J. Phys. Chem. 1995, 99, 7875-7877
7875
Structure of the Twisted-Intramolecular-Charge-TransferExcited Singlet and Triplet States of 4-(Dimethy1amino)benzonitrile As Studied by Nanosecond Time-Resolved Infrared Spectroscopy Mamoru Hashimoto and Hiro-o Hamawchi" Kanagawa Academy of Science and Technology, KSP East 301, 3-2-1 Sakado, Kawasaki 213, Japan Received: February 27, 1995@
Nanosecond time-resolved infrared spectroscopy has been used to study the structure and dynamics of the excited electronic states of 4-(dimethy1amino)benzonitrile (DMABN) in polar and nonpolar solvents. In a polar solvent butanol, two distinct transient species were observed. The first species with a few nanosecond lifetime has been identified as the twisted-intramolecular-charge-transfer (TICT) singlet excited state and the second species having an oxygen-sensitive lifetime as the TICT triplet state of DMABN. Both the singlet and triplet TICT states show large downshifts (> 120 cm-I) of the C=N stretch frequency, indicating that the charge transfer has a significant effect on the structure of the cyano group. In a nonpolar solvent hexane, only one transient species was detected. This species has been assigned to the non-CT triplet excited state of DMABN from the observed oxygen quenching effect.
Introduction Electronically excited molecules are often highly polarizable owing to the accessibility to the closely-lying polar excited states. The high polarizability leads to a large variety of solventinduced photoprocesses. The dual fluorescence from a flexibly bonded donor-acceptor molecule, 4-(dimethy1amino)benzonitrile (DMABN), is a representative photophysical process of this kind.' DMABN exhibits two distinct fluorescence bands in polar solvents but only one in nonpolar solvents.2 The shorter-wavelength band, which is common to polar and nonpolar solvents, shows a mirror-image relationship with the absorption band and is therefore assigned to the "normal" excited singlet state produced by the photoexcitation. The longerwavelength band appears only in polar solvents and exhibits an unusually large Stokes shift. This fluorescence band is thought to arise from the twisted-intramolecular-charge-transfer (TICT) singlet state, in which a full charge transfer occurs from the dimethylamino group to the benzonitrile moiety and in which a 90" torsion of the dimethylamino group occurs with respect to the phenyl ring.3 The TICT state of DMABN serves as an excellent model for the solvent-induced intramolecular charge transfer and a subsequent molecular structural change occurring in the excited state. Though the concept of TICT is now widely accepted,' information on the structure of TICT state is scant. For the TICT state of DMABN, the only known structural parameter is the dipole The reported dipole moments (13.722 D) are consistent with a substantial intramolecular charge transfer but are considerably smaller than the value 25 D, which is expected for a full charge transfer from the dimethylamino group to the cyano group. This fact has been interpreted in terms of the delocalization of the negative charge in the phenyl ring with the cyano group being not much affected by the charge transfer. Time-resolved vibrational spectroscopies, Raman and infrared, are now well established as powerful tools for studying the structure of short-lived electronically excited molecules.8-'0 Time-resolved Raman spectroscopy, however, is not suitable to the strongly fluorescent TICT state of DMABN. Time'3'
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Abstract published in Advance ACS Abstracts, May 1, 1995.
resolved infrared spectroscopy should therefore be the choice. We have recently developed a nanosecond dispersive timeresolved infrared system having 50 ns time resolution and M I A sensitivity for the full 4000-700 cm-' wavenumber region of the infrared.'' In this Letter, we report the first timeresolved infrared spectra of the TICT singlet as well as triplet excited states of DMABN and discuss their structural implications.
Experimental Section A time-resolved infrared system has been constructed which employs the same ac coupling detection scheme as that reported previously.' 1 , 1 2 The system consists of a modified dispersive infrared spectrometer (Jasco DS701G; 40 cm focal length, 10 x 10 cm gratings), a photovoltaic MCT detector (Kolmer Technologies, Inc., KVl03-1-A-l-SMA), a preamplifier (Kolmer KA020-A1, 100 Hz-20 MHz bandwidth), a main amplifier (NFElectronic Instruments 5305; dc -10 MHz bandwidth), a digital oscilloscope (Tektronix TDS520A), and a host computer (NEC PC9801). The time response (fwhm) of the system to a 15 ns laser pulse was 70 ns. The fixed slit width of 5 mm gave the wavenumber resolution of 16 cm-' at 2300 cm-I, 11 cm-' at 2000 cm-I, and 35.5 cm-' at 1900 cm-I. The hydrogen Raman-shifted third harmonic (309 nm, 0.80.4 &/pulse) of a pulsed Q-switch Nd:YAG laser (Spectron SL801; 15 ns pulse width, 50 Hz repetition rate) was used for the photoexcitation. The sample of DMABN was purchased from Aldrich and was purified by recrystallization from hexane. The solvents were all of special research grade and were used as received. The sample solution was circulated by a roller pump through a CaF2 flowing cell (100/500 pm path length for butanolhexane solution) with continuous bubbling with nitrogen or oxygen in the reservoir. The sample solution was bubbled for at least 30 min before the measurement.
Results and Discussion Photoexcitation of DMABN fist generates the lowest excited singlet state (hereafter called the S1 state) which, in polar solvents, decays into the TICT singlet state with a time constant of a few tens of picosecond^.'^ The S I state is too short-lived
0022-3654/95/2099-7875$09.00/0 0 1995 American Chemical Society
7876 J. Phys. Chem., Vol. 99, No. 20, 1995
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Wavenumber /cm-' 2300
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Wavenumber /cm" Figure 1. Time-resolved infrared absorption spectra of DMABN in butanol (5 x mol dm-3) with nitrogen bubbling: (a) -100-0 ns, (b) 0-100 ns, (c) 100-200 ns, (d) 200-300 ns, (e) 300-400 ns, (0 500-600 ns, (g) 1.0-1.1 ps, and (h) 1.5-1.6~safter photoexcitation. -
Figure 3. Time-resolved infrared absorption spectra of DMABN in mol dm-3): (a) 0-100 ns after photoexcitation, butanol ( 5 x nitrogen bubbling; (b) 100-200 ns, nitrogen bubbling; (c) 0-100 ns, oxygen bubbling: (d) 100-200 ns, oxygen bubbling. 1 " " 1 " " 1 " " 1 " 1
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Wavenumber /cm-' -5x103 I , , , , , , , . . . . , , . , . , I 0.0 0.5 1 .o 1.5
Time ips Figure 2. Temporal behaviors of the three transient bands of DMABN in butanol: (a) 2040, (b) 2096, and (c) 2216 cm-I. to be detected in the present experiment. However, we expect to observe the TICT singlet state, which has a few nanosecond lifetime, and any longer-lived species that are formed subsequently. Figure 1 shows the time-resolved infrared difference spectra mol dm-3) in the region of of DMABN in butanol (5 x 2300-1940 cm-I. Two positive peaks at 2096 and 2040 cm-' and a negative peak at 2216 cm-' are observed in the 0-100 ns spectrum [Figure 1b). The positive peaks correspond to the infrared absorptions of photolytically generated species, and the negative band corresponds to the depletion of the ground state of DMABN. The spectra in Figure 1 therefore indicate that two transient species are generated by the photoexcitation of DMABN in butanol. The temporal behaviors of the three transient bands are given in Figure 2. The time profile for the 2096 cm-I band is nearly the same as the apparatus response function, while that for the 2040 cm-' band shows a decay in the submicrosecond time scale. It is known that the lifetime of the TICT singlet state of DMABN is 2.2 ns in butan01.I~ In
Figure 4. Time-resolved infrared absorption spectra of DMABN in mol dm-3): (a) 100-200 ns after photoexcitation, hexane ( 5 x nitrogen bubbling; (b) 100-200 ns, oxygen bubbling.
the present experiment, such a short lifetime is convoluted by the apparatus response and is expected to give a time profile that is practically the same as that of the apparatus response function. Therefore, we assign the 2096 cm-' band to the TICT singlet state of DMABN. The 2040 cm-' band shows a profound oxygen effect (Figure 3). The intensity of this band decreases to about one-tenth with oxygen bubbling, while the 2096 cm-' band is affected much less. On the basis of this finding, we assign the 2040 cm-' band to a triplet transient species. The same time-resolved infrared experiment was carried out for a nonpolar solvent, hexane (Figure 4). Only one positive band, instead of two, is observed around 2000 cm-' with an unusually large half-width. The same negative band as that in butanol is observed at 2216 cm-'. The decay curves (not shown) of the positive and negative bands agree with each other, indicating that the transient species relaxes directly back to the ground state. The 2000 cm-I band disappears with oxygen bubbling (Figure 4b). Therefore, we assign the 2000 cm-I band to another triplet species that is distinct from the one showing the 2040 cm-I band in butanol. The newly observed infrared bands give new insight into the structure of the excited electronic states of DMABN. The
J. Phys. Chem., Vol. 99, No. 20, 1995 7877
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a large change of the dipole moment is associated with the CEN stretch vibration. Of the two distinct triplet species, the one showing the 2040 cm-' band in butanol is assigned to the TICT triplet state, because this species is observed only in a polar solvent. A further downshift of 56 cm-I (2096 2040 cm-I) indicates a further decrease of the CFN bond order on going from the TICT singlet to the triplet state. This decrease of the CN ' bond order can be interpreted in terms of more localization of the transferred electron in the CEN antibonding orbital in the TICT triplet state. The other triplet species found in hexane is assignable to the non-CT excited triplet state with a planar structure. It is not clear whether this non-CT triplet species exists in a polar solvent, because the intensity of the 2000 cm-' band is much smaller than that of the 2040 cm-' band. A previous nanosecond timeresolved visible absorption study gave similar spectra for polar and nonpolar solvents.I6 A preliminary experiment in this laboratory indicated the same result. It seems that both the TICT and non-CT triplet species are formed in polar solvents and that only the non-CT triplet species exhibits strong transient visible absorption, resulting in similar transient absorption spectra both for polar and nonpolar solvents. There is a possibility that the TICT and non-CT triplet states are in an equilibrium in polar solvents. Finally, the parent-daughter relationship between the TICT singlet and triplet states need be mentioned. If the TICT triplet state forms from the TICT singlet, the rise curve of the 2040 cm-' band should show a delay with a time constant of 2.2 ns compared with the rise of the 2096 cm-I band. Owing to the slow response of the spectrometer, we were not able to detect a clear difference in the observed rise curves of the two bands (Figure 2). However, there is a clear indication that the TICT triplet state is formed from the TICT singlet state. Preliminary mixed solvent (butanolhexadecane) experiments showed that both the 2096 and 2040 cm-' band intensities increase with increasing the butanol concentration, keeping the intensity ratio constant. This means that the amount of the TICT triplet state formed is proportional to the amount of the TICT singlet. In any case, time-resolved infrared spectroscopy provides an unique means to study the structure as well as the dynamics of the TICT singlet and triplet excited states of DMABN.
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Time Ins Figure 5. Observed and deconvoluted temporal profiles of the 2096 cm-' band of the TICT singlet state. The dotted line is the observed temporal profile, the solid line the best fitted curve with the convolution of a proper detector response and a single-exponential decay with 2.2 ns lifetime, and the broken line the deconvoluted temporal profile with a 1/20 scale.
observed C E N stretch frequency (2096 cm-I) of the TICT singlet state is very close to the frequency of the same vibration of the benzonitrile anion radical (2093 cm-I).l4 The C s N bonding of the TICT singlet state of DMABN is thus proved to resemble very much that of the benzonitrile anion radical. This fact is considered as strong evidence for a full charge transfer from the dimethylamino group to the benzonitrile moiety. It is also considered that the anionic benzonitrile moiety is electronically isolated from the cationic dimethylamino part. These thoughts are fully consistent with the TICT proposition and the previously reported transient absorption re~u1t.l~ The C=N stretch frequency shows a large downshift of 120 cm-I (2216 2096 cm-') on going from the ground state to the TICT singlet state. This means that the electronic structure of the CEN bonding is greatly changed with the charge transfer and that the CEN bond order is decreased very much on going from the ground state to the TICT singlet state. It is most likely that the electron transferred into the benzonitrile moiety is delocalized into an antibonding orbital of the C=N group. The CEN part of DMABN is much affected by the charge transfer in the TICT singlet state, contrary to what has been supposed so far.'" The intensity of the 2096 cm-' band also gives information on the electronic structure of the CEN group in the TICT singlet state. The observed intensity of the 2096 cm-' band is less than one-fourth of that of the 2216 cm-' band (see Figure 1). This apparent intensity, however, is affected by the slow response of the spectrometer. If the response time of the spectrometer (-50 ns in the present case) is much longer than the lifetime of the detected transient species (2.2 ns for the TICT singlet state), the apparent transient infrared signal is much less than the intrinsic value. The correction for this intensity distortion can be done by deconvoluting the observed temporal profile (Figure 2) by a proper response function. After this correction is made, the intensity of the 2096 cm-l band becomes 8.5 times as large as that of the 2216 cm-l band. The absorption change of the ground state depletion is 0.0022, while that of the TICT singlet state is 0.019 in the corrected temporal profile (Figure 5). A large infrared intensity of the CGN stretch band means a large dipole moment change with the change in the CGN bond distance. The transferred electron in the TICT singlet state is thought to delocalize within the whole benzonitrile moiety and moves back and forth between the phenyl ring and the CEN part with the change of the C S N bond distance. Then, with a positive charge fixed at the dimethylamino group,
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References and Notes (1) Grabowski, Z. R. Pure Appl. Chem. 1993, 65, 1751. (2) Lippert, E.; Liider, W.; Boos, H. In Advances in Molecular Spectroscopy; Mangini, A., Ed.; Pergamon Press: Oxford, 1962; p 443. (3) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; Bauman, W. Nouv. J . Chim. 1979, 3, 443. (4) Suppan, P. J . Lumin. 1985, 33, 29. ( 5 ) Baumann, W.; Bischof, H.; Frohling, J.-C.; Brittinger, C.; Retting, W.; Rotkiewicz, K. J. Photochem. Photobiol. A: Chem. 1992, 64, 49. (6) Rettig, W.; Braun, D.; Suppan, P.; Vauthey, E.; Rotkiewicz, K.; Luboradzki, R.; Suwinska, K. J. Phys. Chem. 1993, 97, 13500. (7) Jonker, S. A.; Warman, J. M. Chem. Phys. Lett. 1991, 195, 36. ( 8 ) Time-Resolved Vibrational Spectroscopy V; Takahashi, H., Ed.; Springer-Verlag: Berlin, 1991. (9) Time-Resolved Vibrational Spectroscopy VI; Lau, A,, Siebert, F., Wemcke, W., Eds.; Springer-Verlag: Berlin, 1993. (10) Hamaguchi, H. In Vibrational Spectra and Structure; During, J. R., Ed.; Elsevier: Amsterdam, 1987; Val. 16, p 227. (11) Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Appl. Spectrosc. 1994, 48, 684. (12) Iwata, K.; Hamaguchi, H. Appl. Spectrosc. 1990, 44, 1431. (13) Wang, Y . ;Eisenthal, K. B. J. Chem. Phys. 1982, 77, 6076. (14) Juchnovski, I.; Tsvetanov, C.; Panayotov, I. Monatsh. Chem. 1969, 100, 1980. (15) Okada, T.; Mataga, N.; Baumann, W. J. Phys. Chem. 1986, 91, 760. (16) Kohler, G.; Grabner, G.; Rotkiewicz, K. Chem. Phys. 1993, 273, 275. JF'950549Y
