A compound of Formula I or a pharmaceutically acceptable salt thereof, wherein, Rh is H or methyl, preferably H; Rg is CH, -C-Rf or N, preferably CH; Rf is methyl or halogen (such as F, Cl, Br or I); m is 0, 1, 2 or 3, preferably 0 or 1, more preferably 0; Re is an electron-withdrawing group selected from the group consisting of tertiary amine cations (-N + R' 3, wherein R' is independently selected from H and C 1 -C 6 alkyl), nitro (-NO 2), trihalomethyl (-CX 3, X=F, Cl, Br or I), halogen (such as F, Cl, Br and I), formyl (-CHO), acyl (-CO-C 1-4 alkyl), carboxyl (-COOH), cyano (-CN) and sulfonic acid group (-SO 3 H); Rd is alkenyl or alkynyl, for example, having 2, 3, 4, 5 or 6 carbon atoms; Ra, Rb and Rc are selected from the following combinations: Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl); Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are attached together to form C 2 -C 4 alkylene (e.g. C 2 -C 3 alkylene, such as ethylidene, 1,3-propylidene); Ra is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Rb and Rc together with the N atom to which they are attached form a 5- or 6-membered saturated heterocyclic ring containing a N atom; Rc is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Ra and Rb together with the N atom to which they are attached form a 5- or 6-membered saturated heterocyclic ring containing a N atom. Rh is H or methyl, preferably H; Rg is CH, -C-Rf or N, preferably CH; Rf is methyl or halogen (such as F, Cl, Br or I); m is 0, 1, 2 or 3, preferably 0 or 1, more preferably 0; Re is an electron-withdrawing group selected from the group consisting of tertiary amine cations (-N + R' 3, wherein R' is independently selected from H and C 1 -C 6 alkyl), nitro (-NO 2), trihalomethyl (-CX 3, X=F, Cl, Br or I), halogen (such as F, Cl, Br and I), formyl (-CHO), acyl (-CO-C 1-4 alkyl), carboxyl (-COOH), cyano (-CN) and sulfonic acid group (-SO 3 H); Rd is alkenyl or alkynyl, for example, having 2, 3, 4, 5 or 6 carbon atoms; Ra, Rb and Rc are selected from the following combinations: Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl); Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are attached together to form C 2 -C 4 alkylene (e.g. C 2 -C 3 alkylene, such as ethylidene, 1,3-propylidene); Ra is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Rb and Rc together with the N atom to which they are attached form a 5- or 6-membered saturated heterocyclic ring containing a N atom; Rc is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl); Rb is C 1 -C 4 alkylene (e.g. C 1 -C 3 alkylene, such as methylene, ethylidene, 1,3-propylidene), and Ra and Rc are attached together to form C 2 -C 4 alkylene (e.g. C 2 -C 3 alkylene, such as ethylidene, 1,3-propylidene); Ra is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Rb and Rc together with the N atom to which they are attached form a 5- or 6-membered saturated heterocyclic ring containing a N atom; Rc is hydrogen or C 1 -C 6 alkyl (e.g. C 1 -C 4 alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl), and Ra and Rb together with the N atom to which they are attached form a 5- or 6-membered saturated heterocyclic ring containing a N atom.