Study on the pH Dependence of the Photophysical Properties of a Functionalized Perylene Bisimide and Its Potential Applications as a Fluorescence Lifetime Based pH Probe

Abstract

In this work, we have investigated the dependence on the pH of the photophysical properties of a functionalized perylene bisimide (PBI) and its potential pH sensing applications. Observed was the presence of aggregates which diminishes at acid pH values and low concentrations, without totally disappearing until a temperature of 80 °C was reached. At basic pH, significant changes in the absorption spectrum were observed, which were associated with more strongly coupled aggregates. The 1 H NMR spectra of the PBI dye in D 2 O/TFA also showed the dependence of aggregation on concentration and temperature. PBI fluorescence intensity and lifetime were also sensitive to pH values. The maximum fluorescence intensity and lifetime were observed in acid medium, in which protonation of the secondary amines on the PBI side chains likely hinders the formation of strongly coupled aggregates. On the contrary, the fluorescence intensity significantly decreased in basic medium, due to deprotonation of the amine groups and the formation of stronger aggregates. Density functional theory calculations corroborated that π-stacked aggregates of PBI derivatives are stable in the protonated state, but their supramolecular structure changes. In the aggregate, monomeric units slide over their adjacent ones and increase the intermolecular distance upon the protonation. Intermolecular hydrogen bonds can help maintain the stability of the protonated aggregate. Fluorescence lifetime showed a sigmoidal dependence on pH, with a linear response range between pH 6 and 8, both in Tris•HCl buffered solutions and in a synthetic buffer mimicking the intracellular environment. The biocompatibility of the PBI dye was tested in C3H10T1/2 mesenchymal cells. The cellular uptake was confirmed by confocal fluorescence microscopy. No significant effects on cellular viability and morphology were observed at the conditions in which compound 1 can be used as a fluorescent probe. This work supports the idea that PBI derivatives can be suitable dyes for fluorescence lifetime sensing applications.

Understanding the Driving Mechanisms of Enhanced Luminescence Emission of Oligo(styryl)benzenes and Tri(styryl)‐s‐triazine

“…Effective control of molecular assembly has promisinga pplications in differentt echnological and scientific fields. For instance, someb iomedical applications, such as biosensing, bioimaging, [14][15][16][17][18][19] and gene and drug delivery, [2,20,21] are based on molecular self-assembled systems. In addition, the crystal packing adopted by p-conjugatedc ompounds determines their electronic andp hotophysicalp roperties and their later use in light-emitting electronic devices, [1,14,15] chemiresistives ensors, [15,22] agricultural films, [23] among others.…”

Perylene Diimide-Based Fluorescent and Colorimetric Sensors for Environmental Detection

“…The colorimetric sensing thus observed is due to the electronic change of the cyclobutene skeleton, which in turn results in dramatic electronic change in PDI core through direct conjugation between the cyclobutene and PDI. This pH sensing mechanism is reminiscent of the commonly used pH indicators [32][33][34][35][36][37][38]. Other molecular design strategies (some uncommon) have also been brought into this field.…”

“…Sensors for transition metal ions from IB group (Cu 2+ ), IIB group (Zn 2+ , Cd 2+ , Hg 2+ ) and VIIIB group (Fe 3+ , Ni 2+ , Pd 2+ ), and alkaline earth metals IIA group (Ba 2+ ), and boron family IIIA group (Al 3+ ) have been studied in both aqueous and binary water-organic solutions . Besides metal ions, sensors for other cations like proton (pH detection) have also been developed, particularly for the bio-relevant systems [32][33][34][35][36][37][38]. Similarly, sensors have also been studied for the non-metal anions such as F − , CN − , ClO 4 − and PO 4 − ions, etc.…”

“…In view of the strong tendency of aggregation PDIs molecules through stacking interactions between the π-conjugated skeletons in aqueous media, most studies of PDI-based chemosensors are in organic-aqueous composite systems to maintain the molecular dispersion of PDI sensors [7,8,[15][16][17][18][19][20][22][23][24][25][26][27][29][30][31]. There still remain challenges to study the sensing performances in pure water solutions [10,21,[32][33][34][35][36][37][38][39]. Most importantly, the fluorescence variations of PDIs are pH-dependent and thus the corresponding sensors should be operated under well-controlled pH of the working solution, for which addition of buffer reagent like (2-[4-(2-hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid) (HEPES) is usually necessary [15,16,20].…”

“…As seen above, solution pH plays crucial role in affecting the assembly-disassembly and PET-controlled fluorescence sensing of PDIs. Especially for the detection of non-metal anions, most of the PDI-based fluorescent sensors used are relied on pH dependent (H + variation) PET process [32][33][34][35][36][37][38]. In these cases, water-solubility is a primary requirement for design and synthesis of PDIs sensor molecules.…”