Marina L. Dekhtyar

Ultrafast Excited-State Dynamics of Donor−Acceptor Biaryls: Comparison between Pyridinium and Pyrylium Phenolates

The excited-state dynamics of two donor-acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck-Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from a normal to an overcritical merocyanine evidenced by steady-state spectroscopy.

Merocyanines: polyene–polymethine transition in donor–acceptor-substituted stilbenes and polyenes☆

Three series of donor–acceptor-substituted conjugated compounds, namely, stilbenes, the open-chain polyenes of equivalent length, and the species of intermediate structure (polyenes terminated with only one phenyl ring) have been studied by the AM1 and HMO methods to elucidate and compare the structural prerequisites of the ideal polymethinic state (‘‘cyanine limit’’). The transition from polyenic to polymethinic properties has been traced in terms of bond-length (bond-order) alternation using the variation of terminal donor and acceptor substituents. Stilbenes manifest themselves as notably ‘‘retarded’’ polyenes since a larger electronic asymmetry is necessary for them to reach the same degree of polymethinic character. The ground and the excited state have been shown to differ much more strongly for stilbenes than for polyenes with respect to the position of the bond equalization point on the scale of donor–acceptor difference. For the compounds containing one phenyl ring, the features revealed are intermediate between stilbenes and polyenes. The large S0–S1 discrepancy in terms of bond alternation is a general property of aromatic ring-terminated chains (stilbenes) and is related to the influence of the aromatic character which can be quantified in this way. In this context, the most relevant definition for the cyanine limit (based on the bond invariance upon excitation) was selected from the existing definitions. The major trends revealed in the polyenic/polymethinic behaviour of the molecules can be interpreted on a topological basis within HMO or even simpler models with some additional influence due to the interelectronic repulsion which is taken into account in the AM1 treatment. 2003 Elsevier Science B.V. All rights reserved.

Polyenic/polymethinic relationships for donor–acceptor substituted stilbenoids: Structural, electronic and spectroscopic aspects

A selection of donor–acceptor-substituted stilbenoids of strongly differing polarities was investigated by the AM1 method with full geometry optimization in the ground and the excited state. The results obtained including structural parameters, electronic density distribution, molecular orbital (MO) energies and localizations and S0–S1 energy gaps were rationalized using topological approaches, the Huckel MO (HMO) and the long chain approximation. In this context, a number of analytical expressions were deduced for the molecular characteristics of an arbitrarily terminated conjugated chain and they were employed to make a correlation between two series of model compounds with varied asymmetry, open-chain polyenes (ACH–(CHCH)5–CHD, I) and their ring-terminated analogues (AC5H4–CHCH–C5H4D, II), where A and D represent acceptor and donor moieties. As a result, the relationship of donor–acceptor-substituted stilbenes to polymethines and polyenes was elucidated, and the special role of the aromatic character in π-electron-based stilbenoid peculiarities was interpreted.

Photochemical switching through protonation in merocyanines

Abstract The ground- and excited-state energies for the planar and various 90-degree twisted structures of the merocyanine Me 2 N–CHCH–CHCH–CHO and its protonated form + Me 2 N CH–CHCH–CHCH–OH have been calculated by the AM1 method involving complete geometry optimization. On successively twisting bonds in the polymethine chain, the energies of the resulting twisted forms display a pronounced alternation which is opposite in the ground and the excited state. This trend in computed energy values as well as the reversed alternation pattern for the two types of polymethine compounds can be anticipated from simple qualitative MO considerations and in the topological long-chain approximation.

Application of the quasi-long chain approximation to structural perturbations in polymethine dyes

A topological approach is presented for the treatment of the optical and chemical behaviour of polymethine dyes, and which can be regarded as a theory for structural perturbations of a polymethine chain caused by arbitrary end-groups. The theoretical basis of the method is discussed in terms of Greens functions, and various alternative ways to the formalism developed are reviewed briefly. Special attention is paid to a simple computational technique for estimating certain molecular parameters. A variety of applications of the method are considered, namely in the search for stable infrared polymethine dyes, the design of structural modifications to yield desired spectral effects, and the determination of the electrophilic and nucleophilic reaction centres in heterocycles.

Origin of states connected with twisted intramolecular charge shift in polymethine cations: a simple analytical treatment

Abstract Based on the Hueckel method and simple model of long-range interelectron repulsion, the tendency for polymethine cations to form twisted structures in the excited state is elucidated. Changes in energy level positions and populations as well as the intramolecular charge transfer occurring on twisting are simulated in terms of π-decoupling of the corresponding conjugated system. The charge transfer between the fragments formed is shown to depend on the end-group nature and to alternate in direction for rotations of successive bonds in the polymethine chain. It is also reversed on switching from the ground to the excited state. The energy advantage of certain excited-state twistings over the planar form can be understood by taking into account the long-range Coulomb interaction of electrons in a quasi-one-dimensional system. On this basis, electron density transfer from the longer to shorter fragment is preferable and can compensate the general energetic disadvantage of π-decoupling upon twisting. Using the [Me2N–(CH)13–NMe2]+ cation as an example, it is inferred that the rotation around the 2–3 bond in the excited state is highly probable for long streptopolymethines, whereas twisting the 1–2 bond is improbable. The reverse predictions are found for boron-containing polymethines.

Electronic properties of polymethine systems. Part 4: Electronic structure of polymethine chain

On the basis of the topological concept of electron donor ability, the electronic structure is considered for two main classes of symmetric linear polymethine systems, polymethine dyes and hetarylpolyenes. Quantum-chemical evidence and analytical relations are presented which account for substantial distinctions between polymethines and polyenes in the π-electronic density distribution over atoms and bonds, both in the ground and excited states. Existence conditions for so-called ideal polymethinic and polyenic states are expressed in terms of electron donor abilities defined for end-groups, and for a polymethine system as a whole.

Small S0–S1 energy gaps for certain twisted conformations of unsymmetric polymethine dyes: quantum chemical treatment and spectroscopic manifestations

The AM1–CI (Austin model 1 configuration interaction) method has been used to calculate energy gaps between the ground state (S0) and the lowest biradicaloid (BR) excited state as a measure for roughly estimating the probability of reaching an S0–BR conical intersection by bond twisting of one of the bonds along the polymethine chain of unsymmetric cyanine dyes. Invoking qualitative concepts for interpreting the computational data, a pronounced alternation of successive bonds is shown and explained. The magnitude of the S0–BR energy gap was found to be governed by the position of the corresponding bond with respect to the polymethine chain ends and by the degree of electronic asymmetry of the chain, i.e. by the difference in electron donor abilities of its end groups. These results are compared to experimental photophysical data for a series of differently bridged asymmetric cyanine dyes and suggest that the magnitude of the energy gap for a given twisted bond can be modulated by bridge-induced perturbations. Nonreactive low-lying minima caused by the accessibility of several twisted excited-state conformers have to be invoked in order to explain the experimentally observed ‘‘ inverse-loose-bolt’’ effect of the nonradiative decay rates and should be taken into account in the treatment of possible pathways of fluorescence quenching and in the design of high-quantum-yield fluorescent dyes.

Nonequilibrium molecular transport photoinduced by potential energy fluctuations

The mechanism of directed substrate-parallel motion of molecules caused by photoinduced potential energy fluctuations is investigated. Unlike simplistic models (e.g., an on-off ratchet), the approach suggested implies that the necessary asymmetry of the potential energy can arise not only from the asymmetry of the substrate potential but also from an asymmetric distribution of the fluctuating charge density in the molecule. The thus induced asymmetry of the potential energy governs the direction motion and enables, under certain conditions, its reversal at some frequencies of resonant laser pulses or temperature. These inferences are exemplified by the model charge distributions in the molecule and substrate, and the charge density fluctuations which are obtained by quantum chemical calculations for the realistic molecule of a substituted phenylpyrene compound on a model substrate.

Two heterocyclic merocyanine classes and their optical properties in relation to the donor-acceptor strength of end substituents

The relationship between the first electronic transition energy of heterocyclic merocyanines and donor-acceptor properties of their end substituents has been quailtatively treated and analytically substantiated in terms of the classical valence bond model and the biradicaloid theory. A simple graphical technique has been suggested to classify donor-acceptor-substituted compounds in relation to their electronic structure and to predict the effect of donor-acceptor strength on their optical behavior. In this context, merocyanines can be divided into two classes which differ in charges and electron populations of their donor and acceptor moieties and exhibit mirror-like responses to the variation of donor-acceptor strength. Two families of new heterocyclic merocyanines have been synthesized and their experimentally observed spectroscopic properties have been rationalized using the approach developed.