4222
J . Phys. Chem. 1986, 90, 4222-4224
Photophysics and Photochemistry of Lepldopterene: Ground-State Processes James Ferguson, Raymond J. Robbins,+ and Gerard J. Wilson* Research School of Chemistry, The Australian National University, Canberra, A.C.T. 2601, Australia (Received: May 27, 1986)
Further studies of the photodissociation of lepidopterene in fluid and glassy media have identified two cycloreversion products (conformations), one observed at room temperature and somewhat below, the other observed at low temperature in a viscous glass. Molecular mechanics calculations have been used to calculate structures of the two conformations. Schematic and calculated potential energy hypersurfaces for the ground and excited states are given.
Introduction Becker and co-workers’” carried out a systematic study of the spectroscopic and photochemical properties of the lepidopterene chromophore and, more recently, we reported rates and activation barriers for the various ground- and excited-state processes of lepidopterene and its dimethyl d e r i ~ a t i v e . ~Contrary to the suggestion of Becker and Sandros5 we observed that the decay of the exciplex does not smoothly regenerate lepid~pterene.~ It appeared that the decay of the exciplex leads to an “eclipsed” product which can be trapped in viscous media. We have therefore carried out a further study of the ground-state products, using a molecular force field program to analyze the results. This new work establishes that the decay of the excliplex leads to a stable eclipsed conformation which can either return to lepidopterene by passing over a low barrier (about 18 kJ mol-I) or escape, over a larger barrier (about 40 kJmol-I), to form another stable conformation, the cycloreversion product of Becker et aL4 Results Descriptions of chemicals and equipment have been given in our previous report.7 Becker et aL4 noted that prolonged irradation of lepidopterene led to a gradual buildup of a room temperature cycloreversion product (A60), which was characterized by its anthracene-like absorption and emission spectra. The presence of this product can be conveniently monitored by measuring excitation spectra in the region 380-450 nm. Low-temperature irradiation of the lepidopterene solution (below about 110 K) led to an increase in the rate of formation of the cycloreversion product and, furthermore, its removal was found to be very fast. It became clear that this product (A180) was different from that obtained at room temperature. A typical buildup and decay profile, measured at 100 K, is shown in Figure 1. The buildup and decay profiles change rapidly over a small temperature range (15-20O) and measurements were made with the sample at 90, 100, and 110 K. At these temperatures only one cycloreversion product (A180) was present and its buildup behavior can be expressed simply as I ( t ) = C / h ( l - exp(-At)) The first-order rates of decay of A180 to lepidopterene were then found to have the following values: 9.5 X (90 K), 7.0 X lo4 (100 K), 9.2 X s-I (1 10 K). An Arrhenius plot of these values provided an activation barrier of 18.7 kJ mol-’. Above about 120 K A180 decays very rapidly to form lepidopterene, while A60 becomes trapped at much higher temperatures, at or below about 270 K. The fluorescence spectrum of the cycloreversion product, which is rapidly produced below 110 K, is very similar to the fluorescence spectrum of the cycloreversion product which is present in the solution, prior to photolysis. There is, however, a significant difference in their intensity distributions, as shown in Figure 2 . Present address: Kodak Research Laboratories, Coburg, Victoria 3058. *Present address: School of Physics, University of New South Wales, Kensington, N.S.W. 2033.
0022-3654/86/2090-4222%01.50/0
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It seemed likely that the two cycloreversion products (A60 and A180) are different conformations with very different reactivities, so molecular force field calculations were used to explore the physical properties of the ground-state surface.
Molecular Mechanics Calculations Lepidopterene photpdissociation involves breaking the Clo-Clcu and the C15-C9, bonds (Scheme I), with the formation of an excited-state molecule containing an anthracene and a 9methylene-9,lodihydroanthracene chromophore, linked by a single methylene bridge, in a topologically favorably arrangement for exciplex formation. The next stage is the adiabatic or diabatic dissociation of this exciplex which involves, initially, an increase in the anthracene-ethylene separation and a corresponding flattening of the hydroanthracene chromophore (Le., an increase in the “butterfly angle” a). Two coordinates are important to describe these changes, one (dl) a rotation about the C9-CI5, bond, the other (d2) a rotation about the Clo,-C15,bond. The various stable conformations arising from these rotations are conveniently defined in terms of the dihedral angle comprising atoms ClW-CI5Tc9.The conformation having maximum overlap is then Becker, H-D.; Sandros, K. Chem. Phys. Lett. 1978, 55, 498. Becker, H-D.; Sandros, K.; Arvidsson, J. Org. Chem. 1979, 44, 1336. Becker, H-D.; Andersson, K.; Sandros, K. J . Org. Chem. 1980, 45, 4549. (4) Becker, H-D.; Sandros, K.; Andersson, K. Chem. Phys. Lett. 1981, 77,
246. (5) Becker, H-D.; Hall, S. R.; Skelton, B. W.; White, A. H. A u s f .J . Chem. 1984, 37, 1313. (6) Becker, H-D. Pure Appld. Chem. 1982, 54, 1589. (7) Ferguson, J.; Robbins, R. J.; Wilson, G.J. J . Phys. Chem. 1984, 88,
5193.
0 1986 American Chemical Society
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23000 27 00 WAVENUMBER (ern" I Figure 2. Corrected fluorescence spectra of the cycloreversion products products measured prior to photolysis (- - -) and after 12 min of irradiation (-) in methylcyclohexane/ispentane at 100 K. The spectra have be=ennormalized at 21 000 cm-I so as to show more clearly their different intensity distributions. designated A180, while that in which the two halves of the molecule are fully rotated away from each other is denoted by AO. Initially, a rotation about dl was chosen to represent the reaction coordinate and the force field driving calculations involved a rigid rotation around 4' in 15' increments, accompanied by minimization at each intermediate geometry. However, for values of 4, < 120' the growing importance of dz considerably increased the minimization times. Fully minimized structures were finally obtained after carrying out several minimization cycles on each intermediate geometry. Since the structure A1 20 represents an energy maximum, it was decided to map the region from 120' < dl < 180' by driving from the minimized A180 structure, while the region from 0' < dl < 120' was mapped by driving from the minimized A60 structure. A plot of the total energy (Ern)together with the nonbonded (Enb),dihedral angle (E4),and bond angle (E,) contributions as a function of the dihedral angle 4' is given in Figure 3. If one looks first at the gross features of Etot,it is clear that d1plays a major role in determining the position of the maxima and minima on the ground-state surface, whereas the relative energies of the two stable conformations, A180 and A60, are determined by their various nonbonded interactions.
Discussion Absorption of light by lepidopterene leads to the formation of the intramolecular exciplex, which then decays either radiatively or nonradiatively to a region of the ground-state surface which, in terms of the coordinate dl, is located a t a local minimum, characterized by the structure A180 (Figure 3). At room temperature the majority of the molecules easily surmount the low
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Figure 4. Proposed ground- and excited-state potential energy hypersurfaces for lepidopterene. The complex reaction coordinate contains variation of the "butterfly angle" a,the mean anthracene-ethylenedistance d, and the dihedral angle $Jl. The vertical arrows denote spectroscopic transitions while the horizontal arrows show possible adiabatic interconversion pathways (thermally activated). The wavy arrow denotes photodissociation from vibrationally excited states of lepidopterene. barrier (about 18 kJ mol-') to form lepidopterene. A small fraction manages to escape by rotating around dl to give the A60 structure, thereby giving the gradual buildup of the cycloreversion product observed by Becker et a1.: although they assumed a geometry corresponding to AO. The barrier height for the removal of A60 to form lepidopterene was confirmed to be 70 J mol-', as reported by Becker et al.? while the barrier in going from A180 to A60 was estimated to be about 40 kJ mol-'. On cooling the solution, the initial population of A180 becomes trapped because the effective barrier to rotation around dl has been increased by the increased viscosity of the solvent medium. We are now able to propose ground- and excited-state potential energy hypersurfaces for lepidopterene which embody our earlier
J . Phys. Chem. 1986, 90, 4224-4233
4224
excited-state results and the present ground-state results. These are given in Figure 4. The surfaces in Figure 4 help to document the complex photochemical and photophysical properties of lepidopterene but they do not provide the insight into the very large spectral shift which characterizes the anthracene-ethylene exciplex, the region covered by the butterfly angle, a,and the mean interchromophore distance, d, being only schematic. This large shift is a puzzle, particularly as the absorption and fluorescence spectra of the A180 conformation show such good spectral overlap. Also, we have carried
out studies of the absorption and fluorescence of [2.2]- and [3.3](9,10)anthracenoparacyclophanes and we found little or no Stokes shift for these materials.8 Perhaps the lack of two constraining bridges in the lepidoterene exciplex allows a much closer approach of the ethylene chromophore than is possible for the cyclophanes.
(8) Ferguson, J.; Puza, M.; Robbins, R. J.; Wilson, G. J., submitted for publication to J . Phys. Chem.
FEATURE ARTICLE Molecular Aspects of Ionic Hydration Reactions G. W. Robinson,*+P. J. Thistlethwaite, Department of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
and J. Lee Picosecond and Quantum Radiation Laboratory, Texas Tech University, Lubbock, Texas 79409 (Received: February 18, 1986)
Photokinetic experiments on ultrafast time scales have suggested that the integrity of the quasi-tetrahedral oxygen structure of liquid water sets the stage for both electron and proton hydration in aqueous media. Acid dissociation, and the attendant proton hydration, produces the much discussed H904+ ion as a direct kinetic product. The parallel electron process gives rise to a similarly constituted ion, H804-. This may be the hydration product of a distorted water anion H20-, such as an OH--H30 semi-ionic pair, and bears on the solvated-electronproblem in radiation chemistry. In Eigen's rate measurements of acid/base neutralization, the reactions were diffusion controlled. With newly developed mixed-solventmethods, the reverse dissociation process can span the gap between a 'diffusion-controlled" regimetranslation of water molecules to satisfy the local concentration requirement-and a "reaction-controlled" (*hydration-controlled")regimerotational diffusion of water molecules to satisfy the local structural requirement. Hydration rates of these 'elementary ions" in the hydration-controlled regime parallel dielectric and spin-lattice relaxation, shear viscosity, and other physical phenomena in pure liquid water where large amplitude rotations of water molecules play a dominant role. The necessary hydrogen reorientational motions thus limit the rates of these electron and proton hydration processes to STD-', where T~ is the Debye relaxation time. The role of the longitudinal relaxation time T~ (((713) in ion hydration is also discussed. Comparisons between photon-initiated acids, such as the excited states of 1- and 2-naphthol, and normal weak acids are made. These proton precursors are further compared with electron precursors. Free energy diagrams are introduced to help understand these correlations, with emphasis on the entropic contribution, which often dominates enthalpy terms. Equilibrium populations of states in the transition region quantitatively explain the rate phenomena. New absolute rate expressions for acid dissociation/recombination processes, which incorporate 7D-I and thermodynamic data, directly follow. A hydrogen/deuterium isotope rate factor of approximately 2.511 arises from purely entropic effects because of the stiffly structured nature of the hydrated proton or electron.
1. Introduction
The existence of "electrically conducting particles" in aqueous solutions of acids, bases, and salts has held a fascination for chemists since the early 1800s.'~* Modern concepts of spontaneous ionic dissociation were launched into their current form 100 years ago with the theory of A r r h e n i ~ s . ~Yet, the molecular nature of ion hydration reactions, particularly the structural role played by the surrounding solvent and the relevant scales of time, is still a subject of considerable speculation. The electron and the proton are ubiquitous chemical entities. The electron in water has engendered considerable research activity because of its importance in oxidation-reduction reactions4 and in radiation ~hemistry,~' while the proton in water is a major entity in analytical,* i n ~ r g a n i co, ~r g a n i ~ , ~and , ' ~electrochemistry."-'3 Permanent address: Texas Tech University.
These elementary ions also play a central role in biological energy transport.14 Current reasoning attributes specific s t r u c t ~ r e s , ' ~ J ~ - ~ ~ (1) Faraday, M. Experimental Researches in Electricity; 1839, Vol. I ; 1844, 1849, Vol. 2; 1855, Vol. 3. (2) For a good historical review, see: Partington, J. R. A History of Chemistry; Macmillan: London, 1964; Vol. 4. (3) Arrhenius, S. Z . Phys. Chem. 1887, 1, 630-648 (1887); In Nobel Lecutres in Chemistry, 1901-1921; Elsevier: Amsterdam, 1966; pp 43-61. (4) Taube., H. In Bioinorganic Chemistry ZI; Raymond, K. N., Ed.; American Chemical Society: Washington, DC, 1977, Ado. Chem. Ser. No. 162, pp 127-144. (5) Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley-Interscience: New York, 1970. (6) Papers presented at Colloque Weyl IV, Michigan State University, East Lansing,MI, June 30-July 3,1975; J. Phys. Chem. 1975,79,2789-3079. Proceedings of the International Conference on Electrons in Fluids, Banff, Canada Sept. 5-11, 1976; Can. J . Chem. 1977,55, 1795-2277. (7) Freeman, G. R. Annu. Rev. Phys. Chem. 1983, 34, 463.
0022-3654/86/2090-4224$01 SO10 0 1986 American Chemical Society
