Vibrational energy transfer through molecular chains

Intramolecular vibrational energy flow in bridged azulene-anthracene compounds is investigated by time-resolved spectroscopy. The bridges consist of molecular chains of the type (CH₂)_m with m ≤ 6 as well as (CH₂OCH₂)_n (n = 1,2) and CH₂SCH₂. With a short laser pulse excited molecules are formed where the excess vibrational energy is localised initially at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side is followed by probing the energy content of the azulene and/or the anthracene chromophore with a second delayed laser pulse. The corresponding time constants τ_IVR for short bridges increase with the chain length. For longer bridges consisting of more than 3 elements, however, τ_IVR is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, τ_IVRis independent on the length.