Klaas A. Zachariasse

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 charge transfer in the excited state Kinetics and configurational changes

Abstract The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9′-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.

Intramolecular excimer formation with diarylalkanes as a microfluidity probe for sodium dodecyl sulphate micelles

Abstract Diphenylpropane and dipyrenylpropane have been used as pseudomonomolecular probes for microfluidity measurements in aqueous micellar solutions with sodium dodecyl sulphate. These probes can be used in small concentration (10 −5 M or lower) and they do not suffer from the problems arising from the distribution statistics of the solute molecules which occur with intermolecular probes. The results with dipyrenylpropane agree well with fluorescence depolarisation measurements. Diphenylpropane shows a clearly lower microfluidity.

Photoinduced intramolecular charge transfer and internal conversion in molecules with a small energy gap between S-1 and S-2. Dynamics and structure

Abstract A discussion is presented of the fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles. The occurrence or absence of this ICT reaction is shown to be determined by the magnitude of the energy gap ΔE (S 1 ,S 2 ) between the two lowest excited states. The strong decrease of the ICT efficiency in the series P8C to P3C of 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, is attributed to the increase with decreasing ring size of the barrier for the configurational change of the amino nitrogen from pyramidal to planar, which is considered to be an important reaction coordinate in the ICT process. Dual fluorescence does not occur with P3C nor with NC in diethyl ether. Application of a pressure of 6000 bar leads to a threefold increase of the CT/LE fluorescence quantum yield ratio for the 4(dialkylamino)benzonitrile DMABN (methyl) in diethyl ether, whereas this ratio only slightly increases for DDABN ( n -decyl) in n -hexane. This difference is attributed to the increase of the solvent dielectric constant with pressure, which effect is much larger for diethyl ether than for n-hexane. These results mean that a large amplitude motion does not occur during the ICT reaction of DMABN and DDABN, in accordance with the model of planar intramolecular charge transfer for dual fluorescence. Dual fluorescence is observed with the rigidized aminobenzonitrile NMC7, showing that a full perpendicular twist of the amino group is not essential for the CT state. I -(Dimethylamino) naphthalene ( I DMAN) undergoes a fast thermally activated internal conversion (IC), for which the activation energy increases with solvent polarity. This IC reaction depends on the magnitude of the ΔE (S 1 ,S 2 ) energy gap and is similar in this respect to the ICT reaction of DMABN.

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.

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.

Dual fluorescence and fast intramolecular charge transfer with 4-(diisopropylamino)benzonitrile in alkane solvents

Abstract Dual fluorescence and fast intramolecular charge transfer (ICT) is observed with 4-(diisopropylamino)benzonitrile (DIABN) in alkane solvents. The rate constant ka for the reaction from the locally excited (LE) to the ICT state has a value of 3.4×1011 s−1 in n-hexane at 25°C, with an activation energy Ea of 6 kJ mol−1. Efficient intersystem crossing with a yield of 0.94 takes place from the ICT state. With 4-(dimethylamino)benzonitrile, in contrast, dual fluorescence is not observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap ΔE(S1,S2), in accordance with the PICT model for ICT in aminobenzonitriles.

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.

ABSENCE OF DUAL FLUORESCENCE WITH 4-(DIMETHYLAMINO) PHENYLACETYLENE. A COMPARISON BETWEEN EXPERIMENTAL RESULTS AND THEORETICAL PREDICTIONS

Abstract From ab initio electronic structure calculations it has been predicted that 4-(dimethylamino)phenylacetylene (DACET) will undergo an exothermic intramolecular charge transfer (ICT) reaction with dual fluorescence even under isolated-molecule conditions. Photostationary and time-resolved fluorescence measurements reveal however that an ICT reaction does not occur with DACET under any condition of solvent polarity or temperature studied. DACET is similar to 4-(methyl)amino-benzonitrile, which shows only fluorescence from a locally excited state, but is clearly different from the dual fluorescent 4-(dimethyl)aminobenzonitrile (DMABN). The energy gap Δ E ( S 1 , S 2 ) of DACET is larger than that of DMABN, supporting the importance of this gap in these ICT reactions.