https://onlinelibrary.wiley.com/doi/pdf/10.1002/jps.2600600902

Pharmaceutical Applications of Solid Dispersion Systems

Phosphorescence has rarely been observed in pure organic chromophore systems at room temperature. We herein report efficient phosphorescence from the crystals of benzophenone and its derivatives with a general formula of (X-C6H4)2C═O (X = F, Cl, Br) as well as methyl 4-bromobenzoate and 4,4′-dibromobiphenyl under ambient conditions. These luminogens are all nonemissive when they are dissolved in good solvents, adsorbed on TLC plates, and doped into polymer films, because active intramolecular motions such as rotations and vibrations under these conditions effectively annihilate their triplet excitons via nonradiative relaxation channels. In the crystalline state, the intramolecular motions are restricted by the crystal lattices and intermolecular interactions, particularly C−H···O, N−H···O, C−H···X (X = F, Cl, Br), C−Br···Br−C, and C−H···π hydrogen bonding. The physical constraints and multiple intermolecular interactions collectively lock the conformations of the luminogen molecules. This structural rigi...

Crystallization-Induced Phosphorescence of Pure Organic Luminogens at Room Temperature

Triplet excitons in organic molecules underscore a variety of processes and technologies as a result of their long lifetime and spin multiplicity Organic phosphorescence, which originates from triplet excitons, has potential for the development of a new generation of organic optoelectronic materials and biomedical agents However, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions in solution, which severely restricts its practical applications In the past few years, room-temperature-phosphorescent systems have been obtained based on organic aggregates Rapid advances in molecular-structure design and aggregation-behaviour modulation have enabled substantial progress, but the mechanistic picture is still not fully understood because of the high sensitivity and complexity of triplet-exciton behaviour This Review analyses key photophysical processes related to triplet excitons, including intersystem crossing, radiative and non-radiative decay, and quenching processes, to illustrate the intrinsic structure–property relationships and draw clear and integrated design principles The resulting strategies for the development of efficient and persistent room-temperature-phosphorescent systems are discussed, and newly emerged applications based on these materials are highlighted Advances in molecular-structure design and modulation of the aggregation behaviour have enabled much progress in the observation of room-temperature phosphorescence from organic aggregates This Review analyses key photophysical processes related to triplet excitons, illustrating the intrinsic structure–property relationships and identifying strategies to design efficient and persistent room-temperature-phosphorescent systems

Room-temperature phosphorescence from organic aggregates

The theory of Part I is applied to nonradiative transitions from the lowest triplet state to the ground state of aromatic hydrocarbons. A previously communicated empirical relation between triplet energy, triplet lifetime, and the relative number of hydrogen atoms per molecule is substantiated and its physical implications are discussed. It is transformed into a relation between the Franck‐Condon factor of the transitions and the triplet energy. In this form it can be compared with the theoretical expressions derived in Part I. No satisfactory theoretical representation of the empirical formula could be obtained on the basis of a harmonic‐oscillator description of the normal modes of the molecules. However, introduction of anharmonicity leads to excellent agreement between theory and experiment. A one‐parameter formula is derived which accounts with good accuracy for the dependence of the triplet lifetime on the triplet energy and the number of carbon and hydrogen atoms in the molecule. This formula shows that the Franck‐Condon factors relevant to the radiationless triplet‐ground‐state transition are governed by CH or CD stretching modes, which behave in this respect as completely degenerate for a given molecule. The single adjustable parameter is related to the anharmonicities of these modes in the ground and triplet state. The analysis confirms that the purely radiative triplet lifetime of most if not all aromatic hydrocarbons is close to 30 sec and that the Franck‐Condon factor is the only parameter in the expression for the nonradiative triplet decay constant which varies considerably between different aromatic hydrocarbons. Finally the temperature dependence of the Franck‐Condon factors is considered. A theoretical treatment indicates a very small temperature dependence below 400°K. This result seems to be borne out by an analysis of recent experiments, leading to the conclusion that the observed temperature effects are associated with bimolecular processes. A notable exception is benzene, where the triplet lifetime is temperature‐dependent down to very low temperatures, possibly due to a Jahn‐Teller distortion.

Radiationless Transitions in Polyatomic Molecules. II. Triplet‐Ground‐State Transitions in Aromatic Hydrocarbons

3. Ab Initio Calculations 2543 C. Dynamics 2550 1. Group Theory 2551 2. H-Bond Network Rearrangement (HBNR) 2552 3. Intramolecular Vibrations 2557 4. Intermolecular Vibrations 2558 III. Experimental Data 2558 A. Condensed-Phase Environments 2560 1. Matrix-Isolation Spectroscopy 2560 2. Inorganic Host Complexes 2561 3. Water Trimer in Liquid Helium Droplets 2561 B. Gas-Phase Spectroscopy of the Free Water Trimer 2561

The water trimer.

As a general introduction to a subsequent series of papers on pseudorotation and five‐membered rings, the two‐ring puckering modes of a five‐membered ring are discussed in general terms. Five cases are separated depending upon the nature of the potential function for each mode. Correlation diagrams connecting the various cases are given. Consistent with our basic policy, we have gone directly from fundamental theory to an arithmetical development using large digital computers. The classical section of this development is outlined. The quantum‐mechanical section is developed in the following paper.

Ring Puckering in Five‐Membered Rings. I. General Theory

The magnitude and the direction of the dipole moment of formic acid were determined through the Stark effect of rotational transitions. The dipole moments of HCOOH and HCOOD are 1.415±0.01 D and 1.39±0.01 D, respectively. The direction of the dipole moment lies between C–H and C=O bonds making an angle of 42.4°±2° with the C=O bond.

Dipole Moment of Formic Acid, HCOOH and HCOOD

Intramolecular energy transfer between triplet states of weakly interacting chromophores. I. Compounds in which the chromophores are separated by a series of methylene groups

The fluorescent intensity, phosphorescent intensity, and phosphorescent lifetime of naphthalene and naphthalene‐ds, dissolved in durene, were measured as a function of temperature from 77° to 325°K. These measurements indicate that radiationless transitions from the first excited singlet state are temperature‐independent but radiationless transitions from the lowest triplet are strongly temperature‐dependent. Analysis of the data for the decay of the triplet state indicates that only one activated state is responsible for this temperature dependence. The activation energy and pre‐exponential factor for decay from this state are 3559±79 cm—1 and (4.2±1.5)×107 sec—1 for naphthalene, and 3362±35 cm—1 and (1.30±0.24)×107 sec—1 for naphthalene‐ds. These data can be interpreted as corresponding to the excitation of one of the intramolecular CC stretching modes to its second excited state or a combination of two CC fundamentals.

Radiationless Triplet—Singlet Transitions in Naphthalene

The extinction coefficients and oscillator strengths were measured for the first strong triplet↔triplet transition in naphthalene, phenanthrene, and triphenylene. The results are: naphthalene, emax=14 000 liter/mole·cm, f=0.05; phenanthrene, emax=27 000 liter/mole·cm, f=0.11; triphenylene, emax=7000 liter/mole·cm, f=0.05.

Richard A. Keller

Papers

Ring Puckering in Five‐Membered Rings. I. General Theory

Dipole Moment of Formic Acid, HCOOH and HCOOD

Intramolecular energy transfer between triplet states of weakly interacting chromophores. I. Compounds in which the chromophores are separated by a series of methylene groups

Radiationless Triplet—Singlet Transitions in Naphthalene

Extinction Coefficients of Triplet↔Triplet Transitions in Aromatic Compounds