(a) Spectral trail of a single TBT molecule in PIB at T = 7 K; number of frequency positions (steps) in scan, 500; time per frequency point, 20 ms. (b) and (c) Time traces of the ZPL intensity at fixed laser frequencies ν 1 and ν 2, respectively (cf. part (a)).
Distributions of the on- (a) and off- (b) periods in Fig. 1(b) in semi-logarithmic representation with exponential fits (straight lines). Details are given in the text.
Spectral trails of two different molecules, which do not reveal an obvious origin of fluorescence blinking. The examples are shown with very different characteristic lifetimes of the “dark” state. The system is TBT in PIB at T = 15 K.
Illustration of the procedure of determining the on- and off-time intervals of fluorescence blinking in a single-molecule spectral trail. (a) Spectral trail of a blinking single TBT molecule in PIB at T = 13 K. The horizontal double frequency and time axis on the top corresponds to the experimental procedure: The SM spectrum is recorded by scanning the laser excitation frequency within a 36 GHz range with 2000 steps and 5 ms exposure time per step. (b) Part of the spectral trail comprising scans 6–9 [between the horizontal lines in (a)]. The dead times (τ return = 3 s each) between two consecutive spectra (indicated by vertical gray bands) are due to tuning the laser frequency back to the beginning of the scan range. (c) SM spectrum in a single scan together with a Lorentzian profile (smooth line) which was chosen as a threshold to discriminate between on- and off-intervals of fluorescence emission. The dashed vertical lines separate “wings” and “center” of the SM spectrum; they are separated by the full width at half-maximum of the spectral envelope. (d) “Random telegraph function” representing the blinking of the spectrum. The last 1.2 s of the scan with poor signal-to-noise ratio (cross-shaded area) were excluded from further data analysis (see text for details).
Illustration of the algorithm used in the Monte Carlo simulation of the fluorescence excitation spectrum of a blinking emitter. (a) Random telegraph function for a two-state process with fixed transition rates. (b) The same function multiplied with a wide Lorentzian profile. (c) Signal after smoothing with a window of 10 frequency points and (d) with white noise added.
Simulated distributions of the on- (a) and off-interval lengths (b) as obtained with the technique described in the text.
Spectral trails (2D-plots, left panels) and distributions of the fluorescence on- (open circles, right panels) and off-interval lengths (squares, right panels) for a specific SM at various temperatures. In all spectral trails the vertical axis represents the number of scan; the top axis represents laser frequency detuning in GHz; the bottom axis represents the corresponding measurement time in the scan in seconds. The solid lines in the distributions are fitting curves representing double-exponential decay laws.
Temperature dependence of the characteristic time constants for the fast and slow process in the on- and off-time distributions shown in Fig. 7. The dependences for the on-intervals are different at different frequency positions within the SM absorption line: The fast processes take place in the center and the slow processes in the wings of the spectrum.
Sketch of the processes causing the fluorescence blinking of a single TBT molecule in PIB. There are at least two processes which switch the chromophore into dark (off-) states: (i) Thermally activated transitions of a tunneling two-level system of PIB in the local surroundings of the chromophore, (ii) light-induced transitions into a dark state of different nature. The time constants of the processes and their dependences on excitation light intensity are different (see text).
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