Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. The long-lived excited state of these clusters and their ability to participate in electron transfer is integral to their function as light-harvesting cells. The excited state behaviour of GSH protected clusters (Au25GSH18, Au18GSH14, Au15GSH13, and Au10-12GSH10-12) is herein explored using ultrafast (femtosecond) transient absorption spectroscopy (fsTAS) and nanosecond transient absorption spectroscopy (nsTAS) to assess if cluster excited-state behaviour can be modulated by varying cluster size. Through investigation, a strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced. The photoactivity of the clusters shows increasing photocatalytic reduction yield with decreasing size, such that Au18GSH14 has the highest potential as a photosentizer, based on the quantum yield of electron transfer and good visible light absorption properties.