Twist and Return−Induced Ring Strain Triggers Quick Relaxation of a (Z)-Stabilized Cyclobisazobenzene

Abstract

Continuous irradiation of the thermodynamically stable ( Z, Z)-cyclobisazobenzene does not lead to accumulation of a ( Z, E) or ( E, E) isomer as one might expect. Our combined experimental and computational investigation reveals that Z → E photoisomerization still takes place on an ultrafast time scale but induces large ring strain in the macrocycle, which leads to a very fast thermal back-isomerization, preventing photostationary accumulation of ( E)-isomers.

Stereoselective Excited-State Isomerization and Decay Paths in cis-Cyclobiazobenzene

“…Previously combined experimental and theoretical work believe that the individual AB unit in CBA still undergoes the ultrafast Z-E photoisomerization in spite of the associated large steric hindrance and internal strain. 52 Our current nonadiabatic dynamics simulations support such viewpoint from dynamical perspective. In terms of our trajectories, the excited-state decay time is estimated to be ca.…”

“…Compared with that of isolated AB molecule, the oscillator strength of the 1 nπ* singlet state in CBA is comparably enhanced, which is consistent with recent experimental observation. 52 Two S1/S0 minimum-energy conical intersections (MECIs) relevant to the cis-trans photoisomerization of the N3'-N4' azo group of CBA are optimized at the OM2/MRCI level and are referred to as S1S0-1 and S1S0-2 in Figure 2. These two MECI structures are all related to the N3'-N4' azo group; however, they have different C2'N3'N4'C5' dihedral angles, which are 90.3°…”

Strain visualization for strained macrocycles

“…A similar method was previously reported for mechanochemistry where unstrained molecules are stretched and the tension that appears upon stretching is analyzed. 25 Attempts were made to apply this method to macrocycles by comparing stretched and unstretched macrocycles as well as conformational changes that induce stretching, 26,27 however, the inherent strain was not addressed. StrainViz can nd this elusive strain energy.…”

Starazo triple switches – synthesis of unsymmetrical 1,3,5-tris(arylazo)benzenes

“…After the successful synthesis of (E)-N-(3-nitro-5-(phenyldiazenyl)phenyl)acetamide (8), the reduction of the nitro group was attempted. However, the literature-known procedure of using sodium hydrosulfide did not yield the desired (E)-N-(3amino-5-(phenyldiazenyl)phenyl)acetamide (9).…”

“…Even subtle interactions, such as London dispersion in alkyl-substituted ABs, can have a significant influence on their isomerization properties [3,4]. Also, the incorporation of AB units into cyclic [5] or macrocyclic structures can control the switching, depending, i.a., on symmetry and ring strain [6][7][8][9]. By combining these approaches, half-lives can be tuned from milliseconds to years.…”