  Entanglement control of qubits in a photoisomerizing molecule is studied in
the context of an NMR quantum computer by taking azobenzene as an example.
Azobenzene has two different isomers, {\it{}trans}-azobenzene (TAB) and
{\it{}cis}-azobenzene (CAB), which can be interconverted by photoisomerization.
Changing molecular structure leads to change in the spin-spin coupling
constant, and hence leads to change in entangling operation time. We first
obtain stable structures of TAB and CAB by {\it ab initio} calculation. Then,
we calculate the NMR spectra of these isomers and verify that they reproduce
the chemical shift obtained experimentally with a good precision. Our result
indicates that the coupling strength between a $^{15}$N and a $^{13}$C nuclei
in the molecule changes by a large amount under photoisomerization.
