Intramolecular rotation

Fig. 3 Ground-state and excited-state energies in the planar and twisted configurations of a TICT molecule. Photon absorption usually elevates the molecule to the LE state in the planar configuration, where the angle of intramolecular rotation, cp0, is close to zero. From the LE state, the molecule can return to the ground state with a rate kf, or it undergoes intramolecular rotation with a rate ka. The twisted state is characterized by a larger intramolecular rotation angle

Fig. 4 Ground-state and excited-state energies of the TICT complexes thioflavin T (a) and 9-(dicyanovinyl)-julolidine (DCVJ) (b) as a function of the intramolecular rotation angle (data from Stsiapura et al. [13] and Allen et al. [14]). In both cases, energy levels were determined by quantum mechanical simulations. For thioflavin T, the energy difference between Si and S0 corresponds to approximately 400 nm in the planar state and 470 nm in the twisted state. In the case of DCVJ, the energy differences correspond to 310 and 960 nm, respectively. The DCVJ energy levels reflect a rotation around the vinyl double bond...

Replacing the nitrile group by a benzothiazole produces an important subclass of fluorescent compounds represented by thioflavin T (25, Fig. 10). It is not clear if this compound undergoes deactivation via intramolecular rotation that would meet the criterion for a molecular rotor. The steady-state absorption and emission properties of thioflavin T has been attributed to micelle formation [53, 54], dimer and excimer formation [55, 56], and deactivation through intramolecular rotation [57]. 

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... 

Rettig and Gleiter81 have studied the dependence of intramolecular rotation in 4-cyano-IVTV-dialkylanilines 6-12 on the twist angle by fluorescence, UV absorption and PE spectroscopic measurements. The twist angles were determined from the split of the first and the third IP. While in molecules 6, 8 and 9, 11 and 12 the twist of the amino group... 

Rhodamine derivatives are widely used as laser dyes, and intramolecular rotational motion of a dialkylamino group leading to a nonemissive TICT state has been... 

W. Rettig and R. Gleiter, Dependence of intramolecular rotation in p-cyano-N,N-dialkylanilines on the twist angle. A fluorescence, UV absorption, and photoelectron spectroscopic study. J. Phys. Chem. 89,4676(1985). 

Recent results show large variations in intramolecular rotations of tryptophan residues in proteins on the nanosecond time scale, ranging from complete absence of mobility to motions of considerable angular amplitudes. Among native proteins with internal tryptophan residues, wide angular amplitude rotations were observed only in studies of azurin,(28 29) where the correlation time of the rapid component was x = 0.51 ns.(28) The existence of... 

The results of fluorescence polarization studies of proteins were discussed above. Time-resolved anisotropy measurements often permit, without any additional variation of experimental conditions, intramolecular rotations to be distinguished from rotation of the whole protein molecule and characterized,... 

There should exist a correlation between the two time-resolved functions the decay of the fluorescence intensity and the decay of the emission anisotropy. If the fluorophore undergoes intramolecular rotation with some potential energy and the quenching of its emission has an angular dependence, then the intensity decay function is predicted to be strongly dependent on the rotational diffusion coefficient of the fluorophore.(112) It is expected to be single-exponential only in the case when the internal rotation is fast as compared with an averaged decay rate. As the internal rotation becomes slower, the intensity decay function should exhibit nonexponential behavior. 

Hosomi H, Ohba S, Tanaka K, Toda F (2000) The first in situ observation of intramolecular rotation and cyclization of anilide by an X-ray study partial single-crystal to single-crystal photocyclization of A-methyl-A- ( )-methylmethacrolyl anilide in inclusion crystals. J Am Chem Soc 122 1818-1819... 

Here P(rfy,t) is the conditional probability of finding an interparticle vector in df about f at time t if it was in df0 about f0 at time zero, and P(f0) is the probability of finding the interparticle vector initially in df0. The interacting particles are of two types those within a given molecule, the inter-nuclear vectors of which are changed by molecular rotation, and those external to the molecule. The interparticle vectors between a nucleus inside a molecule and particles external to the molecule are changed mainly by diffusion. The sum in Equation 7 may be thus separated into inter-molecular (diffusion) and intramolecular (rotation) parts, and for the moment we ignore the latter. 

Fig. 3.9. The free enthalpy barrier of an intramolecular rotation or inversion A B AG° is zero if A and B are degenerated in energy.

Intramolecular rotations in carboxylic and carbonic acid amides have been thoroughly investigated by 1H NMR [108]. The thermodynamic data for rotation in N,N-di-methyltrichloroacetamide as obtained by 1H and 13C 1H NMR agree quite well CH AG294 = 76.5 kJ/mol 13C AG BB = 73.6 kJ/mol) [109]. 

Evidence for the existence of such equilibria comes from the eight-line spectrum obtained for the parent compound in 4-cyano-4 - -pentylbiphenyl, an inert liquid crystal. Bailey and Yesinowski35 found the only tenable rationalization for the fewer than expected 12-line spectrum was a rapid intramolecular rotation of one chelate ring with respect to the other, amounting to a degenerate cis-trans isomerization. 

In principle, there may also be a Curie relaxation contribution of the contact-type whenever there is chemical exchange or intramolecular rotation to modulate the coupling. The contribution to R2M would then be... 

Azomethine derived ferroelectric liquid crystals As DOBAMBC, many ferroelectric LC s were prepared utilizing amyl alcohol as the chiral source. The reason for the small spontaneous polarization of DOBAMBC is the separation between the C=0 dipole moment and the chiral carbon. This effect can be explained by the intramolecular rotation or vibration of the carbonyl dipole moment relative to the chiral carbon, because they are not adjacent. There are some rules linking the molecular structure and the direction of the spontaneous polarization (minus or plus). In order to reduce the separation between the carbonyl dipole moment and the chiral carbon, ferroelectric LC s were synthesized utilizing a secondary alcohol as the chiral source. Ferroelectric LC s with large spontaneous polarizations have large dipole moments at the chiral centre. Ferroelectric LC s with halogen or nitrile units connected directly to the chiral carbon were synthesized from chiral lactic acids or amino acids. 

In 1969, Suppan reviewed experimental data about dipole moment changes in excited states of substituted aromatic molecules and suggested a theoretical approach according to which charge transfer occurs if the lowest vacant orbitals are very close in energy.41 In 1978, Birks introduced the term horizontal radiationless transition, which was applied to intramolecular rotation in stilbene and polyene derivatives.37 In this... 

At short times, before equilibration, the population of A rises with a rate approximately given by the TICT formation rate kBA, and the decay of B is governed by the same rate. It is this time range that is most interesting for direct studies of the intramolecular rotational relaxation. 

In calculating molecular Sis, Burt and Richards allowed intramolecular rotations about the torsional bonds of moving molecules [110], Thus, the number of parameters in the object function was six plus the number of torsional rotations allowed. To exclude conformations with unreasonably large energies, they used a weighted SI in the optimization procedure ... 

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