A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered...

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Competitive photoprocesses were found in a novel biphotochromic dyad with 3‐styrylbenzo[f]quinoline (SBQ) photochromes ‐ fluorescence and unexpectedly irreversible [2+2] photocycloaddition (PCA) resulting in formation of a cyclobutane derivative with two benzo[f]quinoline (BQ) substituents. No SBQ trans‐cis photoisomerization was observed. Quantum‐chemical DFT calculations predict π‐stacking of the SBQ units in the dyad that hinders trans‐cis photoisomerization but favors a photoactive excimer formation with following PCA. Calculations predict also π‐stacking of the BQ units in the cyclobutane that explains experimentally observed exciton absorption and excimer emission of the latter. The unique property of the cyclobutane excimer is its photoinertness: cyclobutane does not undergo photocycloreversion with ring opening. The photoinert excimer is observed for the first time. Possible reasons of difference between chemical properties of photoactive and photoinert excimers are discussed.

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.

Using molecular logic gates (MLGs) for information processing attracts attention due to perspectives of creating molecular computers. Biphotochromic dyads are suitable models of photonic MLGs. However, they suffer from one weakness: the activity of one of the photochromes is often quenched because of Förster resonance energy transfer (FRET). Herein, we designed a dyad with reduced FRET, in which both photochromes keep their photoactivity thanks to spectral and spatial separation, allowing MLG switching between different states. This novel dyad reproduces the functionality of the full set of 16 two-input gates, as well a reversible gate-dual inverter, all gates are photonic.