Measuring NIR-II quantum yield

The fluorescence quantum yield of CH-4T and complexes were measured in a similar manner as described in previous publications^1,2. Briefly, HiPCO SWCNTs were utilized as a reference fluorophore due to their high aqueous stability. The quantum yield of HiPCO SWCNTs has previously been determined as 0.4% based on an IR-26 quantum yield of 0.5% in DCE (please see Discussion for further details on IR-26 quantum yield). A serial dilution of five solutions of HiPCO SWCNTs as well as CH-4T and complexes with an OD <0.1 at 808 nm was measured to confirm absorbance values at 808 nm and the fluorescent emission spectrum was collected on a wavelength-corrected NIR-II spectrometer in a 1 cm quartz cuvette (Starna) in the manner specified above. The fluorescent emission spectrum was integrated and plotted against the OD value at 808 nm and a linear fit was applied to verify the linearity between fluorescent brightness and concentration. For the brightest samples, inter-filter effects were seen ∼OD 0.1 at 808 nm, thus lower concentration ranges were utilized. By comparing the slope of the linear fit between HiPCO SWCNTs and CH-4T and complexes, the quantum yield was determined based on the following supporting equation (1):

where QY_sample is the QY of CH-4T and complexes, QY_ref is the QY of HiPCO SWCNTs in water (0.4% based on IR-26=0.5% in DCE¹), n_sample and n_ref are the refractive indices of HiPCO SWCNTs, CH-4T and complex solutions which are both water (1.33) in this case.