Exatecan derivatives, linker-payloads, and conjugates and thereof

A compound or a pharmaceutically acceptable salt thereof, wherein the compound has the structure of formula (i):

• wherein,
• a is 0 or 1;
• the carbon atoms marked with p1* and p2* each is asymmetric center, and the asymmetric center is S configured, R configured or racemic;
• L1 is selected from C1-6 alkylene, which is unsubstituted or substituted with one substituent selected from halogen, —OH and —NH2;
• M is —CH2—, —NH— or —O—;
• L2 is C1-3 alkylene;
• R1 is Cl;
• and
• R2 is F.