Knowledge Bank

The appearance of intense absorption bands in molecular complexes formed of an electron donor molecule and an electron acceptor molecule is well $known.1$ Such absorption bands have been interpreted by R. S. $Mulliken1$ as being due to \emph{inter-molecular charge-transfer transitions}. Similar bands are expected to occur in molecules where an electron acceptor group and an electron donor group are parts of the same molecule. By analogy, transitions which give rise to such bands are called \emph{intramolecular charge-transfer transitions}. Several transitions of this type have been reported to occur in a variety of $molecules.2,3,4$ Tri-1-naphthylboron provides an interesting example where an intramolecular charge-transfer transition is expected to occur. Here the boron atom acts as the electron acceptor group and the naphthalene acts as the electron donor group. The room temperature absorption spectrum of tri-1-naphthylboron in methylcyclohexane shows an intense absorption band with a maximum absorption of 3525 \AA $(ϵmax=1.92\times 104)$. The rest of the spectrum closely resembles that of naphthalene except that the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}L\mu$ state of naphthalene appears at shorter wavelengths. The absorption at 3525 \AA is interpreted as an intramolecular charge transfer transition and results from the interaction of the highest filled orbital of naphthalene $\Phi N$ with the lowest vacant orbital of trimethylboron $\Phi B$. The energy of the transition may be calculated using the formula: [ \Delta E= {(E_{N} - E_{B})^{2} +4;C^{2}{N};C^{2}{B};\beta^{2}}^{1/2} ] where $EN$ and $EB$ correspond to the energies of $\Phi N$ and $\Phi B$ respectively; $CN$ and $CB$ are the coefficients of the appropriate atomic orbitals in $\Phi N$ and $\Phi B$ respectively, and $\beta$ is the resonance integral between their atomic orbitals. The blue shift of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ transition of naphthalene can be explained as resulting mainly from the interaction of the charge transfer state with the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$ state. Electronic spectra of other aryl boron compounds will be discussed, together with experimental evidence for the above interpretations.