Photoinduced Electron Transfer in Solution: Exciplex and Radical Ion Pair Formation Free Enthalpies and their Solvent Dependence
About: This article is published in Zeitschrift für Physikalische Chemie.The article was published on 1982-01-01. It has received 940 citations till now. The article focuses on the topics: Photoinduced electron transfer & Radical ion.
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TL;DR: Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors.
Abstract: For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757
1,721 citations
TL;DR: Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068 6.1.1 Photochemical Extrusion of Small Molecules 1067 6.2.2 Photochemical Rearrangings 1061 4.4.3.
Abstract: 2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068
1,046 citations
TL;DR: In this paper, the feasibility of photoinduced electron transfer reaction for the nucleobase-specific quenching of fluorescent dyes is investigated by the calculation of the standard free energy changes with the Rehm−Weller equation.
Abstract: Intermolecular static and dynamic fluorescence quenching constants of eight coumarin derivatives by nucleobase derivatives have been determined in aqueous media. One common sequence of the quenching efficiency has been found for the nucleobases. The feasibility of a photoinduced electron transfer reaction for the nucleobase-specific quenching of fluorescent dyes is investigated by the calculation of the standard free energy changes with the Rehm−Weller equation. A complete set of one-electron redox potential data for the nucleobases are determined electrochemically in aprotic solvents for the first time, which are compared with values obtained by various other methods. Depending on the redox properties of the fluorescent dyes, the sequences of the quenching efficiencies can be rationalized by the orders of electrochemical oxidation potentials (vs NHE) of nucleosides (dG (+1.47 V) < dA < dC ≈ dT < U (≥ +2.39 V)) and reduction potentials (dG (< −2.76 V) < dA < dC < dT < U (−2.07 V)). The correlation between...
941 citations
TL;DR: The energetic-dynamic relations, in conjunction with the energetic data for d*/d- and for B/B+, determine the realization of the two distinct mechanisms in different hole donor systems, establishing the conditions for "chemistry at a distance" after charge transport in DNA.
Abstract: We explore charge migration in DNA, advancing two distinct mechanisms of charge separation in a donor (d)–bridge ({Bj})–acceptor (a) system, where {Bj} = B1,B2, … , BN are the N-specific adjacent bases of B-DNA: (i) two-center unistep superexchange induced charge transfer, d*{Bj}a → d∓{Bj}a±, and (ii) multistep charge transport involves charge injection from d* (or d+) to {Bj}, charge hopping within {Bj}, and charge trapping by a. For off-resonance coupling, mechanism i prevails with the charge separation rate and yield exhibiting an exponential dependence ∝ exp(−βR) on the d-a distance (R). Resonance coupling results in mechanism ii with the charge separation lifetime τ ∝ Nη and yield Y ≃ (1 + δ Nη)−1 exhibiting a weak (algebraic) N and distance dependence. The power parameter η is determined by charge hopping random walk. Energetic control of the charge migration mechanism is exerted by the energetics of the ion pair state d∓B1±B2 … BNa relative to the electronically excited donor doorway state d*B1B2 … BNa. The realization of charge separation via superexchange or hopping is determined by the base sequence within the bridge. Our energetic–dynamic relations, in conjunction with the energetic data for d*/d− and for B/B+, determine the realization of the two distinct mechanisms in different hole donor systems, establishing the conditions for “chemistry at a distance” after charge transport in DNA. The energetic control of the charge migration mechanisms attained by the sequence specificity of the bridge is universal for large molecular-scale systems, for proteins, and for DNA.
657 citations
TL;DR: The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer and a fullerene derivative were studied and it is shown that in these films nanocrystalline PCBM clusters are formed at high PCBM content.
Abstract: The electro-optical properties of thin films of electron donor−acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor−acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V OC) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (er = 4.0) compared to that of the polymer (er = 3.4), stabili...
566 citations
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256 citations
TL;DR: In this article, the magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N, Ndimethylaniline (Py/DMA), pyrene 3,5,dimethoxy•N,Ndimethynylinox•N and pyrene DMDMA, and the perdeuterated system Py•d10/dMA•d11.
Abstract: The magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N,N‐dimethylaniline (Py/DMA), pyrene/3,5‐dimethoxy‐N,N‐dimethylaniline (Py/DMDMA), and the perdeuterated system Py‐d10/DMA‐d11. The magnetic field dependence characterized through its B1/2 value is found to be dependent on the sum of the hyperfine coupling constants in the radical pair in agreement with previous theoretical predictions. A drastic reduction of the B1/2 value is observed with the perdeuterated system. By means of measurements of the radical ion and triplet absorption signals with nanosecond time resolution, the influence of the solvent on the geminate singlet and triplet recombination yields is investigated. Complementary measurements of exciplex lifetimes and quantum yields are carried out in a series of solvents with different polarities in order to determine the rate constants of fluorescence e...
126 citations
TL;DR: The structureless fluorescence spectra of more than one hundred heteroexcimers (unsymmetrical molecular complexes which are stable in the excited state) were measured in hexane or other inert solvents.
Abstract: The structureless fluorescence spectra of more than one hundred heteroexcimers (unsymmetrical molecular complexes which are stable in the excited state) were measured in hexane or other inert solvents. The energy (AvJ?TM*) corresponding to the fluorescence maximum is found to decrease as the ionization potential [or the oxidation potential E(D/D+)] of the donor decreases and the electron affinity [or the reduction potential E(A~/A)] of the acceptor increases. It is shown that the simple relation hvf** = e (DID*) E (A-1A) + const and the deviations therefrom lead to a semi-empirical classification of heteroexcimers which can be rationalized in terms of differences in the nature of the hetero-excimer bond. Zusammenfassung Die unstrukturierten Fluoreszenzspektren von mehr als hundert HeteroExcimeren (unsymmetrische, im angeregten Zustand stabile Molekülkomplexe) wurden in Hexan oder anderen inerten Lösungsmitteln gemessen. Die dem Maximum der Hetero-Excimeren-Fluoreszenz entsprechende Energie (AvTMax) ist um so kleiner je geringer das Ionisierungspotential [oder das Oxydationspotential E(D/D+)] des Donors und je größer die Elektronenaffinität [oder das Reduktionspotential E(A-/A)] des Akzeptors ist. Es wird gezeigt, daß, ausgehend von der einfachen Beziehung Ävmax = e(D/D+) E(A-/A) + const, eine semiempirische Klassifizierung der Hetero-Excimeren möglich ist, die sich durch die unterschiedliche Art des Bindungszustands bei Hetero-Excimeren begründen läßt. 184 Dieter Rehm und Albert Weller Einleitung Wie vor einigen Jahren gezeigt werden konnte1-2, tritt in den Fluoreszenzspektren aromatischer Kohlenwasserstoffe (^4) in Lösungsmitteln geringer Polarität bei Zugabe von Dialkylanilin (D) eine neue, bei längeren Wellen liegende unstrukturierte Fluoreszenzbande auf (vgl. Abb. 2), deren Intensität mit wachsender Konzentration an Dialkylanilin zunimmt, während gleichzeitig die der ursprünglich vorhandenen Fluoreszenz abnimmt. Diese neue Fluoreszenzbande, deren Auftreten von keiner entsprechenden Veränderung im Absorptionsspektrum begleitet ist, wurde einem angeregten Molekülkomplex (II) HA-D+) (II) zugeschrieben, der nur im angeregten Zustand stabil ist (vgl. Abb. 1) und dessen polare Natur aufgrund der mit zunehmender Polarität des Lösungsmittels erfolgenden Rotverschiebung des Fluoreszenzspektrums nachgewiesen werden konnte3-4. Ähnliche Beobachtungen von Mataga et al.5-6 und von Chandross und Ferguson7 sowie weitere. Fluoreszenzuntersuchungen mit zahlreichen substituierten und unsubstituierten aromatischen Verbindungen8-11 haben gezeigt, daß die Fluoreszenzbande des Molekülkomplexes bei um so längeren Wellen liegt, je geringer das Ionisierungs1 H. Leonhardt und A. Weller, Ber. Bunsenges. physik. Chem. 67 (1963) 791. 2 H. Knibbe, D. Rehm und A. Welleb, Vortrag auf dem 8. Europ. Mol. Spektr. Kongress in Kopenhagen, August 1965. 3 H. Beens, H. Knibbe und A. Welleb, J. chem. Physics 47 (1967) 1183. 4 . Welleb, Nobel-Symposium 5, Fast Reactions and Primary Processes in Chemical Kinetics (S. Claesson Ed.), Almqvist & Wiksell, Stockholm 1967, p. 413; Pure appi. Chem. 16 (1968) 115. 6 N. Mataga, K. Ezumi und T. Okada, Molecular Physics 10 (1966) 201, 203. — . Mataga, T. Okada und . Yamamoto, Bull. chem. Soc. Japan 39 (1966) 2562. ß . Mataga, T. Okada und H Oohari, Bull. chem. Soc. Japan 39 (1966) 2563. 7 E. A. Chandross und J. Ferguson, J. chem. Physics 47 (1967) 2557. 8 H. Knibbe, D. Rehm und A. Weller, Z. physik. Chem. Neue Folge 56 (1967) 95. — H. Knibbe, Dissertation, Freie Universität, Amsterdam 1969. 9 H. Knibbe und A. Weller, Z. physik. Chem. Neue Folge 56 (1967) 99. H. Beens und A. Weller, Chem. physics Letters 2 (1968) 140. 10 H Beens und A. Weller, Acta physica polon. 34 (1968) 593. 11 H. Beens, Dissertation, Freie Universität, Amsterdam 1970. Bindungszustand und Fluoreszenzspektren von Hetero-Excimeren 185 potential (IP) des Donors und je größer die Elektronenaffinität (EA) des Akzeptors ist. Für die Festlegung von Akzeptor und Donor im Komplex ist dabei deren „molekulare Elektronegativität\", also die Summe von Ionisierungspotential und Elektronenaffinität in der Form (IP + EA)a > (IP + EA)D (1) maßgebend. Die im Grundzustand instabilen und daher auch als HeteroExcimere bezeichneten angeregten Molekülkomplexe unterscheiden sich demnach nur graduell von denjenigen, die durch direkte Anregung der im Grundzustand stabilen Elektron—Donor—Akzeptor-(EDA)Komplexe12 erhalten werden und deren Bindungszustand sowohl im angeregten wie im Grundzustand durch die (verschieden gewichtete) Kopplung (I) der beiden als ,, -bond\"und als Charge-Transfer-(CI7T)Zustand bezeichneten Konfigurationen HAD) -~ i(A-D+) (I) zu beschreiben ist13. Andererseits entspricht die Bildungsweise der Hetero-Excimeren derjenigen der Excimeren14, deren Bildung zuerst von Förster und Kasper15 als Ursache für den von Ihnen an Pyrenlösungen beobachteten Konzentrationsumschlag erkannt worden ist. Dazu kommt, daß auch zur Beschreibung des Bindungszustands der Excimeren CT-Zustände (M~M+) mit herangezogen werden müssen. Ähnlich wie bei Excimeren ist auch bei den gemischten Excimeren, die zwischen aromatischen Verbindungen sehr ähnlicher Konzentration gebildet werden16, der Bindungszustand durch die Kopplung (III) des CT-Zustands mit lokal angeregten Zuständen gekennzeichnet HAD) -hHA-D+) -hHAD). (III) 12 G. Briegleb, Elektron—Donator—Akzeptor-Komplexe, Springer-Verlag, Berlin, Göttingen, Heidelberg 1961. 13 R. S. Mulliken, J. Amer. ehem. Soc. 72 (1950) 600; J. ehem. Physics 56 (1952) 801. 14 . ßß , D. Rehm und A. Welleb, Ber. Bunsenges. physik. Chem. 73 (1969) 839 sowie unveröffentlichte Messungen. 15 Th. Föbsteb und K. Kaspeb, Z. physik. Chem. Neue Folge 1 (1954) 275. 18 T. M. Vembeb und A. S. Cherkasov, Optics and Spectroscopy 6 (1959) 148. — J. B. Brass und L. G. Chbistophobou, Nature [London] 196 (1962) 33. . K. Selinger, Nature [London] 203 (1964) 1062. I. E. Obyknovennaya und A. S. Chebkasov, Optics and Spectroscopy 22 (1967) 22; Bull. Acad. Sei. USSR, Phys. Ser. 32 (1968) 1315. 186 DrETER Rehm und Albert Weller Auf diese zwischen angeregten EDA-Komplexen und Hetero-Excimeren einerseits, sowie zwischen Heteroexcimeren und gemischten Excimeren andererseits bestehenden Zusammenhänge ist von Beens und Weller10 bereits hingewiesen worden. Wie die an einer Reihe von Donor—Akzeptor-Systemen in inerten Lösungsmitteln durchgeführte Bestimmung der Dissoziationsenthalpie * der Heteroexcimeren AH (vgl. Abb. 1) gezeigt hat14, kann deren
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