N,O-boron spirocyclic complexes: How electronic and spatial factors modulate photophysical and physicochemical properties

Abstract

This study presents the synthesis of new spirocyclic boron-coordinated (SBCs) fluorophores based on a simple and facile procedure and their optical properties. The structure of the complexes was confirmed by ¹H, ¹³C, ¹¹B NMR spectra, high resolution mass spectra and single crystal X-ray diffraction data. The effect of the substituents in the terminal aromatic cycles and the spirocycle size on the photophysical properties was investigated by a combined experimental and computational approach. The absorption and fluorescence emission properties of the SBCs were investigated and the results obtained revealed evident relationships between the electronic nature of the substituents and their photophysical properties. These differences clearly divide the spirocomplexes into two groups with significant differences in absorption and emission properties. SBCs with an electron acceptor substituent in cycle A and an electron donor substituent in cycle B have an emission maximum in the shorter wavelength region (416–451 nm) and a quantum yield of 2.3–12.5 %. In another group of compounds with the opposite set of substituents in cycles A and B, the emission maximum is located at 469–560 nm, the quantum yield is much lower (0.1–0.4 %), but the Stokes shift (8114-8160 cm⁻¹) increases significantly. These differences were explained using state-of-the-art quantum chemical calculations, which show that SBCs differ in the values of the electronic charges transferred in ESs (S_1v and S_1r) and their localisation, and have different conjugated systems. SBCs exhibited increased quantum yield with increasing medium viscosity, under mycelial formation conditions, and significant aggregation-enhanced emission behaviour in THF-H₂O and DMSO-H₂O mixtures, along with excellent solid-state photophysical properties. Cellular uptake and cytotoxicity studies using Vero cells showed that SBCs are biocompatible and selectively accumulate in cell lipid droplets, endoplasmic reticulum, Golgi apparatus and mitochondria.