Tetraguanidino-functionalized phenazine and fluorene dyes: synthesis, optical properties and metal coordination

Abstract

In this work the first phenazine derivatives with guanidino substituents were prepared and their structural and electronic properties studied in detail. The guanidino groups decrease the HOMO-LUMO gap, massively increase the quantum yield for fluorescence and offer sites for metal coordination. The yellow-orange colored 2,3,7,8-tetraguanidino-substituted phenazine shows intense fluorescence. The wavelength of the fluorescence signal is strongly solvent dependent, covering a region from 515 nm in Et2O solution (with a record quantum yield of 0.39 in Et2O) to 640 nm in water. 2,3-Bisguanidino-substituted phenazine is less fluorescent (maximum quantum yield of 0.17 in THF), but exhibits extremely large Stokes shifts. In contrast, guanidino-functionalized fluorenes emit only very weakly. Subsequently, the influence of coordination on the electronic properties and especially the fluorescence of the phenazine system was analysed. Coordination first takes place at the guanidino groups, and leads to a blue shift of the luminescence signal as well as a massive decrease of the luminescence lifetime. Luminescence is almost quenched completely upon Cu(I) coordination. On the other hand, in the case of Zn(II) coordination the fluorescence signal remains strong (quantum yield of 0.36 in CH3CN). In the case of strong zinc Lewis acids, an excess of metal compound leads to additional coordination at the phenazine N atoms. This is accompanied by significant red-shifts of the lowest-energy transition in the absorption and fluorescence spectra. Pentanuclear complexes with two phenazine units were isolated and structurally characterized, and further aggregation leads to chain polymers.

1,2,5,6‐Tetrakis(guanidino)‐Naphthalenes: Electron Donors, Fluorescent Probes and Redox‐Active Ligands

“…The quantum yields for protonated 5 were determined to be approximately 2% for (5+ +2H)(PF 6 ) 2 (l ex = 300 nm) and 15 %f or (5+ +4H)Cl 4 (l ex = 280 nm). The drastic increase in the fluorescencei ntensity with the degree of protonation becomes evident from the photos in Figure 9a.T he increasing energetic separation between electronic ground and excited states might be responsible for as uppression/attenuation of thermalr elaxation pathways, explaining the onset of fluorescence upon protonation.C hanges in the fluorescencei ntensity with the degree of protonation (amplification as well as attenuation of fluorescence)h ave already been observed for guanidines that are fluorescent in their unprotonated form, [29,30] but such ad rastic switching effect from fluorescent silent to fluorescent upon protonation has not been reported previously.I nr ecent years, excited-state intermolecular proton transfer (ESPT) has been evidenced for some protonated guanidines. [31,32] However,p rotonated 5 does not display ESPT,g iven that changes in the degree of protonation do not lead to shifts in the energy of the fluorescence signal.…”

“…Crystals suitable for structural characterisation by singlecrystal X-ray diffraction were grown by diffusion of diethyl ether into as aturated acetonitrile solution. C, H, Na nalysis (%) for C 30 1,2,5,6-Tetrakis(dimethylethyleneguanidino)-naphthalene (5): 1,2,5,6-Tetraamino-naphthalene tetrahydrochloride (150 mg, 0.45 mmol) was suspended in degassed acetonitrile (15 mL) at 0 8C. Then 2-chloro-1,3-dimethylimidazolinium chloride (317.5 mg, 1.89 mmol) in acetonitrile (5 mL) was added dropwise to the resulting grey suspension, and the reaction mixture was stirred for 15 min at room temperature.…”

“…Crystals suitable for an X-ray analysis were grown from as aturated acetonitrile solution. C, H, Na nalysis (%) for C 30 Compound 5(SbF 6 ) 2 :C ompound 5 (20 mg, 0.03 mmol) was dissolved in dichloromethane (5 mL). Then Ag(SbF 6 )( 17 mg, 0.06 mmol) was added in one portion.…”

“…Crystals of (5+ +2H)(BF 4 ) 2 suitable for structural characterisation by single-crystal X-ray diffraction were grown by diffusion of diethyl ether into as aturated solution of 5(BF 4 ) 2 in dichloromethane. C, H, Na nalysis (%) for C 30 (2.51 10 4 ), 301 nm (2.10 10 4 ). IR (KBr): ṽ = 3357m, 2947m, 2882m, 2848m, 1629s, 1604s, 1574m, 1489w,1 443w,1 423w,1 405m, 1390m, 1323w,1 299m, 1287s, 1242m, 1202w,1 142w,1 123w, 1040m, 1021m, 965w,9 14m, 879w,8 33vs, 787m, 766m, 742w, 712w,677w cm À1 .…”

“…The compound is extremely hygroscopic and quickly picks up H 2 O. C, H, Na nalysis (%) for C 30 13 CNMR spectroscopic shifts are not given because of low solubility.U V/Vis (CH 3 CN): l max (e in Lmol À1 cm À1 ) = 268 (2.25 10 4 ), 304 nm (0.47 10 4 ). IR (KBr): ṽ = 2930m, 2877m, 2801m, 2660m, 1591vs, 1479m, 1401m, 1373s, 1336w,1 296s, 1127w,1 080w,1 037m, 970m, 913m, 834w,7 85w, 686m cm À1 .…”