Knowledge Bank

Calculations by $Pariser1$ have shown azulene to have excited state dipole moments opposed to that of the ground state. If so, one might expect polar solvents to shift the azulene absorption spectrum to shorter wave-lengths and the compression of such solutions to result in additional blue shifts. To test these ideas the spectrum of azulene has been recorded in both polar and nonpolar solvents at temperatures down to $-80\circ C$ and at pressures up to several thousand atmospheres. For comparison the spectrum of naphthalene was recorded under similar conditions. Results are only partly as anticipated. The three azulene transitions, $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb$, $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bb$ shift to the blue upon solution in polar solvents. However, upon compression the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}LBb$ bands undergo progressive red shifts with increasing solvent density. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ shows a blue shift from vapor to solution in nonpolar solvents and an additional blue shift with increasing optical polarizability the solvent density. Upon decreasing the temperature of the polar solvent, the rate of red shifts with solvent density is less for the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bb$ than in the compressed solvent, as is the rate of blue shift of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb$. The characteristic rates of shift of the various transitions with solvent density tend to verify Pariser’s assignment of the band falling at 4.05 e.v. as a separate transition from the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bb$.