Selected Abstracts

Fig. 1. Chemical structures of the compounds studied and discussed. A: hemicyanine dyes, for substitution and bridging pattern, see Table 1; B: styryl dyes/bases with a pyridine/ium acceptor (Table 2); C: styryl dyes/bases with a benzothiazole/ium acceptor (Table 3); D: styryl base related stilbene dyes (Table 4); E: miscellaneous styryl dyes/bases and receptor units.

In the present contribution, the results and implications of various experimental and theoretical mechanistic studies of the ground and excited state photophysics of a series of dyes shown in Fig. 1 and Tables 1 to 4 and differing in both (donor ac-ceptor) substitution as well as single and/or double bond bridging pattern are dis-cussed. The resulting knowledge of the underlying photochemical mechanisms can be utilized to gain a more systematic access to the understanding of the complexa-tion behaviour of receptor substituted cyanine and styryl based fluoroionophores for future rational probe design.

F. Evers, J. Giraud-Girard, S. Grimme, J. Manz, C. Monte, M. Oppel ,W. Rettig, P. Saalfrank, P. Zimmermann, „Absorption- and Fluorescence Excitation Spectra of 9-(N-carbazolyl)-anthracene: Effects of Intramolecular Vibrational Redistribution and diabatic transitions involving electron transfer“ J. Phys. Chem. 105 (2001) 2905-2911
The absorption and fluorescence excitation spectra of 9-(N-carbazolyl)-anthracene (C9A) in vibronically excited S1 states are measured and calculated by means of a simple model. Accordingly, C9A is excited from torsional states |0j> of the electronic ground-state S0 to diabatic torsional states |1l> of the bright electronically excited state S1, which are coupled to states |2l> of the dark electronically excited-state S2. In addition, all torsional states are coupled to the other vibrations of C9A. The model parameters are adapted from our previous papers yielding good agreement of the experimental and theoretical fluorescence emission spectrum and fluorescence lifetimes of C9A. The present additional agreement for the experimental and theoretical absorption and fluorescence excitation spectra confirms the simple model, which implies rather weak couplings of the torsional bright state S1 but strong coupling of the dark state S2 to the other vibrations of C9A, respectively. This points to different electronic structures of these excited states. This conjecture is confirmed by quantum chemical calculations based on density functional theory (DFT) that reveal the covalent structure of S1, in contrast
with the TICT (twisted intramolecular charge transfer) behavior of S2.

V. Kharlanov, W. Rettig "MULTIPLE EMISSION OF N-(1-NAPHTHYL)-PYRIDINIUM“ J. Photochem. Photobiol. A.: Chemistry, 141 (2001) 127-132
The trimethyl derivative of N-(1-naphthyl)-pyridinium shows temperature dependent multiple fluorescence. Quantum chemical calculations indicate that several minima on the S1-hypersurface are responsible for this behaviour. Locally excited, resonance-type charge shift and biradicaloid charge shift states could be characterized.

W. Rettig, S Lutze "Mechanistic Considerations for the Dual Fluorescence of Dimethylaminobenzonitrile: A Fluorescence Anisotropy Study "
Chem. Phys. Lett. 341 (2001) 263-271
Fluorescence polarization experiments on DMABN and derivatives bearing stronger acceptor substituents, and of amino-bridged model compounds have been conducted at low temperature. They allow the classification into two groups differing in the nature of the LE state. From the comparison with these and further observations, the suitability of different mechanistic models can be assessed. The TICT model (twisted amino group) presents no conflict with the experiments. The RICT model (cyano bending) fails partially. The PICT model (planar CT structure) fails to account for all four key observations considered.

V.A. Kharlanov, W. Abraham, W. Rettig "PHOTOPHYSICS OF ARYLSUBSTITUTED TROPYLIUM IONS", J. Photochem. Photobiol. A: Chem., 141 (2001) 127-132
Phenyl tropylium ions bearing different donor substituents were investigated both experimentally (absorption and fluorescence) and theoretically (semiempirical and ab initio calculations for ground and excited state structures and transitions). Generally, an increased donor strength of the substituent leads to a favouring of the planar geometry in the ground state. In the S1 state, however, energetic minima develop for orthogonal structures, which are characterized by strong charge shift (electron transfer from the substituted aryl to the tropylium fragment), forbidden transition moments and near-zero singlet-triplet energy gaps. These results can serve as a basis to interprete the completely nonemissive behaviour of the tropylium derivatives with strong donor substituents.

A. Szemik-Hojniak, W. Rettig, I. Deperasinska "The forbidden emission of protonated proton sponge" Chem. Phys. Lett. 343 (2001) 404-412
The forbidden and strongly red shifted emission of monoprotonated proton sponge (PS.H+) in water and in acidified acetonitrile is reported. An explanation consistent with both the strongly forbidden nature of the emission as well as its highly redshifted character is proposed to involve an overlap forbidden transition connected with two approximately orthogonal orbitals localized in different parts of the molecule. The corresponding relaxation coordinate leading to the fluorescing charge transfer state is linked with pyramidal to nonpyramidal interconversion of the dimethylamino group of PS.H+ and results in a significant energetic lowering of the CT state.

M. Sczepan, W. Rettig, A.I. Tolmachev, V.V. Kurdyukov, "The role of internal twisting in the photophysics of stilbazolium dyes", Phys. Chem. Chem. Phys. 3 (2001) 3555-3561
The synthesis of selectively bridged stilbazolium dyes related to DASPMI is described. The comparison of steady-state and time-resolved fluorescence studies as a function of temperature allows to develop a kinetic model on the basis of which the high photostability and absence of photoisomerization for these dyes can be understood. There are two possible photochemical deactivation channels: nonradiative decay through single bond twisting which does not lead to a distinguishable photoisomer and through double bond twisting leading to trans-cis photoisomerization. The latter process is more than one order of magnitude slower than single-bond twisting for these compounds and is only observed in the compound where both single bonds are bridged. Lifetime maxima observed at low temperature indicate a further early structural relaxation not connected with bond twisting but with bond length changes.

Matthias Beinhoff, Wilfried Weigel, Martin Jurczok, Wolfgang Rettig,* Claudia Modrakowski, Irene Brüdgam, Hans Hartl, and A. Dieter Schlüter* "Synthesis and Spectroscopic Properties of Arene Substituted Pyrene Derivatives as Model Compounds for Fluorescent Polarity Probes",
Eur. J. Org. Chem. 2001, 3819-3829
The goal of this long-term project is to synthesize spherical dendrimers where photoinduced directed charge transfer takes place from the interior to the exterior along the dendritic wedges. The intention is to create systems with a relatively long lifetime of the charge separated photoexcited state to mimic charge separation processes in biological photosystems. If an electron donor is chosen as the core and the dendritic wedges are equipped with electron acceptors at defined positions (generations), several electron transfer (ET) steps should be possible relaying the electron from acceptor to acceptor, similar to the multistep ET in the photosynthesis provided there is a free energy gradient that provides the necessary driving force. This gradient could be based on polarity. The first step is to clarify whether it is possible to generate and quantify a solvent induced polarity gradient that can be reached either by taking structural measures or by exploiting the position dependent solvation. If the density gradient of a dendrimer decreases from its interior to its exterior as, e.g., SANS measurements and computational studies indicate for certain representatives, local polarity created by penetration of polar solvents would be expected to increase in this direction. The quantification of this effect could be achieved by analyzing the fluorescence of polarity probes based on solvatochromism which are site specifically incorporated. In this project, acceptor substituted pyrene derivatives that are covalently bound in specific generations of the dendrimer will be applied as fluorescence probes. Due to the bulkyness of the probes in comparison to the dendrimer skeleton, our concept involves so-called volume dummies that are incorporated into those generations which do not carry the probes in order to maintain the same sterical and chemical microenvironment in each generation.
Here we describe the synthesis of a set of 1-pyrenyl arenes as well as of two model compounds for volume dummies and their spectroscopic properties in a homogeneous solvent environment. They do not yet carry functional groups for dendrimer synthesis but are supposed to serve as model compounds for a rational design of optimized fluorescent polarity probes and dummy building blocks to be used in the synthesis of the required set of dendrimers.

M. Jurczok, T. Gustavsson, J.-C. Mialocq, W. Rettig, "Electron Transfer and Solvation in 9,9'-Bianthryl and Derivatives: A subps Fluorescence Upconversion Study", Chem. Phys. Lett. 344 (2001) 357-366
The time-evolution of the fluorescence spectra of 9,9'-bianthryl and 9-carbazolyl-anthracene in ethanol and butanol have been studied with subpicosecond time-resolution by the fluorescence upconversion technique. For both compounds, an initial phase (< 1 ps) with a structured anthracene-type fluorescence spectrum could be observed which transforms into an unstructured redshifted emission spectrum further shifting to the red on the time scale of the slow component of solvent relaxation. The results are compared to recent transient absorption studies and are interpreted as evidence for a subpicosecond relaxation process (ca 0.4 ps) occurring prior to electron transfer in these compounds.

M. Maus, W. Rettig "Comparison of the Bandshape and Lifetime Data Analysis of Temperature-Dependent Fluorescence Measurements. A new Approach to Reveal an Excited State Conformational Equilibrium.", Phys. Chem. Chem. Phys 3 (2001) 5430-5437
Steady-state and time-resolved fluorescence measurements are performed to gain information about the temperature influence on excited state conformational relaxations of three differently twisted donor-acceptor biphenyls (I-III) in nonpolar and polar solvents. The band-shape analysis related to Marcus theory and the determination of the fluorescence rate constants both as a function of temperature indicate that the relaxation towards a planar structure in II and III is not stopped at low temperature in the liquid phase of the solvents. Only compound III in a medium polar solvent (diethyl ether) exhibits a nonlinear dependence of fluorescence quantum yields (?f), reorganization (?s+?i’) and charge recombination (?G-CT) energies on temperature, which is due to a temperature dependent population ratio of two conformationally different charge transfer species CT and CTR.

M. Glasbeek, H. Zhang, M.M. Martin, P. Plaza, P. Changenet, W. Rettig "Femtosecond studies of intramolecular bond twisting in solution", in "Femtochemistry", Wiley-VCH (Eds. De Schryver, De Feyter, Schweitzer) 2001
Femto- and picosecond fluorescence up-conversion and transient absorption studies of the excited-state dynamics of a few organic chromophores containing single-bonded phenyl groups, in liquid solution, are presented. For one of the investigated molecules, auramine, i.e., 4,4’-(imidocarbonyl)-bis(N,N-dimethylaniline) monohydrochloride, the excited-state dynamics has been investigated as a function of the viscosity of the ethanol solvent. When increasing the viscosity by lowering the temperature (300 K > T > 170 K) a slowing down of the initial picosecond Stokes shift and a concomitant drop of the integrated fluorescence intensity are found. The results are discussed assuming that diffusional torsional motions of the phenyl groups induce relaxation of the population density along the excited-state potential energy surface. The latter is argued to be characteristic of two adiabatically coupled excited states. These states involve the locally excited emissive state and a non-emissive (“dark”) state. On the basis of the proposed model the temporal dependence of the band emission, in low- and high-viscosity solutions, can be satisfactorily simulated. In addition, for a few unbridged and bridged ionic styryl dyes the results of (sub)picosecond fluorescence studies are discussed. Typically, for both the bridged and unbridged compounds, the initial dynamics (on a time scale of a few picoseconds) is controlled by the solvation of the solutes in the solvent. We observe a drastic shortening of the lifetime of the styryl dyes, by at least an order of magnitude, when the chromophore contains at least one single-bonded phenyl group. We propose that the relaxed excited-state energy is in a region where the excited-state potential energy surface, as a function of the torsional mode coordinate, shows conical intersection with the potential enrgy surface of the ground state.

Sabine Kapelle, Wolfgang Rettig and René Lapouyade "Aniline Dimers and Trimers as Model Compounds for Polyaniline: Steric Control of Charge Separation Properties", Chem. Phys. Lett. 348 (2001) 416-424
The photophysical properties of several derivatives of 4-aminodiphenylamine, model compounds of aniline dimers and trimers are investigated. Several compounds show dual fluorescence with a charge transfer component with a significantly reduced fluorescence rate constant which can be suppressed by bridging and enhanced by sterically hindering substituents, in close similarity to the compounds showing Twisted Intramolecular Charge Transfer. The relation to polyaniline conductivity is also discussed.

Publications 2002

Publications 2003

Vladislav Papper, Vladimir Kharlanov, Sandra Schädel, Dieter Maretzki and Wolfgang Rettig, "New fluorescent probes for visual proteins. Part II. 5-(oxo)penta-2,4-dienyl-p-(N,N-dimethylamino)benzoate", Photochem. Photobiol. Sciences 12 (2003) 1272-1286.
A new dual-fluorescent compound, 5-(oxo)penta-2,4-dienyl-p-(N,N-dimethylamino)benzoate (1), a derivative of dimethylaminobenzoic acid, has been synthesised and studied photophysically. This compound continues the series of potential fluorescent probes for visual and proton-pumping opsin proteins. The photophysical behaviour of this molecule, including charge-transfer interaction in the ground state and dual-fluorescence emission, is similar to that of the previously studied analogue cis-3-(oxo)propenyl-p-(N,N-dimethylamino)benzoate (cis-2). The presence of several theoretically calculated conformers of compound 2 was suggested to be responsible for the observed strongly red-shifted absorption and excitation wavelength dependence. These photophysical anomalies were also observed for molecule 1, though the models put forward to explain them in the cases of 1 and 2e.g. protonation, chemical or biochemical reaction, the charge-transfer absorption disappears, which leads to a dramatic increase in the fluorescence quantum yield.

W. Rettig, M. Dekhtyar "Merocyanines: polyene-polymethine transition in donor-acceptor-substituted stilbenes and polyenes",
Chem. Phys. 293 (2003) 75-90.
Three series of donor–acceptor-substituted conjugated compounds, namely, stilbenes, the open-chain polyenes of equivalent length, and the species of intermediate structure (polyenes terminated with only one phenyl ring) have been studied by the AM1 and HMO methods to elucidate and compare the structural prerequisites of the ideal polymethinic state ("cyanine limit"). The transition from polyenic to polymethinic properties has been traced in terms of bond-length (bond-order) alternation using the variation of terminal donor and acceptor substituents. Stilbenes manifest themselves as notably "retarded" polyenes since a larger electronic asymmetry is necessary for them to reach the same degree of polymethinic character. The ground and the excited state have been shown to differ much more strongly for stilbenes than for polyenes with respect to the position of the bond equalization point on the scale of donor–acceptor difference. For the compounds containing one phenyl ring, the features revealed are intermediate between stilbenes and polyenes. The large S₀–S₁ discrepancy in terms of bond alternation is a general property of aromatic ring-terminated chains (stilbenes) and is related to the influence of the aromatic character which can be quantified in this way. In this context, the most relevant definition for the cyanine limit (based on the bond invariance upon excitation) was selected from the existing definitions. The major trends revealed in the polyenic/polymethinic behaviour of the molecules can be interpreted on a topological basis within HMO or even simpler models with some additional influence due to the interelectronic repulsion which is taken into account in the AM1 treatment.

W. Weigel, W. Rettig, M. Dekhtyar, C. Modrakowski, M. Beinhoff, A.D. Schlüter "Dual fluorescence of phenyl and biphenyl substituted pyrene derivatives" J. Phys. Chem. A, 107 (2003) 5941-47.
The photophysical properties of several acceptor substituted 1-arylpyrene derivatives were investigated. The fluorescence spectra strongly depend on the nature of the aryl moiety and the position and number of methoxycarbonyl acceptor groups. Dual fluorescence, originating from a locally excited and a charge transfer state, was observed for the diester derivatives. The solvent dependence of the dual fluorescence and the slightly curved solvatochromic plots indicate a change of the character of the excited states from solvents of low to high polarity. The rate constants for fluorescence and nonradiative decay were calculated to reveal the nature of the excited-state relaxation, that is, the increase of the mesomeric interactions by geometrical flattening or stabilization of the CT state by further twisting toward perpendicularity. Flattening is the major relaxation pathway of the diester phenylpyrene derivative in nonpolar solvents whereas in highly polar solvents the low value for the fluorescence transition dipole moment (M_f) indicates stabilization of the charge transfer state by further twisting. The fluorescence of the diester biphenylpyrene derivative originates from a locally excited state (LE) in nonpolar solvents. The low value for M_f in polar solvents and the change of the relative intensity of the dual fluorescence signals with the temperature indicate that the red-shifted fluorescence can be assigned to a twisted intramolecular charge transfer state.

K. Rotkiewicz, W. Rettig, N. Detzer, A. Rothe " Substituent-Induced Coupling of the Two Lowest Excited Singlet States of 2-Methoxy- Derivatives of 4-(N,N-Dimethylamino)- and 4-(N-Methylamino)benzonitrile", Phys. Chem. Chem. Phys. 5 (2003) 998-1002.
The title compounds both exhibit strong features of interaction of their two lowest excited states, but only one of them, the dimethylamino derivative, exhibits dual fluorescence. This is a direct indication that state interaction and dual fluorescence are not directly related.

Martin Sczepan, Wolfgang Rettig and Alexei I. Tolmachev, "Spectral and Photophysical Characteristics of Unsymmetric Polymethine Dyes as Model Compounds for the Colour Shift of Visual Pigments", Photochem. Photobiol Sciences, 2 (2003) 1264-1271.
Selectively bridged unsymmetric cyanines (dimethylamino-vinyl-benzothiazolium dyes) have been studied with respect to their photophysical properties using steady-state and time-resolved fluorescence. The nonradiative deactivation channels strongly depend on the bridging pattern and can be enhanced or slowed down, with the changes mainly connected to the Arrhenius pre-exponential factor, reflecting the photochemical pathways. The spectral shifts, on the other hand, are independent of the bridging pattern and are correlated with the compound's position on a donor�acceptor axis including ideal polymethines. Using this model, the colour shift of visual pigments can be explained as an effect of the internal (protein-induced) electric field on the retinal Schiff base, likewise an unsymmetric polymethine dye system.

S. A. Kovalenko, J. L. Pérez Lustres, N. P. Ernsting, W. Rettig, Photoinduced Electron Transfer in Bianthryl and Cyanobianthryl in Solution: The Case for a High-Frequency Intramolecular Reaction Coordinate, J. Phys. Chem. A.; (Communication); 107 (2003) 10228-10232.
Photoinduced intramolecular electron transfer (ET) in symmetric 9,9'-bianthryl (BA) was studied by femtosecond pump-supercontinuum-probe spectroscopy in solvents covering the whole polarity scale. Results are compared with those obtained for the nonsymmetric 10-cyano derivative (CBA). The behavior exhibited by both compounds is qualitatively the same, which suggests that symmetry plays no essential role. Our measurements provide novel aspects: (i) electron-transfer proceeds in CBA within a time on the order of 10 fs and therefore along a high-frequency coordinate, (ii) the observation of both educt and product bands allows an estimate of the ET equilibrium, which shifts through solvent relaxation. We find no evidence of dynamics along the intramolecular twist coordinate before ET.

Maged El-Kemary, Wolfgang Rettig "Multiple emission in coumarins with heterocyclic substituents", Phys. Chem. Chem. Phys. 2003, 5221-5228
The photophysical behavior of three coumarin dyes with heterocyclic substitution in 3-position and an electron-withdrawing cyano group in 4-position, Scheme 1b, have been studied in solvents of different polarity. The results are compared with those for rotation-inhibited coumarin 153 and coumarin 6, Scheme 1a. It is concluded that the dyes undergo an excited-state equilibrium reaction from an emissive intramolecular charge transfer (ICT) state to a twisted intramolecular charge transfer (TICT) state with forbidden emission in strongly polar solvents. In coumarins I and II this reaction acts as nonradiative decay involving the rotation about the bond joining the coumarin and the heterocyclic substituent in the 3-position because the TICT state reached is nonemissive. In contrast, coumarin III shows emission from two species with the longer wavelength emitting species being preferred in polar solvents. This is characteristic for an emissive TICT state. The donor strength of the heterocyclic substituent in 3-position plays a major role in increasing the CT character of coumarins I and II when compared to C153. Coumarin III possesses significantly larger Stokes shifts and a correspondingly higher excited state dipole moment than the comparable coumarin 6 lacking the electron-withdrawing cyano group in 4-position.

Publications 2004

T. Yatsuhashi, S.A. Trushin, W. Fuß, W. Rettig, W.E. Schmid, S. Zilberg, "Ultrafast charge transfer and coherent oscillations in 4-piperidino-benzonitrile", Chem. Phys. 296 (2004) 1-12.
The L_a-like S₂ state of 4-(1-piperidino)-benzonitrile and the corresponding 3,5-dimethyl-piperidino derivative was excited at 270 nm in the gas phase at 130 °C. Nonresonant multiphoton ionization at infrared wavelengths (810 and 1300 nm) with mass-selective detection of the ion yields then probed the subsequent processes. Two groups of time constants were found: in the range <2 ps and in the 100 ps range. In addition, a coherent oscillation with period 1.06 and 1.18 ps for the unsubstituted and substituted species were monitored until about 10 ps. The sub-picosecond time constants can be assigned to relaxation from S₂ through a conical intersection to both S₁ (L_b-like) and a charge transfer (CT) state. These two states then equilibrate within about 1.9 ps. The oscillation must be a vibration in L_b, because this is the only state still populated at 10 ps. It is suggested to assign it to the amino-group wagging (planarization/pyramidalization). It indicates a component of the direction into which the conical intersection is displaced from the L_b minimum. The ultrafast relaxation times suggest, however, that additional coordinates are involved in the process, probably amino-group twist and bond stretching in the benzene ring.

Leonid Belau, Yehuda Haas, Wolfgang Rettig "Dual Emission of 4-(1H-Pyrrol-1-yl)benzonitrile Clusters with Acetonitrile in a Supersonic Jet and Its Absence in Phenylpyrrole Clusters", J. Phys. Chem. A 2004, 108, 3916-3925.
The fluorescence of clusters of 4-(1H-pyrrol-1-yl)benzonitrile (PBN) and of phenylpyrrole with acetonitrile (AN) was studied in a supersonic jet. Two separate bands are found in the case of PBN/AN clusters, one centered at 305 nm, the other at 450 nm. The blue- and red-shifted bands are assigned to locally excited (LE) and charge transfer (CT) states, respectively, in analogy with solution spectra of PBN in acetonitrile. The CT emission is observed only when mass spectra obtained upon resonance-enhanced multiphoton ionization (REMPI) of the clusters contain cluster ions in which at least four acetonitrile molecules are attached to a PBN molecule. In contrast, only a single emission band centered at 300 nm (assigned to a LE state) is observed from clusters of 1-phenyl-1H-pyrrole (PP) with acetonitrile prepared under similar conditions, regardless of cluster size. REMPI-induced mass spectra measured in coincidence with the fluorescence reveal a similar distribution of cluster sizes for the two phenylpyrrol derivatives. A possible rationalization for the different emission profiles is discussed in terms of excited-state dynamics.

"Ultrafast Temporary Charge Transfer in Pyrrolidinyl-Benzonitrile and Pyrrolyl-Benzonitrile in the Gas Phase", W. Fuß, W. Rettig, W.E. Schmid,

S.A.

Trushin, T. Yatsuhashi, Faraday Discuss., 127 (2004) 23–33.
4-(N-pyrrolidinyl)benzonitrile and 4-(N-pyrrolyl)benzonitrile were excited by an ultrashort pulse at 270 nm to their L_a(S₂) states and then probed by ionization at long wavelengths. Parent and fragment ion signals show components with time constants <100 fs which we attribute to ultrafast relaxation to the L_b(S₁) state. From this short time we infer a conical intersection between the L_b and L_a surfaces. The wave packet can branch there, one part going temporarily to a strongly displaced state. Its shift was concluded from an anisotropy observed only there. The only excited state known to have a large displacement is the charge-transfer (CT) state. The positive anisotropy indicates that the CT state belongs to the 2A species for both molecules. For pyrrolylbenzonitrile, this is in contrast to previous assignments. The anisotropy, and a coherent oscillation observed in pyrrolidinylbenzonitrile, support the idea that the amino-group twist is an important component of the CT reaction coordinate.

H. Wang , H. Zhang , W. Rettig , A. I. Tolmachev and M. Glasbeek “Femtosecond dynamics of the S₂ and S₁ fluorescence of ionic styryl dyes in polar solvents” Phys. Chem. Chem. Phys., 2004, 6 (13), 3437 – 3446.

Femtosecond fluorescence upconversion and picosecond time-correlated single-photon counting fluorescence experiments for bridged and unbridged ionic styryl dye compounds in polar solvents are reported. The measured fluorescence transients reveal S₂S₁ internal conversion (IC) with a typical time of 300 fs, independent of bridged or unbridged structure. The lifetime of the relaxed emissive S₁ state differs considerably for the bridged and unbridged structures: when the styryl-group single bonds are unbridged, the S₁-state lifetime is only about 20 ps and the non-radiative decay to the ground state is very effective. When both single bonds of the styryl dye are chemically bridged and only the double bond is free to rotate, the fast decay is suppressed and the lifetime becomes about 2 ns. In addition, the fluorescence of the ionic styryl dyes shows picosecond transient behavior that is attributed to vibrational cooling in the excited S₁ state. Finally, a qualitative discussion is given of the influence of the donor–acceptor strength of the styryl dye compounds on the polymethine and stilbenoid character of the S₁ state and how this affects the effectiveness of the single- and double-bond twisting relaxation pathways for this state.

Publications 2005

M. Beinhoff, W. Weigel, W. Rettig, I. Bruedgam, H. Hartl, A.D. Schlüter, “Phenylene Alkylene Dendrons with Site-Specific Incorporated Fluorescent Pyrene Probes 2” J. Org. Chem. 70 (2005) 6583.
This report deals with the synthesis and the spectroscopic properties of two second generation (G2) dendrons with site-specific incorporated phenyl pyrene derivatives as solvatochromic fluorescent probes. The generations that do not carry the probe are equipped with volume dummies, pyrene moieties that do not show a solvatochromic effect. Two complementary G2 phenylene alkylene dendrons were synthesized using Suzuki-Miyaura cross coupling. Most of the reactions used in the 10-step sequence generating the target compounds proceeded in good yields. The incorporated probes can be selectively photoexcited and show solvatochromic shifts that are of the same magnitude as for the free probes in a homogeneous solvent environment. In addition to the charge-transfer fluorescence, a broad emission band is observed that is assigned to an intramolecular exciplex formation between the aryl pyrene chromophores.

H. El-Gezawy, W. Rettig, R. Lapouyade, “Model studies of spectral and photophysical characteristics of donor-acceptor-polyenes: dimethylamino-cyano-diphenylbutadiene” Chem. Phys. Lett. 401 (2005)140-48

4-Dimethylamino-4’ -cyano-1,4-diphenylbutadiene (DCB) and 4-dimethylamino-2,6-dimethyl-4’ -cyano-1,4-diphenylbutadiene (DMDCB) have been photophysically characterized. Quantum chemical calculations were performed for comparison. The fluorescence quantum yields increase with increasing solvent polarity and decreasing temperature. It is concluded that the lowest excited state is a strongly allowed and highly dipolar p p* state in polar solvents, and nonradiative deactivation is reduced. This occurs similarly in both compounds. More specifically,the two methyl groups in DMDCB do not enhance the nonradiative decay. This suggests that neither a fluorescent nor a nonfluorescent TICT state is involved in DCB and DMDCB.

H. El-Gezawy, W. Rettig, A. Danel, G. Jonusauskas, “Probing the Photochemical Mechanism in Photoactive Yellow Protein” J. Phys. Chem. B 109 (2005) 18699-18705
Selectively bridged model compounds related to the chromophore in photoactive yellow protein have been synthesized where the single bond adjacent to the benzene ring (bond 1) and where both bond 1 and the adjacent double bond (bond 2) are bridged. They were compared to the nonbridged reference compound regarding their photophysical properties using steady-state and time-resolved fluorescence at various temperatures. Quantum chemical calculations were additionally performed and showed that several conformers are populated in the ground state. The neutral model compounds show that the nonradiative deactivation channel is linked to both single- and double-bond twisting. The relative importance of single-bond twisting is increased for the corresponding deprotonated hydroxy compounds with an enhanced donor character. The simultaneous photochemical activity of both single and double bonds explains the ease of photochemical isomerization in the confined environment of the natural PYP protein and also of the primary step in the vision process in rhodopsin.

S Murali, W Rettig, “Meta-positioning effect in sterically hindered N-phenyl-pyrroles: A photophysical study“ Chem. Phys. Lett. 412 (2005) 135-140
The photophysical properties of the dimethyl derivative of N-pyrrolo-4-benzonitrile (DPBN),with a change in the position of the acceptor moiety (m-DPBN), have been investigated and compared with the parent compound (p-DPBN). The values of the Stokes shift and of the excited-state dipole moment indicate that both meta -and para -DPBN possess similar excited-state properties regard-less of the meta -positioning of the cyano group. The low values for the radiative rate constant suggest the presence of a strongly forbidden transition supporting the model of twisted intramolecular charge transfer states.
.

S. Murali, P. Changenet-Barret, C. Ley, P. Plaza; W. Rettig, M.M. Martin and R. Lapouyade, “Photophysical properties of pyrrolobenzenes with different linking and substitution pattern: The transition between charge transfer states with large (MICT) and small (TICT) resonance interaction” Chem. Phys. Lett. 411 (2005) 192-197
Pyrrolobenzenes,with di .erent linking and substitution patterns,2’ -(4-cyanophenyl)-methylpyrrole (MP2-BN) and 2’ -(2,5-cyano-phenyl)-methylpyrrole (MP2-B25CN),are investigated by steady-state and time-resolved UV- Vis spectroscopy and compared to the parent compound N -pyrrolobenzonitrile (PBN). Both the electron donor-acceptor linking sites and the strength of the electron acceptor moiety are found to influence the emission characteristics of these compounds. The large radiative rate constant of MP2-BN indicates an allowed emission due to mesomeric interaction between the donor and acceptor moieties (MICT), whereas in the case of PBN and MP2-B25CN, the reduced radiative rate constant indicates a forbidden emission from a twisted intramolecular charge transfer (TICT)state.

S. Murali, V. Kharlanov, W. Rettig, A.I. Tolmachev, and A.V. Kropachev, “The Tetrafluoro Analogue of DMABN: Anomalous Fluorescence and Mechanistic Considerations“ J. Phys. Chem. A 109 (2005) 6420
Absorption and emission properties of DMABN-F4, the tetrafluoro analogue of DMABN, have been investigated and compared with the parent compound. Unlike in DMABN, this new compound exhibits only a highly solvatochromic and strongly red-shifted charge transfer (CT) fluorescence and is characterized by the absence of an LE band even in nonpolar solvents. This evidences the faster formation of CT in the excited state as compared to DMABN. The low quantum yield values of DMABN-F4 suggest that the high rate of nonradiative decay takes place via internal conversion (IC) rather than intersystem crossing (ISC) as no phosphorescence is observed in rigid glass solvents at 77 K in contrast to DMABN. The emission transition moment and radiative rate constant values of DMABN-F4 in medium and highly polar solvents point to a forbidden emission in the excited state similar to that of DMABN. Electronic structure and twist potentials were also studied by quantum chemical calculations using ab initio and semiempirical methods. In contrast to DMABN, the dimethylamino group in DMABN-F4 is found to be twisted by around 30-5°, but the photophysics are concluded to be analogous to DMABN with the addition of a very fast IC channel.

Publications 2006

H. El-Gezawy, W. Rettig, and R. Lapouyade, “Solvatochromic Behavior of Donor-Acceptor-Polyenes: Dimethylamino-cyano-diphenylbutadiene”
J. Phys. Chem. A 110 (2006) 67-75
4-(Dimethylamino)-4’-cyano-1,4-diphenylbutadiene (DCB) and 4-(dimethylamino)-2,6-dimethyl-4’-cyano-1,4-diphenylbutadiene (DMDCB) have been characterized spectroscopically. Quantum chemical calculations were performed for comparison. Solvatochromic shifts of the fluorescence were strong and showed a linear dependence on the solvent polarity parameters, whereas shifts in the absorption spectra are very weak only correlate better with the polarizability of the solvents. Excited state dipole moments derived from fluorescence using the Onsager model are very large and similar for both compounds. It is concluded that a strongly allowed and highly dipolar p, p*  state is the lowest excited state in polar solvents. The strong difference in absorption and fluorescence solvatochromic slopes suggests that the simple Onsager model with a point dipole approximation is not sufficient here.

Hani El-Gezawy, Wolfgang Rettig , "Model studies of spectral and photophysical characteristics of a series of donor-acceptor-polyenes of different length "
Chem. Phys. 327 (2006) 385-394
The photophysical behaviour of a series of donor-acceptor-polyenes of different length: 4-dimethylamino-4'-cyanostilbene (DCS), 4-dimethylamino-4'-cyano-1,4-diphenylbutadiene (DCB) and 4-dimethylamino-4'-cyano-1,4-diphenylhexatriene (DCH) has been studied by means of steady state and time resolved fluorescence and low temperature measurements. Quantum chemical calculations were performed for comparison. For all three dyes, the fluorescence quantum yields show a maximum in the most polar solvents. A pronounced solvatochromic behaviour is only observed for the emission spectra whereas the absorption spectra remain nearly unaffected. The non-radiative decay decreases in the order DCS >DCB >DCH. Similarly, the radiative rate constant decreases in this order, contrary to the usual behaviour for a lengthening of the p system. The chain-length dependence of the dipole moments of the relaxed excited state l is better described by the Onsager radius derived from molecular length rather than from the density approach. Both calculations and experiments support the conclusion that the emissive state is not a TICT state.

Publications 2007

V. Kharlanov, W. Rettig, "Excited-State Relaxation of Bridged and Unbridged Anilino-Pyridinium Dyes"
Chem. Phys. 332 (2007) 17-26

Several anilino-pyridinium-type hemicyanines including flexible and bridged model compounds are compared using experimental and quantum chemical methods. The flexible compounds show broadened fluorescence band and exhibit strong fluorescence quenching connected with a barrierless viscosity-controlled transition from the planar to the perpendicular geometry. The latter is connected with a small S₁–S₀ and a vanishing S₁–T₁ energy gap, with charge localization and with forbidden emissive character.

Publications 2008

B. Szczepanik, R. Obara, A. Rothe, W. Rettig, M. Stachera, K. Rotkiewicz, “Enhancement of photoacidity by formation of an intramolecular charge transfer state with twisted conformation "
Polish Journal of Chemistry, 82 (2008) 807-829

The synthesis of a pair of biphenyls with a cyano acceptor and a hydroxy (methoxy) donor, which differ in the extent of steric hindrance to planarity, induced by ortho-methyl substituents with respect to the phenyl-phenyl bond is described. It is shown that dual fluorescence can be observed, ascribed to a mesomerically stabilized less polar intramolecular charge transfer (MICT) state of near planar geometry, and a more polar one, with strongly decoupled intramolecular CT resembling a TICT state. It possesses an enhanced dipole moment with respect to the MICT state, but considerably deviates from the perpendicular geometry as judged from the transition moments. This concept is used in conjunction with the excited-state proton transfer (ESPT) mechanism. The resulting photoacidity of the compound forming the enhanced ICT is an order of magnitude larger than for the MICT-compound. The mechanistic concept allows the construction of further compounds with still increased photoacidity.

Abstracts for selected publications

Publications until 1996

Publications 1997-2001

Publications 2002

Publications 2003

Publications 2004

H. Wang , H. Zhang , W. Rettig , A. I. Tolmachev and M. Glasbeek “Femtosecond dynamics of the S₂ and S₁ fluorescence of ionic styryl dyes in polar solvents” Phys. Chem. Chem. Phys., 2004, 6 (13), 3437 – 3446.

Publications 2005

Publications 2006

Publications 2007

Publications 2008

Abstracts for selected publications

Publications until 1996

Publications 1997-2001

Publications 2002

Publications 2003

Publications 2004

H. Wang , H. Zhang , W. Rettig , A. I. Tolmachev and M. Glasbeek “Femtosecond dynamics of the S2 and S1 fluorescence of ionic styryl dyes in polar solvents” Phys. Chem. Chem. Phys., 2004, 6 (13), 3437 – 3446.

Publications 2005

Publications 2006

Publications 2007

Publications 2008

H. Wang , H. Zhang , W. Rettig , A. I. Tolmachev and M. Glasbeek “Femtosecond dynamics of the S₂ and S₁ fluorescence of ionic styryl dyes in polar solvents” Phys. Chem. Chem. Phys., 2004, 6 (13), 3437 – 3446.