Leonard J. Andrews

Bio: Leonard J. Andrews is an academic researcher from Brandeis University. The author has contributed to research in topics: Fluorenone & Internal conversion (chemistry). The author has an hindex of 3, co-authored 3 publications receiving 129 citations.

Photophysical processes in fluorenone

Abstract: Flash-photolytic measurements of photophysical parameters of fluorenone are presented. The flash transient spectrum is given over a wide spectral range in acetonitrile and hydrocarbon solvents, and is assigned to the triplet on the basis of both its decay kinetics and accompanying delayed emission. This corresponds spectrally to prompt fluorescence and is accurately second order in transient concentration. Relative triplet quantum yields (phi/sub T/) are obtained from the dependence of the transient absorption on flash energy. Measurements of florescence lifetimes, quantum yields, and phi/sub T/'s give rate constants for all degradative processes of the lowest excited singlet. In acetonitrile the rates of intersystem crossing and internal conversion to the ground state from the lowest excited singlet are comparable. The absence of phosphorescence and the measured triplet lifetime (approximately 2 ms) in rigid media at 77K give a lower limit for the radiative lifetime (tau/sub o/ > 0.2s) which is compatible with a ..pi..-..pi..* assignment for the lowest triplet.

Kinetic analysis of flash photolysis data for sequential first and second order reactions: The photoreduction of triplet fluorenone by dabco

Abstract: In studying reactions of triplet states by flash photolysis, particularly with laser techniques, one frequently encounters the kinetic problem of an initial pseudo-first order triplet reaction overlaid by a second order decay of products. A simple algorithm is given, adaptable to a desk-top calculator, for treatment of experimental data to obtain rate constants and total product yields (or absorptions) in this situation. The procedure is illustrated in the case of the photoreduction of fluorenone triplet by 1,4-diazabicyclo(2.2.2) octane, a reaction which yields a pair of radical-ions.

Photochemistry and Photophysics of Coordination Compounds: Platinum

Abstract: This chapter provides an overview of the luminescence properties of platinum(II) complexes, exploring how the excited states involved in emission are influenced by the ligands around the metal ion. The square planar nature of d8 Pt(II) complexes has many implications, leading to properties and applications that are not open to d6 complexes. For example, axial intermolecular interactions can lead to new excited states not present in the isolated molecules. This review focuses on complexes containing one or more chelating ligands, of which at least one contains a heterocyclic ring such as pyridine. Thus, we explore the properties of a range of bipyridyl (bpy) and terpyridyl (tpy) complexes, and how they are influenced by the identity of the other ligands that complete the coordination sphere of the Pt(II) ion, such as halide, cyanide, thiolates and acetylides. We consider the sometimes dramatic influence of substituents in the bpy/tpy ligands in producing other excited states that may be much more intensely emissive than those of the parent complexes. The influence of cyclometallation on excited states is discussed, and how it can lead to highly emissive complexes: a range of cyclometallated systems are reviewed, with bidentate and terdentate ligands incorporating one or more metal–carbon bonds. Contemporary applications in areas such as sensors, photoinduced electron transfer, and organic light-emitting devices are highlighted.

The coordination chemistry of dipyridylbenzene: N-deficient terpyridine or panacea for brightly luminescent metal complexes?

Abstract: 1,3-Di(2-pyridyl)benzene (dpybH) structurally resembles the widely-used ligand terpyridine (tpy), with which it is isoelectronic. In this critical review, following a brief overview of synthetic strategies for dpybH and derivatives, we survey the different types of complex that are possible with these ligands. Whilst metals such as ruthenium(II), osmium(II) and platinum(II) give a terdentate N⁁C⁁N binding mode in which cyclometallation occurs at C2, the ions iridium(III), rhodium(III) and palladium(II) favour C4 metallation. The latter process can be blocked by appropriate ligand modification, to allow the N⁁C⁁N mode to be accessed with these metal ions too. The luminescence properties of the complexes are discussed. A huge range of emission efficiencies are encountered amongst Ir(III) complexes containing dpyb derivatives, according to the other ligands present. Trends can be rationalised with the aid of simple frontier-orbital considerations. The Pt(II) complexes of dipyridylbenzenes are also intensely luminescent. Their application to contemporary organic light-emitting device (OLED) technology is discussed, including white light emitters exploiting excimer emission. Their potential as cell imaging agents amenable to time-resolved detection procedures on the microsecond timescale has also been demonstrated (118 references).

Fluorenone-Containing Polyfluorenes and Oligofluorenes: Photophysics, Origin of the Green Emission and Efficient Green Electroluminescence†

Abstract: A series of four new statistical copolymers of 9,9-dihexylfluorene and 9-fluorenone with well-defined structures and a new fluorene−fluorenone−fluorene trimer model compound were synthesized and used to investigate the photophysics, origin of the green emission, and electroluminescence of this class of light-emitting materials. We show that the new oligofluorene trimer with a central fluorenone moiety is an excellent model of the green-emitting chromophore in polyfluorenes. From systematic studies of the steady-state photoluminescence (PL) and PL decay dynamics of solutions of the fluorenone-containing copolymers and oligomer and thin films of the copolymers, we show that the controversial 535-nm green emission band originates from the fluorenone defects in single-chain polyfluorenes and not from intermolecular aggregates or excimers. The green emission, centered at 535 nm, was observed in dilute toluene solutions of all fluorenone-containing copolymers and oligomer; it was long-lived with a single-expone...

Quenching Processes in Hydrogen-Bonded Pairs: Interactions of Excited Fluorenone with Alcohols and Phenols

Abstract: In order to clarify mechanisms of excited state interactions in hydrogen-bonded pairs, we have studied the kinetics of dynamic quenching of singlet and triplet fluorenone by a series of alcohols, phenols, and related compounds, in which hydrogen-bonding power, redox potential, and acidity are systematically varied. In addition, effects of solvent basicity or polarity and deuteration help identify the role of hydrogen-bonding in physical or chemical quenching processes. Alcohols and weak acids, with high oxidation potentials, do not quench the triplet, but quench the singlet at rates which parallel hydrogen-bonding power. This is attributed to a physical mechanism, involving vibronic coupling to the ground state via the hydrogen bond. This is much stronger in the excited state than in the ground state, and provides efficient energy dissipation in the radiationless transition. Phenols, with hydrogen-bonding power comparable to that of the alcohols but with much lower oxidation potentials, quench both single...

Actinometry in monochromatic flash photolysis: the extinction coefficient of triplet benzophenone and quantum yield of triplet zinc tetraphenyl porphyrin

Abstract: The extinction coefficient epsilon/sub T/, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, deltaD/sup 0//sub T/, under demonstrably linear conditions where incident excitation energy, E/sub 0/, and ground state absorbance, A/sub 0/, are both extrapolated to zero The result, 7220 +- 320 M/sup -1/ cm/sup -1/ at 530 nm, validates and corrects many measurements of triplet and radical extinctions and yields, using the energy-transfer method As E/sub 0/ and A/sub 0/ both decrease, deltaD/sup 0//sub T/ becomes proportional to their product In this situation, the ratio R = (1/A/sub 0/) (ddeltaD/sup 0//sub T//dE/sub 0/) = (epsilon/sub T/ /sup -/ epsilon/sub G/)phi/sub T/ Measurements of R, referred to benzophenone, give (epsilon/sub T/ - epsilon/sub G/)phi/sub T/ for any substance, without necessity for absolute energy calibration Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (epsilon/sub T/ - epsilon/sub G/ at 470 nm = 73 x 10/sup 4/ M/sup -1/ cm/sup -1/) give phi/sub T/ = 083 +- 004 6 figures, 2 tables