Spectral Jumps of Single Molecules

One of the main values of single molecule studies is the possibility of focusing on the microscopic environment of the molecule up to a radius of some tens of nanometers. By isolating only one of these regions in a large sample, information is obtained in a very direct way, which avoids all the averaging entailed by classical methods. 

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Figure Cl.5.8. Spectral jumping of a single molecule of terrylene in polyethylene at 1.5 K. The upper trace displays fluorescence excitation spectra of tire same single molecule taken over two different 20 s time intervals, showing tire same molecule absorbing at two distinctly different frequencies. The lower panel plots tire peak frequency in tire fluorescence excitation spectmm as a function of time over a 40 min trajectory. The molecule undergoes discrete jumps among four (briefly five) different resonant frequencies during tliis time period. Arrows represent scans during which tire molecule had jumped entirely outside tire 10 GHz scan window. Adapted from...

In general, the correlation technique as well as the quantum jump technique are powerful tools to unravel complicated molecular photophysical dynamics for a single absorber. This statement is exemplified by the investigation of the chromophore terrylene, for which no kinetical parameters of the triplet state were known from ensemble measurements. The ISC rates presented here were determined solely by experiments on single molecules. Actually, it would be quite difficult to measure absolute rates of photophysical ISC parameters by other techniques when the triplet quant yield is smaller than 10 . Recently, fluorescence correlation spectroscopy was also proposed as an appropriate method for the determination of triplet parameters of fluorophores in solution [75]. Additionally, it is a helpful tool to investigate spectral diffusion of single absorbers as discussed in Sections 1.4 and 1.5. 

Figure 2. Example of spectral jumps of a single pentacene molecule in p-terphenyl, interrupting emission during two scans of the laser (reproduced from Ref 40).

In the standard TLS model, their spatial distribution is random. In real samples, TLS s might be more concentrated around defects or particularly disordered regions of the sample. Microscopy of single molecules, especially with near-field resolution might help correlate TLS s with defects or structures in the sample. In samples with artificial structures, the influence of external parameters on spectral jumps and TLS s could be studied. 

The intensity auto-correlation method has long been in use at high temperature to study dynamics in solutions [88]. Intensity fluctuations can arise from spectral jumps or changes, from rotational or translational diffusion with respect to the exciting beam, and from any process which can modulate the emitted intensity. Since correlation of single molecule fluorescence works at liquid helium temperature as well as at room temperature (see Section 2.1), it probably can cover the whole intermediate... 

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