where 𝐼 ∥ and 𝐼 ' are the vertically and horizontally polarized emission intensities, respectively. The anisotropy of a random ensemble of emitters can, in theory, range between -0.2 and 0.4 depending on (1) the intrinsic dipole moment of the optical transition and (2) depolarization due to mismatch of excitation and emission dipole moments-typically caused by rotational diffusion between the excitation and emission events. The latter effect, while prominent in dichroic dyes , can be regarded as negligible in SC NRs since the rotational diffusion rate (>1 µs), even in lowviscosity solvents at room temperature, is significantly slow compared to the fluorescence lifetime (10-50 ns). Dilute solutions of CdSe/CdS NRs have been experimentally shown to exhibit a peak anisotropy of ~0.2 for near band-edge excitation, and consistently lower values (~0.1) at higher energy excitation (Figure ). 9-11 Samples with no optical anisotropy show values of 𝑟 = 0, whereas an ensemble of randomly oriented, ideal dipoles show 𝑟 = 0.4. Figure shows the ensemble anisotropy of CdSe/CdS NRs recorded as a function of NR concentration. The strong spectral dependence of anisotropy in SC NRs (Figure ) requires that the ensemble anisotropy measurements be carried out using the same excitation wavelength (405 nm) used in the ∆𝐴 𝐴 ! ⁄ measurements. As clearly apparent, the variation of ensemble anisotropy reveals a strong dependence on NR concentration. We observe two regimes of contrasting behavior for samples at low concentration or high concentration. In the dilute regime (≤ 0.5 µM), the anisotropy increases with NR concentration. The positive correlation between anisotropy and concentration suggests that self-assembly of NRs may lead to the formation of mesoscale structures that have more ideal transition dipoles compared to individual NRs. This hypothesis concurs with proposed colloidal-phase NR assemblies in which NRs are aggregated in a side-by-side manner so that their long axes are co-aligned. The decrease in anisotropy observed at higher concentrations can be attributed to multiple scattering and/or multiple reabsorption and reemission events, typical artifacts in ensemble anisotropy measurements when incident photons interact with more than one species before reaching the detector . In order to estimate the anisotropy of the NR colloids independent of these artifacts intrinsic to the measurement, we have extrapolated the trend leading up to 0.5 µM (dashed trace). This extrapolation aids further analysis provided below, though we acknowledge that this approximation is not a rigorous assessment of the true anisotropy, but instead, a rough estimate pertaining to the limiting case where the anisotropy continues to increase with increasing NR concentration. For simplicity, a linear trendline (on a logarithmic x-axis) was chosen to extrapolate the data in the highconcentration regime.