photon counting technique to determine the lifetime of the excited state. Figure 2 (b) shows an example of a time-resolved intensity measurement, which could be fitted with a single exponential decay. Repeating the measurement on several molecules, we obtained lifetimes between 3 . 3 ns and 5 . 7 ns. Then we employed photon correlation measurements using the HBT setup to verify the identification of isolated single DBT molecules. The CW photon au- tocorrelation measurement in Fig. 2 (c) shows a dip in the second order correlation function at delay τ = 0, which corresponds to a reduction of the two-photon detection probability to 0 . 28. This result motivates the use of DBT as a near-infrared single-photon source. 

The performance of a single-photon source is in many cases compromised by fluorescence intermittency. The blinking of semiconductor nanocrystals is a well-known example of this phenomenon [12]. In the case of molecules one has to worry about a long-lived triplet state, populated by intersystem crossing, which can interrupt the continuous stream of photons. To determine the lifetime of the triplet state, we recorded a histogram of the inter-photon arrival times with a pump rate higher than the triplet decay rate [13]. Under this condition the dark intervals in the fluorescence are limited by the triplet lifetime, which can then be extracted from the slow decay in 2 (d). The initial fast decay is a measure for the pump rate. Considering the obtained triplet lifetime of 1 . 5 µ s together with an extremely low intersystem crossing yield of 10 − 7 [10], we can neglect the effect of the triplet state on the efficiency of a DBT single-photon source. Another important property is the brightness, which can be extracted from saturation measurements. Fig.2 (e) shows that we can detect almost to one million photons per second at pump intensities close to saturation. Such count rates are among the highest ever reported [14]. By considering the maximum count rate together with the determined lifetime, we can deduce a total detection efficiency of 0 . 5 %. 

The photostability of DBT molecules embedded in thin crystalline AC films is especially noteworthy, when considering that other molecular emitters typically photobleach after 10 4 − 10 7 photon emissions [15]. We investigated the stability of DBT by irradiating a sample continuously with an intensity of 30 kW/cm 2 and recorded a wide-field image every 20 min over a time period of more than 10 hours (see Fig. 2 (f)). For about 30 out of 40 molecules we could attribute a ’half-life’ of 4 h by fitting an exponential decay to the experimental data. These molecules emitted more than 10 12 photons before photobleaching, assuming the above-calculated detection efficiency of 0 . 5%. The remaining ten molecules, however, did not suffer from any photobeaching, even after more than 10 hours of constant illumination. 

