Efficient energy transfer from Ru(bpy)32+* (bpy = 2,2′-bipyridine, * denotes the excited state) to 3,6-disubstituted tetrazines [R2Tz: R = Ph (Ph2Tz), 2-chlorophenyl [(ClPh)2Tz], 2-pyridyl (Py2Tz)] occurs to yield the triplet excited states of tetrazines (³R2Tz*), which have longer lifetimes and higher oxidizing ability as compared with those of Ru(bpy)32+*. The dynamics of hydrogen-transfer reactions from NADH (dihydronicotinamide adenine dinucleotide) analogues has been examined in detail using ³R2Tz* by laser flash photolysis measurements. Whether formal hydrogen transfer from NADH analogues to ³R2Tz* proceeds via a one-step process or sequential electron and proton transfer processes is changed by a subtle difference in the electron donor ability and the deprotonation reactivity of the radical cations of NADH analogues as well as the electron-acceptor ability of ³R2Tz* and the protonation reactivity of R2Tz*-. In the case of ³Ph2Tz*, which is a weaker electron acceptor than the other tetrazine derivatives [(ClPh)2Tz; Py2Tz], direct one-step hydrogen transfer occurs from 10-methyl-9,10-dihydroacridine (AcrH2) to ³Ph2Tz* without formation of the radical cation (AcrH2̇+). The rate constant of the direct hydrogen transfer from AcrH2 to ³Ph2Tz* is larger than that expected from the Gibbs energy relation for the rate constants of electron transfer from various electron donors to ³Ph2Tz*, exhibiting the primary deuterium kinetic isotope effect. On the other hand, hydrogen transfer from 9-isopropyl-10-methyl-9,10-dihydroacridine (AcrHPr′) and 1-benzyl-l,4-dihydronicotinamide (BNAH) to ³R2Tz* occurs via sequential electron and proton transfer processes, when both the radical cations and deprotonated radicals of NADH analogues are detected by the laser flash photolysis measurements. [ABSTRACT FROM AUTHOR]