Organic light-emitting device (OLED) technology has found numerous applications in the development of solid state lighting, flat panel displays and flexible screens. These applications are already commercialized in mobile phones and TV sets. White OLEDs are of especial importance for lighting; they now use multilayer combinations of organic and elementoorganic dyes which emit various colors in the red, green and blue parts of the visible spectrum. At the same time the stability of phosphorescent blue emitters is still a major challenge for OLED applications. In this review we highlight the basic principles and the main mechanisms behind phosphorescent light emission of various classes of photofunctional OLED materials, like organic polymers and oligomers, electron and hole transport molecules, elementoorganic complexes with heavy metal central ions, and clarify connections between the main features of electronic structure and the photo-physical properties of the phosphorescent OLED materials.

Principles of phosphorescent organic light emitting devices.

Phosphorescence is a phenomenon of delayed luminescence that corresponds to the radiative decay of the molecular triplet state. As a general property of molecules, phosphorescence represents a cornerstone problem of chemical physics due to the spin prohibition of the underlying triplet-singlet emission and because its analysis embraces a deep knowledge of electronic molecular structure. Phosphorescence is the simplest physical process which provides an example of spin-forbidden transformation with a characteristic spin selectivity and magnetic field dependence, being the model also for more complicated chemical reactions and for spin catalysis applications. The bridging of the spin prohibition in phosphorescence is commonly analyzed by perturbation theory, which considers the intensity borrowing from spin-allowed electronic transitions. In this review, we highlight the basic theoretical principles and computational aspects for the estimation of various phosphorescence parameters, like intensity, radiative...

Theory and Calculation of the Phosphorescence Phenomenon

A subazaporphyrin (SubAP), tert-butylated and crowned subphthalocyanines (tBSubPc and SubCRPc), a μ-oxo dimer of tert-butylated SubPc {(tBSubPc)2O}, a subnaphthalocyanine (SubNc), and monosubstituted type unsymmetrical phthalocyanine (Pc) and naphthalocyanine (Nc) analogues have been synthesized. In particular, unsymmetrical Pc's and Nc's have been prepared in moderate yields by the ring expansion reaction of structurally distorted SubPc's and SubNc's with isoindolediimine derivatives in dimethyl sulfoxide−chloronaphthalene (or chlorobenzenes or aromatic hydrocarbons such as toluene and xylene) mixtures. The compounds have been characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence emission, and nuclear magnetic resonance spectroscopy. Both the Soret bands and Q-bands shift to longer wavelength and gain intensity in the order SubAP, SubPc, and SubNc. Fluorescence quantum yields and lifetimes generally decrease with decreasing molecular symmetry. Circular dichroism and NMR ...

Synthesis, Spectroscopy, and Molecular Orbital Calculations of Subazaporphyrins, Subphthalocyanines, Subnaphthalocyanines, and Compounds Derived Therefrom by Ring Expansion1

The homogeneous linewidth of the 0–0 band of the S1←S0 transition of free‐base porphyrin (H2P) in an n‐octane matrix and its temperature dependence (T=1.5–4.2 K) have been measured by means of photochemical hole‐burning. The linewidth extrapolated to T=0 is ∼9 MHz for the two types of sites which have been investigated, and it appears to be determined entirely by the decay time of the emitting state S1 (17 ns). The temperature dependence of the linewidth is strongest for the thermodynamically less stable sites. The increase in linewidth with temperature may arise from a relaxation process involving a low frequency resonance mode (∼5 cm−1) localized at the H2P molecules. At 77 K the homogeneous linewidths are comparable to the inhomogeneous widths of 3–4 cm−1; hence, hole–burning is no longer observed.

Homogeneous linewidth of the S1←S0 transition of free‐base porphyrin in an n‐octane crystal as studied by photochemical hole‐burning

The chlorophyll triplet state as a probe of structure and function in photosynthesis.

Transient signals in the fluorescence intensity induced by microwaves can be used to acquire quantitative information on the molecular rates of populating and depopulating of the lowest triplet state of organic molecules. This method seems especially promising when the molecule emits no detectable phosphorescence. Part of this paper concerns the introduction of an appropriate mathematical model for describing the fluorescence transients. The model is formulated in conjunction with a specific series of experiments from which one may evaluate the molecular quantities in a systematic way. The kinetics of the lowest triplet state of free base porphin (H2P) are determined explicitly by this method. In the discussion the results for H2P are compared with those for Zn porphin. A satisfactory qualitative explanation based on the theory of radiationless transitions can be given for the great difference in behaviour of these two porphyrin molecules. Short attention is given to effects due to the photochemical shift...

W.H. Schoemaker

Papers

The lowest triplet state of free base porphin