Jean‐François Nicoud

The higher second-harmonic efficiency, in powder, of a new organic molecular crystal N-(4-nitrophenyl)-(L)-prolinol (NPP) is reported. Electronic polarizability of NPP molecules relates to that of other para-nitroaniline-like structures such as that of previously reported N-(2,4-dinitrophenyl) methyl alaninate (MAP), and the increase in crystalline nonlinear efficiency by one order of magnitude above the latter is ascribed to an ‘‘optimized’’ crystalline structure rather than to minor changes of the molecular hyperpolarizability. The simultaneous chiral and hydrogen-bonding character of the prolinol electron-donating group leads to a quasioptimal angle, with respect to quadratic phase-matched nonlinear interactions, between the molecular transition dipole moments and the twofold axis of the monoclinic P21 crystal structure. Based on a simple oriented gas description of the quasiplanar structure of the crystal, two possible mutually exclusive second-harmonic phase-matched configurations are evidenced and shown to promote the optimized nonlinear tensor coefficient.

Previous research toward more efficient nonlinear optical organic crystals has aimed at increasing the first-order hyperpolarizabilitiy of individual molecular units. We show that the general features derived from these studies (high conjugation, charge transfer, and molecular asymmetry) leave room for the cancellation of the molecular dipole moment. This can be usefully exploited in two directions: improving the crystal growth conditions and a possibility of acting on the crystal point group structure through otherwise hidden parameters. Pyridine-1-oxide, owing to the electronic ’’push–pull’’ property of the N–oxide bond, allows for substitution in the 4 position of donor as well as acceptor groups, among which the nitro group ensures dipole cancellation. The possibility of charge transfer from or toward the N–oxide bond according to the nature of the radical in the 4 position is investigated and confirmed by means of second-harmonic generation experiments in powder or solution (DCFISH) on a number of compounds. 3-methyl-4-nitropyridine-1-oxide is shown to be the best candidate, among related compounds, for crystal growth. A detailed study of its linear and first-order nonlinear properties is presented. Crystalline samples grown in solution are seen to be of excellent optical quality. Its efficiency compares to that of the best available phase- matchable organic crystals [d = (23±3)×10−9 esu]. Besides, the optical broadband phase-matching potential of the crystal could be useful in a number of applications like frequency doubling of ultrashort pulses in the near infrared.

Fullerene derivatives in which an oligophenylenevinylene (OPV) group is attached to C60 through a pyrrolidine ring have been prepared by 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding aldehydes and sarcosine. Electrochemical and photophysical studies have revealed that ground-state electronic interactions between the covalently bonded OPV moiety and the fullerene sphere are small. The photophysical investigations have also shown that both in dichloromethane and benzonitrile solution an efficient singlet−singlet OPV → C60 photoinduced energy-transfer process takes place, and occurrence of electron transfer, if any, is by far negligible relative to energy transfer. The C60−OPV derivatives have been incorporated in photovoltaic devices, and a photocurrent could be observed showing that photoinduced electron transfer does take place under these conditions. However, the efficiency of the devices is limited by the fact that photoinduced electron transfer from the OPV moiety to the C60 sphere must compete with an efficient energy transfer. The latter process, as studied in solution, leads to the population of the fullerene lowest singlet excited state, found to lie slightly lower in energy than the charge-separated state expected to yield electron/hole pairs. Thus, only a small part of the absorbed light is able to contribute effectively to the photocurrent.

Two-photon photodynamic therapy is a promising therapeutic method which requires the development of sensitizers with efficient two-photon absorption and singlet-oxygen generation. Reported here are two new diketopyrrolopyrrole-porphyrin conjugates as robust two-photon absorbing dyes with high two-photon absorption cross-sections within the therapeutic window. Furthermore, for the first time the singlet-oxygen generation efficiency of diketopyrrolopyrrole-containing systems is investigated. A preliminary study on cell culture showed efficient two-photon induced phototoxicity.

A fulleropyrrolidine derivative bearing an oligophenylenevinylene substituent has been prepared by 1,3-dipolar cycloaddition of an azomethine ylide to C60 and incorporated in a photovoltaic device.<br/><br/>