Catching aromaticity in the act: Direct real-time tracking of how 'excited-state aromaticity' drives molecular shape changes

For the first time, researchers led by Hikaru Kuramochi, Associate Professor at the Institute for Molecular Science/SOKENDAI, have directly tracked how excited-state aromaticity emerges within hundreds of femtoseconds and subsequently drives a molecule's picosecond-scale structural change from a bent to a fully planar shape.

Aromaticity is a foundational concept in chemistry describing the enhanced stability of cyclic molecules whose electrons are delocalized. Although most discussions have focused on molecules in their ground state, the concept of "excited-state" aromaticity has recently been extensively utilized in predicting the structural change and designing the chemical reactivities induced by photoexcitation. While the dynamic properties of excited-state aromaticity have been studied intensively in the past, these have primarily focused on molecules in a "near-equilibrium state," leaving the precise timing and interplay between excited-state aromaticity and structural changes poorly understood. Directly visualizing these ultrafast motions is crucial for designing photoactive materials, such as sensors, adhesives, and switches.

The team used a combination of femtosecond transient absorption and *time-resolved impulsive stimulated Raman spectroscopy* (TR-ISRS) -- an advanced "time-domain" Raman technique that covers vibrational frequencies from terahertz to 3000 cm⁻¹ with femtosecond temporal resolution -- to capture ultrafast snapshots of a newly synthesized cyclooctatetraene (COT)-based "flapping molecule" called TP-FLAP. By exciting TP-FLAP with a femtosecond laser pulse, then probing its evolving vibrational signals, they could see exactly when and how the molecule's central COT ring planarized. Isotope labeling with ¹³C at the central ring allowed the researchers to confirm which specific vibrational mode accompanied the bent-to-planar transition.

Initial measurements revealed a sub-picosecond (≈590 fs) electronic relaxation that imparts aromatic character to the bent molecule's excited state. The molecule then undergoes planarization in a few picoseconds as indicated by a *frequency shift* in the ring's carbon-carbon stretching vibration. With the help of the isotope labeling (¹³C), a telltale shift in the key C=C stretching frequency was unambiguously shown, confirming that the ring's planarization drives the observed vibrational changes. Calculations of aromaticity indices (e.g., nucleus-independent chemical shifts, NICS) further support that the system is "already aromatic" in the bent excited state and becomes *even more* aromatic as it undergoes planarization.

This study provides the first direct observation of nonequilibrium structural changes governed by excited-state aromaticity. It conclusively shows that aromaticity can emerge within hundreds of femtoseconds, preceding -- and then facilitating -- the picosecond-scale flattening of the molecule. Beyond deepening our understanding of fundamental light-driven processes, these insights help guide the rational design of photoactive materials, including molecular sensors, tunable fluorescence probes, and photoresponsive adhesives. The TR-ISRS method's ability to track vibrational modes in real time offers a new avenue for exploring other systems featuring excited-state (anti)aromaticity and complex conformational changes.