Wolfgang Kühnle

Intramolecular charge transfer in aminobenzonitriles: Requirements for dual fluorescence

Dual fluorescence and intramolecular charge transfer (ICT) are observed with aminobenzonitriles when two excited state levels (S 1 and S 2 (CT) in DMABN) have an energy gap sufficiently small for vibronic coupling: a solvent-induced pseudo-Jahn-Teller effect. It is argued that the N-inversion of the amino group acts as a promoting mode. These conclusions are based on a comparison of absorption spectra and photostationary and time-resolved fluorescence data

Intramolecular Excimers withα,ω-Diarylalkanes

The fluorescence spectra of a series of <x,cu-di-(l-pyrenyl)alkanes Py(CH2)7¡P¡/ with re = 2 to 16 and 22, nearly all contain emissions from an intramolecular excimer, when measured in a 1 · -5 M solution in methylcyclohexane at room temperature. It is shown that the emissions do not arise from preformed dimers in the ground state. The intensity ratio of the excimer and monomer fluorescence as a function of chain length, which has a clear minimum for re = 7 and shows an alternating property for larger re, strongly resembles the dependence of the yields of organic ring closure reactions on ring size. For re = 2 and = 4—9 a blue shift of the energy of the excimer emission maximum, relative to re = 3, is observed. Also with an , -diphenylalkane an intramolecular excimer emission is found for longer alkane chains, with re = 13. For l,3-di-(9-phenanthryl)propane, however, an intramolecular excimer could not be observed.

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems

Abstract An extremely large increase of exciplex fluorescence intensity has been observed with photoexcited compounds pyrene-(CH2)n - DMA in acetonitrile when an external magnetic field of several hundred gauss is applied. The effect is readily explained as originating from the radical ion pair which can undergo a magnetic-field-dependent singlet-triplet conversion due to nuclear hyperfine coupling, or an association to form the exciplex. The increasing influence of the exchange interaction in the radical ion pair with decreasing chain length (CH2)n is evident.

Intramolecular single and double proton transfer in benzoxazole derivatives

Abstract The “double” derivatives of benzoxazole, bis-2,5-(2-benzoxazolyl)hydroquinone (II) and bis-3,6-(2-benzoxazolyl)-pyrocatochol (III), have been investigated. In (II), only one proton is transferred in the S 1 state. Primary and tautomeric forms exist in a rapidly established equilibrium. In (III), two tautomers were detected. One is generated in the S 1 state by a double proton transfer without a potential barrier, while the other, generated by a single proton transfer, is already present in trace amounts in the S 0 state.

Investigation of human erythrocyte ghost membranes with intramolecular excimer probes

Human erythrocyte ghost membranes have been investigated using two intramolecular excimer probes, di(1-pyrenyl)propane and di(1-pyrenylmethyl) ether. Values for the viscosity of the direct probe environment in the ghost membranes range from 76 cP at 37 degrees C to 570 cP at 5 degrees C, as reported for di(1-pyrenyl(propane, with liquid paraffin as the reference solvent. For the activation energy of the excimer formation process, determined here mainly by the viscosity of the medium, a value of 37 kJ/mol is obtained. The other probe molecule reports a higher local viscosity, 133 cP at 37 degrees C, as well as a higher activation energy of excimer formation, 54 kJ/mol. Neither thermotropic phase transitions nor temperature hysteresis effects are observed within the temperature range (0 to 40 degrees C) studied. From the vibrational structure of the fluorescence spectrum of di(1-pyrenylmethyl) ether, a polarity of the probe environment close to that of hexanol (epsilon - 13.3) results for the erythrocyte ghost membranes. The polarity measured in egg phosphatidylcholine membranes and in multibilayers of dimyristoylphosphatidylcholine is slightly larger, comparable to that of butanol (epsilon = 17.5), whereas a polarity comparable to that of methanol (epsilon = 32.7) is observed for aqueous micellar solutions of sodium dodecyl sulphate. Further, from the wavelength shifts in the absorption spectrum of di(1-pyrenyl)propane and di(1-pyrenylmethyl) ether, the polarizability of the probe surroundings can be determined, leading to a surprisingly high value for the apparent refractive index. This is attributed to a high local density of the direct environment of the probe, for which a location between the membrane/water interface and the unpolar bilayer mid-plane is deduced.

Intramolecular excimer fluorescence as a probe of fluidity changes and phase transitions in phosphatidylcholine bilayers

Abstract Intramolecular excimer formation of 1,3-di(1-pyrenyl)propane has been used to investigate, as a function of temperature, the fluidity in multibilayer dispersions and sobicated vesicles of dimynstoyl and dipalmitoyl phosphatidylcholine. The main phase transitions are detected at temperatures corresponding to those reported in the literature. The results show that the lipid structure in the ordered phase is strongly perturbed (and hence relatively Quid) in the immediate vicinity of the probe.

Intramolecular monomer and excimer fluorescence with dipyrenylpropanes: double-exponential versus triple-exponential decays☆

Abstract The fluorescence decays of 1,3-di(1-pyrenyl)propane undergoing intramolecular excimer formation can be fitted to a sum of three exponentials, whereas only two exponentials are needed for 1,3-di(2-pyrenyl)propane. It is concluded, from an analysis of the decay parameters, that one monomer and two excimers are involved in the excimer formation for 1,3-di(1-pyrenyl)-propane, in contrast with that for 1,3-di(2-pyrenyl)propane where only one excimer and one monomer are needed in the kinetic scheme. Kinetic and thermodynamic data are presented for both molecules. The significance of the various cases (double and higher) of multi-exponential decay is discussed.

Intramolecular excimer formation with non-planar molecules: 1,3-dibiphenylpropane. Kinetic and thermodynamic results

Abstract Intramolecular excimer formation with dibiphenylpropane in several alkane solvents is reported. By a combination of fluorescence decay time and quantum yield measurements the rate constants describing the excimer formation and deactivation and the values for Δ H and Δ S of the excimer formation equilibrium could be determined. These data are compared with results obtained with di(2-naphthyl) propane and di(1-pyrenyl) propane. It is argued that the difficulty to observe excimer emission with biphenyl under photoexcitation results from a very large value of −Δ S . A comparison between the excimer fluorescence of dibiphenylpropane and [2.2] biphenylophane points to an unusually weak transannular interaction in the phane.

Photophysics of 4-dimethylamino-4'-cyanostilbene and 4-azetidinyl-4'-cyanostilbene. Time-resolved fluorescence and trans-cis photoisomerisation

Abstract The fluorescence decays of trans-4-dimethylamino-4′-cyanostilbene (DCS) and trans-4-azetidinyl-4′-cyanostilbene (ACS) in an unpolar (n-heptane) and a polar (acetonitrile) solvent are single exponential down to a time resolution of around 5 ps and dual fluorescence is not observed. The dipole moment increases in two steps: from 7 D in the ground state via 13 D (DCS) or 10 D (ACS) for the Franck-Condon excited state to 21 D (DCS) or 22 D (ACS) for the relaxed fluorescing CT state. This means that the intramolecular charge transfer (ICT) in the excited state at most involves an intermediate with a subpicosecond lifetime. The presence of the azetidinyl group in ACS does not slow down the ICT reaction, contrary to what has been found for the 4-aminobenzonitriles, showing that inversion of the amino group is not an important reaction coordinate here. The activation energy E tc for trans-cis photoisomerisation is determined from the fluorescence decays in n-heptane (14.0 kJ/mol, DCS) and in acetonitrile (22.7 kJ/mol, DCS and 22.5 kJ/mol, ACS). This increase of E tc with solvent polarity, opposite to what has been found for trans-stilbene, shows that the photoisomerisation transition state is less polar than the CT state.

Picosecond chemical relaxation of excited p-cyano-n, n-dialkylanilines

Abstract New evidence indicate that the dual fluorescence of p -dimethylamino-benzonitrile (I) originates in a chemical relaxation B * → A * , where B * is a state hidden in the first absorption band A * is a highly polar rotamer with NMe 2 group perpendicular to the ring. New derivatives are synthesised, II–V, including those of enforced coplanar (V) or twisted conformation of NMe 2 group (III, IV). All three transition moments, the main responsible for absorption, b and a , are parallel. The relaxation B * → A * is in CH 3 CN slower than the reorientation of the solvent molecules. A * seems to be related to a “Rydberg” state similar to those of aliphatic amines.