Recent studies have reported partial and quasi-complete 27 alignment of SC NRs in the presence of AC electric fields based on the fieldinduced relative change in absorbance. These reports, however, interpret data under the assumption that individual NRs are isolated in solution, and that the ensemble response can be attributed to the collective behavior of the individual, isolated NRs. Yet, growing evidence suggests that more complex interactions may be involved in the dynamic alignment of NRs. For example, colloidal SC NRs have been shown to spontaneously self-assemble into aggregated, microscale bundles in solution at concentrations as low as 1 μM. Moreover, several previous studies indicate that the energetics associated with aligning individual NRs in an externally applied fieldup to field strengths that cause dielectric breakdown of the solvent system-minimally counteract the thermal energy of free rotation, if at all. Instead, synergistic effects between NRs may be partially responsible for the observed alignment. In this alternative picture, bundles or aggregates of NRs in solution are more polarizable by externally applied electrostatic fields compared to individual NRs, thereby enabling the pronounced field-driven response that is observed. Solution phase self-assembly into bundles is also likely to modify the absorption anisotropy and other optical properties of colloidal NRs. For example, Pietra et al. observed increased linear dichroism when solutions of self-assembled NR bundles were placed in strong magnetic fields. The spontaneous aggregation of NRs into bundles-even in the absence of any external fields-complicates interpretations of ordering during optical measurements. Thus, ascertaining the optical anisotropy and related properties of the predominant NR species, i.e., isolated NRs or NR bundles, is critical for accurately quantifying the degree of field-driven ordering.