Mechaisms of Photochemical Reactions in Solution. XXVIII.¹ Values of Triplet Excitation energies of Selected Sensitizers

Abstract

The triplet excitation energies of a number of compounds have been determined by emission spectroscopy. Some of the principles involved in the choice of photosensitizers are discussed.

Photolytic hydration of cyanamides

“…Since direct photolytic hydration of 1 does not occur in pyrex apparatus where energy is cut off at 280 mp the triplet energy transfer from acetophenone to 1 must be taking place in the acidic medium to give the hydration product. In agreement with the mechanism of the triplet energy transfer, pinacol was formed only in small quantity and the majority of acetophenone was recovered (8). In contrast, benzophenone was ineffective in sensitizing this hydration reaction under similar conditions.…”

“…Experiments t o determine the multiplicity of the excited electronic state responsible for the primary photoprocess was then in order. The photosensitized conversion of 1 and 2 could not be effected with 0.1 M benzophenone (ET 69.2 kcal/mole) and acetophonone (ET 76.3 kcal/mole) (8) in the absence of hydrochloric acid @yrex filter). That no triplet energy transfer took place could be shown by the good recovery of the respective pinacols, the expected products from a photoreduction of the ketones (9).…”

“…Though the precise site of the protonation in the ground state of cyanamide is not known, the more resonance stabilized form, 6, can be reasonably assigned to the protonated species. In the direct photolysis 6 is excited to the n -t n* singlet state (7) which crosses over to the triplet excited state (8). Judging from the slow conversion of the cyanamides, the intersystem crossing, [3], may be very inefficient and the singlet excited state 7 may preferentially decay to the ground state by various routes (the reverse reaction of [2]).…”

On the absence of triplet ketene in ketene excited in the region 2800–3660 Å

“…The major requirement for the successful use of the biacetyl sensitized emission technique is that the triplet of the donor be higher in energy than the triplet of biacetyl (15). The triplet of ketene has been given as 61 kcal (16), while the triplet of biacetyl has been given as 55 kcal (17). Thus, if ketene triplets are produced in the presence of biacetyl, one would expect to observe an enhanced emission from biacetyl due to the reaction 3K + BiA -t 3BiA + K Our data indicate that biacetyl emission is reduced in the presence of ketene.…”

Photocyclization Reactions of Aryl Polyenes. IX. The Photocyclization of Stilbenecarboxaldehydes and 1-Formylphenyl-4-phenyl-1,3-butadienes

“…Similarly, irradiation of 1-(m-formylpheny1)-4-phenyl-l,3-butadiene (9) (7) gave a mixture of I-(m-formylpheny1)naphthalene (lo), 5-phenyl-2-naphthaldehyde (ll), and 8-phenyl-1 -naphthaldehyde (12) in 40% yield as shown in Scheme 4. Although the successful photocyclizationoxidation reactions of 1 and 3 to 2 and 4 plus 5, respectively, are surprising, considering the fact that p-acetylstilbene did not photocyclize, several reports in the literature do suggest that aromatic aldehydes and ketones can react differently under photochemical conditions.…”