Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine

International audienceThe excited state properties of protonated ortho (2-), meta (3-) and para (4-) aminopyridine molecules have been investigated through UV photo fragmentation spectroscopy and excited state couple cluster CC2 calculations. Cryogenic ion spectroscopy allows recording well-resolved vibronic spectroscopy that can be nicely reproduced through Franck Condon simulations of the pp* local minimum of the excited state potential energy surface. The excited state lifetimes have also been measured through a pump-probe excitation scheme and compared to the estimated radiative lifetimes. Although protonated aminopyridines are rather simple aromatic molecules, their deactivation mechanisms are indeed quite complex with unexpected results. In protonated 3-and 4-aminopyridine, the fragmentation yield is negligible around the band origin, which implies the absence of internal conversion to the ground state. Besides, a twisted intramolecular charge transfer reaction is evidenced in protonated 4-aminopyridine around the band origin, while excited state proton transfer from the pyridinic nitrogen to the adjacent carbon atom opens with roughly 500 cm-1 of excess energy

Explicit influence of water microsolvation on charge transfer and dynamics in ground and excited electronic states of molecular systems

Modern computational molecular quantum chemical studies, such as the present one, typically employ a wide range of theoretical techniques. The latter are often rather complicated and one should not generally expect that an experimental scientist in the area of physical chemistry, a potential reader of this work, should be familiar with all these techniques. To simplify the reading of the Thesis and to make it self-sufficient, it is supplied with an overview of the employed theoretical methodologies (Chapter 1). The overview explains basic quantum-chemical terminology referred to throughout the Thesis, introduces theoretical foundations of the methods and outlines their properties and limitations. In Part 1.1 of Chapter 1, methods for the solution of the molecular Schrödinger equation are introduced, while in the subsequent Parts 1.2 and 1.3 methods for the solution of the electronic Schrödinger equation are presented to find the ground and excited states, respectively. Part 1.4 is dedicated to basis-set effects which are omnipresent in electronic-structure calculations. It contains a number of unusual insights and concepts proposed by the author and, thus, may be insightful also to experts in quantum chemistry. In Chapter 2, the phenomenon of acetone-water proton exchange catalyzed by tubular as well as amorphous aggregates of calix[4]hydroquinone (CHQ) macromolecules, which has been observed previously in NMR experiments (Ref. D1D), is investigated by means of correlated quantum-chemical methods. The first part of the study (Section 2.3-2.7) considers concerted proton transfer, assisted by several initially neutral OH-groups in the hydrogen-bonded networks of CHQ aggregates. The second part of the study (Section 2.8-2.13) is dedicated to a second mechanism of proton exchange: step-wise proton transfer via formation of ionic intermediates resulting from CHQ pre-dissociation. CHQ application-specific as well as general conclusions, relevant to the main topic of the Thesis (i.e. influence of specific microsolvation on the considered proton transfer processes), are presented in Section 2.14. The phenomenon of dual fluorescence observed in clusters of methyl 4-N,N-dimethylaminobenzoate ester (DMABME) and two water molecules in the gas phase, is studied in Chapter 3. Experimentally, the dual fluorescence was detected in experiments combining optical and ground-state ion-depletion infrared spectroscopies in ultracold molecular beams (Ref. D2D). In Section 3.3, calculated ground-state infrared spectra are presented that allow to identify the structures of those isomers, which are present in the gas-phase, as well as the structure of the isomer responsible for dual fluorescence. To further understand the reaction mechanism of dual fluorescence, excited-state potential energy surfaces of the identified isomers were computed along the relevant twisted intermolecular charge-transfer formation coordinate and the mechanism of energy dissipation in these complexes was investigated (Section 3.4-3.5) (Ref. D3D). A brief summary of the main results of this chapter and conclusions are given in Section 3.6. Finally, in Section 3.7 a complementary benchmark study of the quality of ground-state potential energy surfaces of prototypical hydrogen-bonded systems (ammonia-water and formic acid-water dimers) obtained at the level of BSSE-corrected MP2 combined with moderate basis sets, has been conducted. The quality of potential energy surfaces was tested with respect to basis-set size, level of electron correlation and anharmonicity effects and the applied methodology to identify the IR spectrum of hydrated DMABME complexes (Section 3.3) has been found to be sufficient to uniquely assign the IR spectra.Moderne computergestützte molekulare quantenchemische Studien, wie die vorliegende, verwenden in der Regel ein breites Spektrum theoretischer Methoden. Die letzteren sind oft sehr komplex und man sollte generell nicht erwarten, dass ein praxisorientierter Wissenschaftler im Bereich der physikalischen Chemie – ein potentieller Leser dieser Arbeit – mit all diesen Methoden vertraut sein muß. Um das Lesen dieser Dissertation in einem solchen Fall zu vereinfachen, und sie autark zu machen, ist sie mit einem Überblick über die verwendeten theoretischen Methodologien versehen (Kapitel 1). In diesem Überblick werden quantenchemische Grundbegriffe erläutert, auf die in der gesamten Arbeit immer wieder verwiesen wird, die theoretischen Grundlagen verwendeter Methoden dargelegt und deren Eigenschaften und Beschränkungen skizziert. Im Abschnitt 1.1 werden allgemeine Ansätze zur Lösung der molekularen Schrödingergleichung eingeführt, und in den Teilen 1.2 und 1.3 werden spezifische Ansätze zur Lösung der elektronischen Schrödingergleichung zur Bestimmung des elektronischen Grundzustands und angeregter Zustände präsentiert. Teil 1.4 ist der Beschreibung von Basissatzeffekten, die in Elektronenstrukturrechnungen im Allgemeinen auftreten, gewidmet. Dieser Abschnitt enthält eine Reihe von verschiedenen Einblicken und Konzepten, die im Rahmen dieser Arbeit vorgeschlagen werden, und welche den Experten in der Quantenchemie aufschlußreich sein können. Im Kapitel 2 wird das Phänomen der Katalyse von Aceton-Wasser Protonenaustausch durch selbstaggregierende Calix[4]hydrochinon (CHQ)-Nanotubes sowie durch amorphe Aggregate von CHQ, welche in NMR-Versuchen (Ref. X1X) beobachtet wurde, mit Hilfe von modernen quantenchemischen Methoden untersucht. Der erste Teil dieser Studie (Abschnitt 2.3-2.7) betrachtet den konzertierten Protonentransfer, unterstützt von mehreren ursprünglich neutralen OH-Gruppen innerhalb der wasserstoffgebundenen Netzwerke der CHQ-Aggregate. Der zweite Teil der Studie (Abschnitt 2.8-2.13) ist dem dem schrittweisen Protonentransfer mittels Bildung von ionischen Zwischenprodukten infolge der CHQ- Prädissoziation gewidmet. CHQ-anwendungsspezifische Schlußfolgerungen, sowie allgemeine Aspekte, die für das Hauptthema dieser Dissertation relevant sind (i.e. Einfluß spezifischer Mikrosolvatation auf die betrachteten Protonentransferprozesse), werden im Abschnitt 2.1.4 zusammengefasst. Das Phänomen der dualen Fluoreszenz, das in Komplexen von 4-N,N-Dimethylaminobenzoesäuremethylester (DMABME) und zwei Wassermolekülen in der Gasphase beobachtet wurde, wird im Kapitel 3 untersucht. Im Abschnitt 3.3 werden zunächst berechnete Grundzustandsinfrarotspektren verschiedener DMABME*2H2O Isomere präsentiert, die zum einen die Identifikation aller in der Gasphase vorliegenden Isomeren erlauben, und zum anderen vor allem die Charakterisierung des für das Auftreten der dualen Fluoreszenz verantwortlichen Isomer ermöglichen. Um weiter den Reaktionsmechanismus der dualen Fluoreszenz zu verstehen, wurden die Potentialenergieflächen der relevanten Isomere im angeregten Zustand entlang der sogenannten TICT-Koordinate (TICT: engl. twisted intramolecular charge transfer) berechnet und der Mechanismus der Energierelaxation dieser Komplexe erforscht (Abschnitt 3.4-3.5) (Ref. X3X). Eine kurze Zusammenfassung der wichtigsten Ergebnisse dieses Kapitels und die wichtigsten Schlußfolgerungen finden sich in Abschnitt 3.6. Zum Abschluß wird im Abschnitt 3.7 eine Vergleichsstudie der Qualität der Potentialenergieflächen von prototypischen wasserstoffgebundenen Systemen im elektronischen Grundzustand (Ammoniak-Wasser und Ameisensäure-Wasser) zusammengefasst, in der die Qualität der Potentialenergieflächen hinsichtlich des verwendeten atomaren Basissatzes, der Behandlung der Elektronenkorrelation und der Anharmonizität der Potentialflächen getestet werden. Vor allem wurde festgestellt, dass die zum Studium der IR-Spektren der hydratisierten DMABME-Komplexe verwendete Methode hinreichend genau ist, um die einzelnen Isomere zu unterscheiden und die experimentellen Spektren eindeutig zuzuordnen (Abschnitt 3.3)

Intramolecular Charge Transfer in Solvated Organic Molecules: A Quantum Chemical Study

Photochemical reactions are ubiquitous in nature. Furthermore, they play an important role in organic and inorganic synthesis. In such reactions, light energy is used to induce chemical transformations. Two prime examples of such reactions are photosynthesis – the conversion of light energy to chemical energy, and the cis-trans isomerization reaction in the retina which enables vision. Upon absorption of light, the electrons in a molecule are promoted to higher energy levels; the molecule is elevated from its electronic ground state to an electronically excited state. Once in the excited state, different processes may take place such as the emission of light, interactions with the environment, or chemical reactions. One class of processes often involved in photochemical reactions and of particular importance are charge-transfer processes. A charge transfer can occur intermolecularly i.e., between different molecules or within a molecular complex from a ligand to the center and vice versa. These intermolecular charge- transfer processes, for example, occur in organometal complexes and play a crucial role in numerous catalytic reactions. In intramolecular charge-transfer processes, charge is transferred from one part of a molecule, the donor moiety, to a different part of the molecule, the acceptor moiety. Among those, so-called twisted intramolecular charge-transfer (TICT) processes and the corresponding TICT states are of particular interest. TICT molecules are characterized by the fact that they undergo a rotation about a single bond connecting the donor and acceptor moiety in the excited state. This twisting is accompanied by a charge transfer from the donor to the acceptor, resulting in a highly polar CT state exhibiting a mutually perpendicular orientation of donor and acceptor subsystems. Due to this process, TICT compounds show a very interesting unusual fluorescence behavior, named dual fluorescence, in medium polar and polar solvents. Dual fluorescence means that there are two fluorescence bands observable: one corresponding to the transition from the TICT state and the other from the transition from a planar excited state. In this dissertation, photochemical reactions of different small- and medium-sized organic molecules have been investigated with a particular focus on intramolecular charge-transfer. The well-known and experimentally well-investigated small donor-acceptor TICT compound N- pyrrolobenzonitrile (PBN) and its thiophene derivative 5-(1H-pyrrole-1-yl)thiophenecarbonitrile (TCN) are studied, among others. For this purpose, different quantum chemical electronic structure methods are employed. Especially when used along with spectroscopy, these ab initio methods are powerful tools to study the molecular structure and photochemical reactions. Quantum chemical methods can provide reliable excitation energies, excited state properties, and absorption strengths and allow for the computation of deactivation pathways. In this work, it is shown that the chosen quantum chemical methodology is well suited to describe the photochemical behavior of the considered organic compounds and new insights into the photochemistry of these systems are provided

Intramolecular charge transfer induced by solvent interaction

The effect of solvation on the appearance of a red shifted Twisted Intramolecular Charged Transfer emission is reviewed. Comparison between condensed phase and small cluster red fluorescence is discussed in the particular case of methyl 4-N,N-dimethylaminobenzoate (DMABME). Using a supersonic expansion DMABME-(H2O)n clusters are studied by monitoring at the same time mass spectra and dispersed fluorescence spectra as well as by lifetime measurements. For DMABME-(H2O)n clusters, a clear red shifted fluorescence is observed readily when one or two water molecules are clustered to the molecule. The results of this work are compared with those obtained on the model compound 4- Ν,Ν-dimethylaminobenzonitrile (DMABN), where no TICT emission is observed in small DMABN-(Solvent)n clusters

Specific Microsolvation Triggers Dissociation-Mediated Anomalous Red-Shifted Fluorescence in the Gas Phase

Ion-depletion IR spectroscopy has revealed that at least two water molecules are required in complexes with 4-(dimethylamino)benzoic acid methyl ester (DMABME) for anomalous red-shifted fluorescence to occur in the gas phase. Through the use of high-level quantum-chemical calculations, two experimentally observed isoenergetic isomers are assigned to complexes in which a water dimer is hydrogen-bonded either to the carbonyl oxygen of the ester function or to the amino nitrogen. Surprisingly, computed IR spectra reveal that the N-bonded isomer is responsible for the observed red-shifted fluorescence. For an explanation, the mechanism of twisted intramolecular charge-transfer (TICT) formation and energy dissipation is investigated in detail. In general, for red-shifted fluorescence to occur, the N-bonded complexes must be able to dissipate energy, which in the gas phase can only happen nonradiatively via fragmentation. Arguments are given that only the N-bonded isomer photodissociates rapidly enough into free DMABME and a water dimer as a result of the immediate repulsion between the amino nitrogen and the water dimer in the TICT state. The O-bonded isomer, on the other hand, stays intact because the hydrogen bond is strengthened by additional electrostatic attraction in the ICT state. Furthermore, an experiment to further corroborate that mechanism is suggested