Indirect Intersystem Crossing (S₁ → T₃/T₂ → T₁) Promoted by the Jahn–Teller Effect in Cycloparaphenylenes

Abstract

Vibronic-coupling effects play a key role for excited-state charge- and energy-transfer processes in organic molecular systems. Here, we demonstrate how the Jahn-Teller effect in triplet excited states of highly symmetric cycloparaphenylenes triggers an indirect intersystem crossing deactivation pathway. Strong Jahn-Teller distortion in the doubly degenerate second excited triplet state (T) brings the molecular system energetically close to the lowest triplet state (T), thereby opening the possibility for an extremely rapid internal conversion. Quantum dynamics simulations reveal an initial T → T population decay within 50 fs. Experimental observation of size-dependent intersystem crossing rates of cycloparaphenylenes is explained based on the proposed S → T/T → T mechanism.

Reaction of Oxygen with the Singlet Excited State of [n]Cycloparaphenylenes (n = 9, 12, and 15): A Time-Resolved Transient Absorption Study Seamlessly Covering Time Ranges from Subnanoseconds to Microseconds by the Randomly-Interleaved-Pulse-Train Method

“…In the present reaction system, 1 O 2 * formation via reaction can be ruled out because energy released by ISC is not enough for 1 O 2 * formation. The S 1 –T 1 energy gap ( E ST ) theoretically predicted for [ n ]­CPP increases with n , where E ST = 0.66 eV (S 1 –T 1 ) when n = 15 . Thus, E ST values for [ n ]­CPPs ( n < 15) are obviously less than the excitation energy necessary for the conversion from 3 O 2 to 1 O 2 * (ca.…”

“…The S 1 −T 1 energy gap (E ST ) theoretically predicted for [n]CPP increases with n, where E ST = 0.66 eV (S 1 −T 1 ) when n = 15. 44 Thus, E ST values for [n]CPPs (n < 15) are obviously less than the excitation energy necessary for the conversion from 3 O 2 to 1 O 2 * (ca. 0.98 eV).…”

Unified Hamiltonian Formalism of Jahn–Teller and Pseudo-Jahn–Teller Problems in Axial Symmetries

“…These formalisms are inclusive, yet not enough. Problems with more than two sets of JT/pJT active vibrational modes and of higher axial symmetries, e.g., pentagonal (cyclopentadienyl ring , ), hexagonal (benzene , ), and in general n -gonal (nanotubes, nanohoops, and nanoloops of different sizes − ), await to be addressed. “It is somewhat tedious to evaluate all possible cases of quadratic activity”, this is a statement about deriving second order JT Hamiltonian for the cyclopentadienyl ring in an up-to-date excellent review article on vibronic coupling .…”

A Unified Hamiltonian Formalism of Jahn-Teller and Pseudo-Jahn-Teller Problems in Axial Symmetries

“…48 These formalisms are inclusive, yet not enough. Problems with more than two sets of JT/pJT active vibrational modes and of higher axial symmetries, e.g., pentagonal (cyclopentadienyl ring 49,50 ), hexagonal (benzene 51,52 ), and in general n-gonal (nanotubes, nanohoops, and nanoloops of different sizes [53][54][55][56] ), await to be addressed. "It is somewhat tedious to evaluate all possible cases of quadratic activity", this is a statement about deriving second order JT Hamiltonian for the cyclopentadienyl ring in an up-to-date excellent review article on vibronic coupling.…”