Giant Dipole Moment in a Triad System. Mechanisms of Anisotropic Photoresponse in the Transient dc Conductivity of Dipolar Solutes Sergei N. Smirnov* and Charles L. Braun Department of Chemistry, Dartmouth College, HanoVer, New Hampshire 03755 Scott R. Greenfield, Walter A. Svec, and Michael R. Wasielewski † Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: January 18, 1996; In Final Form: May 15, 1996 X A transient dc conductivity method was used to observe formation of a giant dipole moment for the triad molecule MA-ANI-NI (methoxyaniline-aminonaphthalimide-dimethylphenyl-naphthalenediimide-octyl) in toluene. The independence of the dipole moment on excitation wavelength indicates high efficiency of intramolecular energy or (and) electron transfer. The effect of light polarization on the DC conductivity signal caused by a photoinduced increase in solute dipole moment is considered in detail. It is shown that the time variation of the signal includes information about structural anomalies in the angular distribution function of molecular dipoles and depends on light polarization even for zero ground state dipole moment. Nonzero ground state dipole moment and (or) electric field dependence of the charge transfer rate constants give an additional source for an anisotropic photoresponse signal. Analysis of the photoresponse and its anisotropy for the triad gives ground (µ g ), first (µ 1 ), and second (µ 2 ) excited state dipole moments as follows: µ g ) 12 ( 5 D, µ 1 ) 35 ( 10, and µ 2 ) 87 ( 6 D. The lifetime of the giant dipole state is τ ) 290 ( 10 ns, and the molecule’s rotational time is τ r ) 1.6 ( 0.15 ns. Introduction The main goal in mimicking photosynthesis is to use light to separate charges for chemical energy storage, i.e., to separate charges by distances as large as possible for times as long as possible. Other important features are a broad spectrum of photoactivity and low energy loss between excitation and charge separation. In this paper we study a very promising system: methoxy- aniline-aminonaphthalimide-dimethylphenyl-naphthalenediim- ide-octyl (which we abbreviate as MA-ANI-NI, see Figure 1) in toluene solution. According to transient absorption measure- ments, 1 this triad molecule with ANI as a chromophore, MA as an electron donor, and NI as the acceptor undergoes multistep electron transfer. Excitation at wavelengths longer than 415 nm (primarily of the aminonaphthalimide (ANI) moiety) leads in 8 ps to an electron transfer from the methoxyaniline (MA) group to ANI with >99% yield. In the second step, an electron transfer from ANI - to the naphthalene-diimide (NI) group competes with relaxation to the ground state (5.3 ns) and takes some 0.43 ns in toluene at room temperature. The resultant long range charge separated state is formed with a quantum yield of 0.92 and lasts for about 300 ns. The transient dc conductivity method has been successful in studies of photoinduced dipole moments in a variety of systems including intramolecular 2-5 and intermolecular 6 charge transfer. In this paper we use the transient dc conductivity technique to study charge separation in MA-ANI-NI. For this complicated system, we also had to improve theoretical understanding of the technique. In the previously developed theory of this method, 3 we assumed that the angular distribution function of the dipoles deviated only slightly from equilibrium, hence only first order Legendre polynomials were included. If new dipoles are created with substantial anisotropy, then the general case of an aniso- tropic angular distribution should be considered. Here we consider the general solution for small electric fields in detail and the experimentally relevant case of dipoles formed by linearly polarized light. The anisotropy of the initial distribution is caused by three factors: the light polarization, a nonzero dipole moment of the ground state molecules, and any electric field dependence of the electron transfer. The latter is distinguishable only if the electron transfer occurs within the time scale of molecular rotation. All three factors affect the polarized-light dependence of the photoresponse. The difference between the signals for two perpendicular light polarizations (called the depolarization signal) reflects this anisotropy. As we will see, the depolar- † Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113. X Abstract published in AdVance ACS Abstracts, July 1, 1996. Figure 1. MA-ANI-NI molecule used in this study and diagram of three step charge separation with characteristic times measured by transient absorption in toluene 1 . 12329 J. Phys. Chem. 1996, 100, 12329-12336 S0022-3654(96)00171-2 CCC: $12.00 © 1996 American Chemical Society