Photoisomerization of Stilbene Dendrimers: The Need for a Volume-conserving Isomerization Mechanism¶

Abstract

Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans-cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume-conserving novel mechanism such as hula-twist rather than conventional 180 degrees rotation around the C=C double bond based on fluorescence and isomerization experiments.

Stilbene-cored poly(glutamate) dendrimers

“…In the case of the higher generation dendrimers G2 and wG2, however, photocyclization did not take place during irradiation because of the bulky poly(glutamate) dendrons. These results are consistent with the photochemical cyclization of other series of stilbene-cored dendrimers, 45,49 where the stilbene core in higher generation did not undergo photocyclization. It should be important for photochemical cyclization to take the planar conformation in the excited singlet state, and the bulky dendrons in higher generation probably disturb it (Table 1).…”

“…In this area, the photoisomerization of dendrimers with photoresponsive cores-such as stilbene, [13][14][15][16][17][18][19][20][21] azobenzene, [22][23][24][25][26][27][28][29][30][31][32][33] diarylethene, [34][35][36][37][38] intramolecularly hydrogen-bonded chromophores, [39][40][41] and photolabile chromophores 42-44 -has been extensively investigated. We recently reported that stilbene dendrimers with benzyl ether-type dendrons exhibited a volume-conserving isomerization mechanism and a macromolecular effect (or generation effect) on energy transfer efficiency from the peripheral dendrons to the core stilbene in organic solvents [45][46][47] and aqueous solution. [48][49][50] In these cases, the core stilbene unit played an important role as a photo-trigger for large conformational changes (photoisomerization), and as a fluorescent probe for detecting the environment inside the dendrimer.…”

Supramolecular Effects on Mechanisms of Photoisomerization: Hula Twist, Bicycle Pedal, and One‐Bond‐Flip

“…32 There are also other compounds in the literature where the unique structures appear to require HT to account for the photoisomerized product: the cyclic stilbene -cyclophane 24 33 and a set of stilbene dendrimers. 34 However, formation of high -energy conformers in the primary photochemical process remains to be proven.…”

Different photochemical behavior of bis(biphenyl)ethylenes and ethenes in solution and in the solid-state: Structurally controlled Z/E-photoisomerization in the solid-state

“…Among a number of photochemical molecular transformations, the E/Z-photoisomerization of olefinic compounds is the most fundamental and important process and this photochemical reaction in the confined environments has been extensively studied [1][2][3][4][5][6][7][8][9][10][11][12] in relation to the protein-bound visual chromophore. While the photoisomerization by conventional rotation, i. e., the 180 o rotation around the double bond, is regarded to be difficult within the narrow space such as in a crystal or solid-state, the Hula-twist (HT) mechanism, a space-conserving isomerization process, was proposed by Liu et al to explain such the phenomena [13][14][15][16][17].…”

“…While the photoisomerization by conventional rotation, i. e., the 180 o rotation around the double bond, is regarded to be difficult within the narrow space such as in a crystal or solid-state, the Hula-twist (HT) mechanism, a space-conserving isomerization process, was proposed by Liu et al to explain such the phenomena [13][14][15][16][17]. Not only this hypothesis has sometimes been applied to the numerous examples of E/Z-photoisomerizations in the confined environments [7,[9][10][11][12], but also several investigations have been undertaken to verify this process [18][19][20][21][22]. However, only the 1,2-disubstituted ethylenes were employed in almost all examples previously reported, but, to the best of our knowledge, the tri-and tetra-substituted alkenes have not been used for this type of investigation.…”