Two-Photon Absorption Properties of Alkynyl-Conjugated Pyrene Derivatives

Donor-acceptor substituted phenylethynyltriphenylenes - excited state intramolecular charge transfer, solvatochromic absorption and fluorescence emission

Several 2-(phenylethynyl)triphenylene derivatives bearing electron donor and acceptor substituents on the phenyl rings have been synthesized. The absorption and fluorescence emission properties of these molecules have been studied in solvents of different polarity. For a given derivative, solvent polarity had minimal effect on the absorption maxima. However, for a given solvent the absorption maxima red shifted with increasing conjugation of the substituent. The fluorescence emission of these derivatives was very sensitive to solvent polarity. In the presence of strongly electron withdrawing (-CN) and strongly electron donating (-NMe₂) substituents large Stokes shifts (up to 130 nm, 7828 cm⁻¹) were observed in DMSO. In the presence of carbonyl substituents (-COMe and -COPh), the largest Stokes shift (140 nm, 8163 cm⁻¹) was observed in ethanol. Linear correlation was observed for the Stokes shifts in a Lippert-Mataga plot. Linear correlation of Stokes shift was also observed with E(T)(30) scale for protic and aprotic solvents but with different slopes. These results indicate that the fluorescence emission arises from excited state intramolecular charge transfer in these molecules where the triphenylene chromophore acts either as a donor or as an acceptor depending upon the nature of the substituent on the phenyl ring. HOMO-LUMO energy gaps have been estimated from the electrochemical and spectral data for these derivatives. The HOMO and LUMO surfaces were obtained from DFT calculations.

Synthesis and Fluorescence Emission Properties of 1,3,6,8-Tetrakis(9H-Fluoren-2-yl)Pyrene Derivative

A novel pyrene-based highly pure blue fluorescent and stable molecule, 1,3,6,8-tetrakis[9,9-bis(3-methylbutyl)-9H-fluoren-2-yl]pyrene, was successfully synthesised via a Pd-catalysed Suzuki coupling reaction of 1,3,6,8-tetrabromo-pyrene with 2-[9,9-bis(3-methylbutyl)-9H-fluoren-2-yl]-4,4,5,5-tetramethyl[1,3,2]dioxaborolane. The photoproperties of this pyrene were examined in detail and these data indicated its potential application as blue-emitting materials in organic light-emitting diodes.

Tuning the intramolecular charge transfer of alkynylpyrenes: effect on photophysical properties and its application in design of OFF-ON fluorescent thiol probes

Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.

Synthesis and Spectral Properties of 2,7-di-tert-butyl-4,9-bis(arylethynyl)-and 4,10-bis(arylethynyl)pyrenes

Formylation of 2,7-di- tert-butylpyrene with dichloromethyl methyl ether in the presense of AlCl3 afforded a mixture of 2,7-di- tert-butylpyrene-4,9-biscarbaldehyde and 4,10-biscarbaldehyde in the ratio of 75:25, from which the corresponding bis(arylethynyl)pyrenes were obtained by the Wittig reaction with aryl methyl phosphonium ylides followed by bromination and dehydrobromination.