The utility of dioxetane-based chemiluminescent probes in biosensing and bioimaging is being increasingly recognized. While phenoxy-dioxetane luminophores with fast chemiexcitation kinetics are highly desired, current luminophores suffer from slow chemiexcitation. Herein we describe a rational, computationally-supported design of phenoxy-dioxetanes with fast chemiexcitation kinetics. These new luminophores were designed to contain a substituent that promotes rapid chemiexcitation, emitting light up to 100-fold faster than currently known dioxetanes. We demonstrate the superiority of the new phenoxy-dioxetanes by preparing three chemiluminescent probes for NAD(P)H, which differ from each other in the rate of the chemiexcitation. Comparison of these probes reveals a correlation between the chemiexcitation rate and the probe sensitivity. We anticipate that these new phenoxy-dioxetanes could serve as an ideal platform for designing chemiluminescence probes with enhanced sensitivity for numerous bioassays.