Transient spectral hole burning observed on the single-molecule level in terrylene-doped biphenyl M. Pärs 1 , V. Palm n , J. Kikas Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia article info Article history: Received 30 July 2013 Received in revised form 28 October 2013 Accepted 7 November 2013 Available online 14 November 2013 Keywords: Biphenyl single crystal Incommensurate biphenyl Terrylene Transient spectral hole burning Single-molecule spectroscopy Saturational line broadening abstract We use the method of fluorescence correlation spectroscopy to analyze the single-molecule (SM) spectroscopy data earlier recorded for a special type of terrylene SM impurity center (referred as “spectrally confined unstable molecule”, SCM) in an incommensurate single crystal of biphenyl. The SCM's SM line seems to be chaotically jumping around within a broad “spectral envelope” and was first considered being subject to a peculiar spectral diffusion behavior. However, our correlation analysis reveals that all the features observed for SCM at 1.8 K are consistent with an assumption that this SM center participates in a process of reversible (transient) spectral hole burning (THB) earlier observed for terrylene-doped polycrystalline biphenyl. No observations of THB processes on SM level have been so far reported for this impurity system, partially due to a low concentration of relevant impurity centers. Another reason making searching for such centers experimentally challenging is an unusual SM line behavior: the photoinduced transition to a metastable “dark state” leads to the SM line saturational broadening, which is much stronger than the triplet broadening. Hence required prolonged observation is often prevented by an SM act of persistent spectral hole burning. & 2013 Elsevier B.V. All rights reserved. 1. Introduction About a decade ago, the first spectral hole burning and single- molecule (SM) spectroscopy experiments with terrylene (TR) molecules embedded in a low-temperature biphenyl (BP) matrix have been reported by our group [1]. The chosen impurity system (TR:BP) is of a special interest due to the incommensurate modulation in the host crystal [2], which breaks the translational symmetry and causes the spatial variation on nanometer scale in local environments. This results in a very broad distribution of spectral and dynamical properties of individual (photochemically stable, but very sensitive to the local environment) TR impurity molecules (further referred as SM TR impurity centers). In parti- cular, effective non-photochemical persistent hole burning (PHB) processes with strongly non-exponential kinetics (indicating a very large variety of photoinduced transition probabilities among the impurity centers), as well as processes of transient (reversible) hole burning (THB) with hole lifetimes on the order of a second, have been observed for polycrystalline TR:BP samples at tempera- tures T below 2.2 K across the whole  10 nm broad inhomo- geneous band of the purely electronic TR transition [1]. SM spectroscopy experiments on single-crystalline low-con- centration TR:BP samples have revealed the broad variety of SM dynamical behavior, including irreversible spectral jumps of SM lines of the centers involved in PHB processes [3]. Although in certain conditions a reliable identification of an SM center as one being involved in a THB process should be possible [4–8], obtain- ing such a result for TR:BP system has not yet been reported, remaining an experimental challenge. Among several observed behavioral types of SM TR impurity centers, a specific type called as “spectrally confined unstable molecule” (SCM) has been described in Ref. [3], referring to an SM line that seemingly chaotically jumps around within a broad “spec- tral envelope” and was initially considered being subject to a peculiar spectral diffusion (SD) behavior. Among several potential SCM-type SM centers we observed for TR:BP, only one (further referred as SM1) was not involved in any PHB process for several hours, which allowed performing different types of measurements in order to collect data necessary to analyze its presumed SD dynamics. In addition to fluorescence excitation spectra obtained for SM1 with various scanning rates at temperatures between 1.8 and 2.1 K, the fluorescence intensity has been recorded within extended time periods with various time resolutions for several laser excitation frequency positions within the nearly 2.5 GHz broad “spectral envelope”. Here we present results of the correlation analysis of the data recorded for SM1 at T ¼ 1.8 K with time resolution of 13.5 ms, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.033 n Corresponding author. Tel.: þ372 7374664; fax: þ372 7383033. E-mail address: viktor@fi.tartu.ee (V. Palm). 1 Present address: Experimental Physics IV, University of Bayreuth, 95440 Bayreuth, Germany. Journal of Luminescence 152 (2014) 121–124