Photo credit: www.sciencedaily.com
Recent scientific advancements have unveiled the first real-time observation of the emergence of “excited-state aromaticity” occurring within mere hundreds of femtoseconds. This phenomenon initiates a rapid transformation in molecular structure, shifting from a bent to a planar form within picoseconds. By utilizing a combination of ultrafast electronic and vibrational spectroscopies, the research team successfully captured these transient structural changes on a molecular scale, demonstrating that the appearance of aromaticity occurs prior to and facilitates the planarization of the molecule. These findings are essential for the development of enhanced photoactive materials, such as sensors and light-driven molecular switches, which harness the benefits of excited-state aromaticity.
This groundbreaking work, led by Hikaru Kuramochi, an Associate Professor at the Institute for Molecular Science/SOKENDAI, marks a significant step in our understanding of excited-state aromaticity. For the first time, researchers have directly monitored the rapid emergence of this phenomenon and its influence on the structural alterations of a molecule at picosecond timescales.
Aromaticity is a central principle in chemistry that describes the heightened stability found in cyclic molecules due to the delocalization of electrons. While much research has concentrated on molecules in their ground state, increased attention has turned towards “excited-state” aromaticity, particularly in relation to predicting structural changes and altering the chemical reactivity induced by photoexcitation. Though previous studies have explored the dynamic attributes of excited-state aromaticity, investigations have often been limited to molecules in a “near-equilibrium state,” which inadequately captures the timing and interaction between excited-state aromaticity and subsequent structural alterations. Understanding these ultrafast movements is crucial for advancing the design of photoactive materials used in various applications, including sensors, adhesives, and switches.
The research team employed a sophisticated approach, combining femtosecond transient absorption with time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS), a cutting-edge technique that provides femtosecond temporal resolution across vibrational frequencies from terahertz to 3000 cm⁻¹. This approach allowed them to capture ultrafast snapshots of a newly synthesized “flapping molecule,” TP-FLAP, which is based on cyclooctatetraene (COT). By exciting TP-FLAP with a femtosecond laser pulse and subsequently probing its evolving vibrational signals, the team was able to ascertain the precise timing and mechanisms underlying the molecular planarization of the central COT ring. The use of isotope labeling with ¹³C in the central ring enabled the researchers to identify specific vibrational modes linked to the transition from bent to planar structure.
The initial findings indicated a sub-picosecond (approximately 590 fs) electronic relaxation, which bestowed aromatic characteristics onto the bent molecule’s excited state. Following this, the molecule transitioned to a planar structure over a few picoseconds, shown through a frequency shift in the carbon-carbon stretching vibrations of the ring. The isotope labeling with ¹³C provided clear evidence of a significant shift in the key C=C stretching frequency, conclusively linking the ring’s planarization to the observed vibrational changes. Calculations of aromaticity indices, such as nucleus-independent chemical shifts (NICS), reinforce the claim that the system exhibits aromatic properties even in the bent excited state and becomes increasingly aromatic during the planarization process.
This investigative study represents the first direct observation of nonequilibrium structural transformations driven by excited-state aromaticity. It confirms that aromaticity can develop within a matter of hundreds of femtoseconds, preceding and aiding the flattening of the molecule on a picosecond timescale. In addition to enhancing our understanding of fundamental light-driven mechanisms, these insights are pivotal in directing the systematic design of advanced photoactive materials, which could include molecular sensors, tunable fluorescence probes, and photoresponsive adhesives. The TR-ISRS methodology’s capacity to monitor vibrational modes in real-time opens new avenues for investigating a wider range of systems exhibiting excited-state (anti)aromaticity and intricate conformational changes.
Source
www.sciencedaily.com