Sequential Multistep Excited‐State Structural Transformations in N,N′‐Diphenyl‐dihydrodibenzo[a,c]phenazine Fluorophores

Abstract

We demonstrate that a single polycyclic π‐scaffold can undergo sequential multistep excited‐state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room‐temperature solution. By installing a methyl or trifluoromethyl group on the ortho‐site of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well‐resolved triple bands. The ultra‐broadband triple emissions of ortho‐substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho‐substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho‐substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited‐state structural relaxation.

Dual Stimuli‐Responsive [2]Rotaxanes with Tunable Vibration‐Induced Emission and Switchable Circularly Polarized Luminescence

“…To gain a better understanding of such interesting tunable VIE behaviors of these DPAC‐functionalized [2]rotaxanes, femtosecond transient absorption (TA) spectra were performed under 360 nm excitation in THF (Table 1). Notably, the femtosecond transient absorption (TA) of sole DPAC unit in cyclohexane has been reported, [14] and its excited‐state structure evolution has been analyzed and its multiple fluorescence emission has been assigned. Since the VIE behaviors of [2]rotaxanes and the corresponding axle compounds in our study were investigated in THF, the TA of sole DPAC unit in THF was first tested.…”

Dual Ratio and Ultraprecision Quantification of Mitochondrial Viscosity in Ferroptosis Enabled by a Vibration-Based Triple-Emission Fluorescent Probe

“…The compound DPAC and PPAC were prepared according to previous references. − The synthetic route of the probe PPAC-C4 is presented in Scheme S1 and the related molecular structures have been characterized by 1 H NMR, 13 C NMR, and HRMS. The optimized structures and the corresponding Cartesian coordinates of PPAC-C4 are provided in the calculation results and analysis of Supporting Information in detail.…”

“…Vibration-induced emission (VIE) is a new luminescence mechanism proposed by Tian and co-workers. − DPAC (N, N-disubstituted-dihydrodibenzo­[a,c]­phenazines) as a classic fluorophore exhibits the viscosity-dependent double emissions from the bent and twisted molecular configuration upon excitation. Very recently, our research group found that by regulating the substituent steric hindrance, the single VIE fluorophore has the opportunity to appear bent, planar, and distorted states with successively increasing emission wavelengths, respectively .…”

Dual Stimuli‐Responsive [2]Rotaxanes with Tunable Vibration‐Induced Emission and Switchable Circularly Polarized Luminescence

“…To gain a better understanding of such interesting tunable VIE behaviors of these DPAC-functionalized [2]rotaxanes, femtosecond transient absorption (TA) spectra were performed under 360 nm excitation in THF (Table 1). Notably, the femtosecond transient absorption (TA) of sole DPAC unit in cyclohexane has been reported, [14] and its excited-state structure evolution has been analyzed and its multiple fluorescence emission has been assigned. Since the VIE behaviors of [2]rotaxanes and the corresponding axle compounds in our study were investigated in THF, the TA of sole DPAC unit in THF was first tested.…”