Vibrational fingerprint of localized excitons in a two-dimensional metal-organic crystal

M Corva,M Dell’Angela,Z Feng,G Rupprechter,C Rameshan,N Seriani,M Rinaldi,R Costantini,M Roiaz,A Ferrari,G Comelli,E Vesselli,G Pastore

doi:10.1038/s41467-018-07190-1

Abstract

Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.

Highlights

Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage
We show that nonlinear optical sum-frequency generation (SFG) spectroscopy is a unique tool for this kind of investigation as it mimics a pumpand-probe experiment, paving the way for simultaneous controlled generation and characterization of localized excitons in a
By exploiting IR-Vis Sum-Frequency Generation spectroscopy (IR-Vis SFG), we monitored the evolution of the C–O stretching intensity as a function of the computed the (CO) pressure at room temperature, following the FePc carbonylation process in situ

Summary

Introduction

Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. When light is exploited to induce the singlet–triplet transition as in the LIESST process, the triplet generation process may be associated with intersystem crossing or singlet fission mechanisms, involving a short-lived, intermediate singlet excited state that relaxes through a cooperative path involving adjacent chromophores[9]. In the latter case, light absorption generates a spin-singlet exciton that converts within few hundreds of femtoseconds into a pair of spin-triplet excitons residing at neighboring sites[3]. We show that nonlinear optical sum-frequency generation (SFG) spectroscopy is a unique tool for this kind of investigation as it mimics a pumpand-probe experiment, paving the way for simultaneous controlled generation and characterization of localized excitons in a

Methods

Results

Conclusion