\subsection{Solar radiation exposure}
\label{solarexpose}
Aqueous solutions containing Bip and H$_{2}$Bip in quartz cells (0.5 cm optical path length) were exposed to outdoor conditions on a horizontal platform alongside the fiber optic sensor of the Avantes spectrometer (see Subsection \ref{hrspectrometer}). The measurement routine was mainly performed for clear sky days at solar noon, being the moment of maximum intensity and a minimal optical path crossed by light. The quartz cells were aligned toward sun's axis, until achieving the minimum shadow. 

The attenuation of the solar intensity is strongly dependent on zenith angle, latter related to local time. Accordingly, we carried out an identical procedure using ensembles of samples exposed along the hours for a clear sky day. Also the temporal variation of the intensity was taken into account month by month. The quartz cells position were always aligned following the sun trajectory during the day. This system of sun tracking was designed so that the non-exposed volume of a given sample was always negligible with respect to the total volume.