Thermo-Solvatochromism of Merocyanine Polarity Probes – What Are the Consequences of Increasing Probe Lipophilicity through Annelation?

Merocyanines: Electronic Structure and Spectroscopy in Solutions, Solid State, and Gas Phase

Merocyanines, owing to their readily tunable electronic structure, are arguably the most versatile functional dyes, with ample opportunities for tailored design via variations of both the donor/acceptor (D/A) end groups and π-conjugated polymethine chain. A plethora of spectral properties, such as strong solvatochromism, high polarizability and hyperpolarizabilities, and sensitizing capacity, motivates extensive studies for their applications in light-converting materials for optoelectronics, nonlinear optics, optical storage, fluorescent probes, etc. Evidently, an understanding of the intrinsic structure-property relationships is a prerequisite for the successful design of functional dyes. For merocyanines, these regularities have been explored for over 70 years, but only in the past three decades have these studies expanded beyond the theory of their color and solvatochromism toward their electronic structure in the ground and excited states. This Review outlines the fundamental principles, essential for comprehension of the variable nature of merocyanines, with the main emphasis on understanding the impact of internal (chemical structure) and external (intermolecular interactions) factors on the electronic symmetry of the D-π-A chromophore. The research on the structure and properties of merocyanines in different media is reviewed in the context of interplay of the three virtual states: nonpolar polyene, ideal polymethine, and zwitterionic polyene.

Reproducing the Solvatochromism of Merocyanines by PCM Calculations

Polarizable continuum methods (PCM) have been widely employed for simulating solvent effects, in spite of the fact that they either ignore specific interactions in solution or only partially reproduce non-specific contributions. Examples of three solvatochromic dyes with a negative, a positive and a reverse behavior illustrate the achievements and shortcomings of PCM calculations and the causes for their variable success. Even when qualitatively mimicking non-specific solvent effects, departures of calculated values from experimental data may be significant (20-30%). In addition, they can utterly fail to reproduce an inverted behavior that is caused by significant specific contributions by the solvent. As shown through a theoretical model that rationalizes and predicts the solvatochromism of phenolate merocyanines based on DFT (Density Functional Theory) descriptors in the gas phase, PCM shortcomings are to be held responsible for its eventual failure to reproduce experimental data in solution.

Computational Quantification of the Zwitterionic/Quinoid Ratio of Phenolate Dyes for Their Solvatochromic Prediction

Solvatochromic dyes are utilized in various chemical and biological media as chemical sensors. Unfortunately, there is no simple way to predict the type of solvatochromism based on the structure of the dye alone, which restricts their design and synthesis. The most important family of solvatochromic sensors, pyridinium phenolate dyes, has the strongest solvatochromism. Using a natural population analysis (NPA) of the natural bond orbitals (NBO) of the phenolate group in the frontier molecular orbitals, it is possible to calculate the relative polarity of the ground state and excited state and, thus to develop a model that can predict the three types of solvatochromism observed for this family: negative, positive, and inverted. This methodology has been applied to thirteen representative examples from the literature. Our results demonstrate that the difference in the electron density of the phenolate moiety in the frontier molecular orbitals is a simple and inexpensive theoretical indicator for calculating the relative polarity of the ground and excited states of a representative library of pyridinium phenolate sensors, and thus predicting their solvatochromism. Comparing the results with the bond length alternation (BLA) and bond order alternation (BOA) indices showed that the NPA/NBO method is a better way to predict solvatochromic behavior.

Anionic merocyanine dyes based on thiazol-2-hydrazides: reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts

Reverse solvatochromism, preferential solvation and multiparametric approaches to spectral shifts of (4-nitro/cyanophenyl)-substituted thiazol-2-hydrazide colored merocyanine dyes are presented.

Solvatochromism in perylene diimides; experiment and theory

We report an experimental and computational investigation into the solvatochromism of a perylene diimide derivative. The alkyl swallowtail substituents allowed solubility in many solvents of widely differing polarity, with a complicated resultant behaviour, illustrating both negative and positive solvatochromism as a function of dielectric constant. Luminescence quantum yield and optical absorption linewidth displayed an inverse correlation, indicating varying degrees of intermolecular aggregation, and a remarkably similar trend was found between the peak absorption wavelength and the solvent boiling point, illustrating the dependency of aggregation on the solvent interactions. These outline trends may be parameterised by an empirically derived dimensionless quantity, as a tool to be used in more sophisticated future models of solvatochromism in small molecule chromophores.

Structure–behavior study of a family of “hybrid cyanine” dyes which exhibit inverted solvatochromism

The reverse solvatochromism of twenty “hybrid cyanine” dyes reflects their different responses to some properties of the solvent.

The solvatochromism of phenolate betaines: comparing different cavities of a polarized continuum model

Two variations of the polarized continuum model employing default ("PCM model") and SMD radii ("SMD model") were compared for the reproduction of the solvatochromic behavior of Reichardt's betaine dye, and of eight other phenolate betaines that exhibit a negative, positive or an inverted solvatochromic behavior. Molecules were optimized at the CAM B3LYP/6-31+G(d,p) level of theory, and transition energies were calculated with the TD-DFT method. The PCM model failed to reproduce the negative and the inverted solvachromism of these dyes in protic solvents. The SMD model, though not entirely accounting for hydrogen-bond effects in small, polar hydroxylic solvents, should be recommended as a better alternative for the theoretical simulation of the solvatochromism of phenolate betaines in medium to highly polar solvents. Graphical Abstract A comparison of two polarized continuum models ("default PCM" and "PCM/SMD") for reproducing the solvatochromism of phenolate betaines, with nine examples of negative, positive, and inverted behavior.

Self-aggregation and optical absorption of stilbazolium merocyanine in chloroform

Dipolar aggregation is in many cases detrimental for the functioning of optical materials. In this study we investigate self-aggregation and optical absorption of stilbazolium merocyanine (SM) in chloroform solution by performing classical Molecular Dynamics (MD) simulations under ambient conditions. The reversal solvatochromic shift, the large bathochromic shift, and the structured absorption band presented by SM in chloroform solution are all aspects of its optical absorption behavior for which the existence of self-aggregation is yet not completely understood. Moreover, the spectroscopic properties of SM oligomers and their occurrence in solvent of low polarity remain a relevant topic that deserves to be investigated. Our analysis of the aggregation behavior of SM in chloroform verified that the majority of the chromophores are involved in the formation of oligomers in solution, where the whole dimer and part of the trimer populations present a stable π-stacking structure. The optical properties of the monomers and oligomers in solution were evaluated by means of a discrete polarizable embedding quantum mechanical/molecular mechanical (PE-QM/MM) response scheme where the quantum part is described at the level of density functional theory. The visible absorption spectrum of SM in chloroform is simulated using time average values obtained for the monomeric and oligomeric forms of SM from the PE-QM/MM calculations performed on uncorrelated configurations extracted from the classical MD simulations. This study shows that the self-aggregation of SM in chloroform may exist, but it is not essential for reproducing the reversal solvatochromic shift in chloroform and that the process does not contribute to enhance the bathochromic shift nor explain the structure observed in its absorption band. Moreover, it is verified that since the electronic transitions of the monomer and oligomers are close together, changes in the interplane separation between the monomeric units of the stacked oligomers substantially affect the spectral resolution of their contribution to the optical absorption spectrum.