J . Phys. Chem. 1989,93, 39-43
+
HCl**( 14n-4p) may have avoided crossings with He(23SI) HCl at r(H-Cl) larger than 1.4 A, but these channels are expected to be less significant because the nuclear wave function is small around the avoided crossing located at large r(H-Cl). On the other hand, the Rydberg states with 11 2 converging to the 14Zstate are possibly located near 19.8 eV (see Results, subsection B), and the corresponding He HCI** states are expected to have intersections with the He(23S1) + HCl state at r(H-Cl) C 1.4 A. However, these states are not expected to be significant exit channels in the case of the sideway attack, because the 32Z+state does not mix with the 14Z-state on account of different symmetry. In the case of collinear attack, the He + HC1**(14Z--nl) states have symmetries different from the symmetry of He(23S1) HC1, and therefore, they cannot be the exit channels. The potential curves of HCl+ intersect with each other in a complicated manner (see Figure 1). This indicates that the
+
+
39
corresponding potential curves of the Rydberg states, HCI** (22Z+-4s) and HCl**( 14n--4p), intersect with other Rydberg states with different ion-core states. Therefore, the electronic state of the ion core and the quantum numbers n and 1 are expected to change in the course of the dissociation of HCl**. These intersections are considered to govern the branching ratio of the products, H * C1 and H C1*, and the n distribution of H*.
+
+
Acknowledgment. We acknowledge the Computer Centers of the University of Tokyo and the Institute for Molecular Science, Okazaki National Research Institutes, for the use of HITAC M-680H and S-810 computers. The present study was supported by a Grant-in-Aid for Scientific Research by the Ministry of Education, Science, and Culture of Japan and the Joint Studies Program of the Institute for Molecular Science. Registry No. He, 7440-59-7;HCI, 7647-01-0.
Dual Fluorescence of p #’-Disubstituted 1,6-Diphenyl-1,3,5-hexatrienes: Evidence of a Twisted Intramolecular Charge Transfer State C. T. Lin,* H. W. Guan, R. K. McCoy, and C. W. Spangler Department of Chemistry, Northern Illinois University, DeKalb, Illinois 601 15 (Received: November 23, 1987; In Final Form: April 21, 1988)
The photophysical properties for a series of p,p’-disubstituted 1,6-diphenyl-1,3,5-hexatrienes (referred as D,A-DPH) are investigated, where D and A are the electron-donating and -accepting groups of -OCH3, -N(CH3)2, and -NOz. In all solvents used, a dual fluorescence is observed for D,A-DPH containing the internal rotation groups of -N(CH3)2 and/or -NO,, suggesting that the a* fluorescence is originated from a twisted intramolecular charge transfer (TICT) state. A phosphorescence emission is also detected for the locally excited state associated with the b* fluorescence. The relative intensity of two fluorescences is sensitive to the solvent environments and concentration variances. It is found that the twisted structure of D,A-DPH exists in the ground-state surface. The intermolecular charge interaction is shown to be responsible for the stability of the TICT state.
I. Introduction A detailed study of the electronic spectra of linear diphenyl polyenes’ is essential for an understanding at the molecular level of many photophysical processes of this series of molecules and their related systems. The important processes include the cistrans photoisomerization in simple polyene solution^^.^ as well as in more complex visual pigments4 and the mechanism of charge transport in conducting A conjugated polymer with polaronic ferromagnetism may be possible when a pair of electron-donating (D) and/or -accepting (A) groups is “built-in” along the polymer backbone. The simplest unit of the proposed conducting polymers has a structure D-Ph-( CH=CH),-Ph-A, Le., a p,p’-disubstituted linear diphenyl polyene. The electron push-pull type of polyene has been suggested* as potential soliton switching devices. Recently, the lasing and laser gain properties for a giant dipolar molecule, (CH3)2N-Ph-(CH=CH)2Ph-NO,, were i l l ~ s t r a t e d . ~This molecular is also expected to have a high optical nonlinear susceptibilitylOJ1because of its large permanent dipole moment induced by the substituents, and it may possibly be used for nonlinear optical devices. The photophysical properties of several p,p’-disubstituted linear diphenyl polyenes (e.g., D and A are both -fluoro, -chloro, -methyl, -isopropyl, or -methoxy) have been reported,12 where D and A possess no restricted internal rotations. The investigations were
* Author to whom correspondence
should be addressed.
centered on the identification of the low-lying ‘Ag*(Sl)and ‘B,*(S2)states and the possible intensity borrowing of the forbidden SI state from the allowed S2state. In this paper, we are interested in the possible molecular conformation changes such as the E-Z isomerizations and the twisted intramolecular charge transfer (TICT) rotamer formations. The information on the geometrical changes can provide us the electronic delocalization and ion-pair formation in linear polyenes. The twisted conformation could result in a giant permanent dipole moment for D-Ph-(CH=CH),-Ph-A. To elucidate this, we synthesized (1) Hudson, B. S.; Kohler, B. E. J . Chem. Phys. 1973,59, 4984. (2) Said, M.; Maynau, D.; Malrieu, J. P. J . Am. Chem. SOC.1984, 106, 580. (3) Langkilde, F. W.; Jensen, N. H.; Wilbrandt, R. J. Phys. Chem. 1987, 91, 1040. (4) Birge, R. R.; Pierce, B. M. J. Chem. Phys. 1979, 70, 165. ( 5 ) Fukutome, H.; Takahashi, A,; Ozaki, M. A. Chem. Phys. Lett. 1987, 133, 34.
( 6 ) Gagnon, D. R.; Capistran, J. D.; Karasz, F. E.; Lenz, R. W.; Antoun, S . Polymer 1987, 28, 567. (7) Obrzut, J.; Karasz, F. E. J . Chem. Phys. 1987, 87, 2349. (8) Carter, F. L. Physica D (Amsterdam) 1984, IOD,175. (9) Terauchi, M.; Kobayashi, T. Chem. Phys. Lett. 1987, 137, 319. (10) Kothari, N. C.; Kobayashi, T. IEEE J. Quantum Electron. 1984, QE-20, 418. (1 1) Kobayashi, T.; Kothari, N. C.; Uchiki, H. Phys. Reu. 1984, A29, 2727. (12) Alford, P. C.; Palmer, T. F. Chem. Phys. Lett. 1986, 127, 19.
0022-3654/89/2093-0039$01.50/00 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
Lin et al.
a set of p,p'-disubstituted 1,6-diphenyl-l,3,5-hexatrienes(Le., D-Ph-(CH=CH),-Ph-A, referred as D,A-DPH), where D and A involve groups of free internal rotation (e.g., -OCH3) and groups of restricted internal rotations (e.g., -N(CH3)2 and -NO2). The electronic spectra and photophysical properties of D,A-DPH in polar and nonpolar solvents are investigated. A dual fluorescence is observed for D,A-DPH containing the restricted internal rotation groups. Analogous to the 4-(dimethylamino)benzonitrile, which is ~ e l l - k n o w n ' ~ -to~ *show a dual luminescence in many solvents, we assign the observed a* fluorescence to a highly polar TICT excited state of D,A-DPH. 11. Experimental Section
Five compounds of D,A-DPH were synthesized by using modifications of known compound I, D = +CH3 and A = -OCH3, is a light yellow crystal; compound 11, D = -N(CH3)2 and A = -N(CH3)2 is a yellow crystal; compound 111, D = -NO2 and A = -NO2, is a red-brown crystal; compound IV, D = -OCH3 and A = -NO2 is an orange crystal; and compound V, D = -N(CH3)2 and A = -NO2, is a dark-bronze crystal. All samples were recrystallized several times from chloroform or N,N-dimethylformamide. All solvents used for preparing the solutions were of UV spectroscopic grade (Aldrich Chemical Co.) and used with no additional purification and drying procedures. Absorption and excitation spectra were recorded on a Varian 2290 UV-vis spectrophotometer and a Model 1902 Spex Fluorolog. The emission spectra and emission lifetimes were measured by use of 1 X lo-" to 1 X lo4 M solutions sealed in quartz ampules under a helium atmosphere. The sample is studied at 298 and 77 K. The 3130-A band of an Oriel 200-W Hg(Xe) lamp is isolated by using a 10-cm light path of an aqueous solution of NiS04.6H20 (200 g/L) with an UG11 Schott glass filter and is used to excite the sample. For selective excitation, a 1/4-m monochromator is placed in front of the light source and used to select the excitation wavelengths. The emission spectra are detected by an EM1 9789QB photomultiplier tube in conjunction with a '/2-m Jarrell-Ash spectrometer. The phosphorescencedecay is recorded with a Fabri-Teke series 1062 signal averager. An electromechanical shutter that closes in
