Ultrafast sorting: Excimeric π-π stacking distinguishes pyrene-N-methylacetamide isomers on the ultrafast time scale.

Pyrene based molecules are inclined to form excimers through self-association upon photoexcitation. In this work, the pyrene core is functionalized with the N-methylacetamide group at the position 1 or 2 to develop pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2), respectively. Upon photoexcitation with 345 nm, a portion of molecules in PyMA1 and PyMA2 solutions at ≥1.0 mM have formed static excimers. The steady state spectroscopic measurements suggest that, whether it is the dimerization of molecules in the ground state (GS) or in excimer formation, characteristic signs are more pronounced in PyMA1 than its isomeric counterpart, PyMA2. The shift of the excimer band in their respective emission spectra suggests that the extent of overlap in π-π stacking is greater for PyMA1 than for PyMA2 in the excited state. The optimized geometry of dimers in toluene shows that the overlapping area between the pyrene moieties in π-π stacking between the dimers is greater for PyMA1 than for PyMA2 in GS. The natural bond orbital analysis with the optimized GS geometries shows that the stabilization/interaction energy between the dimers in π-π stacking is higher in PyMA1 compared to PyMA2 in toluene. The transient absorption (TA) measurements in toluene over the fs-ps regime (fs-TA) showed that the formation of static excimers with pre-associated dimers in PyMA1 happens in ∼700 fs whereas the excimers for the pre-associated dimers in PyMA2 have formed in slightly slower time scale (∼1.95 ps). Contrary to what was observed in solution, the extent of overlap in π-π stacking is lower for PyMA1 dimers (∼17%) than for PyMA2 dimers (∼37%) in single crystals.