Method Of Photoselection Research Articles

The Best Photo Selection Method by Sharpness Comparison

The Best Photo Selection Method by Sharpness Comparison

Highly Emissive Nano Rod-in-Rod Heterostructures with Strong Linear Polarization

We report the synthesis of CdSe/CdS rod in rod core/shell heterostructures. These rods, synthesized using a seeded-growth approach, show narrow distributions of rod diameters and lengths and exhibit high emission quantum efficiencies and highly polarized emission. The degree of polarization is controlled by the inner core rod dimensions, and it is equal or up to 1.5 times higher than the polarization of equivalent sphere in rod systems. Using the method of photoselection we measure the polarization anisotropy at different excitation wavelengths and study the interplay between electronic contribution and dielectric effects in determining the absorption and emission polarization.

Polarization of the T 1 → S 0 phosphorescence and S 0 → S n phosphorescence excitation of aromatic hydrocarbons prototype for π,π* states. A reappraisal

The polarization of phosphorescence and phosphorescence excitation in benzene, naphthalene, phenanthrene and their corresponding perdeuterated forms in EPA at 77 K were examined using the photoselection method with both vertically and horizontally polarized light in conjunction with a delay between excitation and detection of emission. Based on the results, polarization in the spectral regions examined is consistent with a situation where the excitation and emission are at an angle near to 90° (i.e., where the polarization it is close to −0.33); this confirms that the phosphorescence of the compounds studied is polarized in the plane normal to the molecular plane. Consequently, any vibronic mixing and singlet-state mixing mechanisms potentially involved in the strengthening of T 1 → S 0 transitions in these compounds must posses a polarization normal to the molecular plane.

Environmental effects on the photophysics of thienyl ketones investigated by transient absorption and phosphorescence emission in polarized light

Environmental effects on the triplet states of some thienyl ketones have been investigated by nanosecond laser flash photolysis, as well as stationary and time-resolved phosphorescence techniques. The compounds studied were the 2,3 ′-di-thienyl, 2-thienyl-4 ′-pyridyl, 3,3 ′-di-thienyl and 3-thienyl-4 ′-pyridyl ketones. Triplet properties (absorption spectra, lifetimes, quenching by oxygen and hydrogen abstraction), determined in ethanol and 3-methylpentane, indicated that the n, π ∗/ π, π ∗ character of the lowest triplet is modulated by the solvent and by structural characteristics. The spectral distribution and lifetime of phosphorescence emission in a rigid matrix at 80 K were affected by the nature of the matrix, rate of freezing, excitation and emission wavelengths and sample concentration. While the matrix influences the excited state energy and therefore the nature of the emitting state, concentration changes are responsible for aggregation. The latter effect is explained in terms of the formation of a triplet excimer (for the 3-thienyl-4 ′-pyridyl ketone) and a ground state dimer/aggregate (for the 2,3 ′-di-thienyl ketone) formation. Measurements of phosphorescence and phosphorescence excitation in polarized light, carried out by the photoselection method at 80 K in a matrix of suitable viscosity (1/4 iso-pentane-3-methylpentane, η=6.2×10 10 P), provided valuable information about the polarization of the electronic transitions involved.

Emitting states of benzyl, p-fluorobenzyl and p-cyanobenzylradicals

Polarization studies of the fluorescence emission and excitation spectra have been carried out by the photoselection method for the benzyl, p-fluorobenzyl and p-cyanobenzyl radicals in poly (vinyl alcohol) films at 77 K. The emitting states have been assigned to the 1 2A 2 electronic state for the benzyl and p-fluorobenzyl radicals and the 2 2B 2 state for the p-cyanobenzyl radical. Flu lifetimes have been determined to be 1.22±0.10, 0.36±0.04 and 0.43±0.04 μs for the three radicals, respectively.

Determination of the iron-carbonyl bond geometry of carboxyprotoheme in solution using picosecond infrared-optical photoselection

The iron-carbonyl bond geometry in carboxyprotoheme in ambient temperature solution has been investigated. Polarized visible and infrared beams have been used to monitor the change in infrared absorbance of the bound CO stretch bands on photodissociation of the ligand. A theoretical description of the picosecond infrared-optical photoselection method is presented in which the anisotropic change in the infrared absorption may be expressed in terms of either the photoselection anisotropy or the polarization ratio, which may be used to deduce the properties of the FeCO geometry in carboxyprotoheme in terms of 〈 P 2(cos α)〉, where α is the angle between the ligand bond axis and the normal to the heme plane. Various models for the equilibrium distribution of the angle are considered. The results for cos −1 (〈cos 2α〉) 1 2 in the solutions studied lie in the range 17–26° which correspond to geometries significantly different from the linear and perpendicular configuration observed in single-crystal studies of similar compounds. Additionally, geminate recombination kinetics are observed following photodissociation in high viscosity solvents.

Linear polarized birefringence photoselection method and the nature of hydrogen bonding in dibucaine

Linear polarized birefringence photoselection method and the nature of hydrogen bonding in dibucaine

Detection of Hidden1Lb-1A1Transition of Anthracene Disulphonates in Glycerol Glass at 77K by Photoselection Technique

Abstract Attempts are made to detect the weakly absorbing 1Lb bands in anthracene 1,5- and 1,8-disulphonates in glycerol glass by the method of photoselection. The fluorescence and absorption (excitation) spectra are separated into two component spectra based on short and long axis polarized transition dipoles and their vibrational analysis are made. The vibrational bands around 29762 cm−1 in 1,5-AS and 30303 cm−1 in 1,8-AS in absorption spectra are mainly due to 1Lb band.

Two-photon photoselection in rigid solutions: A study of the B2u←A1g transition in benzene

The first detailed study of simultaneous two-photon photoselection is reported, using the benzene B2u←A1g two-photon forbidden transition to illustrate this new technique. Polarized emission from the B2u fluorescent state is used to probe the symmetries of excited molecular states reached by two-photon absorption in an initially isotropic solution. Three independent polarization measurements are reported for each of the e1u, e2u, and b2u false origins. These transitions span all possible D6h molecular symmetry species which are two-photon allowed from the ground state in a single color experiment. It will be seen how the photoselection method complements the conventional circular/linear polarization measurements in fluid solutions by providing empirical symmetry assignments for the nontotally symmetric transitions. The partial randomization parameter ε, commonly used in one-photon photoselection studies to rationalize nonideal polarization values, is introduced into the two-photon theory. This considerably improves the agreement between experimental and theoretical two-photon polarization ratios.

Restriction of motion of protein side chains during the photocycle of bacteriorhodopsin.

Linear dichroism was measured during the photocycle of bacteriorhodopsin. The anisotropy of the sample was produced by the photoselection method. The measurements on purple membrane fragments embedded in agar gel were performed at room temperature with 200 microseconds time resolution at several wavelengths in the 240- to 550-nm spectral region. The induced anisotropy of the retinal chromophore remained constant after the formation of the photocycle intermediate M. The anisotropy was also time independent at the characteristic peaks of the UV absorption change. These experimental data suggest that the direction of the retinal transition dipole moment remains unchanged. Moreover, the affected aromatic protein side chains also do not show any rotational motion when they are in the perturbed or ground states during the photocycle. Our data render it possible to calculate the restricted range of sudden chromophore rotations that might be coupled to the appearance and decay of the M intermediate.

Absorption and Fluorescence Spectra of Anthracenecarboxylic Acids. II. 1- and 2-Anthroic Acids

Abstract The absorption and fluorescence spectra of 1- and 2-anthroic acids (1- and 2-anthracenecarboxylic acids) in alcoholic solvents have been observed as a function of solute concentration. It was revealed that 1- and 2-anthroic acids form hydrogen-bonded dimers in their ground states. The equilibrium constants for dimer formation are : 1.2×104 mol−11 for 1-anthroic acid in methanol and 2.0×104 mol−11 for 2-anthroic acid in ethanol. Owing to dimer formation, the lowest frequency absorption band of 1-anthroic acid was uniformly displaced to the red, while that of 2-anthroic acid was distinctly resolved into two bands. The band assignment was confirmed by the method of photoselection. No excimer-type fluorescence was observed in 1- and 2-anthroic acids. The lack of mirror-symmetry relationship between the absorption and fluorescence spectra of 1-anthroic acid is explained by the geometrical relaxation in its excited state.

Photoselection with intense laser pulses

The method of photoselection uses a polarized photon beam to create a preferentially oriented population of excited state molecules. This population is examined using polarized absorption or emission techniques in order to obtain information about the symmetries of transition moments. Prelaser photoselection was limited in application to exceptionally long-lived states (or stable photoproducts) in perfectly rigid matrices or to strongly fluorescent states in moderately viscous media. With lasers, extension is possible to almost any state with a lifetime of picoseconds or longer, making photoselection a very powerful, general technique. This paper rederives the expected experimental behavior and extends the calculations to include saturation effects, which are prominent in practical laser experiments but were not important in classical studies. Furthermore, it shows that in the nonlinear regime, information on transition moment symmetry is available even from a single beam experiment. One very simple experiment is reported which verifies that one can distinguish ’’planar’’ from ’’linear’’ symmetry in absorbers using absorption methods in very fluid media. These results are most timely for picosecond mode-locked laser experiments but apply also to nanosecond Q switched or even intense continuous wave laser studies.

ChemInform Abstract: A PHOTO‐SELECTION STUDY OF THE CHARGE‐TRANSFER PHOSPHORESCENCE

The polarization of the charge-transfer phosphorescence has been studied by the method of photo-selection. It has been measured relative to the electric vector of the excitation light corresponding to the charge-transfer band in wavelength for the complexes of tetracyanobenzene, o-, m-, and p-dicyanobenzenes, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors, and methyl-substituted benzenes as electron donors, in organic glass at 77 K. It has been shown that the degree of polarization through excitation into the charge-transfer band is almost constant in the phosphorescence region, while it gradually decreases as the ionization potential of the donor increases for any of the acceptors. These observations can reasonably be interpreted in terms of a model in which the charge-transfer phosphorescence is polarized normal to the molecular plane, irrespective of the complex, and in which the change in the polarization degree of the charge-transfer phosphorescence is responsible for the...

ChemInform Abstract: POLARIZATION MEASUREMENT AND CONFIGURATION ANALYSIS OF THE TRIPLET‐TRIPLET ABSORPTION SPECTRA OF SUBSTITUTED NAPHTHALENES

Abstract The polarization of triplet-triplet absorption spectra for 1- and 2-methoxynaphthalene, 1,5-, 1,8-, 2,7-, and 2,3-naphthalenediols, and 1,4-dimethoxynaphthalene was measured by the photoselection method. Calculations of the triplet states of these molecules were made by the Pariser-Parr-Pople method, and the resulting wavefunctions were subjected to the configuration analysis. The T-T absorption spectra of the β-substituted naphthalenes are relatively simple and closely resemble that of naphthalene. In the α-substituted naphthalenes, the T-T absorption spectra are not only remarkably different from that of naphthalene but also different from one another. Comparison of experimental and theoretical results leads to reasonable assignments of the individual absorption bands involving some modification of previous assignments. The apparent complexity of T-T absorption spectra of the α-substituted naphthalenes can be interpreted in relation to the enhanced 3 B 1g + ← 3 B 2u + and 3 A g − ← 3 B 2u + transitions of naphthalene.

A Photo-selection Study of the Charge-transfer Phosphorescence

Abstract The polarization of the charge-transfer phosphorescence has been studied by the method of photo-selection. It has been measured relative to the electric vector of the excitation light corresponding to the charge-transfer band in wavelength for the complexes of tetracyanobenzene, o-, m-, and p-dicyanobenzenes, tetrachlorophthalic anhydride, and phthalic anhydride as electron acceptors, and methyl-substituted benzenes as electron donors, in organic glass at 77 K. It has been shown that the degree of polarization through excitation into the charge-transfer band is almost constant in the phosphorescence region, while it gradually decreases as the ionization potential of the donor increases for any of the acceptors. These observations can reasonably be interpreted in terms of a model in which the charge-transfer phosphorescence is polarized normal to the molecular plane, irrespective of the complex, and in which the change in the polarization degree of the charge-transfer phosphorescence is responsible for the change in the direction of the transition moment of the charge-transfer absorption band.

On the lower excited electronic states of glyoxal

The polarization of both nπ* absorption bands of glyoxal has been measured in a glass matrix of 2-methyltetrahydrofuran by the photoselection method. The second absorption band in the 30 000 cm −1 region has been assigned to a 1A g → 1B g nπ* transition. Its intensity is mainly induced by interaction with the solvent. An absorption band at about 43 000 cm −1 has been ascribed to a charge transfer transition in complexes of glyoxal and 2-MTHF.

ChemInform Abstract: ORIENTATION EFFECTS IN TRIPLET‐TRIPLET ENERGY TRANSFER FROM BENZOPHENONE TO PHENANTHRENE BY PHOTOSELECTION STUDIES

The method of photoselection has been used to measure the degrees of polarisation for benzophenone and phenanthrene phosphorescence in poly(methyl methacrylate)(PMMA) at room temperature and 77 K. These measurements are compared with the degrees of polarisation of benzophenone phosphorescence and sensitised phenanthrene phosphorescence obtained upon selective excitation of benzophenone in a mixture of benzophenone and phenanthrene in PMMA. The studies indicate that if an orientation effect exists for triplet–triplet energy transfer it is a slowermoving function than the square of the sine of the angle between the CO axis of benzophenone and the normal to the molecular plane of phenanthrene. The results of photoselection and magnetophotoselection studies are compared with studies of intramolecular energy transfer between chromophores of known molecular orientation.

Polarization measurement and configuration analysis of the triplet-triplet absorption spectra of substituted naphthalenes

The polarization of triplet-triplet absorption spectra for 1- and 2-methoxynaphthalene, 1,5-, 1,8-, 2,7-, and 2,3-naphthalenediols, and 1,4-dimethoxynaphthalene was measured by the photoselection method. Calculations of the triplet states of these molecules were made by the Pariser-Parr-Pople method, and the resulting wavefunctions were subjected to the configuration analysis. The T-T absorption spectra of the β-substituted naphthalenes are relatively simple and closely resemble that of naphthalene. In the α-substituted naphthalenes, the T-T absorption spectra are not only remarkably different from that of naphthalene but also different from one another. Comparison of experimental and theoretical results leads to reasonable assignments of the individual absorption bands involving some modification of previous assignments. The apparent complexity of T-T absorption spectra of the α-substituted naphthalenes can be interpreted in relation to the enhanced 3B 1g + ← 3B 2u + and 3A g − ← 3B 2u + transitions of naphthalene.

Orientation effects in triplet–triplet energy transfer from benzophenone to phenanthrene by photoselection studies

The method of photoselection has been used to measure the degrees of polarisation for benzophenone and phenanthrene phosphorescence in poly(methyl methacrylate)(PMMA) at room temperature and 77 K. These measurements are compared with the degrees of polarisation of benzophenone phosphorescence and sensitised phenanthrene phosphorescence obtained upon selective excitation of benzophenone in a mixture of benzophenone and phenanthrene in PMMA. The studies indicate that if an orientation effect exists for triplet–triplet energy transfer it is a slowermoving function than the square of the sine of the angle between the CO axis of benzophenone and the normal to the molecular plane of phenanthrene. The results of photoselection and magnetophotoselection studies are compared with studies of intramolecular energy transfer between chromophores of known molecular orientation.

Polarization of singlet-singlet electronic transitions in substituted naphtahlenes

The polarization of three low energy-singlet-singlet electronic transitions of several mono-substituted naphthalenes have been measured relative to the transition moment of fluorescence by the method of photoselection. It is shown that substitution of a hydroxy or a methoxy group at the 2 (orβ)-position of naphthalene leads to mixing between the 1L b and 1L a states, resulting in the short-axis polarization of the electronic transition to the former state. 1 (or α)- substituted compound exhibit no such mixing, and the transition to the 1L b state is polarized largely along the long axis of the molecule. Substitution of a cyano group at the 2-position also leads to short-axis polarization of the L b band. These results are consistent with the predictions based on molecular orbital calculations.