Carbon nanorings with inserted acenes: breaking symmetry in excited state dynamics

Abstract

Conjugated cycloparaphenylene rings have unique electronic properties being the smallest segments of carbon nanotubes. Their conjugated backbones support delocalized electronic excitations, which dynamics is strongly influenced by cyclic geometry. Here we present a comparative theoretical study of the electronic and vibrational energy relaxation and redistribution in photoexcited cycloparaphenylene carbon nanorings with inserted naphthalene, anthracene, and tetracene units using non-adiabatic excited-state molecular dynamics simulations. Calculated excited state structures reflect modifications of optical selection rules and appearance of low-energy electronic states localized on the acenes due to gradual departure from a perfect circular symmetry. After photoexcitation, an ultrafast electronic energy relaxation to the lowest excited state is observed on the time scale of hundreds of femtoseconds in all molecules studied. Concomitantly, the efficiency of the exciton trapping in the acene raises when moving from naphthalene to anthracene and to tetracene, being negligible in naphthalene, and ~60% and 70% in anthracene and tetracene within the first 500 fs after photoexcitation. Observed photoinduced dynamics is further analyzed in details using induced molecular distortions, delocatization properties of participating electronic states and non-adiabatic coupling strengths. Our results provide a number of insights into design of cyclic molecular systems for electronic and light-harvesting applications.

Strain‐Promoted Reactivity of Alkyne‐Containing Cycloparaphenylenes

“…[46] Weak shoulders centered at 400 nm for 1,and less pronounced for 2 (392 nm) and 3 (387 nm), could be assigned to partially activated highest occupied molecular orbitallowest unoccupied molecular orbital (HOMO-LUMO) transitions,w hich we ascribe to symmetry breaking caused by introduction of the alkyne unit into the polyphenylene backbone (Supporting Information). [47] Notably,f or all compounds 1-3 the frontier molecular orbitals exhibit asignificant contribution from alkyne-centered orbitals,which denotes the point of highest reactivity in this study (Figure 3b). …”

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

“…It is interesting to note that such concerted in-phase coherent vibronic dynamics is observed across the entire ensemble of trajectories with slow decay for well over 100 fs at room temperature for all considered systems and others 52 , 54 , overcoming effects of thermal fluctuations, solvent viscosity, and disorder. We mention spectroscopic observations of “coherent phonons” persisting up to picoseconds (e.g., in the case of carbon nanotubes 11 ), when the entire ensemble of molecules undergoes in-phase vibrational motion.…”

“…The signatures of such concerted vibronic dynamics can be followed by analyzing common descriptors for both vibrational and electronic degrees of freedom. Bond-length alternation, BLA (see Methods) is a typical parameter for monitoring C–C stretches 52 . Fig.…”

Theoretical Insights for Materials Properties of Cyclic Organic Nanorings

“…[ 274 ] The introduction of acene units such as naphthalene, (nCPPN), anthracene (nCPPA), and tetracene (nCPPT), leading thus to chiral systems, also breaks the symmetry of pristine CPPs with the lowest singlet excited‐state S 1 being now optically allowed and blue‐shifted upon increasing size. [ 275–279 ] Generally speaking, the interplay of π‐conjugation, bending (or strain effects), and chirality (or symmetry effects) in fact determines all photophysics of CPP‐based systems, [ 280 ] which can also be fine‐tuned with functionalization strategies.…”