Nanographenes (NGs) are a class of two dimensional nanomaterials based on graphene laterally confined to a spatial scale of a few nanometers. They have attracted significant interest in the recent literature [1] due to the possibility of controlling their chemical structure with atomic precision, and due to their remarkable optical properties and potential applications in various fields. On the other hand, the fundamental photophysics of NGs is still poorly understood, and some NGs are known to display non-standard photophysical behaviour [2], deviating from the optical response of typical molecular dyes.

In this study, a pyrene-based nanographene (NG) is investigated by means of various spectroscopic techniques capable of providing a comprehensive view of the optical response of the NG initiated by photoexcitation. We employed steady-state absorption and fluorescence spectroscopy to analyze the basic optical properties as well as time-resolved nanosecond spectroscopy to observe the excited-state depopulation. To delve deeper into the ultrafast dynamics of pyrene, we then conducted femtosecond transient absorption (FTA) spectroscopy, which was capable of reconstructing the earliest stages of the photocycle.

The absorption and emission spectra of the NG were found to vary depending on whether the compound was in solution or solid state, and these environmental variations significantly affect its optical properties and relaxation dynamics. The FTA measurements provided insight into the excited-state dynamics and revealed interesting features both in solution and solid state. In particular, one of the most intriguing findings was the observation of coherent nuclear oscillations launched in the NG by excitation with femtosecond pulses. The observation of coherent oscillations in the femtosecond transient absorption signal, rarely observed in nanographenes or other carbon-based compounds, adds a new dimension to our understanding of the excited-state dynamics of nanographenes and opens up new possibilities for their application in advanced photonic and optoelectronic devices.

[1] Drummer et al. Photosynth Res 151, 163–184 (2022)

[2] M. Reale et al. Carbon 206, 45 (2023)