The role of amide (n,π∗) transitions in polypeptide clusteroluminescence

Non-conjugated fluorescent materials have attracted much research attention in recent years due to their great theoretical significance and practical value. But the working mechanism for this extraordinary photophysical phenomenon is still unclear and in debate. In this work, three non-conjugated polypeptides exhibit excitation-independent clusteroluminescence (CL) with an emission peak at 440 nm. Experimental and theoretical analyses show that the (n,π∗) transition of the amide unit plays a crucial role in the CL. Notably, the processes of polymerization and clusterization are proposed as effective ways to activate the (n,π∗) transition, which could increase the CL efficiency. This work not only gains further insights into the CL mechanism but also provides effective ways to design and manipulate non-conjugated fluorescent materials.