Kasha's Exciton Research Articles

Twisted Phosphors that Violate Kasha's Exciton Model in Organic Systems

Comprehensive SummaryKasha's exciton model proposes that T1 energy levels of organic compounds are insensitive to molecular aggregation and microenvironment change because of negligible small transition dipole moments of T1 states. This model holds true in most organic systems till now. Here we report the fabrication of twisted organic phosphors with intramolecular charge transfer characters and flexible molecular structures. When doped into different organic matrices, the twisted phosphor adopts different conformation, exhibits distinct phosphorescence colors and T1 energy levels, which violates Kasha's exciton model in organic system. Given that the change of phosphorescence colors and maxima can be readily distinguished by human eyes and conventional instrument, the twisted phosphors would be exploited as a new type of molecular probe, which would exhibit potential application in optical sensing and stimuli‐responsive systems.

TD‐DFT analysis of the excitation of H‐dimers of cationic dyes in an aqueous solution using functionals without additional dispersion correction

AbstractThis work is a development and extension of the previous one (DOI: 10.1039/d3cp00882g). Here, H‐dimers of acridine (acridine orange—AO and proflavine—PF), thiazine (methylene blue—MB and thionine—TH), and oxazine (brilliant cresyl blue—BCB and Nile blue—NB) dyes were modeled using hybrid functionals with a large proportion of exact Hartree–Fock exchange and long‐range correction. It turned out that nine functionals (LC‐ωHPBE, M06HF, M052X, M062X, M08HX, M11, MN15, SOGGA11X, and ωB97XD) reliably stabilize these molecular aggregates in both the ground and excited states. In addition, these functionals ensure that the conditions for transition moments (M(dimer) ≈ M(monomer) from strong coupling theory for H‐aggregates) and absorption maxima (λmax(dimer) < λmax(monomer) from Kasha exciton theory) are met. The S2 excited state stabilizes the H‐dimers more strongly than the ground state, while the S1 state stabilizes even more than S2. This is due to the large overlap between the corresponding molecular orbitals (LUMO > HOMO−1 > HOMO). When calculating the vibronic absorption spectra, the best agreement with the experiment for AO2, PF2, and NB2 showed the M08HX functional, and M11—for MB2 and BCB2. For dye monomers, these functionals gave the worst agreement, and MN15 demonstrated the closest similarity to the experiment. Vibronic absorption spectra for AO2, MB2, BCB2, and NB2 were calculated for the first time. The exciton splitting is calculated, which for MB2 is in good agreement with the experimental value.

Photoactivation Properties of Self-n-Doped Perylene Diimides: Concentration-dependent Radical Anion and Dianion Formation.

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R•–) and dianions (D••2–). We observe R•– formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D••2– formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R•– to the strongly reducing doublet excited state 2[R•–]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state 1[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R•– and D••2– formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

Bis(merocyanine) Hetero‐Folda‐Dimers: Evaluation of Exciton Coupling between Different Types of π‐Stacked Chromphores

Exciton coupling between different types of chromophores has been rarely investigated. Herein, a systematic study on the exciton coupling between merocyanine chromophores of different conjugation length with varying excited state energies is presented. In this work well-defined hetero-dimer stacks were obtained upon folding of bis(merocyanine) dyes in nonpolar solvents. They show distinctly different absorption properties in comparison with the spectra of the single chromophores, revealing a significant coupling between the different chromophores. The simulated absorption spectra obtained from time-dependent density functional theory (TD-DFT) calculations are in good agreement with the experimental spectra. Our theoretical analysis based on an extension of Kasha's exciton theory discloses strong coupling between the dyes' transition dipole moments despite of an excited-state energy difference of 0.60 eV between the chromophores.

Concerted Interplay of Excimer and Dipole Coupling Governs the Exciton Relaxation Dynamics in Crystalline Anthracenes.

A combined theoretical and experimental investigation into the role of concerted long- (dipole coupling) and short-range (orbital overlap mediated excimer) electronic interactions in modulating the emission of six crystalline acetylanthracenes (1-3) is reported. Friedel-Crafts acylation of anthracene rendered crystalline acetylanthracenes with discrete close packing, varied orbital overlap, and resultant distinct emission (blue-green-yellow) from cooperative excimer and dipole coupling. Time-resolved emission spectroscopy (TRES) studies and the Kasha's exciton theory based quantitative estimation of dipole coupling (mean-field approximation) substantiates the exciton dynamics in crystalline 1-3. Extension of the Kasha's exciton model beyond the traditional nearest-neighbor approach, and consistent agreement among the computed spectral shifts and TRES temporal components, corroborate a holistic approach to decipher the exciton relaxation dynamics in the molecular assembly of novel photonic materials.

Davydov-type excitonic effects on the absorption spectra of parallel-stacked and herringbone aggregates of pentacene: Time-dependent density-functional theory and time-dependent density-functional tight binding.

Exciton formation leads to J-bands in solid pentacene. Describing these exciton bands represents a challenge for both time-dependent (TD) density-functional theory (DFT) and for its semi-empirical analog, namely, for TD density-functional tight binding (DFTB) for three reasons: (i) solid pentacene and pentacene aggregates are bound only by van der Waals forces which are notoriously difficult to describe with DFT and DFTB, (ii) the proper description of the long-range coupling between molecules, needed to describe Davydov splitting, is not easy to include in TD-DFT with traditional functionals and in TD-DFTB, and (iii) mixing may occur between local and charge transfer excitons, which may, in turn, require special functionals. We assess how far TD-DFTB has progressed toward a correct description of this type of exciton by including both a dispersion correction for the ground state and a range-separated hybrid functional for the excited state and comparing the results against corresponding TD-CAM-B3LYP/CAM-B3LYP+D3 results. Analytic results for parallel-stacked ethylene are derived which go beyond Kasha's exciton model [M. Kasha, H. R. Rawls, and A. El-Bayoumi, Pure Appl. Chem. 11, 371 (1965)] in that we are able to make a clear distinction between charge transfer and energy transfer excitons. This is further confirmed when it is shown that range-separated hybrids have a markedly greater effect on charge-transfer excitons than on energy-transfer excitons in the case of parallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functional and TD-lc-DFT calculations lead to negligible excitonic corrections for the herringbone crystal structure, possibly because of an overcorrection of charge-transfer effects (CAM refers to Coulomb attenuated method). In this case, TD-DFT calculations with the B3LYP functional or TD-DFTB calculations parameterized to B3LYP give the best results for excitonic corrections for the herringbone crystal structure as judged from comparison with experimental spectra and with Bethe-Salpeter equation calculations from the literature.

Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized Solar Cells

V-shaped squaraine dyes with carbazole cores in the center and indolenine groups at the end were designed to broaden the light-harvesting range of dye-sensitized solar cells (DSSCs) by taking advantage of intramolecular exciton coupling. V-shaped squaraines containing carboxy group on the indolenine moiety and 4-phenyl-2-cyanoacrylic acid groups on the carbazole cores as anchoring groups for chemical adsorption onto TiO2 were synthesized by the utilization of Stille cross-coupling as a key reaction. Although conventional squaraine dyes show typically narrow absorption bands, V-shaped squaraine dyes exhibited split absorption bands due to the intramolecular exciton coupling and thereby enabled the absorption of a broad range of light. The exciton coupling between squaraine chromophores obliquely installed in the molecule was theoretically supported by the Kasha exciton coupling model, in which the coupling energy was well-fitted with the experimental absorption spectra. As the result of intramolecular exciton coupling, the DSSCs based on exciton-coupled squaraine dyes exhibited a spectral response in a much wider range compared to that of a conventional linear counterpart, giving the higher energy conversion efficiency in comparison to that of the single chromophoric counterpart.

Exciton Coupling of Merocyanine Dyes from H- to J-type in the Solid State by Crystal Engineering.

A key issue for the application of π-conjugated organic molecules as thin film solid-state materials is the packing structure, which drastically affects optical and electronic properties due to intermolecular coupling. In this regard, merocyanine dyes usually pack in H-coupled antiparallel arrangements while structures with more interesting J-type coupling have been rarely reported. Here we show that for three highly dipolar merocyanine dyes, which exhibit the same π-scaffold and accordingly equal properties as monomers in solution, the solid-state packing can be changed by a simple variation of aliphatic substituents to afford narrow and intense absorption bands with huge hypsochromic (H) or bathochromic (J) shifts for their thin films and nanocrystals. Time-dependent density functional theory calculations show that the energetic offset of almost 1 eV magnitude results from distinct packing motifs within the crystal structures that comply with the archetype H- or J-aggregate structures as described by Kasha's exciton theory.

Configuration of organic dye excimers in nanoporous SiO2 matrices

The effect of cyanine dye 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO) and benzimidazole dye 4-dimethylamino-1,8-naphthoylene-1′,2′-benzimidazole (DNBI) accumulation in nanoporous silica matrices on the dyes luminescence properties has been studied. For both dyes, ground state dimer formation with perpendicular transition dipoles at high dye concentrations has been considered as a result of restricted geometry of the nanoscale pores. The dimer excitation leads to excimer formation revealing by appearance of new long-wavelength luminescence band and shortening the dye luminescence lifetime. In the excimer luminescence excitation spectra two additional bands have been observed, one of which is bathochromically shifted relatively to the absorption band and another one is hypsocromically shifted. Using the Kasha exciton model it was shown that the excimers possess oblique transition dipoles configuration.

Spectroscopic study of ordered hybrid complexes formation between dye aggregates and ReVO4:Eu3 + (Re = Y, Gd, La) nanoparticles

For the first time we report on ordered nano-sized hybrid complexes formation between fluorescent aggregates of potential-sensitive mitochondria probe 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolocarbocyanine iodide (JC-1) and ReVO4:Eu3+ nanoparticles (NPs) in aqueous solutions. To study the efficiency and driving forces of the dye aggregates/NPs complex formation, we analyzed the effects of the dye molecular structure on its aggregation behavior using JC-1 derivatives with different alkyl chain lengths and functional groups (JC-1/C4, JC-1/C3S). The aggregation behavior of the dyes in the solutions either with or without NPs has been analyzed within the Kasha exciton model framework. It was found that depending on the dye structure herringbone-type aggregation (for JC-1) and J-type aggregation (for JC-1/C4 dye) can be observed. The interactions with ReVO4:Eu3+ NPs take place due to Coulomb forces and provoke changes of the aggregate parameters such as the angle between polarization axes of the unit molecules and the line of molecular centers (θ), angle between transition dipole moments of the individual dye molecules (α) and center-to-center distance between molecules (R). The effect of NP shapes (spherical, spindle-like and rod-like) on the structure of hybrid complexes is studied. For the first time, using a microspectroscopic technique we demonstrate the JC-1 aggregates/NPs complex accumulation within the hepatocyte nucleus region in vitro experiment rather than a typical JC-1 specific accumulation in cell mitochondria.

All-organic chromophores for dye-sensitized solar cells: A theoretical study on aggregation

We report a computational study on the aggregation of organic chromophores used in dye-sensitized solar cells. More specifically, the effects of dimerization on the optical properties are discussed. Ground state properties have been obtained at Density Functional Theory level including dispersion corrections in the case of the dimers, while excited states have been obtained at Time-Dependent Density Functional Theory level. Several stable dimers conformations were obtained, and the influence of the relative inter-monomer angle as well as of substituents has been evidenced. Absorption spectra and excited states topology were investigated for both monomers and dimers. Dimers excited states have been interpreted in the framework of the Kasha exciton theory. Finally, we thoroughly described the photo-induced charge transfer occuring upon excitation. The formation of excitons and the split of electronic density between the two monomers can strongly affect the electronic injection in solar cells devices, since it modifies the nature of the excited states.

Aggregative properties of Rhodamine dyes in polyacrylamide hydrophilic gel media

The visible absorption spectra of two Rhodamine dyes (R6G and RB) in aqueous solutions, and in the polyacrylamide hydrogel matrix with different composition were studied at room temperature. The spectral properties of the dye-loaded hydrogel were also investigated. The transport and the solute–solute interactions of the ionic dyes in aqueous solutions across the hydrophilic gels were calculated. The monomer–dimer equilibrium of these ionic dyes in water and in different composition of hydrogel environment with different soaking time has been investigated by means of UV–Vis spectroscopy. The natures of the interacting pairs in these dyes were discussed using the Kasha exciton theory.

Hydrophilic gel composition effect on the molecular association and spectroscopic behavior of rhodamine B

Electronic absorption spectra of rhodamine B dye in aqueous solutions and in a polyacrylamide hydrogel matrix with different structural compositions were studied at room temperature. The transport and the solute–solute interactions of rhodamine B in aqueous solutions across the hydrophilic gels were investigated via exploration of the spectral properties of the dye-loaded hydrogel. The nature of the dye pair interactions in these media was discussed using the Kasha exciton theory. In addition, the monomer–dimer equilibrium of rhodamine B molecules in hydrogels with different compositional percentages was studied by means of UV–vis spectroscopy and least-squares fitting methods.

Bis(Diquaternary Ammonium)Salts: Synthesis, Effect of Spacer on Surface Activities and Aggregation Properties of Reactive Red 198 in Aqueous – Surfactants Solutions

Abstract A series of bis-diquaternary ammonium salts (cationic Gemini surfactants) with different length in spacer were prepared and characterized. The surface properties such as the critical micelle concentration CMC, minimum surface tension γCMC, effectiveness of the surface tension reduction πCMC, maximum surface excess Γmax and surface area per molecule Amin have been determined by means of surface tension measurements. As expected, these surfactants have good surface activities than those conventional monomeric ones (dodecyl trimethyl ammonium chloride (DTAC)) and the spacer plays an important role in the surface properties. Under experimental conditions, surface parameters decreases with the increase of carbon number of the spacer the except analogue with a benzene ring due to its inflexibility. The aggregation ability of some practically used reactive azodyes (C.I. Reactive Red 198) in aqueous solutions and the prepared surfactants was investigated using UV-visible spectroscopic methods. The spectroscopic studies were extended in order to determine the effect of molecular structure of the surfactants on the aggregative properties of C.I. Reactive Red 198 (RR 198). The aggregation behaviour of the RR 198 in solutions and in the presence of additives is related to their apparent deviations from Beer's law and aggregation number (N). The aggregation constant (Kn) of the monomer/dimer equilibrium was calculated. The aggregate structures and the nature of the interacting pairs in this dye were discussed using Kasha exciton theory.

The Photophysics of a Polar Molecule in a Nonpolar Cryogenic Glass - The Effects of Dimerization on (1-Butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP)

The first excited state of BIDP was shown in a previous communication to exhibit an ultrafast decay in fluid nonpolar, nonprotic solutions due to the presence of a S(1)/S(0) conical intersection (CI). In frozen polar and nonpolar glasses a strong fluorescence was observed, rationalized by the hindering of the internal torsion required to reach the geometry of the CI. Complete analysis of the data was hampered by some unusual observations in nonpolar glasses. In this paper we show that they can be explained by assuming dimer formation, with a formation constant of K(eq) = (4 ± 3) × 10(5) M(-1) at 83 K and ΔH(dim) = 7 ± 2 kcal/mol. A complete analysis of the spectra is presented, and fluorescence quantum yields of the monomer and dimer are reported. Efficient self-quenching is found, with a Stern Volmer constant, K(SV) = (1.5 ± 0.1) × 10(6) M(-1), assigned to static quenching. The dimer absorption spectrum was extracted from the data and is compared to Kasha's exciton model and to quantum chemical (QC) calculations. The basic features of exciton splitting are reproduced by quantum mechanical calculations, but complete quantitative agreement of the QC computations with the experimental results is not attained. The previous analysis of the monomer spectra using the displaced harmonic oscillator model is extended to the more demanding conditions prevailing at cryogenic temperatures. The derived ΔH(dim) is in good agreement with other dimers formation enthalpies and with the quantum mechanical calculation presented. The new analysis corrects τ(f) in MCHIP to 2.9 × 10(-13) s, somewhat smaller than the value reported in polar solvent in a previous communication, thereby strengthening the assumption that polarity can reduce the efficiency of CI.

Effect of polyacrylamide hydrophilic gel composition on photo-physical behavior of Oxazine 750

Electronic absorption spectra of Oxazine 750 dye in aqueous solutions and in polyacrylamide hydrogel matrix with different structural composition were studied at room temperature. The transport and the solute–solute interactions of the ionic dye in aqueous solutions across the hydrophilic gels were investigated via exploring spectral properties of the dye-loaded hydrogel. The nature of the dye pair interactions in these media was discussed using the Kasha exciton theory. In addition, the monomer–dimer equilibrium of Oxazine 750 in hydrogels with different compositional percentage has been determined by means of UV–vis spectroscopy and least square fitting methods.

Effect of hydrophilic gel composition on photo-physical behavior of Oxazine720

The visible absorption spectra of Oxazine 720 dye in aqueous solutions and in polyacrylamide hydrogel matrix with different compositions were studied at room temperature. The spectral properties of the dye-loaded hydrogel were also investigated. The transport and the solute–solute interactions of the ionic dye in aqueous solutions across the hydrophilic gels were calculated. The nature of the interacting pairs in this dye was discussed using the Kasha exciton theory. Also the monomer–dimer equilibrium in this ionic dye in water and in different compositions of hydrogel environment has been investigated by means of UV–Vis spectroscopy.

Controllable Adsorption and Ideal H-Aggregation Behaviors of Phenothiazine Dyes on the Tungsten Oxide Nanocolloid Surface

The monomer-aggregate equilibrium of four phenothiazine (PN) dyes, containing thionine (TH), methylene blue (MB), new methylene blue (NMB), and 1,9-dimethylmethylene blue (DMB), in the tungsten(VI) oxide (WO(3)) nanocolloid solution has been investigated by means of UV-vis spectroscopy. Addition of PN dye into the WO(3) nanocolloid solution brought about significant changes in the absorption spectrum, suggesting the formation of H-type (face-to-face fashion) trimer on the WO(3) nanocolloid surface. The adsorptivity of PN dyes onto the WO(3) nanocolloid surface was diminished by the raising the ionic strength, indicating the evidence of the electrostatic interaction between cationic PN dye and negatively charged WO(3) nanocolloids. The detail analysis of each spectral data provided insight into the effect of molecular structure of PN dyes on the adsorption and aggregation behaviors on the WO(3) nanocolloid surface. Moreover, in situ measurement of PN dye aggregation using the centrifugal liquid membrane (CLM) technique revealed that the aggregation of PN dyes on the WO(3) nanocolloid surface proceeded in a two-step three-stage (monomer --> dimer --> trimer) formation. The aggregation mechanism of PN dyes on the WO(3) nanocolloid surface was discussed on the basis of Kasha's exciton theory.

Photophysical properties of long rodlike meso-meso-linked zinc(II) porphyrins investigated by time-resolved laser spectroscopic methods.

The molecular design of directly meso-meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413-414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.