Fitting (pseudo) first order reaction

Hello guys,

I want to ask for some help regarding the fitting of pseudo first order reactions. The problem is, if I am fitting my data to an exponential equation namely I=A1*exp(t/T1)+y0 the fit works very well and R^2 is also acceptable.

But when I use the natural logarithm on my integrals and try linear fitting, the error explodes and R^2 turns to shit.

Does anyone have experience with the fitting of kinetic data or knows what the problem could be? I'm grateful for any hints and suggestions.