Measurement of Electromagnetic Radiation Emitted during Rapid Intramolecular Electron Transfer

We report direct observation of intramolecular electron transfer (ET) by measuring the electromagnetic (EM) waveform broadcast by the charge-transfer process itself. Intermolecular and intramolecular ET each have a rich history and play a central role in chemistry, biology, and physics. For example, photosynthesis, artificial photosynthesis, redox chemistry, photography, xerography, and other processes all rely on ET. By directly monitoring the emitted waveform, both forward and back charge-transfer rates may be measured on a time scale of 0.1-10 ps without relying on the photoluminescent properties of the donor or acceptor species. It is known that an accelerating charge generates EM radiation.1 This is the underlying basis for light sources as specialized as free-electron lasers and synchrotrons, or as mundane as radio transmitters. This is, in fact, the basis for generation of terahertz (THz) pulses by photoconductive antennas.2 THz pulses are nearly half-cycle pulses of EM radiation that are a few hundred femtoseconds in duration. A photoconductive antenna is created by applying an electric field across the face of a semiconductor wafer such as GaAs. Upon photoexcitation, electrons are promoted into the conduction band and accelerated by the bias field, thereby generating EM radiation. It has been shown that pulses generated from biased GaAs in this fashion are related to the dynamics of the charge carriers.3 It follows that molecules undergoing coherent ET will radiate an EM pulse that is directly related to the dynamics of the charge-transfer event. The method described here involves orienting a solution of polar molecules with an electric field and photoinitiating ET with a short (∼100 fs) laser pulse. Each molecule then acts as a tiny antenna, broadcasting its dynamics by emitting an EM transient as charge moves between donor (D) and acceptor (A) sites. Dynamical information is contained in the resulting time dependent electric field, which is measured with subpicosecond temporal resolution by free-space electrooptic sampling.4 The EM transients are measured in the far-field with no focusing elements to avoid pulse reshaping. The experimental setup is described elsewhere,5 and only minor modifications have been made. To probe ET in this manner, the only fundamental requirements are that the molecule can be oriented (most easily if it possesses a ground state dipole moment), and that upon photoexcitation there is a change in dipole moment along the molecular axis of orientation. One expects the signal strength to increase linearly with the degree of orientation, number of photoexcited molecules, and change in dipole moment. It should also vary as the cosine of the angle between the molecular orientation axis and excited state dipole. The sign of the radiated field depends on whether the change in dipole is positive or negative, and thus the direction of the ET can be obtained directly from the measurement. We report EM generation from betaine-30 and compare it to generation from biased GaAs (Figure 1). Betaine-30, [2,6diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate] also known as Reichardt’s dye, was purchased from Aldrich, dissolved in CHCl3, and used without further purification. Its dipole moment decreases and changes direction upon photoexcitation (15 D in the ground state to -6 D in the excited state).6 Dye solutions with concentrations of roughly 4 mM were placed in a 1 mm path length quartz cuvette. High voltage pulses (∼0.1 ms duration, 500 V to 3 kV) were applied across 0.75 mm thick metal electrodes inside the cuvette with 3 to 4 mm separation to generate fields from 1.25 to 10 kV/cm. Typically, a laser pulse energy of 100 μJ/pulse was used. Figure 1 compares the THz pulses generated from biased GaAs and betaine-30. In each case the applied field is in the same direction. The pulse generated from betaine-30 has a polarity opposite that of the EM pulse generated from biased GaAs, which is a direct result of the fact that the ET in this molecule is in the opposite direction of the applied field. Furthermore, the shapes of the resultant pulses are different, reflecting their respective dynamics. In contrast, upon photoexcitation at 400 nm of the dye 4-dimethylamino-4′-nitrostilbene (DMANS) dissolved in toluene, the dipole moment increases. We have verified that the signal generated from photoexciting DMANS in an applied field has the same general characteristics as the signal generated from betaine-30, but with opposite polarity. The time-dependent properties of betaine-30 have been well studied7 and can be used to characterize our new technique. In the ground state, S0, an electron has already been transferred form the donor (D) pyridinium group to the acceptor (A) phenolate group. Upon photoexcitation at 800 nm, ET occurs instantaneously from Af D+. The first excited state, S1 (D A), has a smaller dipole moment than the ground state and points in the opposite direction (Scheme 1). Relaxation from S1 f S0 proceeds via internal conversion with rate constant k2 and results in movement of the electron from D back to A. This back ET rate is highly solvent-dependent, with rate constants7 ranging from 0.2 to 1 ps-1. * To whom correspondence should be addresed. E-mail: charles. schmuttenmaer@yale.edu. (1) Jackson, J. D. Classical Electrodynamics; John Wiley & Sons: New York, 1962. (2) (a) Smith, P. R.; Auston, D. H.; Nuss, M. C. IEEE J. Quantum Electron. 1988, 24, 255-260. (b) Fattinger, C.; Grischkowsky, D. Appl. Phys. Lett. 1989, 54, 490-492. (3) Leitenstorfer, A.; Hunsche, S.; Shah, J.; Nuss, M. C.; Knox, W. H. Physica B 1999, 272, 348-352. (4) Lu, Z. G.; Campbell, P.; Zhang, X. C. Appl. Phys. Lett. 1997, 71, 593595. (5) Beard, M. C.; Turner, G. M.; Schmuttenmaer, C. A. Phys. ReV. B, accepted for December 15, 2000, issue. (6) Liptay W. Angew. Chem., Int. Ed. Engl. 1969, 8, 177-188. (7) (a) Levinger, N. E.; Johnson, A. E.; Walker, G. C.; Barbara, P. F. Chem. Phys. Lett. 1992, 196, 159-165. (b) Walker, G. C.; Akesson, E.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. J. Phys. Chem. 1992, 96, 3728-3736. (c) Akesson, E.; Johnson, A. E.; Levinger, N. E.; Walker, G. C.; Dubruil, T. P.; Barbara, P. F. J. Chem. Phys. 1992, 96, 7859-7862. Figure 1. (a) EM generation from biased GaAs (upper curve) and betaine-30 (× 100) dissolved in CHCl3 (lower curve). For both betaine30 and GaAs the applied field was in the same direction; however, the emitted fields have opposite polarities. (b) Experimental setup. Polarization of laser pulse (thick arrow) is parallel to applied field, as is polarization of generated pulse. 11541 J. Am. Chem. Soc. 2000, 122, 11541-11542