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The light-induced trans-cis isomerization of azobenzene and its derivatives is of great scientific and technological interest as these molecule-sized photo-switches can be used for nano-scale sensing [1] or information storage [2]. Particularly, 3,3’,5,5’-tetra-tert-butylazobenzene (mTBA) adsorbed on Au(111) has seen much interest, being studied at low coverages (< 0.1 ML) by scanning tunneling microscopy (STM) [3] and near monolayer coverage (0.9 ML) by two-photon photoemission [4]. However, missing is a single-molecule level insight into how photo-switching behaviour compares at different coverages. Here, we combine STM with a self-designed computer-vision algorithm to study photoisomerization induced by pulsed laser radiation at high surface coverage (0.6 ML to 0.9 ML). This approach can track the isomerization state of thousands of individual mTBA molecules and enables us to detect subtle variations in switching yields among densely packed, similarly oriented molecules. Further, at these high surface coverages we identify two packing phases that exhibit different photo-isomerization rates, over a variety of excitation wavelengths.
[1] V. Ferri, et al. Angew. Chem. Int. Ed., 47 (2008) 3407-3409.
[2] Z. F. Liu, K. Hashimoto, A. Fujishima. Nature, 347 (1990) 658-660.
[3] M. J. Comstock, et al. Phys. Rev. Lett. 99 (2007) 038301.
[4] S. Hagen, et al. Chem. Phys. Lett. 444 (2007) 85-90.