Dynamics of Single-Molecule Stokes Shifts: Influence of Conformation and Environment

Single-Molecule Ultrafast Fluorescence-Detected Pump-Probe Microscopy

We introduce fluorescence-detected pump-probe microscopy by combining a wavelength-tunable ultrafast laser with a confocal scanning fluorescence microscope, enabling access to the femtosecond time scale on the micrometer spatial scale. In addition, we obtain spectral information from Fourier transformation over excitation pulse-pair time delays. We demonstrate this new approach on a model system of a terrylene bisimide (TBI) dye embedded in a PMMA matrix and acquire the linear excitation spectrum as well as time-dependent pump-probe spectra simultaneously. We then push the technique toward single TBI molecules and analyze the statistical distribution of their excitation spectra. Furthermore, we demonstrate the ultrafast transient evolution of several individual molecules, highlighting their different behavior in contrast to the ensemble due to their individual local environment. By correlating the linear and nonlinear spectra, we assess the effect of the molecular environment on the excited-state energy.

White light employing luminescent engineered large (mega) Stokes shift molecules: a review

Large (mega) Stokes shift molecules have shown great potential in white light emission for optoelectronic applications, such as flat panel display technology, light-emitting diodes, photosensitizers, molecular probes, cellular and bioimaging, and other applications. This review aims to summarize recent developments of white light generation that incorporate a large Stokes shift component, key approaches to designing large Stokes shift molecules, perspectives on future opportunities, and remaining challenges confronting this emerging research field. After a brief introduction of feasible pathways in generating white light, exemplifications of large Stokes shift molecules as white light candidates from organic and inorganic-based materials are illustrated. Various possible ways to design such molecules have been revealed by integrating the photophysical mechanisms that are essential to produce red-shifted emission upon photoexcitation, such as excited state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), excited state geometrical relaxation or structural deformation, aggregation-induced emission (AIE) alongside the different formations of aggregates, interplay between monomer and excimer emission, host-guest interaction, and lastly metal to ligand charge transfer (MLCT) via harvesting triplet state. Furthermore, previously reported fluorescent materials are described and categorized based on luminescence behaviors on account of the Stokes shifts value. This review will serve as a rationalized introduction and reference for researchers who are interested in exploring large or mega Stokes shift molecules, and will motivate new strategies along with instigation of persistent efforts in this prominent subject area with great avenues.

Stokes shift microscopy by excitation and emission imaging

In this contribution, we present a new method, based on a tunable excitation laser source and a robust common path interferometer in the detection channel. Its purpose is to image spectral excitation and emission information on a monochrome complementary metal oxide semiconductor (CMOS) camera. This allows us to spatially obtain both excitation and emission spectra of the whole imaged area and create derived images such as red-green-blue (RGB), excitation and emission maxima, and Stokes shift images. Our presented method is a further development of hyperspectral imaging that usually is limited to recording spatially resolved emission spectra. Taking advantage of the full camera chip should speed up the acquisition versus line scan or pointwise hyperspectral imaging.

Single-molecule excitation-emission spectroscopy

Single-molecule spectroscopy (SMS) provides a detailed view of individual emitter properties and local environments without having to resort to ensemble averaging. While the last several decades have seen substantial refinement of SMS techniques, recording excitation spectra of single emitters still poses a significant challenge. Here we address this problem by demonstrating simultaneous collection of fluorescence emission and excitation spectra using a compact common-path interferometer and broadband excitation, which is implemented as an extension of a standard SMS microscope. We demonstrate the technique by simultaneously collecting room-temperature excitation and emission spectra of individual terrylene diimide molecules and donor-acceptor dyads embedded in polystyrene. We analyze the resulting spectral parameters in terms of optical lineshape theory to obtain detailed information on the interactions of the emitters with their nanoscopic environment. This analysis finally reveals that environmental fluctuations between the donor and acceptor in the dyads are not correlated.