Studies on Magnetic Field Effects in Fluorescence of Intramolecular Exciplex Systems and Application

Abstract

A photoexcited molecule could be quenched by a ground state quencher via electron transfer, yielding two quenching products: a transient emissive complex (exciplex) and a spin-correlated radical ion pair (RIP). The exciplex and RIP could interconvert to each other reversibly. If the RIP undergoing singlet-triplet transition by hyperfine interaction, an external magnetic field could alter the coherent interconversion, increasing exciplex emission. Magnetic field effects (MFEs) on the intramolecular exciplex emission could be an important tool for studying spin dynamics and mapping nano- and micro-magnetic fields with fluorescence microscopy. In the first part of this dissertation, a quantitative magnetic field distribution imaging was carried out using a pyrene-based magnetosensing exciplex fluorophore, pyrene-(CH2)12-O-(CH2)2-N,N-dimethylaniline (Py-12-O-2-DMA) on a conventional fluorescence microscope system with an off-the-shelf LED lamp but without continuous sample supply. The solvent condition (anisole/DMF, v/v=50/50) was carefully optimized as monitoring the brightness of the exciplex fluorescence and the extent of modulation caused by the external magnetic field. The exciplex emission from Py-12-O-2-DMA increases by ca. 1.5 times under the external magnetic field of 50 mT and it is ca. 24.7 times brighter than previously reported phenanthrene-based magnetosensing fluorophore, phenanthrene-(CH2)12-O-(CH2)2-N,N-dimethylaniline (Phen-12-O-2-DMA) under the same condition. Moreover, Py-12-O-2-DMA can be excited by longer wavelength excitation light source (up to 375 nm) than that by which Phen-12-O-2-DMA can. Combination of these advantages allows magnetic field images with high S/N ratio in milder condition: low illumination power, reduced sample concentration and simpler optical setup, which will attract a wide range of potential users for magnetic field distribution imaging. We also demonstrated its feasibility for a 3D magnetic field distribution image by two-photon fluorescence microscopy. In the second part of this dissertation, the MFE in exciplex emission has been studied for decades, but it only has been observed to occur in solvents with a limited range of polarity. This limitation is mainly because the reversible interconversion between two quenching products of photo-induced electron transfer process, the exciplex and magnetic field-sensitive RIP, collapses beyond that polarity range. In a non-polar solvent, the formation of RIPs is suppressed, whereas in a polar solvent, the probability of their re-encounter forming the exciplexes reduces. In this study, we developed new exciplex-forming (phenyl-Phenanthrene)-(phenyl-N,N-Dimethylaniline)-peptoid conjugates (PhD-PCs) to overcome this limitation. The well-defined peptoid structure allows precise control of the distance and the relative orientation between two conjugated moieties. Steady-state and time-resolved spectroscopic data indicate that the PhD-PCs can maintain the reversibility, which allows MFEs in exciplex emission regardless of the solvent polarity. Subtle differences between the exciplex emissions of the PhD-PCs were observed and explained by their exciplex geometries obtained through time-dependent density functional theory (TD-DFT) calculations. In the third part of this dissertation, the exciplex is a transient-sandwiched complex composed of an electron-donor and an electron-acceptor. The exciplex emission shows a structureless emission band with a longer wavelength region due to large charge separation. We synthesized and characterized two new chain-linked exciplex-forming systems, pyrene-(CH2)12-O-(CH2)2-Julolidine (Py-12-O-2-Julolidine), and pyrene-COO-(CH2)11-O-(CH2)2-N,N-dimethylaniline (PyCOO-11-O-2-DMA). The exciplexes of the two chain-linked systems exhibit an emission band in a longer wavelength region as compared with that of Py-12-O-2-DMA. The lifetime of exciplex emission decreases in the order of Py-12-O-2-DMA, Py-12-O-2-Julolidine, and PyCOO-11-O-2-DMA. Based on the behaviors of exciplexes formed from the modified electron-donor (Julolidine) or electron-acceptor (pyrene-COO-), it could be concluded that the degree of charge-separation of the exciplex increases in order Py-12-O-2-DMA, Py-12-O-2-Julolidine, and PyCOO-11-O-2-DMA. The mixture of Py-12-O-2-DMA and Py-12-O-2-Julidine showed independent exciplex formation and luminescence processes, respectively, under 355-nm illumination.