The electronic structure of pyracene: a spectroscopic and computational study

Area of Science:

• Photochemistry and Photophysics
• Computational Chemistry
• Spectroscopy

Background:

• Pyracene is a polycyclic aromatic molecule featuring aliphatic five-membered rings fused to a naphthalene core.
• Understanding its electronic structure and excited-state dynamics is crucial for applications in materials science and organic electronics.

Purpose of the Study:

• To synthesize pyracene via an improved route.
• To investigate the ground-state and excited-state properties of pyracene using spectroscopic and computational methods.
• To characterize the electronic transitions and vibrational dynamics of pyracene and its dimer.

Main Methods:

• Improved synthetic route for pyracene.
• Gas-phase infrared (IR) and solid-state Raman spectroscopy.
• Resonance-enhanced multiphoton ionization (REMPI) spectroscopy.
• Sorbitol-corrected coupled-cluster with singles and doubles approximation (SCS-ADC(2)) calculations.
• Vibrational analysis and excited-state geometry optimization.

Main Results:

• An improved synthesis of pyracene was achieved.
• The ground-state structure was confirmed as D2h symmetry.
• The first excited singlet state (S1) has an adiabatic excitation energy of 30,798 cm⁻¹ (3.818 eV), with computational agreement.
• Low-frequency vibrational modes in the S1 state indicate a flat potential energy surface.
• Lifetimes for vibronic bands in the S1 state ranged from 43 ns to 76 ns.
• The S2 state exhibits a non-planar geometry and broad transitions, with lifetimes between 60 ns and 70 ns.
• Pyracene dimer formation was observed, with a stable crossed geometry and rapid ion signal rise (<10 ps).

Conclusions:

• Pyracene exhibits interesting photophysical properties, including a flat S1 potential energy surface and distinct vibrational signatures.
• The study provides a comprehensive understanding of pyracene's electronic structure and excited-state behavior.
• The formation and characteristics of the pyracene dimer highlight potential for excimer interactions.