Molecular fluorescent pH-probes based on 8-hydroxyquinoline

Abstract

Three 5,7-pi-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and 5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent pH-probes in nonaqueous solution. Absorption and photoluminescence spectra of the introduced compounds also including the starting material 8-benzyloxy-5,7-dibromoquinoline as well as their N-protonated counterparts were recorded and the results were rationalized by quantum-chemical calculations. A pronounced red shift of the emission occurred upon protonation of the non halogenated derivatives, while the dibromo-derivative is hardly emissive and is virtually not protonated under the conditions used. The diphenyl- and the bis(biphenyl)-derivative especially show promising photoluminescence quantum yields both in the parent and the protonated state making them candidates for the active component in pH sensing applications.

Synthesis and Photophysical Properties of 2-Aryl-6,8-bis(arylethenyl)-4-methoxyquinolines

“…Application of dichlorobis-(triphenylphosphine)palladium(II) (PdCl 2 (PPh 3 ) 2 as Pd(0) catalyst source, on the other hand, led to incomplete conversion of 4a to afford the cross-coupled product 6a in 55% yield after 18 h. The prolonged reaction times and reduced yield prompted us to use PdCl 2 (PPh 3 ) 2 –tricyclohexylphosphine catalyst complex in DMF-water (3:1, v/v) and an excess of arylboronic acid (2.5 equiv.) in the presence of K 2 CO 3 as a base in analogy with the literature precedents [ 23 , 24 , 27 ] and we achieved complete conversion of 4a to 6a within 3 h. Alkylphosphine ligands, are known to coordinate with palladium and increase its electron density more so than arylphosphines and, in turn, accelerate the oxidative addition and reductive elimination steps in the catalytic cycle [ 28 , 29 ]. Extension of these reaction conditions to other dibromoquinolines 4 using 4-substituted phenyvinylboronic acids as coupling partners afforded the corresponding 2-aryl-6,8-bis(2-arylethenyl)-4-methoxyquinolines 6a – l in high yield without the need for column chromatography ( Scheme 4 ).…”

“…and phenylbromide to afford 6,8-diphenylquinoline in 94% yield [ 23 ]. Dibromoquinolines such as 5,7-dibromoquinoline and 8-benzyloxy-5,7-dibromoquinoline, on the other hand, undergo Suzuki cross-coupling with less or no selectivity compared to many of the other dibromoheteroaromatics [ 24 , 27 ]. Our initial trial to effect the Suzuki-Miyaura cross-coupling of 6,8-dibromo-4-methoxy-2-phenylquinoline ( 1a ) and 4-fluoro-phenylvinylboronic acid (1 equiv.)…”

Organoiridium Quinolinolate Complexes: Synthesis, Structures, Thermal Stabilities and Photophysical Properties

“…Removal of the benzyloxy protecting group was achieved by treatment with BF 3 ·Et 2 O and KI in CH 3 CN as reported recently. [13] Compounds 5a-5j were characterised by 1 H-and 13 C-NMR spectroscopy, absorption and fluorescence measurements as well as matrix-assisted laser desorption ionisation time-of-flight (MALDI-TOF) MS. Furthermore, the thermal behaviour and the impact of ligand modifications on the thermal stability were probed by combined DSC/TGA measurements.…”

Studies of one-pot double couplings on dibromoquinolines

“…10 (Scheme 2) The other reported a symmetrical double coupling of both bromides under standard Suzuki conditions. 11 …”