This paper describes measurements of the sensitized charge transfer efficiency of h e r s compared to monomers in a rigid monolayer structure of cyanine dye molecules on the surface of p-chloranil single crystals. Relations for monomer/dimer ratios are derived from absorption spectra and action spectra of the photocurrent.Comparison yields a minor but comparable injection probability for dimers with respect to monomers. The results are discussed in terms of energy relations, separation of reaction sites and steric arrangements and are compared with those for dyes in solution, the sensitizing efficiency of which is governed by their adsorption kinetics rather than by their electrochemical photoreactivity at the electrode. From competing quenching measurements at an adjacent metal electrode a rate constant for the sensitized charge transfer, kc^ 10'O s-l, is estimated.Analysis of photochemical reactivity involves the study of reaction mechanisms and structure-reactivity relationships. Determination of discrete steps in a given photoreaction results in the identification of intermediates. One of the competing reactions of electronically excited molecules is charge transfer and when they are adsorbed at phase boundaries of non energy-quenching electrodes, such as organic semiconductors, this step can be investigated by electrochemical methods.2In such cases two parameters are usually responsible for a high reaction rate (1) the energetic relationships on both sides of the phase boundary have to allow a charge transfer, i.e. donor and acceptor terms have to be at equal levels, including vibrationally excited states 2 . as treated by theories of mechanisms of charge transfer;(2) in accordance with the high yields of stereoselective products observed in thermal-and photo-reactions at the surface of organic or inorganic crystals steric relationships of adsorbed molecules with respect to the surface as well as to each other are of importance4Both parameters are usually altered by the mode of adsorption, and whether dimerization or aggregation can O C C U~,~ in a way different from that observed under equilibrium conditions in homogeneous solution, which causes a change in reactivity.The present work allows comparison of the sensitizing efficiency of dimers as compared to monomers by changing the steric conditions introduced by the experimental arrangement. Because of the difficulty of describing the kinetics of adsorption and of dimer formation at a phase boundary * only limited conclusions about surface coverage can be drawn from adsorption measurements in solution. A promising technique is to impose a known structure of dye molecules on to the surface of a molecular crystal. This is possible by deposition of monolayers of cyanine dye onto * There are only a few cases that can be described by adsorption isotherms.6
Extrinsic photocurrents in anthrncene and p-chloranil crystals resulting from either electron photoemission from a metal contact or sensitized electron injection from excited adsorbed betracene molecules are investigated. At field strengths, where space charge effects are unimport.ant, i.e. where currents usually are assumed to be saturated, their field and temperature behaviour can be quantitatively described assuming that both generation processes initially yield hot electrons boundto the positive countercharge by Coulombic attraction. Direct escape, determined by scattering in the Coulombic potential well, is a temperature independent process giving rise to an exponentially field dependent photocurrent component. The scattering mean free path in anthracene is found to be
The first part of the paper deals with the non-radiative decay modes of n x* triplet excitons a t the interface between a p-chloranil crystal and an aluminum contact studied by means of extrinsic photocurrents. Utilizing the technique of inserting fatty acid monolayers between crystal and metal, which allows variation of the interaction distance, it is shown (i) that the dominant exciton decay process is a fast charge transfer reaction and (ii) that its rate constant decreases exponentially with increasing reaction distance. I n the second part of the paper a model is presented to treat exciton dissociation processes a t a metal surface in general. It is based on the concept of configuration interaction between the bound molecular exciton state and a continuum of ionized states.Der erste Teil der Arbeit beschaftigt sich mit Photostrom-Messungen zur Untersuchung der strahlungslosen Zerfallsmoglichkeiten von n n*-Triplett-Exzitonen an der Grenzflache zwischen einem p-Chloranil-Kristall und Aluminium. Der Wechselwirkungsabstand wird mit Hilfe zwischengezogener monomolekularer Fettsaureschichten variiert. Es zeigt sich erstens, da13 der Exzitonenzerfall im wesentlichen durch eine schnelle Charge-transferReaktion bestimmt wird und zweitens, da13 ihre Geschwindigkeitskonntante exponentiell mit wachsendem Reaktionsabstand abflllt. I m zweiten Teil der Arbeit wird ein Modell zur Behandlung von Exzitonen-Dissoziationsprozessen an Metalloberflachen im allgemeinen vorgelegt. Es griindet sich auf das Konzept der Konfigurationswechselwirkung zwischen dem gebundenen molekularen Exzitonenzustand und einem Kontinuum von ionisierten Zustanden.
The current-field dependence of contact-limited photocurrents resulting from dissociation of Frenkel excitons at the interface between molecular crystals and a metal or a fatty acid layer is investigated. I n the field range 3 X lo3 Vcm-l< F < lo5 Vcm-l both photocurrent and dark current follow a j = ,jo (1 + bF) relationship with no indication of a field-independent limiting current. An explanation is given in terms of the Onsager mode1 for thermally activated diffusional escape of a carrier inside the crystal from the attractive Coulombic potential of its countercharge. At higher fields the Frenkel-Poole effect becomes operative. It is concluded that exciton dissociation initially produces a thermalized carrier inside the crystal.Es wird die Feldstarkenabhangigkeit von kontaktbestimmten Photostroinen untersucht, die von der Dissoziation von Frenkel-Exzitonen an der Grenzflache zwischen Molekulkristalleii und einer Metall-bzw. Fettsaureschicht herriihren. Dabei ergibt sich, daB im Feldstarkenbereich 3 x lo3 Vcm-l < F < lo5 Vcm-l sowohl Photostrome als auch Dunkelstrome der Beziehung j = jo (1 + bF) genugen. Ein feldunabhangiger Grenzstrom tritt nicht auf.Dies laBt sich anhand des Onsager-Modells verstehen, das die diffusionsbedingte Trennung eines unter gegenseitiger Coulombscher Anziehung stehenden Ladungstragerpaares a n der Phasengrenzflache beschreibt. Bei hoheren Feldstarken macht sich der Frenkel-Poole Effekt bemerkbar. Aus den Ergebnissen folgt, daB Exzitonendissoziation primar zur Bildung eines thermalisierten Tragers im Kristall fiihrt.
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