Hirayama rule

Chandross and Dempster (16) provided further proof for the necessity of the symmetric sandwich configuration, since 1-(a-naphtyl)-3-(6-naphtyl)-propane did not show excimer fluorescence, it should be noted that they observed a weak excimer emission In 1,4-bls (a-naphtyl)-butane, as a first exception to the Hirayama rule. 

The typical synthetic vinyl polymers prepared by the free radical polymerisation of vinyl monomers are shown in Figure 4.3. They have a linear chain structure in which the substituents R and R are separated by three carbons. If the substituents R are aromatic chromophores, the vinyl polymers will be rich in excimer structures and strong bimolecular quenching of the fluorescence will occur. This is in agreement with the Hirayama rule [20], which says that if the chromophores are separated by 3 carbon atoms, the probability of excimer formation is the highest, assuming that the chain is flexible enough to rotate... 

Further confirmation for the necessity of a sandwich configuration was obtained In the paracyclophane series (18-19) in which the 4,4-paracyclophane showed an excimer fluorescence while the 4,5-paracyclophane did not. In the 2,2- and the 3,3-paracyclophane, appreciable Interaction between the chromophores in the ground state is observed in the ultraviolet spectra. Therefore in these systems, the two chromophores are held in a sandwlchllke configuration already in the ground state, they are therefore Irrelevant to check Hirayama s rule. 

On the basis of these results De Schryver and co-workers reformulated Hirayama s rule as follows the possibility of intramolecular complex formation is limited by the probability to reach, within the lifetime of the excited state involved, a favorable conformation and by the extent of stabilization in the complex, covalent bond formations being the extreme. 

The emission properties of some carbazole double molecules [l,n-bis(JV-carbazoyl)alkanes] have been examined and one compound in particular, 1,3-bis-(N-carbazoyl)propane (1,3-BCP), was found to be a useful model for poly-(/V-vinylcarbazole). Measurements of the temperature dependence of monomer and excimer decay constants have provided useful kinetic and thermodynamic information on this system. The binding energy for the intramolecular excimer of 1,3-BCP was shown to be 2.76 kcal mol-1, a rather low value. Measurements on other carbazole double molecules showed that Hirayama s n = 3 rule is obeyed and that the preferred geometry of the intramolecular excimer is sandwichlike.161... 

Transient spectra showed that these benzophenone phosphorescence decays were due only to the benzophenone triplet , and the irradiation intensity independence ruled out the T-T annihilatitm under the experimental conditions used Since the deviation was not observed at temperatures below T of each acrylic polymer or T of polystyrene and polycarbonate, this quenching is considered not of the static character as featured in the Inokuti-Hirayama theory, but of a dynamic type associated with the collision of functional groups and the energy transfer with some activation energy. 

The chapters in this book by Langsam, Xu et aL, Hirayama et aL, Fritsch, and Maier et al focus on polymer structure modification to improve the performance of gas separation membranes relative to the upper bound tradeoff relations. Mahajan et al, describe characteristics of hybrid inorganic/organic membranes as a route to break the simple rules that result in equations 8 and 9, possibly resulting in materials with properties which are above and beyond the upper bound lines. Koval et al and Eriksen et al, describe facilitated transport membranes. They seek to strongly enhance solubility selectivity for penetrant pairs i,e, olefin/paraffin) where... 

Studies of intramolecular excimer formation in bichromophoric systems, as described by Professor De Schryver in this publication, have been invaluable in provision of information relevant to understanding of the photophysics of macromolecules and in the early years led to the generalisation of Hirayama C3] known as the "n = 3 rule". Whilst the n=3 rule is not strictly obeyed it has focussed the thoughts of workers seeking elucidation of macromolecular photophysics especially for the case of vinyl aromatic polymers. 

It was recognised that intramolecular excimer configurations might be expected to be attained as a result of interactions between adjacent chromophores upon the polymer backbone which, in head to tail placements, satisfy the n=3 rule" of Hirayama [3] (which states that excimer formation is expected between aromatic substituents attached to an alkane chain only when the chromophores are separated by three carbon atoms. The limitations of the rule are briefly noted in section 2). 

It is noteworthy that the emission profile of oligomer 1 of n=2 is in sharp contrast to what is expected from Hirayama s rule [95], which predicts that a molecular... 

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