Coherent Modified Redfield theory (CMRT) is employed to describe photoinduced proton-coupled electron transfer. Traditional Redfield theory is restricted to the treatment of very weak system-bath coupling strengths. CMRT on the other hand includes the diagonal part of the system-bath coupling into the zeroth order Hamiltonian and extends the approach to capture weak to moderate system-bath coupling strengths. The reduced density matrix equations are secular and can capture non-Markovian dynamics. We consider a model Hamiltonian consisting of a donor and an acceptor electronic state which are coupled to the proton and to a phonon-bath. The electron is photo-induced from the ground electronic state to a donor state. We study the proton-deuterium isotope effect, coherent to incoherent transition in the population dynamics. At small bath reorganization energies, system parameters such as the energy bias between the donor and acceptor sites and overlaps of the vibronic states play a crucial role in influencing the population decay and isotope effect.