Experimental evidence for OOP-CI pathways from ultrafast electron diffraction (UED) studies is only indirect, as the observable is biased towards the significantly larger geometric changes from the ring-opening pathways. Nevertheless, there exists evidence for the presence of additional pathways from the experimentally observed relative signal levels in αTp and αPh, which can serve as a measure for the ring-opening quantum yield. The two experiments were conducted consecutively in the same experimental setup within one week, yielding highly comparable experimental conditions. The two molecules have similar absorption cross-sections at the pump wavelength. Thus, a given pump pulse energy can be expected to yield comparable photoexcitation yields. The observed photoinduced signatures were weaker for αPh. Fitting simulated diffraction signatures, which incorporate the predicted ring-opening quantum yield, to the experimental signatures of αTp obtained with 4 μJ pump pulse energy suggests an excitation fraction of 1.56 %.