Ronald P. Steer

Excitation pulse‐shape mimic technique for improving picosecond‐laser‐excited time‐correlated single‐photon counting deconvolutions

The determination of accurate subnanosecond fluorescence lifetimes by the time‐correlated single‐photon counting technique is often limited by difficulties in obtaining the correct instrumental response function f(λ,t) to the excitation pulse. These difficulties are increased when a grating monochromator is used to disperse the emission due to the introduction of an additional temporal broadening of f(λ,t). A technique for determining the correct f(λ,t) at the wavelength of sample emission, λem, is described. The technique consists of using a very short‐lived fluorophore to ‘‘mimic’’ the excitation pulse shape at λem, yielding the convoluted mimic decay function Cm(λem,t), and then to computationally extract f (λem,t) from Cm(λem,t). The technique is experimentally and computationally simple and yields the desired instrumental response function at λem which eliminates problems due to the sensitivity of f(λ,t) to λ (color shift artifact). The photomultiplier tube receives spatially equal illumination when ...

Sonoluminescence of argon saturated alkali metal salt solutions as a probe of acoustic cavitation

Emission from argon saturated aqueous alkali metal salt solutions is observed during insonation at 460 kHz. It is postulated to arise from de‐excitation of excited alkali metal atoms formed by free radical reduction processes. In addition to the emission resonance lines, diffuse bands are observed at ∼554 nm and ∼740 nm for Na and K, respectively. The latter are due to emission from alkali metal–argon exciplexes and are known to occur when mixtures of alkali metal vapor and argon are rapidly compressed. An estimate of the cavitational temperatures and pressures is obtained by comparison of experimental emission band parameters with those derived theoretically.

Mechanisms of Low-Power Noncoherent Photon Upconversion in Metalloporphyrin−Organic Blue Emitter Systems in Solution

The mechanisms of noncoherent photon upconversion that involve triplet-triplet annihilation (TTA) in solution have been investigated for two model systems. ZnTPP (meso-tetraphenylporphine zinc) is used as the model visible light-absorbing metalloporphyrin because its S(1) fluorescence intensity can be used to monitor the initial rate of porphyrin triplet state production and because its S(2) fluorescence intensity can be used as a direct measure of the rate of porphyrin TTA. When perylene, which has a triplet energy lower than that of ZnTPP, is added as a signaling blue emitter (BE), the mechanism of photon upconversion involves triplet energy transfer from the porphyrin to the BE followed by TTA in the BE to form the fluorescent perylene S(1) state. The kinetics of this process have been characterized and are unremarkable. When coumarin 343 (C343), which has photophysical properties similar to those of perylene except that it has a much higher triplet energy than ZnTPP, is added as the signaling BE, emission from the ZnTPP S(2) state is quenched and fluorescence from the C343 grows in. Contrary to previous suggestions, the mechanism of photon upconversion in this system does not involve singlet energy transfer from the porphyrin S(2) state to the BE. Instead, ground-state C343 complexes with the ZnTPP triplet to form a triplet exciplex, which then undergoes TTA with a second ZnTPP triplet to give the fluorescent state of the BE in a three-center process.

Photophysical properties of lumichromes in water.

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 micros have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 micros has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, phi(Delta), are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D(2)O.

Radiationless decay of the second excited singlet states of aromatic thiones: Experimental verification of the energy gap law

Abstract S2 → S0 fluorescence quantum yields and S2 lifetimes of eight aromatic thiones in inert perfluoroalkane solutions at room temperature have been measured using picosecond laser techniques. Photostable, structurally rigid thiones undergo S2 → S1 internal conversion at rates consistent with the energy gap law of radiationless transitions. An average electronic coupling matrix element of 1.9 × 102 cm−1 is found.

Photophysics of soret-excited tetrapyrroles in solution. II. Effects of perdeuteration, substituent nature and position, and macrocycle structure and conformation in zinc(II) porphyrins.

The steady-state absorption, fluorescence, and excitation spectra and upper excited-state temporal fluorescence decay profiles of 11 tetrapyrroles in several fluid solvents are presented and analyzed to ascertain the factors that control their S2 population decay times. The S2 lifetimes, which vary by more than 2 orders of magnitude, are controlled exclusively by their rates of radiationless decay. The only important electronic relaxation path is S2-S1 internal conversion, the efficiency of which is near 1.0 in all compounds studied (except CdTPP where it is 0.69). The rate of S1 population rise equals the rate of S2 population decay in all cases. Among the compounds studied, only MgTPP exhibits S2-S1 decay behavior that corresponds to the weak coupling limit of radiationless transition theory; all zinc metalloporphyrins exhibit intermediate to strong coupling. Perdeuteration of ZnTPP produces no significant change in the rate of S2 decay or in the quantum yield of S2-S0 fluorescence, indicating that in-plane C-C and C-N vibrations are the accepting modes in S1 with the largest Franck-Condon factors. The initial vibrational energy content of the S2 states (0 < E(vib) < 3500 cm(-1) over the range of compounds) plays no significant role in determining their overall population decay rates in solution. The S2 population decay rates of these tetrapyrroles are controlled by two factors: the Franck-Condon factor, which is inversely proportional to the exponent of the S2-S1 electronic energy spacing and the S2-S1 coupling energy. The S2-S1 electronic energy spacing is determined in solution by the difference in the polarizabilities of the S2 and S1 states and can be controlled by varying the polarizability of the solvent. The S2-S1 coupling energy is influenced by the nature, location, and effect of the substituents, with beta-alkyl substitution and reduction of symmetry in the tetrapyrrole--for example by loss of planarity--increasing the interstate coupling energy.

Spectral and photophysical properties of 9,10-diphenylanthracene in perfluoro-n-hexane: the influence of solute-solvent interactions

Abstract The photophysical properties of 9,10-diphenylanthracene (DPA) are found to be unusual in perfluoro-n-hexane (PFH) compared with other solvents. Low values of the fluorescence quantum yield, φF = 0.58, and S1 lifetime, τS1 = 6.7 ns, and relatively large shifts in the absorption and emission spectra in PFH are attributable to changes in the electronic energies of DPA which permit an enhancement of the rate of S1 ⇝ T2 relaxation in this solvent. A large value of the intersystem crossing quantum yield in PFH, φISC = 0.35, supports this conclusion.

Photophysics and spectroscopy of the higher electronic states of zinc metalloporphyrins: a theoretical and experimental study

The photophysics of the S2 and S1 excited states of zinc porphyrin (ZnP) and five of its derivatives (ZnOEP, ZnTBP, ZnTPP, ZnTFPP, ZnTCl8PP) have been investigated by measuring their steady-state absorption and fluorescence spectra, quantum yields and excited state lifetimes at room temperature in several solvents. The radiative and radiationless decay constants of the fluorescent excited states accessible in the visible and near UV regions of the spectrum have been obtained. Despite the similarities in the Soret spectra of these compounds, their S2 excited state radiationless decay rates differ markedly. Although the S2-S1 electronic energies of a given zinc porphyrin vary linearly with the Lippert (refractive index) function of the solvent, the S2 radiationless decay rates of the set of compounds do not follow the energy gap law of radiationless transition theory. Calculations, using time-dependent density functional theory (TDDFT), of the energies and symmetries of the complete set of excited states accessible by 1- or 2-photon absorption in the near UV-visible have also been carried out. Substitution on the porphyrin macrocycle framework affects the ground state geometry and alters the electron density distributions, the orbital energies and the relative order of the excited electronic states accessible in the near UV-blue regions of the spectrum. The results are used to help interpret both the nature of the electronic transitions in the Soret region, and the relative magnitudes of the radiationless transition rates of the excited states involved.

Fluorescence from the second excited singlet state of thiophosgene vapour

Abstract Fluorescence has been observed from the second excited singlet state of thiophosgene vapour. The emission is excited mainly by transitions originating in the out-of-plane bending mode of the ground state and terminating in the lowest vibrational levels of the excited state.

Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).