Nov., 1962
COMMUNICATION TO
obtained by extrapolation of the data of McLaughlin and Tamres7 for the gas phase dissociation of the ether-boron trifluoride complexes a t 65-90’ were combined with the observed equilibrium constants to give an estimate of the standard free energy changes for the reaction B1 BFB +B.L:BFa These estimates are given in Table I. The agree-
+
THE
EDITOR
2279
ment between values obtained with the same base and two reference boron fluoride complexes (see for example tetrahydropyran and methanol in Table I) indicates the validity of the equilibrium method for the determination of relative basicities. Acknowledgment.-We wish to thank the Syracuse University Research Institute for its finanical aid to P. I. S. during the summer of 1960.
COMMUNICATION TO THE EDITOR CONCERNING T H E PRIMARY ABSORPTION ACT IK A ONE-ELECTRON PHOTOOXIIDATION IN A RIGID MEDIUM1
Sir: Very recently,2 a quantitative study was reported of a one-electron photooxidation of N,N,N’,N’tetramethylparaphenylenediamine (TMPD) in a 3-methylpentane glass at 77°K. A principal discovery was that the wave length dependence of the absolute quantum yield for the photooxidation shows considerable structure as well as order of magnitude changes over the smooth, completely unstructured near-ultraviolet absorption band of TMPD. In contrast, the wave length dependence of the relative quantum yields of fluorescence and phosphorescence over this same band is constant. The latter observation is not unexpected and can be regarded as reflecting a uniform efficiency a t all wave lengths for radiationless conversion to the thermally equilibrated first excited singlet state followed either by emission (fluorescence) or internal conversion to the triplet state with consequent phosphorescence. On the other hand the distinctly different behavior of the wave length dependence of the photochemical quantum yield was much more puzzling. Attemxhs to explain the high1.s structured photochemicaf quantum yield spe&& were based upon two very different points of view. One explanation posed the possibility of a very weak solute-solvent charge transfer band (not apparent in absorption) coincidentally underlying the near-ultraviolet solute band The photooxidation then would arise through an electronic transition directly into this band followed by a very efficient electron ejection. The structure in the photooxidation quantum yield excitation spectrum then may reflect structure in the charge transfer band; while the observed low quantum yield .would be a consequence of i.ts low extinction coefficient. The second point of view assumes that the primary absorption act is the same for both emission and photooxidation, namely excitation directly into the first excited singlet state of TMPD. This is the excitation responsible for the near-ultraxiolet band. Following the excitation, (1) This work has been supported, in pert, b y a grant from the National Science. Foundation. (2) W. C. Meyer a n d 8. C. Albreoht, J . P h g s . C h s m . , 66, 1168 (1962).
there is competition between electron ejection and thermal equilibration. It was argued that the structure in the photochemical excitation spectrum would, in this case, reflect a dependence of the electron ejecting efficiency on the nature of the vibronically excited normalmodes of the upper state. Since this structure was not reflected in an inverse sense in the emission excitation spectrum, it was argued that the absolute quantum yields for emission were much higher than those for the photochemistry. The second mechanism, vibronic photochemistry, was adopted on the basis of a number of arguments, none of them alone conclusive. The purpose of this communication is to present conclusive experimental evidence supporting this choice of mechanism. By illuminating rigid solutions with polarized (or unpolarized) light, a non-random sample of solute may be selectively excited. The nature of the nonrandom distribution is fixed by the polarization and orientation of the selecting light and by the orientation of the axis of the primary absorption with respect to the molecular axes. Information regarding the nature of the non-random distribution may be had by observing the polarization of emission from the oriented excited molecules or by observing the orientation of photoproduct molecules (by polarized absorption spectroscopy) which replace the oriented photoselected solute molecules after photochemical ~ h a n g e . ~If the primary absorption act is the same for both luminescence and photochemistry a t all wave lengths, then data obtained by the two techniques should reduce to the same non-randomness for each photoselecting wave length. Polarized photochemistry experiments on TMPD in a rigid glass (1-propanol and isopentane) already have been ~ o m p l e t e d . ~Sow the necessary polarized emission studies have been made which permit the crucial comparison, graphically presented here, which establishes that the primary absorption act is the same for both emission and photooxidation. The experimental details for the polarized emission studies, especially with regard to photoselection, are similar to those for the polarized photochemistry study4 and shall not be repeated here. One change involves the use of a double Glan prism for polarized excitation (instead of a stack of quartz ( 3 ) A. C . Albrecht, J . M o l . Spectry., 6,84 (1960). (4) A. C. Albrecht, J . Am. C h e m . Soc., 82,3813 (1960).
2280
COMMUNICATION TO THE~EDITOR
/N\
CH,
2 90
350
320 WAVE LENGTH,
CH,
mp
Fig. 1.-(a) Fractional transition probability, &(I): X, values obtained from polarized photochemistry (1-propanol, isopentane glass); - - -, values obtained from polarized fluorescence (3-methylpentane g l a d ) . (b) Molar extinction , in 1-propanol, isocoefficient, e(X), a t 77°K.: pentane g l a d ; - - -, in 3-methylpentane.*
--
plates4). Another difference is the use here of 3-methylpentane as the solvent. The fluorescence component of the emission from the photoselected
Vol. 66
sample of TMPD was isolated6and, for each photoselecting wave length, was analyzed with a second double Glan prism employing a Photovolt multiplier photometer (Series 520-M) with a 1P-21 phototube. The raw data were properly corrected for blank effects and handled by methods, described el~ewhere,~ to convert them to the fractional longaxis component in the primary absorption act as a function of exciting wave length. The results are found plotted in Fig. 1, where the observations reported previously4and obtained by polarized photochemistry also are found. It is evident that the two different methods give very similar results. Even were some refinement in the comparison attempted by taking into account in some manner the differences in absorption spectra (which we attribute to solvent differences a t low temperatures), the qualitative agreement would not be changed. We conclude that the primary absorption act leading to one-electron photochemistry and the one leading to fluorescence are one and the same. Furthermore, the primary absorption step must be to the excited state responsible for the near-ultraviolet band of TMPD since the yields for emission are estimated a t greater than 0.1 and, in any case, the emission spectrum is a fairly good mirror image of the absorption band. The original proposal2 of a vibronic photochemical mechanism thus is supported. More complete details of this and especially of related work on both TMPD and other molecules will appear in the near future. ( 5 ) Corning filters 3850 a n d 5970 transmitted the higher energy fluoresceno: (peak a t 380 mp) a n d none of t h e partially overlapping phosphorescence (peak about 470 mp).
DEPARTMENT OF CHEMISTRY A. H. KALANTAR A. C. ALBRECHT CORNELL URIVERSITY ITHACA, NEWYORK RECEIVED SEPTEMBER 13, 1962
