Combined experimental and theoretical studies on the photophysical properties of cycloparaphenylenes

We studied the UV-vis absorption and fluorescence in solution/solid states of [n]cycloparaphenylene ([n]CPP: n = 9, 12, 14, 15, and 16), and conducted theoretical studies to better understand the experimental results. The representative experimental findings include (i) the most intense absorption maxima (λ_abs1) display remarkably close values (338-339 nm), (ii) the longest-wavelength absorption maxima (λ_abs2) are blue-shifted with increasing the ring size (395 → 365 nm), (iii) the emission maxima (λ_em) are blue-shifted with increasing the ring size (494 → 438 nm for longest-wavelength maxima), (iv) the fluorescent quantum yields (Φ_F) in solution are high (0.73-0.90), (v) the fluorescence lifetimes (τ_s) of [9]- and [12]CPP are 10.6 and 2.2 ns, respectively, and (vi) the Φ_F values slightly increase in polymer matrix but significantly decrease in the crystalline state. According to TD-DFT calculations, the longest-wavelength absorption (λ_abs2) corresponds to a forbidden HOMO → LUMO transition and the most intense absorption (λ_abs1) corresponds to degenerate HOMO - 1 → LUMO and HOMO → LUMO + 1 transitions with high oscillator strength. The interesting and counterintuitive optical properties of CPPs (constant λ_abs1 and blue shift of λ _abs2) could be ascribed mainly to the ring-size effect in frontier molecular orbitals (in particular the increase of the HOMO-LUMO gap as the number of benzene rings increases). On the basis of comparative calculations using hypothetical model geometries, we conclude that the unique behavior of HOMO and LUMO of CPPs is due mainly to their lack of a conjugation length dependence in combination with a significant bending effect (particularly to HOMO) and a torsion effect (particularly to LUMO).