Spectroscopic Investigation of the Mechanism of the Intramolecular Heavy Atom Effect on the Phosphorescence Process. I. Naphthalene Emission

The infrared (240–3000 cm^—1), Raman (100–3000 cm^—1) and phosphorescence spectra of eight naphthalene mono‐ and dihalogenated derivatives are obtained and the polarization of the phosphorescence spectra is measured and compared. It is found that each phosphorescence spectrum is composed of two subspectra: Subspectrum I, which correlates with that of naphthalene and involves only totally symmetric C–C modes and Subspectrum II, which is caused to appear by an out‐of‐plane mode. The variation in the appearance of the unpolarized phosphorescence spectra of the different molecules in the series is explained mainly in terms of changes in the relative intensity of the two Subspectra I and II (and not in terms of the Franck—Condon principle). Subspectrum I is polarized perpendicular to the ¹L_a, ¹L_b excitation as in the parent hydrocarbon, while Subspectrum II is polarized parallel to the ¹L_a excitation in the α derivatives but almost depolarized in the β compounds. This strongly indicates that the two subspectra originate from two different coupling schemes. The relative intensities of Subspectra II to I in the unpolarized emission spectra of the α‐halonaphthalenes (which is also a measure of the relative importance of the corresponding two mechanisms involved) is found to increase as the number of the halogens or their atomic number increases. Since the heavy‐atom effect involves orders of magnitude intensification of the phosphorescence emission, and since the intensities of Subspectra I and II are not greatly different in all the different compounds, it is concluded that the halogen enhances the original naphthalene‐type mechanism as well as introduces a comparably important new mechanism of its own. A possible coupling scheme for Subspectrum II is $${\mathrm{(S}}_{{\mathrm{\pi ,\pi }}^{*}}\hspace{0.17em}\mathrm{or}{\mathrm{\hspace{0.17em}S}}_{{\mathrm{\sigma ,\sigma }}^{*}}\mathrm{)\leftrightarrow }{\displaystyle {lim}^{\mathrm{vib}.}}{S}_{{\mathrm{\sigma ,\pi }}^{*}}\leftrightarrow {\displaystyle {lim}^{\mathrm{S.O.}}}{T}_{{\mathrm{\pi ,\pi }}^{*}}\mathrm{,\hspace{0.17em}}\mathrm{or}{\mathrm{\hspace{0.17em}(S}}_{{\mathrm{\pi ,\pi }}^{*}}\hspace{0.17em}\mathrm{or}{\mathrm{\hspace{0.17em}S}}_{{\mathrm{\sigma ,\sigma }}^{*}}\mathrm{)\leftrightarrow }{\displaystyle {lim}^{\mathrm{S.O.}}}{T}_{{\mathrm{\sigma ,\pi }}^{*}}\leftrightarrow {\displaystyle {lim}^{\mathrm{vib.}}}{T}_{{\mathrm{\pi ,\pi }}^{*}},$$ and that for Subspectrum I is $$S_{{\mathrm{\sigma ,\pi }}^{*}}\leftrightarrow {\displaystyle {lim}^{\mathrm{S.O.}}}{T}_{{\mathrm{\pi ,\pi }}^{*}};$$ where σ and σ^* orbitals involve the carbon—halogen electronic system. The spin—orbit (S.O.) coupling can account for Subspectrum I, whereas the vibronic—S.O. interaction explains Subspectrum II.