Robust singlet fission in pentacene thin films with tuned charge transfer interactions.

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings.

Dissecting charge relaxation pathways in CdSe/CdS nanocrystals using femtosecond two-dimensional electronic spectroscopy.

Exciton relaxation dynamics of CdSe and quasi-type-II CdSe/CdS core/shell nanocrystals were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The use of 2DES allowed for determination of structure-specific and state-resolved carrier dynamics for CdSe nanocrystals formed with five, or fewer, CdS passivation monolayers (ML). For CdSe and CdSe/CdS nanocrystals formed with one through three MLs of CdS, excitation using broad bandwidth femtosecond visible laser pulses generated electron-hole pairs among the |X1〉 = 2.14 eV and |X2〉 = 2.27 eV exciton states. For both excitations, the electron is promoted to the lowest energy excited (1Se) conduction-band state and the hole is in the 1S3/2 (X1) or 2S3/2 (X2) valence-band state. Therefore, the relaxation dynamics of the hot hole were isolated by monitoring the-time-dependent amplitude of 2DES cross peaks. The time constant for hot hole relaxation within the CdSe valence band was 150 ± 45 fs. Upon passivation by CdS, this hole relaxation time constant increased to 170 ± 30 fs (CdSe/CdS-3ML). This small increase was attributed to the formation of a graded, or alloyed, interfacial region that precedes the growth of a uniform CdS capping layer. The small increase in hole relaxation time reflects the larger nanocrystal volume of the CdSe/CdS system with respect to the CdSe nanocrystal core. In contrast, the dynamics of larger core/shell nanocrystals (≥4ML CdS) exhibited a picosecond buildup in 2DES cross-peak amplitude. This time-dependent response was attributed to interfacial hole transfer from CdS to CdSe valence-band states. Importantly, the 2DES data distinguish CdSe exciton relaxation from interfacial carrier transfer dynamics. In combination, isolation of structurally well-defined nanocrystals and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.