In the present paper, we synthesized a series of benzophenone (BP)-N-methylphthalimide (MePI) dyads (Cn, n = 3, 6, and 9, where n denotes the number of methylene in the linker) and investigated the photochemical properties and intramolecular triplet-triplet energy transfer from BP(T-1) to MePI. Formation of two different intramolecular complexes was found, that is, a ground-state complex and a singlet exciplex. The formation of the triplet-equilibrium between MeBP and MePI was observed. The triplet-equilibrium constant (1.0 and 1.1 for C6 and C9, respectively) and forward ((3.8+/-1.3) x 10(7) and (3.9+/-1.2) x 10(7) s(-1) for C6 and C9, respectively) and back ((3.8+/-1.3) x 10(7) and (3.6+/-1.2) x 10(7) s(-1) for C6 and C9, respectively) energy transfer rates were estimated from the result of transient absorption measurements. From the van't Hoff plots, enthalpy and entropy change for the equilibrium formation were estimated.