Charge transfer-induced reduction of vibronic coupling for single terrylene molecules adsorbed onto hBN
- Date
- Thursday 24 October 2024
- Time
- Address
-
Gorlaeus Building
Einsteinweg 55
2333 CC Leiden - Room
- BM.1.23
Titus de Haas,a Robert Smit,b Arash Tebyani,b Semonti Bhattacharyya,b Kenji Watanabe,c Takashi Taniguchi,c Francesco Buda,a Michel Orrit.b
a Leiden Institute of Chemistry, Leiden University, 2300 RA Leiden, The Netherlands
b Huygens-Kamerlingh Onnes Laboratory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
c Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan
Abstract
Fluorescence spectra of single terrylene molecules adsorbed on hexagonal boron nitride (hBN) flakes were recorded at cryogenic temperatures. The pure electronic transitions of terrylene molecules are spread over a broad energy scale, from 570 to 610 nm. Surprisingly, the vibronic spectrum corresponding to fluorescence towards the various vibrational levels of the molecule in its ground electronic state presents dramatic variations in intensity from molecule to molecule, by a factor of up to 20. Moreover, the vibronic intensity correlates both with the spectral position of the electronic transition and with the frequency of the longitudinal stretch mode, which varies between 243 and 257 cm-1. Red-shifted molecules tend to show weaker vibronic coupling and higher vibration frequency. We find an extreme case where the Debye-Waller-Franck-Condon factor of the zero-phonon line exceeds 0.8. Through means of extensive DFT calculations, we show that these observations can be explained by terrylene chemisorption on charge donating defect sites. Specifically, we show that the oxygen substitution at the nitrogen site leads to a defect that strongly binds terrylene. Upon adsorption, an electron is transferred from the hBN defect to the terrylene LUMO orbital. This charge transfer significantly reduces the structural changes associated with the terrylene S0 to S1 HOMO-LUMO excitation, leading to a strongly diminished vibronic coupling across the entire spectrum. These insights offer a pathway to designing molecules with minimal vibronic coupling, which would be very attractive for the efficient emission of single photons with narrow lines and for the generation of indistinguishable photons.
Acknowledgements
We are grateful for the funding of this work, provided by NWO Spinoza prize 2017. The calculations in this work were sponsored by NWO – Domain Science for the use of supercomputer facilities.