Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer

Abstract

Replacing conventional dialkylamino substituents with a three-membered aziridine ring in naphthalimide leads to significantly enhanced brightness and photostability by effectively suppressing twisted intramolecular charge transfer formation. This replacement is generalizable in other chemical families of fluorophores, such as coumarin, phthalimide, and nitrobenzoxadiazole dyes. In highly polar fluorophores, we show that aziridinyl dyes even outperform their azetidinyl analogues in aqueous solution. We also proposed one simple mechanism that can explain the vulnerability of quantum yield to hydrogen bond interactions in protonic solvents in various fluorophore families. Such knowledge is a critical step toward developing high-performance fluorophores for advanced fluorescence imaging.

A General Strategy to Enhance Donor‐Acceptor Molecules Using Solvent‐Excluding Substituents

“…We next sought to explore whether the effect of the β‐carbonyl can be expanded to small azacyclic donating groups (Figures a,b; see Table S1 and Figure S1) . Of these donors, azetidine is known to enhance fluorescence quantum yield and brightness by the inhibition of TICT.…”

“…These results suggest the β‐carbonyl acts to prevent solvent‐mediated quenching mechanisms, in particular EC. For the pyrrolidino‐based donors 7 – 9 , which are known to undergo TICT, increasing fluorescence brightness and quantum yield are observed in ethanol ( Φ EtOH =0.10 for 7 and 0.28 for 9 ). In nonpolar 1,4‐dioxane, this trend persists ( Φ dioxane =0.31 for 7 and 0.47 for 9 ), suggesting a potential role of β‐carbonyl to inhibit TICT.…”

“…In nonpolar 1,4‐dioxane, this trend persists ( Φ dioxane =0.31 for 7 and 0.47 for 9 ), suggesting a potential role of β‐carbonyl to inhibit TICT. β‐carbonyl functionalization fails to improve upon the behavior of the piperidino derivatives 10 – 12 , likely because of the low‐energy, twisted conformation stably present in the ground state . In all cases, Beer's Law plot was used to ensure that all compounds were soluble at the concentrations and solvents of measurement, suggesting that the effect of β‐carbonyl was not due to the change of solubility (see Figure S16).…”

“…In recent years, a focus has been placed on achieving the above goal to develop more effective fluorophores with improved fluorescence quantum yields and photostability for application in biological and photoorganic chemistry . In particular, small azacyclic donors have been used to inhibit TICT in D‐A and rhodamine‐based fluorophores, improving their application in super‐resolution microscopy . Additionally, N ‐cyclohexanol‐substituted amino donors have been shown to inhibit solvent–solute interactions with the excited state, attenuating the effects of EC in aqueous solutions .…”

Enhancing the Anti‐Solvatochromic Two‐Photon Fluorescence for Cirrhosis Imaging by Forming a Hydrogen‐Bond Network

“…[7] Redshift is due to matching alignment between the molecules and the solvent that lowers the HOMO-LUMO gap. [9] In accord with the "energy gap law" energy is released through electron transition from excited states to the empty orbitals. [9] In accord with the "energy gap law" energy is released through electron transition from excited states to the empty orbitals.…”