J. Phys. Chem. 1995,99, 14752-14757
14752
Intramolecular Photoinduced Electron Transfer from Nitroxyl Radicals Sarah Green and Marye Anne Fox* Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712 Received: July 5, 1995@ Intramolecular photoinduced electron transfer was observed from a nitroxide radical to a diimide acceptor in N,”-bis(2,2,6,6-tetramethyl-N-oxidopiperidin-4-yl)-1,4,5,8-naphthalenediimide(DTDI). Irradiation of this substrate produced a zwitterion in which the diimide monoanion, with a lifetime of ’200 ps, was detected by transient absorbance spectroscopy. Slow charge recombination was attributed to the different symmetry constraints for forward electron transfer to an n,n* state and back electron transfer from a n,n*state. The restricted motion of the nitroxide group prevents efficient overlap with the ring n system, slowing back electron transfer.
Photochemical oxidation of nitroxides is an attractive goal because it is a possible mode for light-driven charge separation to produce an active redox catalyst. The oxoammonium cation produced by nitroxide oxidation is a relatively stable species that has been used catalytically to selectively oxidize alcohols and amines to aldehydes and ketones,’-3 as well as in oxidative coupling Oxoammonium ions have previously been generated c h e m i ~ a l l y ~ - ~and . ~ -electro~hemically,4.~ ’~ and our results suggest that they may also be usefully produced photochemically. Stable nitroxide radicals, such as 2,2,6,6-tetramethylpiperidine- 1-oxy1 (Tempo), are well-known quenchers of excited singlet,” doublet,I2 and triplet states. Several possible mechanisms have been suggested for intermolecular triplet quenching, including electron-exchange-enhanced intersystem crossing (ISC),I3 electron transfer (ET), and energy transfer (EnT). Although nitroxides can potentially act as either electron acceptors or donors to many organic chromophores, the ions expected from such electron transfer events have proven elusive. Borisevich et al., in examining the dependence of the rate of quenching of the triplet states of a series of cyanine dyes by TEMPO or 4-hydroxy-TEMPO on their redox potentials and on solvent polarity, concluded that charge transfer complexes were sometimes formed, with the nitroxyl radical acting as the electron donor, but with no direct evidence for the purported product ions.I4 Samata and Kamat argued that quenching of fullerene triplets by TEMPO resulted in nitroxyl oxidation but were unable to detect the expected Cm or C70 anions.I5 To our knowledge, the only previous conclusive report of photoinduced electron transfer from nitroxyl radicals has been to an inorganic polyoxometallate, W 1o0324-. l6 Here we demonstrate the generation of an organic zwitterion of N,”-bis(2,2,6,6-tetramethyl-N-oxidopiperidin-4-y1)-1,4,5,8naphthalenediimide,a naphthalenediimidebearing two TEMPO groups (DTDI), by intramolecular electron transfer quenching of a locally excited organic triplet state by the covalently linked nitroxyl radical. Two paramagnetic naphthalene diimide derivatives (the diTEMPO-diimide derivative (DTDI)and an asymmetrically monoalkylated monoTEMPO-substituted diimide (Ndodecyl-N-(2,2,6,6-tetramethyl-N-oxidopiperidin-4-y1)1,4,5,8naphthalenediimide (ATDI)) were prepared by oxidation of their amine precursors (N,N’-bis(2,2,6,6-tetramethylpiperidin-4-y1)1,4,5,8-naphthalenediimide(the dipiperidinyl dimimide, DPDI) and N-dodecyl-K-(2,2,6,6-tetramethylpiperidin-4-y1)-1,4,5,8naphthalenediimide (the alkylated monopiperidinyl diimide, @Abstractpublished in Aduance ACS Abstracts, September 1, 1995.
APDI),respectively). The photophysical properties of these nitroxides were compared with those of a diamagnetic control compound, NN-didodecyl- 1,4,5,8-naphthalenediimide(the dialkylated diimide, DADI).
Experimental Section Equipment and Materials. Absorption spectra were measured on an HP-8451 diode array spectrophotometer, and fluorescence spectra were recorded on an SLM-Aminco 500 fluorometer. Fluorescence lifetimes (with a detection limit of 20 ps) were measured by single-photon counting at the Center for Fast Kinetics Research at the University of Texas at Austin. A Briicker EPR spectrometer was employed to c o n f i i nitroxide spin levels. Electrochemical measurements were made on a BAS 100 electrochemical analyzer for solutions in tetrahydrofuran (THF) containing oven-dried tetrabutylammonium hexafluorophosphate as the supporting electrolyte. NMR spectra of all diamagnetic compounds were measured on a General Electric 300-MHz spectrometer. The most stable conformation of DTDI was calculated with a molecular modeling program employing a computational energy minimization routine (Chem 3D, Cambridge Scientific). Transient absorption experiments were performed on a homebuilt system. Excitation was from the third harmonic (355 nm) of a Continuum Surelite, Q-switched Nd:YAG laser (6-ns pulse width, 3-4 mJ/pulse, -6-mm diameter). Absorption (detection limit = 15 ns) was probed with a pulsed (Kinetic Instruments pulser) 150-W Xe monitoring lamp (Oriel model 6256 lamp, model 66001 housing, and model 68805 power supply) focused
0022-365419512099-14752$09.00/0 0 1995 American Chemical Society
Electron Transfer from Nitroxyl Radicals
J. Phys. Chem., Vol. 99, No. 40, 1995 14753
through a fast shutter (Uniblitz model VS25 with D122 methanol) on silica gel. Mp: 265-270 OC dec. 'H-NMR controller). The probe light was delivered to and from the (CDC13): 6 8.73 (s, 4 H), 5.76 (t, J = 16.0 Hz, 1 H), 4.20 (t, sample to the detection monochromator (FTI) via optical fiber J = 18.0 Hz, 2 H), 3.10 (t, J = 15.0 Hz, 4 H), 1.72 (s, 12 H), bundles (Oriel). An R928 photomultiplier tube driven at 600 1.25 (s, 20 H), 0.80 (t, J = 9.0 Hz, 3 H). I3C-NMR (CDCl3): 6 163.2, 162.7, 131.3, 131.0, 126.8, 126.7, 126.7, 126.6, 45.1, V (Bertan 205B-03R high-voltage supply) was used for detec41.1, 38.4, 31.9, 31.0, 29.7, 29.6, 29.5, 29.4, 28.1, 27.1, 25.4, tion. The output of the PMT was coupled to a Tektronix TDS 25.3,25.30, 22.7, 14.1. FTIR (KBr): 2930, 2880, 1707, 1667, 540 digitizing oscilloscope and transmitted to a personal 1579,1454,1385,1341,1250 cm-I. Mass spectrum: 574.3641; computer. Data analysis was by a combination of locally written calc for C35H48N304, 574.3644. and commercial software. Electronic synchronization and N,K-Bis(2,2,6,6-tetramethyl-N-oxido-piperidin-4-y1)-1,4,5,8control of the lamp, shutter, and laser were achieved with locally naphthalenediimide (DTDI): DPDI (0.25 g, 0.5 "01) was constructed electronics. Purging with nitrogen had virtually no dissolved in CH2C12 and stirred at room temperature during the effect on the solution phase lifetime of DTDI; nitrogen purging slow addition of 2.3 equiv of m-CPBA. After 2 h, the mixture of a solution of DADI did increase the observed lifetime, as was passed through plugs of alumina and silica, before removal expected for a triplet state (see below). of solvent under rotary evaporation, to give a purple powder in 1,4,5,8-Naphthlaenetetracarboxylicacid dianhydride, 4-amino45% yield. Mp: 280 "C. FTIR (KBr): 2976, 1707, 1669, 1579, 2,2,6,6-tetramethylpiperidine, N,N-dimethylacetamide (DMA), 1464, 1452, 1363 (N-0 stretchI7), 1333, 1250 cm-I. m-chloroperbenzoicacid (m-CPBA), and 4-amino-TEMPO were (CH2C12): 362 nm (21 000 f 400 M-lcm-'), 382 nm (22 000 obtained from Aldrich and used as received. Methylene dz 550 M-'cm-'), in reasonable agreement with those reported chloride, 2-propanol, N,N-dimethylformamide (DMF), and acby PenneauI8 and ZhongI9 for related naphthalene diimides. All etonitrile were spectral grade. THF, 2-methyltetrahydrofuran diimide derivatives showed the same characteristic absorption (MTHF), and benzene were freshly distilled immediately before spectrum; in both DTDI and ATDI, a very weak n-n* use. Sucrose octaacetate (SOA, Aldrich) was recrystallized absorbance of the nitroxide was also apparent at %460 nm. Mass twice from ethanol. spectrum (m 1): 575.2883; calc for C32H39N406, 575.2869. Synthesis. N,N'-Bis(2,2,6,6-tetramethylpiperidin-4-y1)-1,4,5,8- The electron paramagnetic resonance (EPR) spectrum of naphthalenediimide (DPDI): 1,4,5&Naphthalenetetracarboxylic DTDI showed the characteristic three-peaked spectrum of acid dianhydride (4.4 g, 16 m o l ) suspended in 30 mL of DMA TEMPO with an identical g value and 2.2 f 0.1 spins per mole was added dropwise over 45 min to a solution of 5.4 g (35 (2.0 spins expected). There was no evidence of coupling "01) of 4-amino-2,2,6,6-tetramethylpiperidinein 30 mL of between the two nitroxide radicals. DMA. Acetic acid (2 mL) was added, and the resulting solution N-Dodecyl-Nf-(2,2,6,6-tetramethyl-N-oxidopiperidin-4-y1)was stirred at 100 "C (boiling water bath) for 2.5 h. The reaction 1,4,5,8-naphthalenediimide (ATDI): APDI was oxidized by the was stopped when the absorption spectrum showed two method described above for PDI, yielding ATDI as a tan characteristic diimide peaks (362 and 382 nm) with no further powder. Mp: 145-147 OC. Mass spectrum: 588.3447; calc changes. The tan product precipitated on cooling with addition for C35b6N305, 588.3437. The EPR spectrum matched that of ether. Filtration, followed by an ether rinse and air drying, of TEMPO, as described above. gave 8.2 g (92%) of product. Mp: 320 OC dec. 'H-NMR Steady State Photolysis. DTDI and ATDI proved to be (CDC13): 6 8.72 (s, 4 H), 5.66 (t oft, J = 13.0 Hz, 13.5 Hz, 2 unstable to prolonged laser or W lamp (200-W Xe) irradiation. H), 2.44 (t, J = 12.5 Hz, 4 H), 1.68 (d, J = 3.0 Hz, 4 H), 1.39 Extended exposure of DTDI or ATDI solutions to ultraviolet (s, 12 H), 1.34 (s, 2 H), 1.25 (s, 12 H). I3C-NMR (deuteriolight (in CH2C12, CH3CN, or benzene, with or without oxygen) trifluoroacetic acid): 6 167.2, 134.8, 129.1, 63.4, 47.7, 39.0, resulted in significant degradation, ultimately leading to the 32.5, 26.1, 19.5. FTIR (KBr): 2974, 1701, 1665, 1579, 1452, formation of unidentified fluorescent products. Therefore, each 1390, 1377, 1331, 1254 cm-I. Mass spectrum (m 1): sample was stirred vigorously and subjected to a maximum of 545.31 16; calc for C32blN404, 545.3128. 20 laser shots before being replaced with fresh solution; a flow N,N'-didodecy 1- It4,5,8-naphthalenediimide (DADI): 1-Dodecell was employed for some experiments. No decomposition camine was used in place of 4-amino-2,2,6,6-tetramethylpipwas noted for DADI. DPDI was not used for photochemical eridine in the above procedure. The product was obtained as studies because of the documented electron transfer reactions white crystals in 98% yield. Mp: 160 OC. 'H-NMR (CDCl3): between excited state naphthalimides and amines. 6 8.76 (s, 4 H), 4.19 (t, J = 7.5 Hz, 4 H), 1.74 (quin, J = 7.7 Results Hz, 4 H), 1.43 (m, 4 H), 1.36 (m, 4 H), 1.25 (m, 28 H), 0.75 (t, J = 7.0 Hz, 6 H). I3C-NMR (dCDCl3): d 162.8, 130.9, 126.7, Cyclic voltammetry of DTDI in THF (Figure 1) showed two 126.7, 41.0, 31.9, 29.6, 29.6, 29.6, 26.5, 26.5, 29.3, 29.3, 27.1, reversible waves at -0.63 and -1.2 V vs SCE, assigned to the 22.7, 14.1. Mass spectrum (m 1): 603.417; calc for one- and two-electron reductions of the diimide group, respecC38H55N204, 603.416. ti~ely.'*.'~ In addition, the nitroxide group shows a reversible oxidation at +0.69 V and an irreversible reduction near -1.8 DADI Anion: DADI was chemically reduced by addition of V. aqueous dithionite to DADI in bubble-deoxygenated DMF. As The fluorescence of both alkyl- and TEMPO-substituted has been reported for related compounds,I8the anion was stable diimide derivatives in solution was extremely weak. In contrast, ('10 min) in the absence of oxygen and showed a strong in either a frozen glass (MTHF, 77 K) or a room temperature absorbance peak at 475 nm and a weaker one at 610 nm (Figure glass (SOA), DADI exhibited intense, structured fluorescence 4). N-dodecyl-N'-(2,2,6,6-tetramethylpiperidiidin-4-yl)-1,4,5,8-naph-at 410 nm, and DTDI showed a matching but much weaker emission (Figure 2). No phosphorescence could be detected thalenediimide (APDI): To a solution of 1-dodecamine (1.6 g, from either solution; however, the addition of ~ 1 0 % ethyl iodide 8 "01) and 4-amino-2,2,6,6-tetramethylpiperidine(1.3 g, 8 to either the frozen or room temperature glasses induced strong m o l ) in DMA was added a suspension of 1,4,5,8-naphthalephosphorescence from DADI at 605 and 665 nm (Figure 1). netetracarboxylic acid dianhydride in DMA. Acetic acid (2 mL) Phosphorescence could not be induced from DTDI by adding was added, and the resulting solution was stirred at 100 "C ethyl iodide. (boiling water bath) for 2.5 h, as above. When the solution was cooled and ether was added, three products precipitated: The observed fluorescence lifetimes of both DADI and DTDI they were separated by flash chromatography (CH2C12 and in methylene chloride were shorter than our detection limit for
+
+
+
Green and Fox
14754 J. Phys. Chem., Vol. 99, No. 40,1995
TABLE 1: First- and Second-Order Rate Constants for the Decay of Transients" DTDI ATDI DADI
CH3CN benzeneb 2-propanol CH3CN 2-propanolc
411 411
CHzClz
1 1 0.5
311 311
2 f l
4fl
4fl
311
a Air-saturated solutions. Rate constants were determined as an average of six or more decay profiles, as described in the text. benzene, an equally good fit was obtained to a monoexponential first order decay with k = 6.8 x lo3 s-'. 'The most consistent fits for 2-propanol were monoexponential; however, inspection of the curve fits showed that the decay rate in the initial 20-40 ms was underestimated by this fit.
Figure 1. Cyclic voltammogram of DTDI (0.2 mM) under nitrogen in anhydrous tetrahydrofuran containing 0.1 M tetrabutylammonium hexafluorophosphate at a platinum foil working electrode referenced to AglAgCl; scan rate = 500 mV/s.
I
I'
P t
of T+DI-, rather than a ketyl radical, is the insensitivity of the transient's lifetime to oxygen. Ketyl radicals are expected to react with 0 2 at diffusion-controlled rates, whereas reduction of 0 2 by the diimide anion is quite slow. We conclude that electron transfer in DTDI produces a charge-separated state which we denote as T+DI- (eq 1).
0.5 0.4
hv
0.3
___t
0.2 0.1
DTDI r
0 350
400
450
500
550
600
660
700
-I-
750
Muelength
Figure 2. Emission spectra of DADI at 77 K: (-) fluorescence in frozen 2-methyltetrahydrofuran; (- - -) phosphorescence in frozen 2-methyltetrahydrofuran containing 10% ethyl iodide. TDI'
emission (20 ps). In MTHF at 77 K, DADI fluorescence showed a biexponential decay with lifetimes of 0.63 f 0.01 ns (69%) and 2.9 f 0.04 ns (31%). DTDI in the frozen MTHF glass had a fluorescence lifetime of
