Activating the fluorescence of a Ni(II) complex by energy transfer

TL;DR

This study demonstrates that fluorescence in Ni(II) complexes can be activated through resonant energy transfer, overcoming quenching mechanisms, with implications for using more abundant transition metals in luminescent applications.

Contribution

The paper introduces a method to activate fluorescence in Ni(II) complexes via energy transfer, bypassing the usual quenching by dark states, using STM techniques and molecular manipulation.

Findings

Resonant energy transfer activates NiPc fluorescence at low temperature.

An activation barrier for intersystem crossing can be overcome with local environment control.

Fluorescence can be induced without overcoming the ISC barrier by molecular proximity.

Abstract

Luminescence of open-shell 3d metal complexes is often quenched due to ultrafast intersystem crossing (ISC) and cooling into a dark metal-centered excited state. We demonstrate successful activation of fluorescence from individual nickel phthalocyanine (NiPc) molecules in the junction of a scanning tunneling microscope (STM) by resonant energy transfer from other metal phthalocyanines at low temperature. By combining STM, scanning tunneling spectroscopy, STM- induced luminescence, and photoluminescence experiments as well as time-dependent density functional theory, we provide evidence that there is an activation barrier for the ISC, which in most experimental conditions is overcome. We show that this is also the case in an electroluminescent tunnel junction where individual NiPc molecules adsorbed on an ultrathin NaCl decoupling film on a Ag(111) substrate are probed. However, when placing an MPc (M = Zn, Pd, Pt) molecule close to NiPc by means of STM atomic manipulation, resonant energy transfer can excite NiPc without overcoming the ISC activation barrier, leading to Q-band fluorescence. This work demonstrates that the thermally activated population of dark metal-centered states can be avoided by a designed local environment at low temperatures paired with a directed molecular excitation into vibrationally cold electronic states. Thus, we can envisage the use of luminophores based on more abundant transition metal complexes that do not rely on Pt or Ir.