Unlocking the Potential of Push-Pull Pyridinic Photobases: Aggregation-Induced Excited-State Proton Transfer

Abstract

The pH effect on the photophysics of three push-pull compounds bearing dimethoxytriphenylamine (TPA-OMe) as electron donor and pyridine as electron acceptor, with different ortho-functionalization (-H, -Br, and -TPA-OMe), is assessed through steady-state and time-resolved spectroscopic techniques in DMSO/water mixed solutions and in water dispersions over a wide pH range. The enhanced intramolecular charge transfer upon protonation of the pyridinic ring leads to the acidochromic (from colorless to yellow) and acido(fluoro)chromic (from cyan to pink) behaviours of the investigated compounds. In dilute DMSO/buffer mixtures these molecules exhibited low pK_a values (≤3.5) and extremely short singlet lifetimes. Nevertheless, it is by exploiting the aggregation phenomenon in aqueous environment that the practical use of these compounds largely expands: i) the basicity increases (pKa≈4.5) approaching the optimum values for pH-sensing in cancer cell recognition; ii) the fluorescence efficiencies are boosted due to Aggregation-Induced Emission (AIE), making these compounds appealing as fluorescent probes; iii) longer singlet lifetimes enable Excited-State Proton Transfer, paving the way for the application of these molecules as photobases (pK_a*=9.1). The synergy of charge and proton transfers combined to the AIE behaviour in these pyridines allows tunable multi-responsive optical properties providing valuable information for the design of new light-emitting photobases.

Keywords: Aggregation-induced emission; Excited-state proton transfer; Photobases; Push-pull compounds; Ultrafast spectroscopy.

Scheme 1

Chemical structures of the investigated push‐pull systems: the dimethoxytriphenylamine (TPA‐OMe) portion acts as the electron donor D (push, in black) while the heterocyclic pyridine as the electron acceptor (pull) and proton acceptor A (in red).

Figure 1

Spectrophotometric (A) and fluorimetric (B) titrations of compound Py‐1 in a large pH range (DMSO/buffer 60/40 %v/v). Fluorescence spectra were obtained by exciting each sample at the isosbestic point (λ_exc=372 nm). Insets: (A) from left to right Py‐1 at pH=9, 4.86, 0.33; (B) from left to right Py‐1 at pH=9, 4.86, 0.33 under blue laser irradiation. pK_a fittings obtained from the spectrophotometric (C) and fluorimetric (D) titrations for compound Py‐1 in DMSO/buffer 60/40 (%v/v).

Figure 2

Frontier molecular orbitals for neutral and protonated Py‐1 at the ground (min S₀) and excited state (min S₁) optimized geometry.

Figure 3

Fs‐FUC (λ_exc=400 nm) results for Py‐1 in DMSO/buffer 60/40 (%v/v) at different pHs. Panel A: spectra obtained at different delay times after excitation. Thicker lines refer to representative spectra corresponding to the explicit time delays (see legend). Panel B: Evolution Associated Spectra (EAS) obtained for the transients provided by Global Analysis with related lifetimes: Solv./S_1,LE (solvation/locally‐excited singlet state, black line), Solv./VC (solvation/vibrational cooling, brown), SR (structural relaxation, green line), and S_1,ICT, (stabilized singlet state with ICT nature, red line). Panel C: Representative kinetics acquired at selected wavelengths to probe the S_1,LE, black, and S_1,ICT, red, deactivation dynamics.

Figure 4

Spectrophotometric (A) and fluorimetric (B) titrations together with relative pK_a fittings (insets) obtained for compound Py‐1 in water dispersion [DMF/buffer (1/99 %v/v)] in a large pH range. Fluorescence spectra were obtained by exciting each sample at the isosbestic point (λ_exc=379 nm).