2526
J. Phys. Chem. B 1997, 101, 2526-2532
Photoinduced Electron Transfer in Donor-Acceptor Aryl Dyads Based on N,N,N′,N′-Tetramethyl-p-phenylenediamine as the Donor Renae D. Fossum and Marye Anne Fox* Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712
Ann M. Gelormini and Anthony J. Pearson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106 ReceiVed: August 30, 1996; In Final Form: NoVember 28, 1996X
The solution-phase photophysics of several electron transfer donor-donor-acceptor assemblies (2 and 3) incorporating N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) derivatized with piperazyl, piperidyl, morpholinyl, and prolinyl groups in the 1- and 4-positions as donors and N,N-dimethyl-4-nitroaniline, anthraquinone, 3,5-dinitrobenzene, and 2,4-dinitrobenzene as acceptors are described. In measurements of the model compounds 1 incorporating only the donor moieties, flash photolysis generated the radical cation and triplet species of TMPD as evidenced by the superposition of their transient absorption spectra. Lifetime measurements reveal, as well, a delayed fluorescence derived from triplet-triplet annihilation. In 2 and 3, electron transfer from the lowest excited singlet state of TMPD to the various acceptors was established by the following: (1) steady-state emission measurements where the fluorescence of TMPD was drastically quenched by the acceptor, (2) the transient absorption spectra of the radical cation and radical anion of the donor and acceptor, and (3) a single-exponential decay profile in 2 and 3, superseding the biexponential decay observed in the model donor.
Introduction
SCHEME 1
Photoinduced electron and energy transfer processes have been intensively studied in the past several years to understand excited-state interactions between covalently attached electronrich donors and electron-poor acceptors.1 These interactions have received much attention because they elucidate the mechanisms of intramolecular electronic couplings both as a model for photosynthesis2,3 and for potential use in molecular electronic devices.4,5 Covalently bound donor-acceptor systems offer a distinct advantage over their bimolecular counterparts in that intramolecular electron and energy transfer may be studied over well-defined distances.6-10 We are interested in donor-acceptor complexes that use N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) derivatives 1-3 as electron donors (Scheme 1). TMPD was chosen as the donor because it is readily oxidized upon laser irradiation to generate a stable radical cation that might interact with an acceptor in close contact.11,12 The donor-acceptor derivatives discussed here have been prepared according to a new synthetic route that covalently links the modified TMPD moiety in 1 to various acceptors in 2 and 3: N,N-dimethyl-4-nitroaniline (DMNA), anthraquinone, 3,5-dinitrobenzene, and 2,4-dinitrobenzene.13 The TMPD is derivatized with piperazyl, piperidyl, morpholinyl, and prolinyl groups at the 1- and 4-positions of the phenyl ring to increase the molecule’s stability (Scheme 1), and these groups may in fact also act as secondary donors.9 These compounds should prove to be interesting with respect to their electronic properties for use in molecular devices.14 In order to assess the donor-acceptor interactions, steady-state and time-resolved transient absorption and emission spectra of 2 and 3 are compared with those of the model donor 1. Steady-State Behavior. The steady-state emission spectra of 1, 2, and 3 in 2-methyltetrahydrofuran (MTHF) at room temperature obtained upon excitation at 290 nm are shown in X
Abstract published in AdVance ACS Abstracts, March 1, 1997.
S1089-5647(96)02663-6 CCC: $14.00
Figure 1, parts A-C. The spectra shown in Figure 1, part A display a superposition of the fluorescence expected of a piperazine (piperidine, morpholine, or proline) ring (λmax at 320-330 nm)6 and of TMPD (λmax at about 410 nm)15 for compounds 1a-c. By comparison, the chemically bound donor-acceptor analogs 2 and 3, shown in Figure 1, parts B and C, show nearly complete quenching of the TMPD fluores© 1997 American Chemical Society
Photoinduced ET in Donor-Acceptor Aryl Dyads
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2527
Figure 1. Steady-state fluorescence for (A) 1a-c, (B) 2a-d, (C) 3a-c, monitored at room temperature in degassed MTHF (10-5 M). Steady-state fluorescence for (D) 1a-c, 2a, and 2b and (E) 2c, 2d, and 3a-c, monitored in degassed MTHF glasses at 77 K (10-5 M). Excitation occurred at 290 nm.
cence at room temperature, and only fluorescence from the respective heterocyclic ring is evident. Spectra measured at low temperature are similar to those obtained at room temperature, except that emission from TMPD (λmax at 420-450 nm) is visible in 2 and 3. The fluorescence at 320-330 nm results from energy transfer from the TMPD heterocycle since neither piperazine, piperidine, morpholine, or proline is directly excited at 290 nm. The maxima between 400 and 420 nm (Figure 1, parts D and E) correspond to those expected of an isolated TMPD moiety.
At 77 K, the TMPD-centered maxima are shifted to about 450 nm and the observed emission corresponds primarily to TMPD phosphorescence. The maxima at 350 nm in Figure 1, part D, for 2b is assigned to emission from the DMNA moiety. The low-temperature emission at 450 nm overlaps with the room temperature fluorescence and thus may be a superposition of low-temperature fluorescence and phosphorescence. When a chopper fitted with light baffles is used to remove the contributions of short lifetime species from the spectra, this blue band at 320 nm disappears, but the red TMPD band does not change,
2528 J. Phys. Chem. B, Vol. 101, No. 14, 1997
Fossum et al.
TABLE 1: Fluorescence Quantum Yields,a OF, at Room Temperature and Total Emission Quantum Yields,b OE, at 77 K. Emission Lifetimesc of 1a-c and 3b at Room Temperatured and 77 Ke compd
φF(rt)f
φF(77 K)f
φF(rt)g
ΦEh(77 K)g
1a
0.24
0.11
0.44
1b
0.21
0.12
1c
0.21
0.08
2a 2b 2c 2d 3a 3b
0.27 0.25 0.08 0.69 0.21 0.28
0.18 0.16 0.14 0.15 0.09 0.09
0.70 0.73 0.71 0.73 0.58 0.67
3c
0.03
0.12
0.70
τ400 (273 K) (ns)
τ490 (273 K) (ns)
0.68
5.60 ( 0.01
5.60 ( 0.02
0.51
0.87
4.74 ( 0.01
0.48
0.85
5.10 ( 0.01
9.52 ( 0.27 (50%) 15.20 ( 0.33 (50%)
4.62 ( 0.06 (92%) 14.4 ( 0.8 (8%) 4.66 ( 0.02
τ370 (77 K) (ns) 2.83 ( 0.02 (65%) 12.80 ( 0.10 (35%) 2.96 ( 0.04 (51%) 10.10 ( 0.10 (49%) 3.28 ( 0.04 (54%) 10.60 ( 0.05 (46%)
1.82 ( 0.06 (34%) 15.50 ( 0.04 (66%)
a
Fluorescence quantum yields are measured relative to that of naphthalene in degassed MTHF at room temperature.37 Excitation at 290 nm, solutions in degassed MTHF (10-5). b Total emission quantum yields at 77 K determined relative to 9,10-diphenylanthracene.37 c Lifetimes determined by single-photon counting upon excitation at 345 nm. d Measured in degassed THF (10-5 M). e Measured in degassed MTHF glasses (10-5 M). f Emission monitored from 300 to 370 nm and assigned to the heterocyclic ring. g Emission monitored from 370 to 540 nm and assigned to the TMPD moiety. h φE ) φF(77 K) + φDF + φP.
suggesting that the observed emission does indeed have a delayed fluorescence component.15 The fluorescence quantum yields for the saturated heterocycle (piperazyl, piperidyl, morpholinyl, or prolinyl, λmax at 320 nm) and TMPD (λmax at 410 nm) components are listed in Table 1 (λex at 290 nm). The fluorescence intensity of the heterocyclic component does not vary appreciably in 1 at room temperature, although it is drastically quenched in 2c and 3c (φ ) 0.08 and 0.03) and it is substantially enhanced in 2d (φ ) 0.69). In the first two cases, direct excitation of the anthraquinone moiety at 290 nm competes with sensitization by TMPD. In 2c and 3c, the TMPD moiety is the active excited state, and electron transfer to the acceptor occurs so rapidly that competing sensitized fluorescence from the piperidyl or prolinyl ring is minimal. In 2d, where the heterocyclic fluorescence is increased, singlet-singlet energy transfer apparently takes place very rapidly from the TMPD moiety since the intersystem crossing yield in anthraquinone is 0.9.16 The total emission quantum yields at 77 K reveal that the fluorescence from the heterocyclic ring generally is quenched relative to that at room temperature. However, the donor-acceptor molecules 2 and 3 exhibited similar emission intensities as do their respective donor models 1, each at 77 K. Emission from TMPD Moiety in 1-3. The fluorescence from the TMPD portion of 2 and 3 (λmax at 400 nm) is completely quenched at room temperature (Table 1). Conversely, at 77 K, the TMPD-localized emission is more intense in 2 and 3 than in 1a-c. Comparing the donor-acceptors 2 and 3 with the corresponding parent compound 1, we found that the emission quantum yields are very similar except that 2a and 2b show some triplet-triplet energy transfer from the TMPD (ET ) 2.64 eV) to DMNA (ET ) 2.38 eV) as evidenced from the appearance of DMNA emission at 330 nm (Figure 1, part D).16 Emission Lifetimes. The fluorescence lifetimes for 1 and 3b in dilute degassed THF at room temperature and in degassed MTHF glasses at low temperature (77 K) obtained using singlephoton counting techniques are summarized in Table 1.17 At room temperature, the singlet lifetime τs for 1 was measured to be 4.75-5.60 ns. We observed that the lifetimes for the TMPD fluorescence increased in the order 1b < 1c < 1a according to the substituent trend: dimorpholinyl < dipiperazyl < dipiperidyl. The fluorescence lifetime for TMPD has been reported as 4.3 ns in nonpolar and 7.1 ns in polar solvents.16 Thus, the heterocycle does not quench the singlet lifetime appreciably.
In 1b at room temperature, there appears to be a minor second component at 490 nm with a much longer lifetime (14.4 ns). At low temperatures, this long component is present in all compounds 1a-c: 35% for 1a and approximately 50% for 1b and 1c. One plausible explanation for this second component is that two conformers are present that are quenched by different relaxation pathways as a result of restricted rotation in a frozen glass. It follows that the measured lifetimes at room temperature represent an average of decay processes of rapidly interconverting species in solution. We also considered the possibility of producing a twisted intramolecular charge transfer state.18,19 However, no charge transfer band could be observed in the steady-state spectrum in a range of nonpolar to polar solvents. The further possibility that this second component might derive from an aggregate was unlikely because samples of 1c measured at higher concentrations (10-3 M) in EtOH, CH3CN, 2-methylcyclohexane, and benzene all displayed single-exponential lifetimes on the order of 5 ns. A final hypothesis is that triplettriplet annihilation might occur producing an excited singlet state which consequently emits after a triplet-lifetime-enforced delay. Triplet-triplet annihilation for TMPD in a rigid glass at low temperatures has been reported previously using flash photolysis techniques;15 the emission reported here was induced in a singlephoton counting apparatus and the incident flux was very weak, making this two-photon process unlikely. We also found that, at low temperatures, the phosphorescence lifetimes of 1a-c could be roughly measured at 490 nm by averaging the light emitted over several seconds. The phosphorescence lifetimes for compounds 1a-c were found to be approximately 2-3 s. Of the donor-acceptor derivatives 2 and 3, only 3b displayed a measurable fluorescence lifetime. Two decay components were detected in 3b at both room temperature and at 77 K (Table 1). Neither component appears to correspond to the same decay observed for the TMPD fluorescence; therefore, it must be the result of an interaction between the proline and the dinitrobenzene halves of the molecule. Because 3 has a flexible linkage between the prolinyl ring and the acceptor, several different conformations would be present that are likely to have different photophysical properties. The two components observed in the fluorescence decay may correspond to an extended and sandwich conformation. The longer-lived component is not affected by a decrease in temperature, but the shorter-lived component decreases in intensity and lifetime, suggesting that the longer-
Photoinduced ET in Donor-Acceptor Aryl Dyads
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2529 TABLE 2: Transient Absorption Lifetimesa of 1-3 at Room Temperature in Degassed CH3CN or THF % 1a
1b
1c Figure 2. Transient absorption spectrum at 24 µs after the laser pulse of 1a, 1b, and 1c. All solutions were 10-5 M in degassed CH3CN at room temperature. Excitation occurred at 355 nm, 8.0 mJ/pulse. 2a
lived emission emanates from the most stable conformer. The contrasting behavior of 3a and 3b is especially perplexing: considering that these compounds differ only by the nitro group substitution relative to the carbonyl; the excited state of 3a does not show any measurable lifetime. The explicit identity of the species exhibiting these lifetimes is unknown at this point. Transient Absorption for 1. Transient absorption spectra were obtained for 1a-c in acetonitrile (Figure 2) and THF at room temperature. These spectra appear to be superpositions of the spectra reported for a TMPD radical cation20 and a triplet-triplet absorption21 displaying maxima at 340, 580, and 620 nm. The triplet-triplet absorption and radical cation spectra of TMPD may be generated separately in different solvents as rigid glasses at low temperatures;22,23 the triplet-triplet absorption has maxima at 300, 560, and 600 nm and the TMPD radical cation at 330, 570, and 630 nm. The TMPD radical cation is formed in solution from photoionization from the singlet by a single-photon process in polar11,24-26 and nonpolar solvents27 at room temperature and by a two-photon process at low temperatures.15 In acetonitrile, the photoionization of TMPD itself occurs from the singlet state to generate an ion pair between the radical cation of the TMPD and the acetonitrile dimer radical anion.26 We confirmed that the spectra of the corresponding radical cations were present in 1a-c by using pulse radiolysis. Pulse radiolysis of 1c in nitroxide-saturated water generated the radical cation of TMPD: its spectrum has maxima at 560 and 610 nm which agrees well with the transient absorption spectra shown in Figure 2. Since the TMPD triplet-triplet and radical cation spectra overlap in our transient absorption spectra, the triplet may be distinguished from the radical cation by its faster decay.28 Therefore, by analogy we assign the long component of the decay of 1 at 580 and 620 nm to the radical cation and the short component of the decay to the triplet-triplet absorption (Table 2). In compounds 1a-c, the radical cation contribution to the observed absorptions at 580 and 620 nm in THF amounts to 25, 38, and 3%, respectively. In acetonitrile, the radical cation is responsible for a much larger percentage: 100, 70, and 43%, respectively. The preference for radical cation formation in CH3CN is expected from the known preference for ion formation in more polar solvents.29 Close inspection of the lifetimes of 1 (Table 2) reveals that the lifetime and percent composition of the major and minor components of the decay at 340 nm do not correspond to the other decay profiles that we observe at 580 and 620 nm.22,23
2b 2c 2d
3ag 3b 3c
τ(CH3CN) (µs) b
100
6.2 ( 0.3
100
7.3 ( 0.3c
100
7.4 ( 0.3d
100
12.0 ( 0.6b
30 70 29 71 49 51 57 43 56 44 100 100
2.5 ( 0.7c 15.5 ( 0.7 4.3 ( 1.4d 15.7 ( 0.2 8.9 ( 0.6b 23.4 ( 0.4 3.8 ( 0.2c 22.2 ( 0.2 3.5 ( 0.2d 21.5 ( 0.1 0.4 ( 0.1b 0.6 ( 0.1c
100
0.5 ( 0.1b
%
τ(THF) (ms)
30 70 77 23 74 26 47 53 60 40 64 36 55 45 97 3 96 4 100 100 100 100 100 100 100 100 100 100 100 100 100
7.3 ( 0.4b 48.1 ( 0.2 1.7 ( 0.1c 44.0 ( 0.4 2.3 ( 0.2d 36.3 ( 0.3 13.0 ( 0.2b 74.3 ( 0.6 7.5 ( 0.9c 73.2 ( 0.1 8.7 ( 0.6d 76.7 ( 0.9 9.2 ( 0.6b 85.2 ( 0.5 1.6 ( 0.01c 12.6 ( 0.3 1.6 ( 0.1d 10.6 ( 0.4 0.4 ( 0.1b 0.6 ( 0.1c 0.5 ( 0.1c 0.4 ( 0.1b 0.4 ( 0.1c 0.4 ( 0.1d 3.4 × 10-3 ( 0.1e 3.3 × 10-3 ( 0.1d 0.5 ( 0.1b 0.6 ( 0.1e 0.5 ( 0.1f 0.5 ( 0.1c 0.6 ( 0.1d
100 100 100 100 100
2.6 × 10-3 ( 0.1b 3.0 × 10-3 ( 0.4i 1.9 × 10-3 ( 0.4e 2.0 × 10-3 ( 0.3c 2.7 × 10-3 ( 0.3d
a Excitation at 355 nm with a laser power of 8 mJ/pulse. Decay monitored at the following: b 330-360 nm. c 580 nm. d 620 nm. e 380 nm. f 540 nm. g Signal too noisy to measure kinetics. h 570 nm. i 630 nm.
The peak at 340 nm for 1a,b in THF has a major component with a lifetime similar in magnitude to that of the radical cation and a minor component with a lifetime slightly longer than that of the triplet-triplet absorption. For 1c, the long-lived component makes up the minority of the decay (45%). In acetonitrile, biexponential behavior is not detected in 1a or 1b. In 1c, biexponential behavior is detected with 8.9 and 23.4 µs components. A possible source for the longer component of the decay trace at 340 nm is triplet-triplet annihilation: two triplets collide to generate a high-lying excited-state singlet which is then photoionized to the radical cation. The shorter component might also arise from triplet-triplet annihilation followed by intersystem crossing and subsequent absorption to result in a transient absorption maxima at the same wavelength as the triplet with an effectively longer lifetime. Delayed fluorescence derived from triplet-triplet annihilation has been observed for TMPD in glasses at low temperatures.15 Furthermore, the measured lifetime of the longer component at 340 nm decreased with decreasing laser power, whereas the lifetime of the shorter component at 340 nm, and both components at 580 and 620 nm, did not, suggesting that triplet-triplet annihilation is the source of the longer component of the decay at 340 nm. Transient measurements for 1c in a MTHF glass at 77 K did not show any decay in the region from 420 to 700 nm: the glass had turned blue and this blue color disappeared only upon
2530 J. Phys. Chem. B, Vol. 101, No. 14, 1997
Fossum et al.
Figure 3. Transient absorption spectrum for (A) 2a and 2b in degassed THF at room temperature 1.2 µs after the laser pulse, (B) 2a and 2b in degassed THF at room temperature 150 ns after the laser pulse, (C) 2c in degassed THF at room temperature at A (0.80 ns) and B (3.3 ns) after the laser pulse, (D) 2d in degassed THF at room temperature at 12 and 59 µs after the laser pulse, (E) 3b in degassed THF at room temperature at A (0.8 ns) and B (1.0 ns) after the laser pulse, and (F) 3c in degassed THF at room temperature at A (0.77 ns) and B (3.3 ns). Excitation occurred at 355 nm, 8.0 mJ/pulse.
warming. This behavior is typical of the TMPD radical cation referred to as Wu¨rster’s Blue, and the blue color results from a solvated electron.30 Transient Absorption Results for 2 and 3. When acceptors are covalently attached to the TMPD derivatives in 2 and 3, electron transfer occurs from TMPD to the acceptor, as established from transient absorption spectra that agree with the corresponding radical cation and radical anions (Figure 3). Steady-state measurements indicate that the TMPD fluorescence
is completely quenched, implying that electron transfer occurs from the TMPD singlet state. In compounds 2a and 2b, weak absorbances were observed in the range of 300-700 nm. The maximum at 340 nm corresponds to the radical anion of DMNA and the bleach at 390 nm is assigned to the ground-state absorbance of DMNA. If spectra are recorded from 450 to 700 nm, the TMPD radical cation maxima are visible with characteristic peaks at 580 and 620 nm (compare Figure 3, part A to part B). Excitation at 355 nm excites both the TMPD and
Photoinduced ET in Donor-Acceptor Aryl Dyads DMNA moieties, but in 2a and 2b, the extinction coefficient is much higher for the TMPD portion. The triplet-triplet absorption maximum for DMNA at room temperature has previously been reported as 460 nm,31 but this absorbance was not observed in 2a, suggesting that efficient electron transfer from the singlet state precludes observable intersystem crossing. Furthermore, the lifetimes (Table 2) for the charge-separated species derived from 2 and 3 are single exponential at all of the maxima. This is in contrast to measurements of the donor 1 where the triplet-triplet absorption and the radical cation overlap to result in a biexponential decay. Finally, the TMPD radical cation lifetime is shortened in the presence of covalently bound DMNA in 2a and 2b (Table 2). The spectrum of 2a at low temperature displayed maxima at 450 and 600 nm with lifetimes of 45.7 and 65.3 µs that correspond to the charge-separated DMNA radical anion and TMPD radical cation. Unlike 1c, 2c did not turn blue upon excitation as is consistent with production of an associated radical cation/radical anion pair. In compounds 2c and 2d, the acceptor is anthraquinone, and charge separation is also observed (Figure 3, parts C and D). The transient absorption spectrum of 2c was done on the picosecond time scale with sole excitation of TMPD at 355 nm. (Note: This is a different excitation wavelength than what was used in the steady-state measurements. An excitation wavelength of 290 nm was used in the steady-state measurements in order to detect the fluorescence of TMPD.) The spectrum reveals broad maxima from 380 to 400 nm and 530 to 650 nm which correspond to an overlap of reported spectra for the TMPD radical cation and the anthraquinone radical anion.20 In 2d, a similar spectrum was acquired, but on a longer timescale than that used for 2c. The transient spectrum for 2d was measured on the nanosecond time scale (Figure 3, part D). Again, the resulting spectrum is an overlap of those of the radical cation of TMPD and radical anion of anthraquinone. Local maxima were obtained at 330 and 620 nm (the TMPD portion) and 380 and 530 nm (the anthraquinone portion). There is also a maximum at 580 nm in which both species contribute. The lifetimes for 2a-2d are given in Table 2; they are single exponential and drastically quenched compared to that of the corresponding donor 1 alone. Transient absorption spectra of the proline derivatives 3a-c also displayed spectral features expected for the radical cation of TMPD in 3a,b and the radical anion of anthraquinone in 3c (Figure 3 parts E and F, respectively). The spectra for 3a-c were weaker than those of the other donor-acceptor compounds as a result of increased nonradiative decay consequent to increased rotational freedom between the proline and the acceptor caused by its flexible methylene linkage. No kinetic data could be obtained for 3a because the absorbances were too weak. It has been found previously in ESR studies of m- and p-nitrobenzyl alcohol that the radical anion is more stable in the para derivative and in fact the meta derviative gives a signal that is too weak to analyze.32,33 It follows that the spectra for the dinitro derivative would be more intense for 3b. An upper limit on the lifetime of the charge-separated pair in 3a can be estimated and must be on the order of 0.5 ns. Conclusion. Photoinduced electron transfer dominates the steady-state and time-resolved spectroscopy of several donordonor-acceptor triads 2 and dyads 3 that incorporate TMPD as the donor. In samples of the donor-donor model 1 alone, energy transfer from the singlet excited state of TMPD to piperazyl, piperidyl, morpholinyl, or prolinyl groups was observed upon steady-state excitation. Transient absorption spectra of 1 revealed that the radical cation of TMPD is formed,
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2531 as well as the lowest triplet excited state. The observed decay profiles also imply that triplet-triplet annihilation is taking place. In 2 and 3, electron transfer from the excited TMPD moiety generates radical cation/radical anion pairs with nanosecond lifetimes. The decay profiles are all single exponential, suggesting that intramolecular photoinduced electron transfer dominates over competing processes observed in 1. Electronic coupling is intramolecular because dilution does not affect the lifetime of the observed radical ion pairs. The transient spectra of the TMPD radical cation was observed for 1a-c, 2a-d, and 3a,b, and the radical anion was observed for DMNA (2a,b) and anthraquinone (2c,d, 3c). Experimental Section Compounds 1, 2, and 3 were prepared as reported earlier.13 Kinetic Methods. Absorption spectra were obtained on a Hewlett-Packard 8451A diode array spectrometer. Fluorescence and phosphorescence measurements were made on an SLM Aminco SPF 500 fluorometer. A phosphoroscope attachment equipped with light baffles and a variable speed chopper (010,000 rpm) was used to differentiate short and long lifetime species. Transient absorption spectra on the nanosecond timescale were obtained with a Q-switched, frequency-tripled (λ ) 355 nm, 8 mJ/pulse) Quantel YG481 Nd:YAG laser and a pulsed high-intensity Xe arc lamp in a 1 cm cell containing a nitrogen bubble-degassed solution with OD ) 0.2-0.4 AU. The decay measurements reported were the averages of 20 laser pulses, and transient spectra were obtained as the average of 3 laser pulses at 10 nm intervals. Transient absorption spectra on the picosecond timescale were obtained with a mode-locked Nd: YAG laser using 30 ps wide excitation laser pulses34,35 in a 1 cm cell containing a nitrogen bubble-degassed solution with OD ) 0.2-0.4 AU. Transient spectra were obtained as the average of 400 laser pulses at 10 nm intervals. Transient emission spectra were obtained using a Continuum Surelite Q-switched Nd:YAG laser (6 ns pulse width, 10-20.5 mJ/pulse) in a 1 cm cell containing nitrogen bubble-degassed solution with OD ) 0.1 AU. The decay measurements and spectra resulted from the average of 50 laser pulses at 10 nm intervals. Emission lifetimes were determined using timecorrelated single-photon counting with a mode-locked, synchronously pumped, cavity-dumped pyridine I dye laser. Emitted photons were collected at 90° from the excitation beam (350 nm) and passed through a monochrometer to a Hamamatsu Model R2809U microchannel plate. Data analysis was made after deconvolution of the instrument response function.17 Signals were obtained from a 1 cm cell containing nitrogen bubble-degassed solutions with OD ) 0.1 AU. Pulse radiolysis measurements were done according to earlier reported procedures using a Van de Graaff accelerator.36 Electron pulses were delivered to NO-saturated aqueous solutions in a quartz flow cell such that fresh sample was injected after each pulse. Spectroscopic grade MTHF was obtained from Aldrich and was freshly distilled from LiAlH4 prior to use. Spectroscopic grade acetonitrile (Burdick and Jansen) and tetrahydrofuran (THF, Burdick and Jansen) were used without further purification. All samples prepared were normalized to the ground-state absorbance at the excitation wavelength and were bubble degassed for several minutes. Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy and by the Texas Advanced Research Program. The authors
2532 J. Phys. Chem. B, Vol. 101, No. 14, 1997 also thank Dr. D. O’Connor for assistance with the single-photon counting and pulse radiolysis measurements which were conducted at the Center for Fast Kinetics Research at the University of Texas at Austin. References and Notes (1) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401. (2) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (4) Metzger, R. M. In Molecular and Biomolecular Electronics; Birge, R. A., Ed.; American Chemical Society: Washington, DC, 1994; Vol. 240, p 81. (5) Fujihira, F. In Molecular and Biomolecular Electronics; Birge, R. A., Ed.; American Chemical Society: Washington, DC, 1994; Vol. 240, p 373. (6) (a) Mes, G. F.; van Ramesdonk, H. J.; Verhoeven, J. W. J. Am. Chem. Soc. 1984, 106, 1335. (b) Halpern, A. M.; Ramachandran, B. R.; Sharma, S. J. Phys. Chem. 1982, 86, 2049. (c) Halpern, A. M.; Gartman, T. J. Am. Chem. Soc. 1974, 96, 1393. (7) Padden-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (8) Shephard, M. J.; Padden-Row, M. N.; Jordan, K. D. J. Am. Chem. Soc. 1994, 116, 5328. (9) Willemse, R. J.; Verhoeven, J. W.; Brouwer, A. M. J. Phys. Chem. 1995, 99, 5753. (10) Wiessner, A.; Hu¨ttmann, G.; Ku¨hnle, W.; Staerk, H. J. Phys. Chem. 1995, 99, 14923. (11) Hirata, Y.; Mataga, N. J. Phys. Chem. 1984, 88, 3091. (12) Weller, A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 197. (13) Pearson, A. J.; Gelormini, A. M.; Fox, M. A.; Watkins, D. M. J. Org. Chem. 1996, 61, 1297. (14) DiBella, S.; Fragala, I. L.; Ratner, M. A.; Marks, T. J. In Molecular and Biomolecular Electronics; Birge, R. A., Ed.; American Chemical Society: Washington, DC, 1994; Vol. 240, p 233.
Fossum et al. (15) Potashnik, R.; Ottolenghi, M.; Bensasson, R. J. Phys. Chem. 1969, 73, 1912. (16) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed; Marcel Decker, Inc.: New York, 1993. (17) O’Connor, D. V.; Phillips, D. Time Correlated Single Photon Counting; Academic Press: London, 1984. (18) Grabowski, Z. R.; Dobkowski, J. Pure Appl. Chem. 1983, 55, 245. (19) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (20) Shida, T. Electronic Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (21) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1. (22) Bagdasaryan, K. S.; Kirjukhin, Y. I.; Sinitsina, Z. A. J. Photochem. 1973, 1, 223. (23) Cadogan, K. D.; Albrecht, A. C. J. Phys. Chem. 1969, 73, 1868. (24) Hirata, Y.; Mataga, N. J. Phys. Chem. 1983, 87, 3190. (25) Hirata, Y.; Mataga, N. J. Phys. Chem. 1985, 89, 4031. (26) Hirata, Y.; Mataga, N. J. Phys. Chem. 1983, 87, 1680. (27) Hirata, Y.; Nogata, A.; Mataga, N. Chem. Phys. Lett. 1992, 189, 159. (28) Yamamoto, N.; Nakato, Y.; Tsubomura, H. Bull. Chem. Soc. Jpn. 1966, 39, 2603. (29) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Mill Valley, CA, 1991. (30) Wu¨rster, C.; Sendtner, R. Ber. Dtsch. Chem. Ges. 1879, 12, 1803. (31) Cowley, D. J.; Pasha, I. J. Chem. Soc., Perkin Trans. II 1983, 1139. (32) Wan, P.; Muralidharan, S. J. Am. Chem. Soc. 1988, 110, 4336. (33) Muralidharan, S.; Wan, P. J. Photochem. Photobiol. A 1991, 57, 191. (34) Atherton, S. J.; Hubig, S. M.; Callan, T. J.; Duncanson, J. A.; Snowden, P. T.; Rodgers, M. A. J. J. Phys. Chem. 1987, 91, 3137. (35) Chambron, J.-C.; Harriman, A.; Heitz, V.; Sauvage, J.-P. J. Am. Chem. Soc. 1993, 115, 6109. (36) Rodgers, M. A. J.; Foyt, D. C.; Zimek, Z. A. Radiat. Res. 1977, 75, 296. (37) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107.
