To investigate the influence of the polymer structure on the optical properties of tethered PPcy, we synthesized another group of polyesters, denoted as PPLz (z represents the degree of polymerization of PLLA on each side of PPcy), by grafting poly (L-lactic acid) (PLLA) from P0 via a similar synthetic scheme of ring-opening polymerization (ROP) (Scheme S1). In contrast to the J-band as observed in PPCx NPs, characteristic H-band appeared in the absorption spectra of PPLz NPs (Figure ). These H-aggregate NPs showed weak to none fluorescence, a typical feature of Haggregation. With the decrease of z from 22 to 10, the absorbance of H-band was gradually enhanced. Similar feature of the absorption spectrum can be observed in an amorphous isomer of PPL10, i.e. poly(D,L-lactic acid) (denoted as PP(DL)L10) (Figure ). While the ARTICLE Please do not adjust margins Please do not adjust margins a Fluorescence decays at 760 nm or 800 nm upon the laser excitation of 635 nm. b Fluorescence quantum yields (Φ) of NPs solution were measured by the standard method using PPcy in chloroform (Φ = 0.69, which was determined by using indocyanine green (ICG) as the standard. mechanism of the different aggregate structures favorable for PPC and PPL remains unclear and warrants further study, it was also observed in some donor-acceptor merocyanine dyes that subtle change of the substituent groups resulted in dramatically different aggregating states (i.e. H-and J-aggregates) in spin-coated thin films on quartz substrates. We were curious whether the formation of J-aggregation of fluorophores for PPC is due to the semicrystalline behavior of PCL. To verify the hypothesis, the amorphous biodegradable polymer polydecalactone (PDL) was chosen to replace PCL to afford PPDy. Due to the limited reactivity of ɛ-decalactone (DL) in ROP, only PPD2 and PPD6 with low degree of polymerization of PDL were obtained. One can see that both PPD2 and PPD6 NPs showed sharp J-bands in their absorption spectra (Figure ). Interestingly, the absorption spectrum of PPD6 NPs with an intense J-band at 785 nm and a shoulder peak at 735 nm is quite similar to that of PPC15, although the chain length of PDL in PPD6 is almost 3-fold shorter than that of PCL in PPC15. These results suggest that PDL with amorphous nature can also induce the J-aggregation of PPcy fluorophores.