The optical properties of the PPCx NPs were then characterized. First, the UV-VIS-NIR absorption spectrum of PPC5 NPs exhibit an intense and sharp J-aggregation band (denoted as J-band) with a maximum absorption wavelength (λmax,abs) of 788 nm and a full width of half maximum (FWHM) of 46 nm (Figure ). The steady-state fluorescence (FL) emission spectrum of PPC5 J-aggregate showed a monomodal peak at 810 nm (λmax,em) and a FWHM of 35 nm, and a FL quantum yield (Φ) (0.65%) 6.5-fold higher than that of the NPs composed of physically blended hydroxyl-bearing PPcy initiator (P0) and PCL (Figure & S2, Table ). With the increase of the numberaverage degree of polymerization (x) from 5 to 25, the J-band in the absorption spectra was gradually attenuated, accompanied with the enhanced absorbance of monomeric PPcy (M-band, Figure ). In the FL emission spectra, an approximate 10 nm blue shift of λmax,em was observed with the increase of x from 5 to 20.