Nanosecond photoexcitation of a bilayer of smooth anatase TiO2 coated with palladium tetrakis(4-carboxyphenyl)porphyrin (PdTPPC) results in a delayed, after-pulse growth of the conductivity over many microseconds as monitored by time-resolved microwave conductivity. This phenomenon is attributed to the slow diffusion of PdTPPC triplet excitons followed by electron injection into the TiO2 conduction band. The temporal form and intensity dependence of this process have been simulated by Monte Carlo (MC) calculations of exciton diffusion and exciton-exciton annihilation. Good agreement between the experimental results and MC simulations are obtained using a triplet exciton diffusion coefficient DE 8 X 10(-11) m(2)/s, exciton lifetime (tauE)greater than or equal to10 mus, effective triplet-triplet annihilation distance R-a = 1. 5 nm and interfacial electron injection efficiency phi.(inj) = 0.44. (C) 2004 Elsevier B.V. All rights reserved.