The Solitonic Nature of the Electronic Structure of the Ions of Linear Conjugated Systems

Achieving Spectral Narrowing by Strengthening the Soliton-Like Characteristics through Peripheral Group Substitution in Aromatic Heterocyclic Emitters

The effect of substituents on the spectral behavior of aromatic heterocyclic systems is a significant yet highly complex issue. Inspired by cyanine dyes, it was found that emission spectrum narrowing can be realized by regulating the terminal substituents to enhance the soliton-like characteristics of emitters. Herein, compared to the parent compound 2AcPh (full-width at half-maximum, FWHM = 0.124 eV), the introduction of fluorobenzene-group substituents in 2AcPh-PhF narrows the emission spectral FWHM to 0.108 eV. Single crystal structures reveal more homogeneous carbon-carbon bond lengths in 2AcPh-PhF compared to 2AcPh. Theoretical calculations show that the charge magnitudes on carbon atoms of 2AcPh-PhF also become homogenized, but the charges on adjacent carbon atoms are opposite. These results suggest that the soliton-like electronic structural characteristics in 2AcPh-PhF are enhanced after introducing peripheral substituents, which is considered to be the reason for the narrower emission spectrum of 2AcPh-PhF. The sky-blue organic light-emitting diode devices based on 2AcPh-PhF demonstrated a maximum external quantum efficiency of 29.17% and a small electroluminescent spectral FWHM of 25 nm.

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.

Achieving Ultra-Narrow-Band Deep-Red Electroluminescence By a Soliton-type Dye Squaraine

Due to the soliton-like electronic structural characteristics, cyanine dyes typically exhibit spectral behaviors such as large molar extinction coefficients, narrow spectra, and high fluorescence efficiency. However, their extensive applications as emitters in electroluminescence are largely ignored due to their serious emission quenching in the aggregation state. Herein, it is reported a squaraine dye (a type of cyanine) SQPhEt. At different solution concentrations, the unusual decrease in full-width at half-maxima (FWHM) with increasing Stokes shift indicates the fluorescence quenching of SQPhEt in the aggregated state is because of the strong self-absorption effect. A sensitized device structure can help to reduce the doping concentration of dye, which can effectively suppress self-absorption. Benefitting from the large molar extinction coefficient of SQPhEt, even at low doping concentrations of 0.1 wt%, efficient Förster energy transfer can be achieved. The corresponding spin-coating sensitized device based on SQPhEt as the dopant exhibits favorable deep-red emission at 668 nm with a small FWHM of 0.10 eV.

Fluorescent pseudorotaxanes of a quinodicarbocyanine dye with gamma cyclodextrin

Spectrophotometric titration of buffered solutions of gamma cyclodextrin (γCD) and 1,1'-diethyl,2,2'-dicarbocyanine (DDI) demonstrates extension of the known 1:2 host:guest complex to form a previously unreported 2:2 complex near the γCD solubility limit. Though DDI is predominantly hosted as a non-fluorescent H-aggregate, both complexes exist in respective equilibria with two secondary complexes hosting unaggregated DDI as 1:1 and 2:1 complexes. The 2:1 complex exhibits significant fluorescence emission, with a quantum yield six times that of DDI in organic solvents, but ten times lower than that of an analogous indodicarbocyanine. Fragment Molecular Orbital calculations suggest that the 2:1 complex has the tail-to-tail conformation, and that solvent access to the dye strongly favors photoisomerization. In the host-guest complex, γCD limits solvent access to the dye and hinders rotation of the quinolyl terminal groups, but nevertheless pairwise rotation of methine carbons within the γCD cavity likely remains as a significant nonradiative relaxation pathway for the excited state.

Unsymmetrical Relaxation Paths of the Excited States in Cyanine Dyes Detected by Time-Resolved Fluorescence: Polymethinic and Polyenic Forms

Novel applications of organic dyes and vast opportunities for their molecular tailoring keep the focus of the scientific community on the issues of symmetry breaking in the systems having different location of uncompensated charge, which has tremendous impact on photoluminescent properties of the dyes. In this article, we provide distinctive experimental evidence of three relaxation paths (one symmetrical and two unsymmetrical) of excited states by analysis of lifetime and spectra of time-resolved fluorescence at low temperature with strong support of quantum-chemical modeling. Importantly, the studied cyanine dye (astraphloxin) in aqueous solution has two different unsymmetrical relaxation paths of excited states in the polymethinic and donor-acceptor polyenic forms, where the last form strongly diminishes in less polar media. The experimental and computational results provide essential fundamental knowledge of molecular electronic relaxations substantially affected by matrix rigidity and polarity for design and photonic applications of elongated π-electronic systems.

Structure and Reactivity of Radical Ions: New Twists on Old Concepts

Electron transfer is the simplest reaction possible, yet it has a profound impact on the structure and reactivity of organic compounds. These changes allow a new look at some of the fundamental concepts that are used to explain organic chemistry, such as symmetry, aromaticity, and bonding. The results from high-level electronic structure calculations are used to analyze the mechanistic differences in the pericyclic reactions of simple hydrocarbons and their radical cation counterparts. The importance of state symmetry correlation, Jahn-Teller distortions, delocalization, and fractional bonding for the reaction pathways of hydrocarbon radical cations is discussed.