Irreversible One‐Way [2+2] Photocycloaddition in Bis‐Styrylbenzo[f]quinoline Dyad: Photoactive and Photoinert Excimers in the Same System

Abstract

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.

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

“…The final structured spectrum 8 (Figure 4) with peaks at 319, 333, and 349 nm is characteristic of the unconjugated BQ aromatic π-system. These spectral variations are analogous to those observed earlier for the SBQ dyads [30,31] and imply that the similar reactions take place, namely, photoisomerization (Scheme 3) and PCA with formation of a cyclobutane derivative CB, Scheme 4.…”

“…All model chromophores M1, M2 and M3 are fluorophores and emit in the same spectral region, Figure 2, therefore, all subunits of the triad, i. e., SBQ1, SBQ2 and NA, can contribute to its fluorescence spectrum. The spectral maxima and fluorescence quantum yields are 383, 402 nm and 0.37 for M1, [31] 424 nm and 0.043 for M2, [34] 411 nm and 0.38 for M3. [31] The fluorescence and fluorescence excitation spectra of all model compounds overlap, and the fluorescence excitation spectra coincide with the absorption spectra.…”

“…The spectral maxima and fluorescence quantum yields are 383, 402 nm and 0.37 for M1, [31] 424 nm and 0.043 for M2, [34] 411 nm and 0.38 for M3. [31] The fluorescence and fluorescence excitation spectra of all model compounds overlap, and the fluorescence excitation spectra coincide with the absorption spectra. The spectral overlapping creates a condition for the Förster resonance energy transfer (FRET) between different subunits in the triad SBQ1-NA-SBQ2.…”

“…Photochemical properties of the model styrylbenzoquinolines M1 and M2 have been studied earlier. [30,31] Both compounds undergo reversible photoisomerization, Scheme 2, quantum yields of trans-cis (ϕ tc ) and cis-trans (ϕ ct ) photoisomerization are ϕ tc = 0.32 and ϕ ct = 0.66 for M1, [31] and ϕ tc = 0.52 ϕ ct = 0.43 for M2. [30] The triad SBQ1-NA-SBQ2 has two SBQ photochromes and, consequently, four possible isomers -EE, EZ, ZE and ZZdepending on the trans-or cis-configuration of the first (SBQ1) or the second (SBQ2) photochrome.…”

“…[27] For example, dyads containing photochromic styrylquinoline or styrylbenzo[f]quinoline (SBQ) groups, in addition to geometric photoisomerization, undergo also [2 + 2] photocycloaddition (PCA) with quantitative formation of a single rctt cyclobutane derivative. [28][29][30][31] The PCA efficiency was explained by a "pre-organizing" action of the o-xylylene or 2,3-naphthylene frameworks, which orient the two SBQ moieties in parallel planes. The dyads in the conformations with closely spaced SBQ moieties are stabilized by π-stacking interactions.…”