Interplay Between Electronic Energy Transfer And Reversible Photoreactions In A Triad Comprising Two Different Styrylbenzoquinoline Photochromes And A ' Hidden ' Quencher

A trichromophoric triad comprising two different 3-styrylbenzo[f]quinoline photochromes (SBQ1 and SBQ2) and a residue of 3-oxy-2-naphthoic acid (NA) was synthesized and studied by steady-state and time-resolved spectroscopy and by DFT calculations. The SBQ photochromes are linked to the NA framework by methylene spacers of different lengths to provide a longitudinal displacement between the photochromes. Under light irradiation, the triad undergoes two competitive reactions, intra-photochrome photoisomerization and inter-photochrome [2+2]-cross-photocycloaddition giving rise to a cyclobutane derivative (CB) bearing two benzo[f]quinoline (BQ) substituents. The photochemical properties of both the triad and CB are affected significantly by intramolecular energy transfer (ET). In the triad, the SBQ1 subunit is quenched via ET to the SBQ2 subunit. Due to unique ratio between the local excited states of NA, SBQs, and BQ, the NA subunit serves as a ' hidden quencher '; it has little influence on the properties of the triad, but after the formation of CB it turns on and quenches both the BQ fluorescence and the cyclobutane ring-opening reaction because of fast ET from BQ to NA.