Single Molecule Spectral Fluctuation Originates from the Variation in Dipole Orientation Connected to Accessible Vibrational Modes

Single molecule FRET (fluorescence resonance energy transfer) is a powerful technique for detecting real-time conformational changes and molecular interactions during biological reactions. In this review, we examine different techniques of extending observation times via immobilization and illustrate how useful biological information can be obtained from single molecule FRET time trajectories with or without absolute distance information.

Vibrational levels of the electronic ground states in dye molecules have not been previously explored at high resolution in solid matrices. We present new spectroscopic measurements on single polycyclic aromatic molecules of dibenzoterrylene embedded in an organic crystal made of para-dichlorobenzene. To do this, we use narrow-band continuous-wave lasers and combine spectroscopy methods based on fluorescence excitation and stimulated emission depletion (STED) to assess indivi...

Direct observation of single-molecule interactions and dynamic configurations in situ is a demanding challenge but crucial for both chemical and biological systems. However, optical microscopy that relies on bulk measurements cannot meet these requirements due to rapid molecular diffusion in solutions and the complexity of reaction systems. In this work, we leveraged the fluorescence activation of pristine hexagonal boron nitride (h-BN) in organic solvents as a molecular sens...

The motion of chemical bonds within molecules can be observed in real time, in the form of vibrational wavepackets prepared and interrogated through ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are commonly performed on large ensembles of molecules, and as such, are limited to the extent that ensemble coherence can be maintained. Here, we describe vibrational wavepacket motion on single molecules, recorded through time-resolved, surface-enhanced, cohe...

Single molecule localization microscopy has the potential to resolve structural details of biological samples at the nanometer length scale. However, to fully exploit the resolution it is crucial to account for the anisotropic emission characteristics of fluorescence dipole emitters. In case of slight residual defocus, localization estimates may well be biased by tens of nanometers. We show here that astigmatic imaging in combination with information about the dipole orientat...

This is the first in a series of two papers investigating the effect of electron-phonon coupling in two-dimensional Fourier transformed electronic spectroscopy. We present a series of one- and two-dimensional nonlinear spectroscopic techniques for studying a dye molecule in solution. Ultrafast laser pulse excitation of an electronic transition coupled to vibrational modes induces a propagating vibrational wave packet that manifests itself in oscillating signal intensities and...

We have developed a general approach for the calculation of the single molecule polarization correlation function C(t), which delivers a correlation of the emission dichroisms at time 0 and t. The approach is model independent and valid for general asymmetric top molecules. The key dynamic quantities of our analysis are the even-rank orientational correlation functions, the weighted sum of which yields C(t). We have demonstrated that the use of non-orthogonal schemes for the ...

Donor-Acceptor (D-A) type thermally activated delayed fluorescence (TADF) emitters which constitute the key functional units in proposed Gen3 Organic Light Emitting Diodes (OLEDs), are sensitive to the rigidity and polarity of their local environment. In particular, the torsional freedom of the D-A dihedral angle and the excited state dipole moments of the occurring charge transfer states, condition a distribution of TADF dynamics over the emitter ensemble, concealed in stand...

We report on time dependent Stokes shift measurements of single molecules. Broadband excitation and emission spectroscopy were applied to study the temporal Stokes shift evolution of single perylene diimide molecules (PDI) embedded in a polymer matrix on the time scale of seconds. The Stokes shift varied between individual molecules as well as for single molecules undergoing different conformations and geometries. From the distribution and temporal evolution of Stokes shifts,...

Single molecule rotational correlation functions are analyzed for several reorientation geometries. Even for the simplest model of isotropic rotational diffusion our findings predict non-exponential correlation functions to be observed by polarization sensitive single molecule fluorescence microscopy. This may have a deep impact on interpreting the results of molecular reorientation measurements in heterogeneous environments.