Exchange interaction between the triplet exciton and the localized spin in copper-phthalocyanine

TL;DR

This study investigates the exchange interaction between triplet excitons and localized spins in copper-phthalocyanine using first-principles density-functional theory, revealing a measurable interaction that could enable optical control of spins.

Contribution

It provides the first detailed computational analysis of exchange interactions in CuPc, highlighting their potential for spin manipulation via optical means.

Findings

Exchange interaction magnitude is around 1.5 meV.

Interaction oscillation period is in the order of picoseconds.

Potential for optical manipulation of localized spins in CuPc.

Abstract

Triplet excitonic state in the organic molecule may arise from a singlet excitation and the following inter-system crossing. Especially for a spin-bearing molecule, an exchange interaction between the triplet exciton and the original spin on the molecule can be expected. In this paper, such exchange interaction in copper-phthalocyanine (CuPc, spin-$\frac{1}{2}$) was investigated from first-principles by using density-functional theory within a variety of approximations to the exchange correlation, ranging from local-density approximation to long-range corrected hybrid-exchange functional. The magnitude of the computed exchange interaction is in the order of meV with the minimum value (1.5 meV, ferromagnetic) given by the long-range corrected hybrid-exchange functional CAM-B3LYP. This exchange interaction can therefore give rise to a spin coherence with an oscillation period in the order of picoseconds, which is much shorter than the triplet lifetime in CuPc (typically tens of nanoseconds). This implies that it might be possible to manipulate the localised spin on Cu experimentally using optical excitation and inter-system crossing well before the triplet state disappears.