Donor and Ring‐Fusing Engineering for Far‐Red to Near‐Infrared Triphenylpyrylium Fluorophores with Enhanced Fluorescence Performance for Sensing and Imaging

Abstract

Fluorescent probes have become an indispensable tool in the detection and imaging of biological and disease‐related analytes due to their sensitivity and technical simplicity. In particular, fluorescent probes with far‐red to near‐infrared (FR‐NIR) emissions are very attractive for biomedical applications. However, many available FR‐NIR fluorophores suffer from small Stokes shifts and sometimes low quantum yields, resulting in self‐quenching and low contrast. In this work, we describe the rational design and engineering of FR‐NIR 2,4,6‐triphenylpyrylium‐based fluorophores (TPP‐Fluors) with the help of theoretical calculations. Our strategy is based on the appending of electron‐donating substituents and fusing groups onto 2,4,6‐triphenylpyrylium. In contrast to the parent TPP with short emission wavelength, weak quantum yields, and low chemical stability, the obtained novel TPP‐Fluors display some favorable properties, such as long‐wavelength emission, large Stokes shifts, moderate to high quantum yields, and chemical stability. TPP‐Fluors demonstrate their biological value as mitochondria‐specific labeling reagents due to their inherently positive nature. In addition, TPP‐Fluors can also be applied to develop ratiometric fluorescent probes, as the electron‐donating ability of the 2,6‐phenyl substituents is closely correlated with their emission wavelength. A proof‐of‐concept ratiometric probe has been developed by derivatizing the amino groups of TPP‐Fluor for the detection and imaging of nitroreductase in vitro and in hypoxic cells.

Evolving a Unique Red-Emitting Fluorophore with an Optically Tunable Hydroxy Group for Imaging Nitroreductase in Cells, in Tissues, and in Vivo

“…In addition, the emission wavelength of LDOH-1 represents a bathochromic shift to 601 nm (Figure 2 and Table S1), probably due to enhancement of the hyperconjugation effects of the σ bond electrons in ethylene. 32 Unfortunately, the S/B ratio of LDOH-1 is still limited and is not ideal for developing activated fluorescent probes via hydroxyl group modulation.…”

“…As shown in Figure S2, the twist angle between rings B and C in LDOH-1 decreased after the hydroxy deprotonation, while the dihedral angle of ACF4 did not change significantly under the same conditions. In addition, the emission wavelength of LDOH-1 represents a bathochromic shift to 601 nm (Figure and Table S1), probably due to enhancement of the hyperconjugation effects of the σ bond electrons in ethylene . Unfortunately, the S/B ratio of LDOH-1 is still limited and is not ideal for developing activated fluorescent probes via hydroxyl group modulation.…”

Natural flavylium-inspired far-red to NIR-II dyes and their applications as fluorescent probes for biomedical sensing

“…16,17 To improve quantum yield and photostability, rigid flavylium dyes (F4–F7) were developed to prevent nonradiative decay. 18–23 Inspired by the fluorescence switching mechanism of rhodamine spirolactone, carboxylic acid functionalization of the flavylium scaffold was developed. The NIR fluorescence of the carboxylic-acid-functionalized flavylium is quenched when the spirol ring closes, and it is restored when the spirol ring opens.…”

Synthesis and Properties of the Doubly Oxonium-Embedded Picenes as Electron-Deficient Polycyclic Aromatic Hydrocarbons

“…Pyrylium is a promising motif for stable oxonium ion because the positive charge is delocalized and stabilized by Hückel aromaticity. Moreover, pyrylium-containing π-conjugated molecules are used as not only synthetic intermediates , but also photofunctional materials such as laser dyes, fluorescent probes, and photoredox catalysts . Indeed, most of oxonium-modified PAHs has a pyrylium skeleton. , However, PAHs incorporating two or more pyrylium rings are scarce because of their synthetic difficulties and instability. − Although Nishihara and co-workers have reported the synthesis of pentacyclic perylene and benzo­[ e ]­pyrene analogues containing two pyrylium rings, , molecular design for multiple pyrylium-containing PAHs has not been established.…”

“…Pyrylium is a promising motif for stable oxonium ion because the positive charge is delocalized and stabilized by Huckel aromaticity. Moreover, pyrylium-containing π-conjugated molecules are used as not only synthetic intermediates 10,11 but also photofunctional materials such as laser dyes, 12 fluorescent probes, 13 and photoredox catalysts. 14 Indeed, most of oxonium-modified PAHs has a pyrylium skeleton.…”