Interplay between Electronic Energy Transfer and Reversible Photoreactions in a Triad Comprising Two Different Styrylbenzoquinoline Photochromes and a ′Hidden′ Quencher

Abstract

A trichromophoric triad comprising two different 3-styrylbenzo [f]quinoline photochromes (SBQ1 and SBQ2) and a residue of 3oxy-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.

[2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward–Hoffmann rules and photoinduced stepwise reverse reaction of the ring openingviapredissociation

“…These maxima are characteristic of the benzoquinoline core and indicate a break of the SBQ π-conjugated system. Taking into account an analogy in properties of the dyad D44B and previously studied supramolecular systems with the SBQ photochromes, 22–25 the final spectrum 10 in Fig. 4 can be attributed to the cyclobutane CB44B , which forms from the dyad D44B as a result of the PCA reaction, Scheme 4.…”

“…We have recently found that supramolecular systems, dyads and triads, with covalently linked styrylbenzoquinoline (SBQ) photochromes are suitable objects for studying the PCA reaction. 20–24 Using a balanced rigid-flexible linker between SBQ units, we observed for the first time both emissive and reactive excimers in the PCA reaction. 25…”

“…19 We have recently found that supramolecular systems, dyads and triads, with covalently linked styrylbenzoquinoline (SBQ) photochromes are suitable objects for studying the PCA reaction. [20][21][22][23][24] Using a balanced rigid-flexible linker between SBQ units, we observed for the first time both emissive and reactive excimers in the PCA reaction. 25 The present paper is aimed at studying photochemical cyclobutane formation and especially at a more detailed study of photoinduced cyclobutane ring opening.…”

“…25 The photochemical properties of the model M1 were studied earlier. 24 This SBQ photochrome undergoes reversible photoisomerization, the quantum yields of trans-cis (j tc ) and cistrans (j ct ) photoisomerization are 0.52 and 0.43, respectively. The spectra of the trans-(E-) and cis-(Z-) isomers are shown in Fig.…”

“…FRET quenches the E-SBQ unit and sensitizes the Z-SBQ unit in the asymmetric EZ and ZE isomers, which leads to a decrease in j tc and an increase in j ct . 24 The exact value of FRET efficiency (j ET ) in asymmetric isomers is unknown. Variation of the j ET parameter shows that taking into account the FRET process reduces j ct and has little effect on the other quantum yields.…”