Ultrafast Processes in Isomeric Pyrene-N-methylacetamides: Formation of Hydrogen Bond Induced Static Excimers with Varied Coupling Strength

TL;DR

This study investigates ultrafast formation of hydrogen bond-induced static excimers in pyrene derivatives with different coupling strengths, revealing rapid formation in one isomer and slower in the other through spectroscopic methods.

Contribution

It demonstrates how positional functionalization on pyrene influences excimer formation dynamics and excitonic coupling strength.

Findings

PyMA1 forms static excimers in approx. 560 fs

PyMA2 forms static excimers in approx. 65 ps

Hydrogen bonding affects excimer coupling strength

Abstract

Pyrene based molecules are inclined to form excimers through self-association upon photoexcitation. In this work, the pyrene core is functionalized with N-methylacetamide group at the position 1 or 2, to develop pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2), respectively. Upon photoexcitation, PyMA1 and PyMA2, at 1.0mM, in toluene, have formed predominantly static excimers. The steady state spectroscopic studies have showed that the excitonic coupling of PyMA1 dimers are much stronger in solution than its isomeric counterpart, PyMA2. The transient absorption (TA) measurements over fs-ps regime (fs-TA) showed that the formation of static excimers with the strongly-coupled pre-associated dimers, in PyMA1, happens in approx. 560fs, whereas, the excimers for the weakly-coupled pre-associated dimers in PyMA2 have formed in much slower time scale (approx. 65ps). The introduction of methylacetamide group at the position 1 or 2 on pyrene ring, was believed to have allowed forming hydrogen bonded excimers with different degrees of excitonic coupling.