Photophysical behavior of acridine with amines within the micellar microenvironment of SDS: a time-resolved fluorescence and laser flash photolysis study

Abstract

The photophysical behavior of acridine (Acr) shows a facilitated water assisted protonation equilibrium between its deprotonated (Acr* ∼ 3.4 ns) and protonated forms (AcrH⁺* ∼ 33 ns) within a confined environment of sodium dodecyl sulphate (SDS) micelles above the critical micellar concentration of 8 mM. The acidic interface of the micelles is capable of protonating Acr whereas deprotonated Acr is partitioned into the hydrophobic core. The time-resolved-area-normalized-emission spectra confirm the presence of both Acr* and AcrH⁺*, while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH⁺* with triethylamine (TEA) results in a linear Stern–Volmer (S–V) plot, whereas non-linearity arises with N,N-dimethylaniline (DMA). Both steady-state and time-resolved quenching results with TEA are explained on the basis of excited state proton transfer (ESPT), however the reasons behind the quenching of excited Acr with DMA are proposed as ESPT followed by a photoinduced electron transfer. Partitioning of DMA at the interface makes it accessible for both Acr* and AcrH⁺* in hydrophobic and hydrophilic regions of micelles respectively. The rate of electron transfer at the interface is found to be slower compared to that in the hydrophobic core. Characterization of transient intermediates formed during ESPT and PET between Acr and amines by laser-flash photolysis also supports the observation obtained during fluorescence studies. The mode of interactions between Acr and amines inside micelles is controlled by the localization of the proton/electron donors and acceptors in different hydrophobic or hydrophilic regions of such nano-confined environments.