Fluorescence ordered molecular systems

Fluorescence polarization techniques and ordered molecular systems... 

Bain, A.J. (2002). Time resolved polarized fluorescence studies of ordered molecular systems. In An Introduction to Laser Spectroscopy, Andrews, D.L. and Demidov, A., eds. Chapter 6, pp. 171-210. Kluwer Scientific London. 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

The LB deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices. The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra give important information on them. We have studied photoresponses, specific interactions, and in-plane and out-of-plane orientation of various chromophores in LB films [3-11], In addition to the change of absorp-... 

The great sensitivity of fluorescence spectral, intensity, decay and anisotropy measurements has led to their widespread use in synthetic polymer systems, where interpretations of results are based upon order, molecular motion, and electronic energy migration (1). Time-resolved methods down to picosecond time-resolution using a variety of detection methods but principally that of time-correlated single photon counting, can in principle, probe these processes in much finer detail than steady-state techniques, but the complexity of most synthetic polymers poses severe problems in interpretation of results. 

The intensity is proportional to the first-order correlation function Gl1) (R, t) which, according to Eq. (35), can be expressed in terms of the molecular dipole operators, or equivalently, in terms of matrix elements of the density operator of the molecular system. Using Eq. (35), we can write the intensities of the observed fluorescence fields on the visible (/ ) and ultraviolet (/ ) transitions as... 

The choice of method depends on the system to be investigated. The methods of intermolecular quenching and intermolecular excimer formation are not recommended for probing fluidity of microheterogeneous media because of possible perturbation of the translational diffusion process. The methods of intramolecular excimer formation and molecular rotors are convenient and rapid, but the time-resolved fluorescence polarization technique provides much more detailed information, including the order of an anisotropic medium. 

The sorbent materials used to construct this type of sensor are widely varied (ion exchangers, adsorbent solids, polymers) and are employed as particles (larger than 30 pm in order to avoid overpressure in the flow system) or films. Most of these sensors are optical and rely on absorption, reflectance or molecular fluorescence measurements. In order to ensure that the sensing microzone is fully compatible with the detector, the sorbent material used must be as transparent as possible (photometry) or give rise to no appreciable light scatter (fluorimetry) so that the baseline (resulting from passage of the carrier) may be as low as possible. 

Fluorescence spectra of pyrene were employed in order to see the polarity of the environment given by the molecular assemblies mentioned so far. The III/I ratio of vibronic spectra of pyrene, a good measure of microenvironmental polarity(194.2Q). is plotted for the DODACl systems as a function of MEGA-n concentrations in Figure 11. 

Perhaps the most convincing evidence for internal conversion is provided by the observations of Hammond et al.sl who find that the quenching of molecular fluorescence by conjugated dienes is not accompanied by either an increase in yield of molecular triplet states (kfa = 0), by any detectable photochemical change (kg = 0), or by the appearance of a characteristic exciplex band (k% = 0). Since the observed quenching constants, KQ, vary by several orders of magnitude it may be assumed that reversible photoassociation is operative in these systems (Section III.C) in which case with (cf. Eq. 33)... 

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