Intersystem crossing pathways in [5]-, [7]-, and [9]cycloparaphenylenes

Area of Science:

• Photochemistry and photophysics of organic molecules.
• Quantum chemistry and computational modeling.
• Spectroscopy of conjugated organic systems.

Background:

• Cycloparaphenylenes ([n]CPPs) are fascinating molecular nanoribbons with unique photophysical properties.
• Understanding excited-state dynamics, including internal conversion and intersystem crossing, is crucial for designing new optoelectronic materials.
• Previous studies have explored the electronic structures of [n]CPPs, but detailed pathways for intersystem crossing remain less understood.

Purpose of the Study:

• To elucidate the intersystem crossing (ISC) pathways in odd-numbered [n]cycloparaphenylenes ([n]CPPs) for n = 5, 7, and 9.
• To investigate the size-dependent energetics and internal conversion dynamics of singlet and triplet excited states.
• To correlate computational findings with experimental observations of fluorescence and emission energies.

Main Methods:

• Quantum wavepacket propagation calculations were employed to simulate excited-state dynamics.
• The linear vibronic coupling framework was utilized to model the interactions between electronic and vibrational states.
• Energetics of singlet (S) and triplet (T) manifolds were analyzed to identify key relaxation pathways.

Main Results:

• Both [5]- and [7]CPPs were found to rapidly relax to the S2 excited state from higher singlet states.
• A decrease in the S2-S1 energy gap with increasing CPP size was observed, leading to faster S2 → S1 internal conversion in [9]CPP.
• Higher triplet states serve as receiver states for ISC from S1 or S2, with subsequent decay to the T1 state via conical intersections.

Conclusions:

• The study identifies specific intersystem crossing pathways in odd-numbered [n]CPPs, highlighting the role of S2 and higher triplet states.
• The calculated size-dependent dynamics and energetics align well with experimental fluorescence and emission data.
• These findings provide valuable insights into the photophysical behavior of cycloparaphenylenes, aiding in the rational design of organic electronic materials.