Zero-Overlap Fluorophores for Fluorescent Studies at Any Concentration

Abstract

Fluorophores are powerful tools for the study of chemistry, biology, and physics. However, fluorescence is severely impaired when concentrations climb above 5 μM as a result of effects like self-absorption and chromatic shifts in the emitted light. Herein, we report the creation of a charge-transfer (CT) fluorophore and the discovery that its emission color seen at low concentrations is unchanged even at 5 mM, some 3 orders of magnitude beyond typical limits. The fluorophore is composed of a triphenylamine-substituted cyanostar macrocycle, and it exhibits a remarkable Stokes shift of 15 000 cm–1 to generate emission at 633 nm. Crucial to the performance of this fluorophore is the observation that its emission spectrum shows near-zero overlap with the absorption band at 325 nm. We propose that reducing the spectral overlap to zero is a key to achieving full fluorescence across all concentrations. The triphenylamine donor and five cyanostilbene acceptor units of the macrocycle generate an emissive CT state. Unlike closely related donor–acceptor control compounds showing dual emission, the cyanostar framework inhibited emission from the second state to create a zero-overlap fluorophore. We demonstrated the use of emission spectroscopy for characterization of host–guest complexation at millimolar concentrations, which are typically the exclusive domain of NMR spectroscopy. The binding of the PF6 – anion generates a 2:1 sandwich complex with blue-shifted emission. Distinct from twisted intramolecular charge-transfer (TICT) states, experiment-supported density functional theory shows a 67° twist inside an acceptor unit in the CT state instead of displaying a twist between the donor and acceptor; it is TICT-like. Inspired by the findings, we uncovered similar concentration-independent behavior from a control compound, strongly suggesting this behavior may be latent to other large Stokes-shift fluorophores. We discuss strategies capable of generating zero-overlap fluorophores to enable accurate fluorescence characterization of processes across all practical concentrations.

Relief of excited-state antiaromaticity enables the smallest red emitter

“…A large Stokes shift of the molecular fluorescence is usually ascribed to ICT 5 , 6 or ESIPT 17 – 19 . In line with this prevailing paradigm, the unusual long-wavelength emission of SBF is often interpreted as de-excitation from the ICT states originating from HOMO–LUMO asymmetry 29 – 32 .…”

“…Molecular light-emitters are finding wide applications from electronic displays to biological imaging 1 – 4 . When developing an optimal fluorophore for a given task, primary considerations are made to the size, wavelength, quantum yield, and synthetic tunability 5 – 10 . Within this context, low molecular weight fluorophores are recently gaining significant interest, as they can easily permeate cells with minimal perturbation of the biological system 11 , 12 .…”

“…In solid-state device settings, small molecules also suffer less from intermolecular interactions and electronic coupling that often lead to luminescence quenching 13 , 14 . A large Stokes shift is another crucial factor in fluorophore design to reduce the inner-filter effect by minimizing spectral overlap between absorption and emission 5 , 6 , 15 .…”

“…Existing strategies to realize large Stokes shift rely on (i) intramolecular charge transfer (ICT) 5 , 6 , 16 , (ii) excited-state intramolecular proton transfer (ESIPT) 17 – 19 , (iii) fluorescence resonance energy transfer (FRET) 20 , 21 , (iv) desymmetrization of the molecular structure 6 , 22 – 24 , (v) excimer/exciplex emission 25 , or (vi) excited-state planarization of the benzannelated 8π system 26 – 28 . These design principles are applicable to known fluorogenic motifs, but often compromise other photophysical properties such as emission quantum yield.…”

Bottom-up Modular Synthesis of Well-Defined Oligo(arylfuran)s

“…Thus the solvent dependence of the emission shows that the excited state is stabilized in more polar solvents, which is due to an intramolecular charge transfer. 57 To our delight, most of the oligofurans complexes exhibit large Stokes shifts, [58][59][60] ranging from 142 nm to 217 nm (Table 2). Analysis of the Stokes shift values in different solvents for the investigated molecules , with an associated energy value (Δν) of ∼15000 cm −1 , which is very useful for small organic fluorophores, 61,62 especially for their imaging applications.…”