Exciton coupling effects and conformational change of perhexyloligosilanes with optically active methyl(1-naphthyl)phenylsilyl terminals

Perhexyloligosilanes (R,R)-(+)-MeNpPhSi*(Hex(2)Si)(n)Si*PhNpMe (n = 2; (R,R)-(+)-4a, n = 4; (R,R)-(+)-6a, n = 6; (R,R)-(+)-8a) with chiral methyl(1-naphthyl)phenylsilyl terminals were synthesized and characterized. The absorption wavelengths lambda(max) by (1)L(a,Ph) transition of phenyl chromophore conjugated with oligosilane units in (R,R)-(+)-4a - (R,R)-(+)-8a show bathochromic shift of about 3-4 nm compared with those of the alpha,omega-phenyl substituted perhexyloligosilanes Ph(Hex(2)Si)(m)Ph (m = 4; 4b, m = 6; 6b, m = 8; 8b) having the same silicon chain length. Longer chain length induces the separated lambda(max) of (1)L(a,Ph) from (1)B(b,Np) of naphthyl chromophore with positive exciton chiralities. In (R,R)-(+)-8a, although the extremum wavelengths lambda(ext) of exciton coupling between (1)B(b,Np) and (1)L(a,Ph) are separated by about 80 nm, the compound retains the positive exciton chirality, which provides definite information on the absolute configuration of terminal chiral silicon atoms. Bulky terminal substituents and lowering the temperature affect the conformation of the main chain, inducing extended silicon backbone structure.     

Evaluation of absolute configuration of naphthylphenyl-substituted oligosilanes by CD exciton chirality method

Absolute configurations of methylnaphthylphenyl-substituted oligosilanes, MeNpPhSi*SiMeR(1)R(2) [2 (R(1), R(2)=Me), 3 (R(1)=Me, R(2)=Ph), 4 (R(1), R(2)=Ph), and 5 (R(1)=Me, R(2)=SiMe(3))] were predicted by circular dichroism (CD) exciton chirality method. The sigma-pi conjugation effect of oligosilylene units (sigma-linkage) with pi-electron systems caused an intense red-shift of (1)L(a,Ph) transition band of the oligosilanes as shown in UV/VIS and made it possible to observe clear CD exciton chirality between the two aromatic chromophores on chiral silicon atom.