Molecular conformation dependence of phosphorescence lifetime in organic aggregates

Organic bulk crystal with room-temperature phosphorescence (RTP) is normally fabricated into nanoparticles (NPs) for biological applications, accompanying with inevitable change in RTP lifetime and molecular conformation. By using molecular dynamics and quantum chemistry methods, the difluoroboron β-diketonate (BF₂bdk) compound (Br-NpCzBF₂) was explored to uncover the molecular conformation-dependent RTP from crystal to NPs. We proposed that from crystal to amorphous aggregates, the distorted BF₂bdk group caused by the weakened intermolecular B–F⋯H–C hydrogen bonds convert the T₁ from hybridized local and charge transfer to locally excited states. Such change strengthens the vibronic coupling effect on the C[dbnd]C stretching vibration, and enormously accelerates the nonradiative decay rate of T₁→S₀ by 1–2 orders of magnitude. Consequently, the RTP lifetime is shortened from 566 μs in crystal to about 10 μs in amorphous aggregates, which is consistent with the experiments. This work provides new insight into the RTP mechanism and opens a novel design avenue for designing organic RTP materials in biological fields.