Influence of UV and combined MW–UV irradiation on
2-
tert-
butylphenol (
2TBP) transformation was investigated in the presence and in the absence of sensitizers with different value of singlet and triplet energy and in the presence of solvents with different polarity. Irradiation by UV and also by combination MW–UV gave three C
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C dimers: 3,3′-di-
tert-butylbiphenyl-
2,
2′-diol (
1) (
ortho–ortho), 3,3′-di-
tert-butylbiphenyl-
2,4′-diol (
2) (
ortho–para) a 3,3′-di-
tert-butylbiphenyl-4,4′-diol (
3) (
para–para). Their ratio depends upon the nature of solvents and sensitizers used. In non-polar solvents (hexane, heptane, benzene, toluene), direct
2TBP radiation led to the formation of two products
1 and
2 where
ortho–para 2 is predominant over
ortho–ortho dimer
1. Irradiation in dipolar aprotic solvent 1,4-dioxane led only to
ortho–ortho 1 and in case of acetonitrile only to
ortho–para dimer
2. In polar solvents (acetic acid, water, methanol/water = 1:1 v/v), it led to the formation of all three products. In methanol, the reaction did not proceed. Influence of solvent polarity (hexane, acetonitrile) to relative product distribution of
2TBP photoreaction was also observed in the presence of sensitizers: acetophenone, benzophenone, naphthalene, acridine and anthracene. Reaction in hexane gave
ortho–ortho 1 and
ortho–para 2 dimers with all tested sensitizers. The acridine-sensitized reaction in acetonitrile was found to give
ortho–para 2 and a greater amount of
para–para 3 dimer. Singlet-sensitized reaction (naphthalene, anthracene) in acetonitrile led to
ortho–para dimer
2. Nonsensitized as well as singlet-sensitized reaction of
2TBP proceeded from singlet state situated under singlet state energy of anthracene (3
20kJmol
−1). Triplet state of
2TBP was found to be under triplet state of benzophenone (
290kJmol
−1).