Degree Of Charge Transfer Character Research Articles

Are all charge-transfer parameters created equally? A study of functional dependence and excited-state charge-transfer quantification across two dye families.

Small molecule organic dyes have many potential uses in medicine, textiles, forensics, and light-harvesting technology. Being able to computationally predict the spectroscopic properties of these dyes could greatly expedite screening efforts, saving time and materials. Time-dependent density functional theory (TD-DFT) has been shown to be a good tool for this in many instances, but characterizing electronic excitations with charge-transfer (CT) character has historically been challenging and can be highly sensitive to the chosen exchange-correlation functional. Here we present a combined experimental and computational study of the excited-state electronic structure of twenty organic dyes obtained from the Max Weaver Dye Library at NCSU. Results of UV-vis spectra calculations on these dyes with six different exchange-correlation functionals, BP86, B3LYP, PBE0, M06, BH and HLYP, and CAM-B3LYP, were compared against their measured UV-vis spectra. It was found that hybrid functionals with modest amounts (20-30%) of included Hartree-Fock exchange are the most effective at matching the experimentally determined λmax. The interplay between the observed error, the functional chosen, and the degree of CT was analyzed by quantifying the CT character of λmax using four orbital and density-based metrics, Λ, Δr, SC and DCT, as well as the change in the dipole moment, Δμ. The results showed that the relationship between CT character and the functional dependence of error is not straightforward, with the observed behavior being dependent both on how CT was quantified and the functional groups present in the molecules themselves. It is concluded that this may be a result of the examined excitations having intermediate CT character. Ultimately it was found that the nature of the molecular "family" influenced how a given functional behaved as a function of CT character, with only two of the examined CT quantification methods, Δr and DCT, showing consistent behavior between the different molecular families. This suggests that further work needs to be done to ensure that currently used CT quantification methods show the same general trends across large sets of multiple dye families.

Pyrene-stabilized acenes as intermolecular singlet fission candidates: importance of exciton wave-function convergence

Singlet fission (SF) is a photophysical process considered as a possible scheme to bypass the Shockley–Queisser limit by generating two triplet-state excitons from one high-energy photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance both theoretically and experimentally. However, their instability prevents them from being utilized in SF-based photovoltaic devices. In search of practical SF chromophores, we use many-body perturbation theory within the GW approximation and Bethe–Salpeter equation to study the excitonic properties of a family of pyrene-stabilized acenes. We propose a criterion to define the convergence of exciton wave-functions with respect to the fine k-point grid used in the BerkeleyGW code. An open-source Python code is presented to perform exciton wave-function convergence checks and streamline the double Bader analysis of exciton character. We find that the singlet excitons in pyrene-stabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrene-fused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrene-stabilized anthracene derivative is considered as a possible candidate for triplet–triplet annihilation because it yields a lower SF driving force than anthracene.

Luminescence spectroscopy of oxazine dye cations isolated in vacuo.

Here we report gas-phase action and luminescence spectra of cationic dyes derived from oxazine: cresyl violet (CV+), oxazine 170 (Ox-170+), nile blue (NB+), darrow red (DR+), oxazine 1 (Ox-1+), oxazine 4 (Ox-4+), and brilliant cresyl blue (BCB+). The first four have a benzofused structure, which results in asymmetric charge distributions along the long axis. The positive charge is also asymmetrically distributed in BCB+ while Ox-1+ and Ox-4+ are symmetric. As the ions are isolated in vacuo, there are no interactions with solvent molecules or counter ions, and the effect of chemical modifications is therefore more easily revealed than from solution-phase experiments. The transition energy decreases in the order: DR+ > CV+ > Ox-4+ > Ox-170+ > BCB+ > Ox-1+ > NB+, and the fluorescence from BCB+ is less than from the others. We discuss the results based on electron delocalisation, degree of charge-transfer character, rigidity of the chromophore structure, and substituents.

A comparison of the metal-ligand interactions of the pentafluorophenylethynyl and trifluoropropynyl ligands in transition metal cyclam complexes

The pentafluorophenylethynyl ligand, C2C6F5, is a commonly used electron withdrawing ligand. In the context of developing complexes with blue-shifted emission for organic light emitting diode applications, it is often necessary to have electron poor ligands that are also relatively high on the spectrochemical series. The trifluoropropynyl ligand, C2CF3, is also electron withdrawing and has never been directly compared to C2C6F5. Herein complexes of the type trans-[M(cyclam)(C2R)2]OTf, where M = CoIII and CrIII and C2R = C2C6F5, are prepared and characterized by X-ray crystallography, cyclic voltammetry, and UV-Vis, IR, and Raman spectroscopy. Emission spectroscopy for the CrIII complex is also reported. These C2C6F5 complexes are compared to the corresponding complexes where C2R = C2CF3, and C2C6H5. For trans-[Co(cyclam)(C2R)2]OTf, the reversible CoIII/II reduction for the C2C6F5 complex occurs 120 mV more cathodic than that for the C2CF3 complex, indicating that C2CF3 is more electron withdrawing, a conclusion that is supported by analysis of CC stretching frequencies. The lowest energy metal-centered (MC) 1A1g → 1T2g absorption band for the C2CF3 and C2C6F5 complexes in CH3CN appear at 425 and 438 nm, respectively, suggesting that C2CF3 is a stronger field ligand. In the region of the LMCT transitions, the absorption spectra for the trans-[Cr(cyclam)(C2R)2]OTf complexes, where C2R = C2C6H5 and C2C6F5, show significant vibronic structure that is also superimposed on the MC transitions, suggesting some degree of charge transfer character in the latter. The CrIII complexes are all emissive in room-temperature fluid solution with the C2CF3 complex showing significant fine structure and the C2C6H5 and C2C6F5 complex emission being broad, structureless, and redshifted by comparison. This suggests that emission in the C2CF3 complex is of 2Eg (Oh) origin and emission for the C2C6H5 and C2C6F5 complexes is of 2T1g (Oh) origin. Emission for all CrIII complexes in 77 K glass shows significant fine structure. The presence of a dominant 0-0′ emission band in the C2C6H5 and C2C6F5 complexes suggests a non-centrosymmetric ground state. This is supported by the crystal structure for trans-[Cr(cyclam)(C2C6F5)2]OTf that shows an angle of 77.8° between the planes defined by the aryl rings.

Controlled Intramolecular Charge Transfer Using a Sulfur-Containing Acceptor Group

A series of donor–acceptor (D–A) and donor–acceptor–donor (D–A–D) molecules containing anthracene chromophores paired with a sulfur-containing acceptor group have been prepared. The oxidation state of the acceptor sulfur atom controls the degree of charge transfer character in the excited state wave function. Steady-state photoluminescence spectroscopy shows more pronounced solvatochromism with increasing sulfur oxidation state. Computational methods (DFT/TD-DFT) are used to show that orbital mixing between donor and acceptor is facilitated as the oxidation state of the acceptor sulfur is increased.

Excited state intramolecular proton transfer (ESIPT) from phenol OH (OD) to adjacent “aromatic” carbons in simple biphenyls

This work reports results of further studies on a new class of excited state intramolecular proton transfer (ESIPT), from phenol OH to adjacent aromatic carbon atoms of suitably designed biphenyl systems. For this purpose, a number of 2-phenylphenols 3– 6 with methyl and methoxy substituents on the adjacent proton accepting phenyl ring were synthesized. In particular, we were also interested in studying the effect of an acetyl (ketone) substituent on the proton accepting ring (biphenyl 7) and the effect on the photochemistry when the ketone is reduced to alcohol (biphenyl 8). All compounds except for 7 were found to undergo deuterium exchange ( Ф ex = 0.019–0.079) primarily at the 2′-position on photolysis in 1:3 D 2O–CH 3CN. This is consistent with a reaction mechanism involving initial ESIPT from the phenol OH to the 2′-position of the adjacent phenyl ring, to generate a biphenyl quinone methide intermediate which rapidly tautomerizes back to starting material. Biphenyl 8 also undergoes a competing photosolvolysis reaction (overall loss of water). Both photosolvolysis and ESIPT reactions react via isomeric quinone methide intermediates and are best interpreted as arising from an excited singlet state that possesses a large degree of charge transfer character, from the phenol ring to the attached phenyl ring. The failure of 7 to react may be due to two possible reasons: (i) high intersystem crossing rate to a non-polarized triplet excited state and/or (ii) a polarized singlet state that is now much more basic at the carbonyl oxygen. The results are consistent with qualitative examination of calculated HOMOs and LUMOs (AM1).

Spectroscopy and Femtosecond Dynamics of Excited-State Proton Transfer Induced Charge Transfer Reaction

Fluorescence spectroscopy and femtosecond relaxation dynamics of 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) and 2-{[2-(2-hydroxyphenyl)benzo[d]thiazol-6-yl]methylene}malononitrile (diCN-HBT) are studied to probe the excited-state proton transfer (ESPT) coupled charge transfer (ESCT) reaction. Unlike most of the ESPT/ESCT systems previously designed, in which ESCT takes place prior to ESPT, both diCN-HBO and diCN-HBT undergo ESPT, concomitantly accompanied with the charge transfer process, such that the ESPT reaction dynamics are directly coupled with solvent polarization effects. The long-range solvent polarization interactions result in a solvent-induced barrier that affects the overall proton transfer reaction rate. In cyclohexane, the rate constant of ESPT of diCN-HBO is measured to be 1.1 ps (9.1 x 10(11) s(-1)), which is apparently slower than that of 150 fs for the parent molecule 2-(2'-hydroxyphenyl)benzoxazole (HBO). Upon increasing solvent polarity to, for example, CH 3CN, the rate of ESPT is increased to 300 fs (3.3 x 10(12) s(-1)). The results are rationalized by the stabilization of proton transfer tautomer, which possesses a large degree of charge transfer character via an increase of the solvent polarity, such that the corresponding solvent-induced barrier is reduced. We thus demonstrate a prototypical system in which the photon-induced nuclear motion (proton transfer) is directly coupled with solvent polarization and the corresponding mechanism is reminiscent of that applied in an electron transfer process.

Metal-to-ligand charge transfer absorption in a rhenium(I) complex that contains a π-conjugated bipyridine acceptor ligand

This Letter describes a study of the absorption and electroabsorption spectroscopy of a complex consisting of the –Re I(CO) 3Cl chromophore coordinated to the 4,4 ′-{bis-[2,5-(dimethoxyphenyl)ethynyl]}-2,2 ′-bipyridine ligand. The objective of the study is to explore whether Re → bipyridine metal-to-ligand charge transfer (MLCT) excitation leads to electron delocalization into the π-conjugated 2,5-(dimethoxyphenyl)ethynylene moieties. The results indicate that the two low-energy absorption bands observed for the complex have a strong degree of charge transfer character; however, the bands are dominated by intraligand (IL) π,π * transitions. The spectroscopic data are consistent with possible configuration mixing between the lowest IL π,π * and MLCT transitions.

The effect of molecular environment on the photochemistry of p-methoxycinnamic acid and its esters

In addition to rapid, reversible trans-cis photoisomerisation, air-saturated solutions of p-methoxycinnamic acid and 2-ethylhexyl- p-methoxycinnamate in polar solvents undergo a slower, irreversible photoreaction when exposed to ultraviolet radiation. However, no such reaction is evident in polar solvents that are deoxygenated or in air-saturated, non-polar solvents. The fluorescence spectra were also dependent on the solvent polarity with a shift to longer wavelengths in polar solvents. This behaviour suggests a degree of charge transfer character in the lowest excited singlet state responsible for fluorescence in a polar molecular environment. The fluorescence yields of solutions of these cinnamoyl molecules were low, ≤1.6 × 10 −3, indicating that there are very efficient non-radiative pathway/s for non-radiative relaxation of the excited singlet states of these molecules.

Photochemistry and photocuring activities of novel substituted 4′-(4-methylphenylthio) benzophenones as photoinitiators

The photoinduced polymerization activities of 19 novel substituted 4′-(4-methylphenylthio)benzophenones (or 4′-(4-tolylthio)-benzophenones) were determined and compared with that of the unsubstituted derivative itself in different monomers and prepolymers using real-time infrared (RTFTIR) and pencil hardness methods. Absorption, fluorescence and phosphorescence analyses, as well as photoreduction/photolysis studies, were undertaken on the compounds, and the data were related to the photopolymerization activities. Solvent shift studies on the absorption maxima of all the compounds indicate the presence of a long-wavelength single ππ ∗ state with some nπ ∗ character. The absorption maxima of the bistolythio, amino, chloro and nitro derivatives are more red shifted than those of the simpler alkyl, alkoxy, phenyl and phenoxy derivatives. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The fluorescence emission maxima are markedly red shifted with increasing solvent polarity, indicating that the lowest excited singlet state exhibits a high degree of charge transfer character. The lowest excited triplet state of all the 4-substituted 4-tolythiobenzophenones is essentially a ππ ∗ configuration, with some nπ ∗ character, as indicated by the phosphorescence lifetime data. The lowest excited triplet state of the unsubstituted derivative, on the other hand, is essentially ππ ∗ in character with a longer emission lifetime and a lower quantum yield. The emission maxima are independent of the nature of the substitution, indicating the presence of a rigid molecular structure. The latter is confirmed by the observation of very high phosphorescence quantum yields. The bistolythio derivatives exhibit shorter phosphorescence emission lifetimes, indicating that their lowest excited triplet state is nπ ∗ in nature. The triplet ππ ∗ character of the amino derivatives is indicated by their increased emission lifetimes. The electron-withdrawing effect of the chloro and nitro groups markedly reduces the phosphorescence quantum yield. These effects are accounted for by the relative spacings of close-lying lowest excited singlet ππ ∗ and second excited triplet nπ ∗ states controlling the competitive processes of internal conversion and intersystem crossing. Compared with the unsubstituted 4-tolylthio derivative, all the alkyl-, alkoxy-, phenyl- and phenoxy-substituted derivatives exhibit higher activities as photoinitiators in the absence of an amine cosynergist. Lengthening or broadening of the alkoxy substituent also gives rise to an increased photoinitiation activity, whereas alkyl groups are less effective. The presence of an amine cosynergist enhances photocuring, giving rise to variable effects depending on the nature of the triplet exciplex interaction. The bistolythio derivatives exhibit the highest photoinitiation activity, whereas the 4-nitro group has a marked deactivating effect. The presence of the 4-amino group also enhances the photoinitiation activity, but only by RTFTIR measurements. The photoreduction/photolysis rates of all the 4-substituted derivatives are greater than that of the unsubstituted derivative and show some relation to the nature of the lowest excited triplet state. The photophysical and photochemical data are discussed in relation to the relative, photoactivities of the compounds as photoinitiators for curing acrylated multifunctional monomers.

Photochemie und Photophysik von 3‐(2‐Isoxazolinyl)‐phenylketonen

Photochemistry and Photophysics of 3‐(2‐Isoxazolinyl)‐phenylketones3‐Benzoyl‐Δ2‐1,2‐oxazolines (1–6) are formed by 1,3‐dipolar cycloaddition between benzoylnitril‐N‐oxide (8) and dihydrofurane 9 or 1,3‐dioxep‐5‐enes (10a–c). The preparative yields are small due to the competitive dimerization of the dipole 8. Two stereoisomers are obtained by using 2‐substituted 1,3‐dioxep‐5‐enes as dipolarophiles. The different steric position of the substituents in 3–6 gives rise to different spectral data. The synthesized ketones possess triplet states with a high degree of charge transfer character. Therefore, the ability to H‐abstraction reaction from alcohols is small. For ketone 2 and methanol as H‐donor a rate constant of k = 4,1 · 102 M−1s−1 is determined. Also by electron transfer reactions with triethylamine and some onium compounds the reactivity of the T1 of the ketones 1–6 is less compared to those of nπ* excited ketones. The photolysis of the ketones takes place very unselectively and leads to a product mixture. The quantum yields for the decay of the ketones are 10−2 to 10−3.

Structure-reactivity relationship for fluorescence quenching of trans-stilbene by tertiary amines

The quenching of rans-stilbene fluorescence by ten amines of formula (CH 3) 2NCH 2R in n-hexane has been examined. There is an excellent linear free energy relationship between the Stern-Volmer quenching constants and the inductive substitutent constants σ 1, giving a slope ϱ of −1.24 and a correlation coefficient of −0.997 for amines with RH, CHCH 2, Ph, CCH, CCCH 3, CO 2Et or CN. The negative and small absolute value of ϱ is indicative of a mechanism involving exciplexes with a moderate degree of charge transfer character (contact ion pairs) with partial positive charge on the amines molecules.

Photochemistry of thioxanthones VI: A polymerization, spectroscopic and flash photolysis study on novel water-soluble substituted 3-(9-oxo-9 H-thioxanthene-2,3-γ-4-yloxy)- N,N,N-trimethyl-1-propanaminium salts

The photopolymerization activity and photochemistry of nine novel water-soluble 3-(9-oxo-9 H-thioxanthene-2,3-γ-4-yloxy)- N,N,N-trimethyl-1-propanaminium salts is examined in water and 2-propanol using absorption, luminescence and conventional microsecond flash photolysis techniques. For both the chloro and the methylsulphonate salts, photopolymerization activity in water follows the order of substitution 4- > 2- > 3-. Methyl substitution in the 1-position deactivates the molecule and this is reflected by a reduction in both the photopolymerization activity and luminescence (fluorescence and phosphorescence) quantum yields and the triplet lifetime. Intramolecular hydrogen-atom abstraction is the main competitive process suppressing photochemical activity. The longest wavelength absorption maxima and extinction coefficients and luminescence quantum yields are similar to those of water-soluble 2-substituted derivatives studied previously, indicating a lowest excited singlet state with strong n̄n̄ * character. On changing the solvent from 2-propanol to water all the fluorescence quantum yields are enhanced and this is consistent with a strong degree of charge transfer character in the lowest excited singlet state. From conventional microsecond flash photolysis results, transient absorptions below 400 nm are associated with the intermediate ketyl radical formed by the lowest excited singlet/triplet states of the thioxanthene molecule abstracting a hydrogen atom from the amine synergist (used in photopolymerization) while absorptions above 400 nm are associated with the radical anion intermediate formed by a concurrent electron abstraction process from the amine. The latter is confirmed through a detailed study on the effect of pH and amine concentration on transient production, as well as the effect of various amines of increasing ionization potential. Transient formation is only partly reduced in aerobic conditions and is consistent with our earlier findings and conclusions that photopolymerization activity is associated, in the main, with electron abstraction by the lowest excited singlet state of the thioxanthone molecule from the tertiary amine synergist. This is confirmed by a correlation between the photopolymerization activity and the ionization potential of different amines.

Free Rotor Styrylcyanine Chromogens

Fluorescence spectra (maximum wavelength λF) and fluorescence quantum yields (φF) were measured for four structurally related styrylcyanine chromogens of the pyridinium and quinolinium type (1–4) in different solvents at ambient temperature and 77 K. The response of λF and φF values to changes in solvent polarity, solvent hydrogen bonding donor strength, viscosity and temperature was a sensitive function of chromogen structure. The sensitivities of the λF and φF values correlate with the degree of charge transfer character of the S1,CT state; Stokes shift of fluorescence was progressively decreased while φF value was enhanced as the CT character of S1,CT state increases. Moreover, a large edge‐excitation red shift was observed in ethanol glass at 77 K. The dominant photophysical features for these dyes are discussed in terms of strong emission from an intramolecular CT state characterized by different solvation sites indicated by the observation of the excitation‐wavelength dependent phenomenon in ethanol at 77 K and an important non‐radiative decay channel involving rotation of the different parts of molecules leading to a more relaxed weakly fluorescent S1,CT created in fluid media. The viscosity dependence of fluorescence properties (a marked increase in φF was observed with increasing viscosity) suggests that these dyes can be useful reporters of microviscosity for different sites in various organized assemblies. Moreover, it was suggested that increasing H‐bonding donor strength of the solvent activates a rotatory non‐radiative decay channel probably by localizing charge densities and decreasing CT nature of the S1,CT state.

On the possibility of protein-chromophore charge transfer in visual pigments.

Abstract— Quantum chemical calculations have been performed on the retinylidene Schiff base assumed to be protonated or hydrogen bonded at the nitrogen and perturbed by a carboxylate ion at its β‐ionone end. The results indicate that weak charge transfer type bands might exist due to electron transfer from the carboxylate ion to the Schiff base and that the intense (π, π*) band in the visible spectrum might acquire a significant degree of charge transfer character.

The effect of bromine substitution on the charge-transfer emission of the complex of phenanthrene and tetracyanobenzene

The absorption spectra of the charge-transfer complexes of sym-tetracyanobenzene (TCNB) with phenanthrene, 9-bromophenanthrene, and 9,10-dibromophenanthrene are measured in chloroform solutions at room temperature. The total emission and phosphorescence spectra of the donors and the complexes are measured at 77 K in rigid glasses. The phosphorescence decay lifetimes are determined for phenanthrene, TCNB, and for the phenanthrene-TCNB complex, and a decrease in the phenanthrene-TCNB complex lifetime relative to the lifetimes of the two components is observed. The luminescence spectra of the complexes exhibit both a red shift and a lack of structure as compared with the donor spectra. The results are interpreted, in agreement with the results of Iwata et al. for the phenanthrene-TCNB complex ( 1), as an indication that there is a considerable degree of charge-transfer character in the lowest triplet state ( T 1). Bromine substitution leads to a decrease in the energy of the phenanthrene triplet state. As a result, the energy gap between the donor molecule triplet state and the complex charge-transfer triplet state decreases from phenanthrene, to 9-bromophenanthrene, to 9,10-dibromophenanthrene. The results suggest that the proximity of these two triplet states in 9,10-dibromophenanthrene and its charge-transfer complex leads to some local donor triplet state character in the emitting complex triplet state.