Molecular Design of UV–vis Absorption and Emission Properties in Organic Fluorophores: Toward Larger Bathochromic Shifts, Enhanced Molar Extinction Coefficients, and Greater Stokes Shifts

Abstract

Understanding the molecular origins of the optoelectronic properties of fluorophores provides rational guidelines for chemists to synthesize better-performing dyes. Factors affecting the UV−vis absorption spectral shift, molar extinction coefficients, and Stokes shift of fluorophores are herein examined at the molecular level, via both (time-dependent) density functional theory-based calculations and the empirical harmonic-oscillator-stabilization-energy (HOSE) and bond-lengthalternation (BLA) models. The importance of these factors is discussed using six coumarin dyes as exemplars. In particular, a special focus is devoted to the Stokes shift, a critical parameter in fluorophores. It is demonstrated that incorporating a "rotational" substituent in a fluorophore molecule with tailored steric hindrance effects and resonance effects leads to a substantial increase in the Stokes shift, not only in coumarins but also in other chemical dye families: boron-dipyrromethenes (BODIPYs), cyanines, and stilbenes. Structure−property relationships concerning the rotational substituent are discussed in detail with examples of several dye families. These findings lead to the proposal of molecular design criteria that enable one to tune the Stokes shift. Such criteria provide a foundation for the molecular engineering of fluorophores with improved optoelectronic properties.

Varied Length Stokes Shift BODIPY-Based Fluorophores for Multicolor Microscopy

“…All of the other fluorophores in the BAA library that exhibited Stokes shifts >30 nm possessed amine groups with lone electron pairs that delocalize into the styryl π-system. This was in agreement with a previously observed, strong effect of an electron-donating aniline on the Stokes shift of BODIPY dyes 25,26 . In the case of styryl and related BODIPYs, such as those described herein, the delocalization of the amine group lone pairs lead to charge-separated structures that were relatively strong contributors to excited state geometries.…”

“…The diverse styryl modifications of the core BODIPY structure extended its π-conjugation resulting in substantially red-shifted absorbance and emission wavelengths 21,22 . The maximum absorbance and emission of each unique BAA fluorophore was influenced by the electron withdrawing or donating tendencies of the aromatic aldehyde used for styryl modification 23,24 .…”

A Multifunctional Chemical Probe for the Measurement of Local Micropolarity and Microviscosity in Mitochondria

“…Our strategy to develop a multifunctional mitochondrial probe, MMP , appropriate for the simultaneous monitoring of microviscosity and micropolarity was to combine a fluorescent phenylquinoxaline-derived molecular rotor that undergoes internal rotation at a rate dependent on local microviscosity 28 (but independent of polarity variations), with coumarin 343, a solvatochromic polarity probe 29 with blocked internal bond rotations that avoid local microviscosity crosstalk. Attaching these two dyes to the termini of a mitochondria-penetrating peptide separates the functional modules of the probe, and thus, the electric dipolar nature and solvatochromic character of the coumarin moiety is purely directed by the polarity of the cellular microenvironment.…”

Transforming Benzophenoxazine Laser Dyes into Chromophores for Dye‐Sensitized Solar Cells: A Molecular Engineering Approach

“…Our previous studies have shown the HOSE model to be a useful tool for correlating bond length patterns to the optical properties of other organic dyes. [29][30][31] Figure 6. Correlation between experimentally determined peak absorption wavelengths λ peak max and the C p Q contribution calculated from the solution-state DFT-generated structures of 1-4 modeled in methanol.…”