Stephan A. Jonker

Bio: Stephan A. Jonker is an academic researcher from Delft University of Technology. The author has contributed to research in topics: Excited state & Singlet state. The author has an hindex of 11, co-authored 16 publications receiving 699 citations.

Excited-state dipole moments of dual fluorescent 4-(dialkylamino)benzonitriles: influence of alkyl chain length and effective solvent polarity

Abstract: Singlet excited-state dipole moments of a number of aminobenzonitriles have been determined in cyclohexane, benzene, and 1,4-dioxane, using time-resolved microwave conductivity (TRMC) and fluorescence spectroscopy techniques. For the 4-(dialkylamino)benzonitriles (methyl, ethyl, propyl, and decyl) intramolecular charge transfer (ICT) occurs in the excited singlet state even in the nonpolar solvent cyclohexane

Sudden polarization in the twisted, phantom state of tetraphenylethylene detected by time-resolved microwave conductivity

Abstract: Photoexcitation of the symmetrical molecules tetraphenylethylene and tetra-p-methoxyphenylethylene dissolved in saturated hydrocarbon solvents results in a transient increase in the dielectric loss of the solutions as monitored using the nanosecond time-resolved microwave conductivity (TRMC) technique. This provides direct evidence for the dipolar, or [open quotes]zwitterionic,[close quotes] nature of the [sup 1]p* phantom state formed from S[sub 1] by rotation around the central carbon-carbon bond. Dipole relaxation occurs mainly by charge inversion between the two energetically equivalent zwitterionic configurations. Z[sub [+-]], on a timescale of several picoseconds. A minimum dipole moment of ca. 7.5 D for the individual Z[sub [+-]] states is found. The fluorescence of TPE in alkane solvents has two decay components, one with a decay time less than 200 ps and a second with a decay time of 1.9 ns. The former ([lambda][sub max] [approx] 490 nm) is assigned to emission from the partially relaxed S[sub 1] state prior to twisting. The latter ([lambda][sub max] [approx] 540 nm) is assigned to emission from a small, ca. 1%, concentration of the relaxed S[sub 1] state in equilibrium with the [sup 1]p* state in saturated hydrocarbon solvents. 27 refs., 2 figs., 2 tabs.

Modeling long-range photosynthetic electron transfer in rigidly bridged porphyrin-quinone systems

Abstract: Photoinduced charge separation as well as subsequent charge recombination is studied in bichromophoric molecules containing a quinone unit (Q) and either a porphyrin (P) or a zinc porphyrin (ZnP) unit that are interconnected by a rigid saturated hydrocarbon bridge that unequivocally determines both the separation and the relative orientation of the chromophores. Across a bridge comprising a separation equivalent to two saturated carbon-carbon bonds (i.e., in P(2)Q), extensive direct overlap between the {pi}-systems of the chromophores is still possible and accordingly very fast photoinduced charge separation (typically on a 30-40-ps time scale) is observed. However, even across a bridge comprising an extended array of six saturated bonds, charge separation times in the order of 100 ps can still be realized if the driving force for this process is optimized by modification of the porphyrin and of the solvent (a minimum charge-separation time of 65 ps was observed for ZnP(6)Q in chloroform). This implies a rate of charge separation comparable to or somewhat higher than that of the charge transfer from pheophytin to quinone in natural photosynthesis in spite of the fact that the interchromophore distance in the latter process is slightly smaller. The time-resolved microwave conductivity method was employed tomore » confirm the occurrence of photoinduced charge separation as well as to measure the rate of charge recombination in ZnP(6)Q.« less

Intramolecular charge separation and recombination in non-polar environments via long-distance electron transfer through saturated hydrocarbon barriers

Abstract: Time-resolved microwave conductivity and fluorescence spectroscopy techniques have been used to monitor the kinetics of charge separation and recombination following photo-excitation of donor-spacer-acceptor (DSA) molecules in which the spacer is a rigid saturated hydrocarbon bridge of length varying from 4.6 to 13.5 A. The solvents used were all completely non-polar saturated hydrocarbons with relative dielectric constants varying from 1.8 to 2.3. The lifetimes of the highly dipolar, charge-separated states formed increase initially with increasing length of the spacer but eventually decrease for distances longer than approximately 9 A. At that point the lifetime becomes sensitive to the dielectric constant of the medium and the temperature which was varied between 175 and 375 K. At the transition distance delayed donor fluorescence is observed. The results are explained in terms of the decrease in the Coulomb energy with increasing distance which raises the energy level of the charge separated state and eventually brings it close to the energy level of the locally excited donor (LED) state. Under these conditions charge recombination occurs preferentially via the LED state by thermally activated back electron transfer. The enegetics underlying this change in recombination mechanism are discussed.

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Abstract: 6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Design principles of fluorescent molecular sensors for cation recognition

Abstract: The main classes of fluorescent molecular sensors for cation recognition are presented: they differ by the nature of the cation-controlled photoinduced processes: photoinduced electron transfer, photoinduced charge transfer, excimer formation or disappearance. In each class, distinction is made according to the structure of the complexing moiety: chelators, podands, coronands (crown ethers), cryptands, calixarenes. The most representative examples are presented in each subclass with special attention given to selectivity.

Nature of biological electron transfer

Abstract: Powerful first-order analysis of intraprotein electron transfer is developed from electron-transfer measurements both in biological and in chemical systems. A variation of 20 A in the distance between donors and acceptors in protein changes the electron-transfer rate by 10(12)-fold. Protein presents a uniform electronic barrier to electron tunnelling and a uniform nuclear characteristic frequency, properties similar to an organic glass. Selection of distance, free energy and reorganization energy are sufficient to define rate and directional specificity of biological electron transfer, meeting physiological requirements in diverse systems.