole‐based push–pull chromophores

A series of imidazolium chlorides for the formation of tridentate CNO‐donor palladium(II) complexes featuring N‐heterocyclic carbene moieties have been developed from cheap and readily available starting materials with high yields. Their palladium complexes were prepared by reactions between the ligand precursors and PdCl2 using K2CO3 as base in pyridine with reasonable yields. These air‐stable metal complexes were characterized using 1H NMR and 13C{1H} NMR spectroscopy and elemental analyses. Heteronuclear multiple bond correlation experiments were performed to identify key NMR signals of these compounds. The structures of two of the complexes were also established by single‐crystal X‐ray diffraction analysis. One of these complexes was successfully applied in the direct C―H functionalization reactions between heterocyclic compounds and aryl bromides, producing excellent yields of coupled products. The coupling reactions were scalable, allowing grams of coupled products to be obtained with a mere 2 mol% of Pd loading. The catalyst system developed allowed the large‐scale preparation of several push–pull chromophores straightforwardly. Photophysical properties based on UV–visible and fluorescence spectroscopy for these chromophores were investigated. Deep blue photoluminescence with moderate quantum efficiency and twisted intramolecular charge transfer excited state were observed for these chromophores. Density functional theory (DFT) and time‐dependent DFT calculations were performed to support the experimental results.

Fluorescence Quenching in BODIPY Dyes: The Role of Intramolecular Interactions and Charge Transfer

The fluorescence properties of the BODIPY dye and its two meso-substituted derivatives, tert-butyl- and phenyl-BODIPY, are rationalized. The non emissive behavior of the latter two are attributed to the energetically accessible low-lying conical intersection between the ground state and the lowest excited singlet state. Both intramolecular non-covalent interactions and excited state charge transfer character are identified as being crucial for “stabilizing” the intersection and prompting the nonradiative decay. Similar crossing was located in the bare BODIPY dye, however, being energetically less accessible, which correlates well with the high fluorescence quantum yields of the parent dye.

Cover Picture: Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer (ChemPhotoChem 5/2017)

Cover Picture: Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer (ChemPhotoChem 5/2017)

Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer

AbstractThe front cover artwork is provided by the group of Prof. Dr. Hiroshi Ikeda at Osaka Prefecture University (Japan). The picture represents the luminescence associated with geometrical changes of a molecule in the photoexcited state. Drops of rainwater from the π‐electron clouds accumulate in a water bottle. When water finally flows out of a tilted bottle, luminescence is observed. Read the full text of the article at 10.1002/cptc.201600028.

Comparing the performance of TD-DFT and SAC-CI methods in the description of excited states potential energy surfaces: An excited state proton transfer reaction as case study.

The performances, in the description of excited state potential energy surfaces, of several density functional approximations representative of the currently most applied exchange correlation functionals' families have been tested with respect to post Hartree-Fock references (here Symmetry Adapted Cluster-Configuration Interaction results). An experimentally well-characterized intermolecular proton transfer reaction has been considered as test case. The computed potential energy profiles were analyzed both in the gas phase and in toluene solution, here represented as a polarizable continuum model. The presence of intermolecular (dark) and intramolecular (bright) charge transfer excited states, whose polarity strongly differs along the reaction pathway, makes clear that only subtle compensation between spurious electronic effects-related to the incorrect asymptotic behavior of the functional-and solvent stabilization of polar states leads to the overall correct description of this excited state reaction when using global hybrids with low percentage of Hartree Fock exchange. © 2017 Wiley Periodicals, Inc.

Remarkable Solvatofluorochromism of a [2.2]Paracyclophane‐Containing Organoboron Complex: A Large Stokes Shift Promoted by Excited State Intramolecular Charge Transfer

AbstractAn organoboron complex containing a [2.2]paracyclophanyl group (2 BF2) exhibits remarkable solvatofluorochromism associated with a large Stokes shift. With increased solvent polarity, the fluorescence (FL) color of 2 BF2 changes from sky blue (λFL=485 nm in cyclohexane (c‐C6H12)) to orange (λFL=576 nm in CH3CN). This behavior derives from the intramolecular charge transfer (ICT) character of the excited state 2 BF2* in which the [2.2]paracyclophanyl group serves as the electron donor while the remaining 2 BF2 structure (apart from the sub‐benzene ring) functions as the electron acceptor. Wave deconvolution of FL spectra in a solvent mixture of CH2Cl2/c‐C6H12, and a plot of the FL emission energy versus the CH2Cl2/c‐C6H12 ratio, suggest that FL of 2 BF2 occurs from two types of 2 BF2* excited states. The results of DFT calculations support this explanation and further emphasize that the ICT character of 2 BF2* is derived from a large conformational change associated with the electron‐donating character of the [2.2]paracyclophanyl group. Moreover, the fact that 2 BF2 is more sensitive than Reichardt's dye to polarity changes in a low polarity CH2Cl2/c‐C6H12 solvent mixture is also of importance.

Efficient blue fluorescent electroluminescence based on a tert-butylated 9,9′-bianthracene derivative with a twisted intramolecular charge-transfer excited state

An orthogonally twisting tert-butylated 9,9′-bianthracene derivative, namely 10,10′-bis(4-tert-butylphenyl)-9,9′-bianthracene (BAn-(4)-tBu), possessing high thermal stability and high luminescence efficiency was synthesized. The BAn-(4)-tBu emitter with a twisted intramolecular charge transfer state characteristics, originating from its twisting 9,9′-bianthracene moiety, enhanced the occurrence of singlet excitons in the fluorescent emitter. The BAn-(4)-tBu device doped with sky blue fluorescent material 4,4-bis[4-(di-p-tolylamino)styryl]biphenyl displayed a maximum singlet generation fraction of >25%, maximum current and external quantum efficiencies of 6.00cdA−1 and 3.22%, respectively. Remarkably, a low efficiency roll-off was observed. These results demonstrated that BAn-(4)-tBu as a host material is advantageous for fabrication of highly efficient blue fluorescent organic light-emitting devices.