Ira M. Brinn

Benzamidoximes: structural, conformational and spectroscopic studies. I

Abstract The synthesis and properties of seven arylamidoximes are reported. 1 H- and 13 C-NMR studies and MO calculations were performed on all seven and an X-ray crystallographic determination was done on one, to determine their structure. The theoretical calculations were done using the AM1 and PM3 methods. From these results it is concluded that, for all of the arylamidoximes studied here, the NH 2 group of the amidoxime ( N -hydroxyamidine) has very little sp 2 character and the aryl ring is not coplanar with the amidoxime group. X-ray crystallographic data for p -chlorobenzamidoxime were compared with the theoretically calculated coordinates. It is interesting to note that, in crystals, the –NH 2 group is coplanar with the CN bond and the aryl ring is out of the plane of the amidoxime group. Molecular mechanics (Biosym) calculations on benzamidoxime yielded better coordinates than either the AM1 or PM3 methods.

Fluorescent symmetric phenazines from naphthoquinones: 3. Steady-state spectroscopy and solvent effect of seven phenazine derivatives: structure–photophysics correlations

Abstract The fluorescence spectra of seven phenazines (two belonging to the C2h point group, two to the C2v point group, two to the Cs point group and one to the C1 point group) in organic solvents of varying polarity show a bathochromic shift in all cases. Fluorescence quantum yields increase as the polarity of the solvent increases for the C2h and the Cs compounds, however, no appreciable changes are noted in the C2v compounds. The unexpectedly strong solvent interactions with the two centrosymmetric phenazines are explained on the basis of a localized excited state. This explanation was supported by excited state dipole moment measurements, which indicated similar moments for the C2h and C2v compounds. Two of the compounds undergo lasing when placed in a laser cavity and pumped with the third harmonic of an Nd-YAG laser.

Coordination complexes of bifunctional compounds ☆: I. Synthesis and properties of bis[5-(2-oxyphenyl)-3-phenyl-1,2,4-oxadiazolyl]copper(II). A fluorescent coordination compound of Cu(II)

Abstract The synthesis of two Cu(II) bis complexes of a substituted 1,2,4-oxadiazole, which acts as part of a bidentate ligand, is reported for the first time. The two coordination compounds, which form an inseparable mixture, are characterized mainly via vibrational and EPR spectroscopy. Room temperature fluorescence is observed from two uncoupled excited states of the complexes, and the respective lifetimes are approximately 600 ps and 5 ns. The fluorescence quantum yields are a function of excitation wavelength, varying between 1.7 and 9.0×10 −3 . These values are slightly higher than those of the isolated ligand.

Excited state acidity of bifunctional compounds. 6. A novel, high fluorescence quantum yield, excited state intramolecular proton transfer compound: 2-hydroxyphenyl-lapazole in non-protic solvents

The synthesis of 2-hydroxyphenyl-lapazole (HPL) is reported for the first time. HPL in the first excited singlet state undergoes excited state intramolecular proton transfer (ESIPT) in cyclohexane, with an overall fluorescence quantum yield of 0.15. This is an order of magnitude greater than that observed for 2-hydroxyphenyl-benzoxazole (HBO), which has been extensively studied and has a similar structure. At room temperature in non-protic solvents, ESIPT in HPL attains equilibrium, whose constant (between normal and tautomer species) is approximately 1.4. These observations suggest that HPL (and its derivatives) is well worth investigating because it is easier to study (and may work more efficiently as a laser dye) than HBO.

Excited-state Acidity of Bifunctional Compounds. 7. Long Distance, Solvent-assisted Excited-state Proton Transfer in Olivacine

Abstract— By use of fluorescence spectroscopy and time‐correlated single photon counting, solvent‐assisted excited‐state proton transfer (ESPT) has been observed for olivacine in two organic solvents, 1,4‐dioxane and methanol. Because of spectral overlap, factor analysis was used to determine the emission spectrum of the proton‐transferred (PT) species. The ESPT is not observed in the nonpolar solvent n‐hexane. For the first time in a system of this type, the individual excited‐state rate constants have been calculated exactly from time‐resolved single photon counting results. Because the PT process involves a path of more than 6 Å, its individual rate constant is extremely low, approximately 107 s−1.

Excited state acidity of bifunctional compounds

2-Hydroxyphenyl-lapazole (HPL) is shown to undergo excited state intramolecular proton transfer (ESIPT) in the protic solvents methanol, propan-2-ol and octan-1-ol at room temperature. Investigation of the kinetics of this process, using time-resolved single photon counting and transient absorption spectroscopy, indicates the presence of three different excited-state species. These results are very different from that already reported for HPL in non-protic solvents, where it was found that the ESIPT process attains equilibrium during the lifetime of the excited state. Factor analysis of the steady state spectra supports the conclusions drawn from the kinetic results. The difference in behavior as a function of solvent is attributed to two factors that depend on the stronger solute–solvent interactions in the case of the protic solvents. (1) The slight slowing down of the process of proton transfer, which prevents equilibrium from being established during the lifetime of the excited singlet state. (2) The weakening of the intramolecular hydrogen bond, which allows rotation of the hydroxyphenyl moiety.

Excited-state acidity of bifunctional compounds

Fluorescence emission and excitation spectra and experimental dipole moments are presented for 5-(2-hydroxyphenyl)-3-phenyl-1,2,4-oxadiazole and 3-(2-hydroxyphenyl)-5-phenyl-1,2,4-oxadiazole. These data indicate that the long-wavelength emission of the former, in non-hydrogen bonding solvents, is due to a tautomeric PT structure. The latter exhibits only the ‘normal’ band. The striking difference in behavior between these two compounds of very similar structure is explained on the basis of coplanarity between the aryl ring in position 5 and the heterocycle ring, the aryl ring in the 3 position not being in the same plane.

The prediction of in-plane fundamental vibrational frequencies based on cis-trans correlation

Abstract Comparisons of in-plane fundamental vibrational frequencies of cis and trans isomers of various stable molecules have been made. It is concluded that a relationship exists which permits the prediction of the fundamental frequencies of one species (either C 2v or C 2h ) if the spectrum of the corresponding species of the other symmetry group is known.

Excited states of naphthols. Part 2.—Molecular orbital calculations on substituted naphthols

Excited singlet state dipole moments are reported for various substituted naphthols and the two unsubstituted naphthols. These results with previously reported pKa values, are compared with the prediction of CNDO/2-type calculations and it is concluded that the latter give good agreement for the ground state and reasonable agreement for the excited state. A model is developed in order to quantitatively compare calculated charge densities with experimental pKas.

Photoswitchable trinuclear transition-metal complexes. Intramolecular triplet–triplet energy transfer from fac-(diimine)ReI(CO)3 chromophores to a stilbene-like bridging ligand

A series of novel trinuclear metal complexes containing fac-(diimine)ReI(CO)3 chromophores and a stilbene-like bridging ligand have been synthesized and in CH3CN exhibit photoswitchable luminescence arising from photoinduced intramolecular energy transfer from the fac-(diimine)- Re(CO)3 chromophores to the bridging ligand.