Access to Triplet Excited State in Core-Twisted Perylenediimide

Solvent-free crystal structure of <i>N,N</i>-bis­(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis­(dicarboximide), PDI-Br₄, obtained by X-ray diffraction reveals the core-twisted perylene motif having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br₄ arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br₄ in toluene reveal ultrafast intersystem crossing (τ_ISC < 110 fs) when excited at 400 nm. Triplet quantum yield (Φ_T) of 19 ± 1% and 105 ± 5% for PDI-Br₄ in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br₄ can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br₄ thin film

Access to Triplet Excited State in Core-Twisted Perylenediimide

Solvent-free crystal structure of <i>N,N</i>-bis­(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis­(dicarboximide), PDI-Br₄, obtained by X-ray diffraction reveals the core-twisted perylene motif having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br₄ arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br₄ in toluene reveal ultrafast intersystem crossing (τ_ISC < 110 fs) when excited at 400 nm. Triplet quantum yield (Φ_T) of 19 ± 1% and 105 ± 5% for PDI-Br₄ in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br₄ can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br₄ thin film

Quantum Dynamics Simulations Reveal Vibronic Effects on the Optical Properties of [<i>n</i>]Cycloparaphenylenes

The size-dependent ultraviolet/visible photophysical property trends of [<i>n</i>]­cycloparaphenylenes ([<i>n</i>]­CPPs, <i>n</i> = 6, 8, and 10) are theoretically investigated using quantum dynamics simulations. For geometry optimizations on the ground- and excited-state Born–Oppenheimer potential energy surfaces (PESs), we employ density functional theory (DFT) and time-dependent DFT calculations. Harmonic normal-mode analyses are carried out for the electronic ground state at Franck–Condon geometries. A diabatic Hamiltonian, comprising four low-lying singlet excited electronic states and 26 vibrational degrees of freedom of CPP, is constructed within the linear vibronic coupling (VC) model to elucidate the absorption spectral features in the range of 300–500 nm. Quantum nuclear dynamics is simulated within the multiconfiguration time-dependent Hartree approach to calculate the vibronic structure of the excited electronic states. The symmetry-forbidden <i>S</i>₀ → <i>S</i>₁ transition appears in the longer wavelength region of the spectrum with weak intensity due to VC. It is found that the Jahn–Teller and pseudo-Jahn–Teller effects in the doubly degenerate <i>S</i>₂ and <i>S</i>₃ electronic states are essential in the quantitative interpretation of the experimental observation of a broad absorption peak around 340 nm. The vibronic mixing of the <i>S</i>₁ state with higher electronic states is responsible for the efficient photoluminescence from the <i>S</i>₁ state. The fluorescence properties are characterized on the basis of the stationary points of the excited-state PESs. The findings reveal that vibronic effects become important in determining the photophysical properties of CPPs with increased ring size