Intramolecular rotation, restriction

Most of the molecules introduced in this chapter are hydrophobic. Even those molecules that have been functionalized to improve water-solubility (for example, CCVJ and CCVJ triethyleneglycol ester 43, Fig. 14) contain large hydrophobic structures. In aqueous solutions that contain proteins or other macromolecules with hydrophobic regions, molecular rotors are attracted to these pockets and bind to the proteins. Noncovalent attraction to hydrophobic pockets is associated with restricted intramolecular rotation and consequently increased quantum yield. In this respect, molecular rotors are superior protein probes, because they do not only indicate the presence of proteins (similar to antibody-conjugated fluorescent markers), but they also report a constricted environment and can therefore be used to probe protein structure and assembly. 

J. Chen, Z. Xie, J.W.Y. Chen, C.C.W. Law, and B.Z. Tang, Silole-containing polyacetylenes. Synthesis, thermal stability, light-emission, nanodimensional aggregation and restricted intramolecular rotation, Macromolecules, 36 1108-1117, 2003. 

Keywords Aggregation-induced enhanced emission (AIEE) Fluorescent organic nanoparticles Ion-association method Organic dye nanoparticles Reprecipitation method Restriction of intramolecular rotation Size-dependent optical properties... 

As shown in Equation (19), l-(2-dimethylaminoethyl)-8,9-dihydropyrano[3,2-e]indoles (44), rotationally restricted phenolic analogues of the neurotransmitter serotonin, were prepared in good yield from 5-hydroxyindoles in a cyclization reaction using an intramolecular variant of the Mitsunobu reaction <91TL3345, 92T1039). The borane complex could be dissociated with CsF and Na2C03 in refluxing EtOH. 

Most of the reported compounds which show this effect are organic molecules (179-184), where restriction of intramolecular rotation is generally accounted for being the main cause for AIE (175,185). The enhancement is therefore mostly related to the reduction of the nonradiative rate constant rising from the vibrational and rotational mode of the appended groups to the chromophores, which in the aggregates are strongly prevented. 

The random occurrence of inelastic collisions may be expected to lead to a complex pattern of displacements of the polymer chain. The overall motion of the chain may be described by a superposition of two forms of displacement Segmental and normal mode motion. The former is usually associated with the movement of a block of the chain relative to the main backbone change from structure (2) to (l). The only restrictions placed on such a motion is that sufficient energy should be available from inelastic collisions for the intramolecular interactions restricting free rotation of the polymer backbone to be overcome. It is implicit in this definition of segmental motion that the relaxation process involves direct activation from the lowest energy state via a cascade mechanism which may lead to excitation of the other thermal vibrational states. 

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This intramolecular interaction restricts bond rotation and leads to a stiff chain. As a result, polyacrylonitrile has a very high crystalline melting point (317 C) and is soluble... 

Chen J, Law CCW, Lam JWY, Dong Y, Lo SMF, Williams ID, Zhu D, Tang BZ (2003) Synthesis, light emission, nanoaggregation, and restricted intramolecular rotation of 1,1-substituted 2,3,4,5-tetraphenylsiloles. Chem Mater 15 1535-1546... 

A static model does, however, not give a complete description of the molecule. It does not tell anything about the intramolecular movements, A more elaborate structure model has to be introduced in order to describe the more or less restricted intramolecular rotation that often takes place, or to describe the molecular vibrations. Force constants determining the intramolecular movements must also be considered as molecular structure parameters, along with the geometrical molecular structure parameters. So far no single... 

To separate die cooling effect from the glass effect, we choose dichlo-romethane (DCM), a liquid wifli much lower melting point, for the PL measurement. The PL intensity of die solution increases with a decrease in temperature in a nearly hnear fashion (Fig. 6B). This enhancement in emission must be due to the restricted intramolecular rotation caused by cooling-induced conformation freezing because the melting point of the solvent (-95°C) is lower than the lowest temperature we tested for this solution (-78°C). Similar to 3, polymers 2,4, and 7 also show much stronger emission when their solutions are cooled. 

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This intramolecular interaction restricts bond rotation and leads to a stiff chain. As a result, polyacrylonitrile has a very high crystalline melting point (317°C) and is soluble in only a few solvents such as dimethylacetamide and dimethylformamide and in aqueous solutions of inorganic salts. Polyacrylonitrile cannot be melt processed since extensive decomposition occurs before any appreciable flow occurs and fibres are therefore spun from solution. In one process, for example, a solution of the polymer in dimethylformamide is extruded into a coagulating bath of glycerol arid the fibre formed is drawn and wound. 

---