The use of photoinitiated polymerization is continuously growing in industry as reflected by the large number of applications in not only conventional areas such as coatings, inks, and adhesives but also high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology. In this Perspective, the latest developments in photoinitiating systems for free radical and cationic polymerizations are presented. The potential use of photochemical methods for step-growth polymerization is also highlighted. The goal is, furthermore, to show approaches to overcome problems associated with the efficiency, wavelength flexibility, and environmental and safety issues in all photoinitiating systems for different modes of activation. Much progress has been made in the past 10 years in the preparation of complex and nano-structured macromolecules by using photoinitiated polymerizations. Thus, the new and emerging applications of photoinitiated polymerizations in the field of biomaterials, surface modific...

Photoinitiated Polymerization: Advances, Challenges, and Opportunities

It has been experimentally discovered that a low magnetic field (less than 500 mT) can substantially change the electroluminescence, photoluminescence, photocurrent, and electrical-injection current in nonmagnetic organic semiconducting materials, leading to magnetic-field effects (M FEs). Recently, there has been significant driving force in understanding the fundamental mechanisms of magnetic responses from nonmagnetic organic materials because of two potential impacts. First, MFEs can be powerful experimental tools in revealing and elucidating useful and non-useful excited processes occurring in organic electronic, optical, and optoelectronic devices. Second, MFEs can lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies. This progress report discusses magnetically sensitive excited states and charge-transport processes involved in MFEs. The discussions focus on both fundamental theories and tuning mechanisms of MFEs in nonmagnetic organic semiconducting materials.

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

We have employed a combination of steady-state and time-resolved spectroscopic techniques to explore the effect of protein binding on the photophysical and photochemical properties of three triaryl...

Effect of BSA Binding on Photophysical and Photochemical Properties of Triarylmethane Dyes

Absorption, steady-state fluorescence, steady-state fluorescence anisotropy, and intrinsic and induced circular dichroism (CD) have been exploited to explore the binding of calf thymus DNA (ctDNA) with three cationic phenazinium dyes, viz., phenosafranin (PSF), safranin-T (ST), and safranin-O (SO). The absorption and fluorescence spectra of all the three dyes reflect significant modifications upon interaction with the DNA. A comparative study of the dyes with respect to modification of fluorescence and fluorescence anisotropy upon binding, effect of urea, iodide-induced fluorescence quenching, and CD measurements reveal that the dyes bind to the ctDNA principally in an intercalative fashion. The effect of ionic strength indicates that electrostatic attraction between the cationic dyes and ctDNA is also an important component of the dye-DNA interaction. Intrinsic and induced CD studies help to assess the structural effects of dyes binding to DNA and confirm the intercalative mode of binding as suggested by fluorescence and other studies. Finally it is proposed that dyes with bulkier substitutions are intercalated into the DNA to a lesser extent.

Binding interaction of cationic phenazinium dyes with calf thymus DNA: a comparative study.

Molecular aspects on the interaction of phenosafranine to deoxyribonucleic acid: Model for intercalative drug–DNA binding

Cationic organic dyes (phenosafranine, safranine T (ST + ), thiopyronine, and methylene blue) are efficient quenchers of the triplet states of aromatic ketones. The triplets of benzophenone (BP), xanthone, thioxanthone, benzil, and N-methylacridone are quenched by the dyes via energy transfer, generating triplet excited dyes (typically kq ) 5.5 10 9 M -1 s -1 for 3 BP * + ST + in acetonitrile solution). Regarding Michler’s ketone, p-(dimethylamino)benzophenone, and 3,6-bis(dimethylamino)thioxanthone, the mechanism of triplet quenching is solvent-dependent. Electron transfer occurs in polar solvents like acetonitrile, and energy transfer is the dominant reaction pathway in less polar solvents, such as dichloromethane. Delayed fluorescence of the dye caused by heterotriplet-triplet annihilation involving ketone and dye was detected upon studying the reaction of the dyes with triplet excited ketones (BP, xanthone, thioxanthone, and benzil). By applying two successive laser pulses (Iex ) 532 and 308 nm, respectively), the rate constant of the reaction 3 BP * + 3 (ST + ) * was determined: kht ) 1.1 10 10 M -1 s -1 .

Photoinduced Energy and Electron Transfer between Ketone Triplets and Organic Dyes

Nucleophilic radicals, such as α-amino and ketyl radicals, produced by reaction of benzophenone triplets with amines and alcohols respectively, react efficiently by electron transfer with cationic organic dyes, such as phenosafranine, thiopyronine, methylene blue, and crystal violet. Time-resolved laser flash photolysis studies reveal a high reactivity of α-amino radicals toward these cationic dyes; in contrast, ketyl radicals are less reactive. The electron transfer rate constants for the reaction between the α-amino radical of triethanol amine and these dyes range from 2.7 × 109 M−1 s−1 (phenosafranine) to 7.5 × 109 M−1 s−1 (methylene blue). In contrast, rate constants ranging from 1.1 × 108 M−1 s−1 (phenosafranine) to 1.2 × 109 M−1 s−1 (methylene blue) were measured for the reaction of the dyes with the diphenyl ketyl radical. In dye mixtures used in conjunction with ketone-amine systems, stepwise electron transfer from an initially formed semireduced dye to a molecule of another dye occurs. For example, the primarily formed crystal violet radical reacts very rapidly (k = 2 × 109 M−1 s−1) with phenosafranine, thus forming the semireduced phenosafranine and regenerating crystal violet.

Photoreduction of organic dyes in ketone amine systems

Benzophenone triplets ( 3 BP*) react very effectively with the cations of crystal violet (CV + , methylene blue (MB + ) and phenosafranine (PS + ) in argon-saturated acetonitrile solution at 23 °C: k q values (in M −1 s −1 ) are (3 ± 1) × 10 10 (CV + ), (7 ± 1) × 10 9 (MB + ) and (5 ± 1) × 10 9 (PS + ). The results suggest that electron transfer occurs in the system 3 BP*—CV + , whereas triplet energy transfer is the dominant reaction route in the other two cases. Substituted methyl radicals generated by the reaction of 3 BP* with several amines (triethanol amine, triethyl amine, dimethyl and diethyl aniline) are effectively oxidized by the three dye cations. Electron transfer rate constants k e ranging from 0.6 × 10 10 to 3 × 10 10 M −1 s −1 were determined. The dye cations are less reactive towards ketyl radicals, e.g. k e [(Ph) 2 C(OH) + CV + ] ≈ 2 × 10 8 M −1 s −1 and k e [(CH 3 ) 2 COH + CV + ] ≈ 1.3 × 10 9 M −1 s −1 .

H.-J. Timpe

Papers

Photoinduced Energy and Electron Transfer between Ketone Triplets and Organic Dyes

Photoreduction of organic dyes in ketone amine systems

On the photoreduction of crystal violet, phenosafranine and methylene blue by benzophenone