J. Phys. Chem. 1993, 97, 3217-3224
3217
Laser Photolysis Studies of Transient Processes in the Photoreduction of Naphthalimides by Aliphatic Amines Attila Demeter, Us216 Bicz6k, and Tibor Wrces’ Central Research Institute for Chemistry, Hungarian Academy of Sciences, Pusztaszeri u. 59/67, 1025 Budapest, Hungary
Veronique Wintgens, Pierre Valat, and Jean Kossanyi Laboratoire de Photochimie Solaire, E.R.241. CNRS, 2-8 Rue H.Dunant, 94320 Thiais, France Received: March 30, 1992; I n Final Form: September 9, I992
The photoreduction of N-phenyl- 1,8-naphthalimide and N-phenyl-2,3-naphthalimideby aliphatic amines has been studied by laser flash photolysis with transient absorption and transient conductivity methods in different solvents. Analysis of transient time profiles establish for most systems the occurrence of a fast primary and a slower secondary reduction process. Primary reduction is ascribed to the reaction between a triplet naphthalimide and an amine, while secondary reduction is assigned to the reaction of an amine-derived a-aminoalkyl radical with a ground-state naphthalimide molecule. In polar solvents, with aliphatic amines both primary and secondary reductions proceed by electron transfer. In solvents of intermediate polarity, hydrogen atom transfer (primary reduction) is succeeded by electron transfer (secondary reduction). Finally, in nonpolar solvents, only primary reduction by hydrogen atom transfer is found to occur. Rate constants are obtained for most of these processes by computer modeling of the transient time profiles. In polar solvents, reaction AH2+ AH2 A H AH3+ (where AH2 and AH are the amine and a-aminoalkyl radical, respectively) is a key reaction in which the a-aminoalkyl radical is formed. Its rate constant is found to decrease by more than 4 orders of magnitudes when AH2 varies from tertiary, through secondary, to primary amine. This is explained by the significant change in the dissociation energy of the C-H bond in the a-position to the nitrogen. The electron transfer between the a-aminoalkyl radical and the naphthalimide molecule is found to occur in polar solvents with a rate close to the diffusion controlled limit, whatever the type of the aliphatic amines.
+
I. Introduction Photoreduction of aromatic ketones by amines has been extensively investigated during the past 2 decades.’-20 Recent laser photolysis investigations of transient proce~ses~-~J5-20 contributed especially to the establishment of the dynamics of the initial steps. Cohen and co-worker~l-~ proposed that quenching of the triplet ketone by the amine consists of an electron transfer from the amine to the carbonyl group, forming a charge-transfer complex. Electron transfer is followed either (i) by a proton transfer to form a ketyl and an a-aminoalkyl radical, or (ii) by spin inversion and back-electron transfer, leading to the groundstate reactants, or (iii) even by dissociation into free, solvated ions. 16.1 7.20 Combined picosecond and nanosecond laser photolysis studiesI8J9 of the photoreduction of benzophenone by amines have shown that the first step of the triplet quenching is an electron transfer to form a contact radical ion pair (CIP). In nonpolar or moderately polar solvents, CIP is removed mainly by very fast proton transfer to form the free ketyl and a-aminoalkyl radicals. In polar solvents, solvation converts CIP into a solvent-separated radical ion pair (SSIP) which then predominantly dissociates intofree,solvatedions.20Thecontribution to theion pair removal by proton transfer, by back-electron transfer, and by dissociation into free ions is shown16J8.19 to be influenced considerably by the amine structure and by specific solvent effects. Photoreduction yields greater than unity were obtained by Cohen and co-workers at high ketone concentrations when quenching triplet carbonyls by amines. These yields were explained in termsof secondary reduction of ground-statecarbonyl molecules by the amine-derived a-aminoalkyl radicals.21-25Direct evidence for the secondary reduction has been obtained in recent laser flash photolysis experiments.17J9J6In these investigations, a relatively slow secondary reduction process (occurring on a
-
+
100-ns or -ps scale) has been detected by monitoring the growth of the absorption of the ketone radical anion formed by electron transfer to the ketone from the a-aminoalkyl radical. In addition, CIDEP experiments provided further support for the secondary reduction reaction.2’ The present paper reports the results of a laser flash photolysis study of transient processes occurring in the photoreduction of naphthalimidesby aliphatic amines in polar and nonpolar solvents. Naphthalimides have been selected because their luminescence and transient properties,28 as well as their triplet formation processes,29 are known in detail from recent flash photolysis investigations. In addition the photochemistry of the aromatic dicarboximides shows certain similarities to other carbonyl compounds.30-32 11. Experimental Section
A. Materials. N-Phenyl-2,3-naphthalimide(N-Ph-2,3-NI) was prepared by refluxing a mixture of naphthalene-2,3dicarboxylicacid and aniline.33 N-Phenyl-l,8-naphthalimide(NPh-1,8-NI) was obtained in the same way from the appropriate dicarboxylic anhydride. 1,4-DiazabicycIo[2.2.2]octane (DABCO) was purchased from EGA Chemie AG. DABCO and the naphthalimides were recrystallized from a 1:1 benzene-hexane mixture and sublimed under vacuum. Triethylamine (TEA) (Loba Chemic), N,N-diethylamine (DEA; Merck), and n-pentylamine (PA, Fluka) were distilled three times before use. The N,N-diphenylethylamine (DPhEA) was prepared as described in the literat~re.3~Di-tert-butyl peroxide (Fluka) was used as received. Acetonitrile (Merck, Uvasol), ethyl acetate (Merck, Uvasol), 1.2-dichloroethane (Merck, Uvasol), and n-hexane (Fluka, for UV spectroscopy) were used without further purification. Butyronitrile (Aldrich), diethyl ether (Merck for
0022-3654/93/2091-3211S04.00/00 1993 American Chemical Society
Demeter et al.
3218 The Journal of Physical Chemistry, Vol. 97,No. 13, 1993
analysis), dibutyl ether (Ferak, purum), and cyclohexane (Ferak, for analysis) were distilled before use. B. Laser Flash Photolysis Technique. The nanosecond laser flash photolysis apparatus was similar to that described elsewhere.35-36 Degassed samples in an 1 X 1 X 4 cm quartz cuvette were irradiated by the 308-nm flash from an EMG 101 excimer laser (80 mJ, 15-ns pulses). Samples were exposed to not more than 8-16 flashes, and the cuvettes were homogenized by shaking after each shot. Transient photocurrent measurements were carried out with degassed solutions in a rectangular quartz cell of 1 X 1 X 4 cm size equipped with gold-plated platinum electrodes spaced approximately 8 mm apart. The electrodes were connected to a 90-V DC power supply. The load resistance in the measuring circuit was 1 kR, and the time constant of the setup was 0.4 ps. Kinetic data were extracted from the experimental transient absorption-time profiles by kinetic model simulation and parameter optimization.35-36 C. Time Resolved Spectroscopy. Transient absorption spectra for naphthalimide radical anions were obtained with carefully degassed acetonitrile solutions of dicarboximides (corresponding to 0.4 absorbance at 308 nm) and 0.001 mol dm-3 TEA. The absorption measurements a t different wavelengths were made with I-ms delay time after the laser flash. The solution was homogenized by shaking the cuvette after each laser flash, and the sample was replaced by a new one after about 40 flashes. Molar absorption coefficients for the radical anions were determined at the absorption maxima using two different methods: (i) Radical anions were produced by laser flash photolysis of the dicarboximides in the presence of mol dm-3 or higher DABCO concentrations. The initial absorbance, after correction for the small contribution from the DABCO radical cation at the appropriate ~avelength,~' was measured against the absorbance of the diphenyl-hydroxymethyl radicals (at 540 nm) obtained in the photolysis of a benzophenone-benzhydrolacetonitrile solution. The molar absorption coefficients of the naphthalimide radical anions were obtained with c = 3500 dm3 mol-' cm-' for the ketyl radical absorption coefficient38 at 540 nm and the known triplet yields for the naphthalimide~.~~ (ii) In accordance with the total depletion method,39tert-butoxy free radicals were produced by the photolysis of di-tert-butyl peroxide in acetonitrile which reacted with TEA to form a-aminoalkyl radicals. These radicals, known as strong one-electron reducing agents,z6-aconverted the naphthalimide molecules quantitatively into naphthalimide radical anions. From the naphthalimide concentrations and the maximum values of the radical anion absorbances, the molar absorption coefficients for the naphthalimide radical anions were derived. The electron spin resonance (ESR) spectrum of the naphthalimide radical anion was obtained with a JEOL JES-FE/3X spectrometer. Degassed acetonitrile solutions of mol dm-3 naphthalimide and mol dm-' TEA were irradiated in the cavity of the spectrometer, using the focused light of a 125-W medium pressure mercury arc. D. Reduction Potentials. The reduction potentials were measured in anhydrous acetonitrilecontaining 1 X 10-3 mol dm-3 naphthalimide and 0.1 mol dm-3 tetrabutylammonium tetrafluoroborate. The solution was bubbled with argon for 15 min. The cyclovoltammograms were obtained by using a PAR 173 type potentiostat with a PAR 175 type programmer coupled to a TGM XY Sefram recorder. Platinum gauze (working), platinum plate (counter), and SCE (reference) electrodes were used. The reactions at the electrodes were reversible, and the redox potential was obtained as the arithmetic average of the cathodic and anodic waves. Thus, EY: = -1.60 V and E$ = -1.35 V were obtained vs SCE for N-P6-2,3-NI and N-Ph-l&NI, respectively.
30000
5
t
20000
E" "
5 Y
ii n m
10000
1
Alnm
Figure 1. Spectra of N-Ph-2,3-NI (broken line) and N-Ph-1,8-NI (full line) radical anions in acetonitrile and that of N-Ph-1,8-NI kctyl radical (dotted line) in n-hexane. Inset: Spectra of N-Ph-2,3-NI (broken line) and N-Ph-1,8-NI (full line) triplets in acetonitrile.
111. Results and Discussion
Photoreduction experiments are carried out with either 2-phenyl- lH-benz[de]isoquinoline-1,3(2H)-dione (N-Ph- 1,8-NI) or 2-phenyl-lH-benzv]isoindole-l,3(2H)-dione (N-Ph-2,3NI): The low triplet yields f0und2~for the investigated 1,2-
N-Ph. I &NI
N-Ph-2.3-NI
naphthalimide derivatives prevented us from studying their photoreduction. A. Naphthalimide Triplet Quenching by Amines. 1. Triplet Spectra. The triplet spectra of naphthalimides were obtained from time resolved 308-nm laser flash photolysis experiments, and the spectra will be described in detail el~ewhere.2~ Triplettriplet absorption spectra of the N-phenyl derivatives in acetonitrile solution are shown in the inset of Figure 1. Absorptions a t the maxima (i.e. 445 and 475 nm for 2,3- and I,g-naphthalimide, respectively) were chosen for monitoring the triplet-state decay in the quenching experiments. 2. Triplet Quenching Rates. The decay of the triplet, formed by laser flash excitation in the presence of an amine, followed mixed first/second-order kinetics as the result of triplet deactivation by amine quenching and triplet-triplet annihilation. The contribution of the latter was kept small by using low laser intensities (5-10 mJ/flash). The slope of the linear plot of the pseudo-first-order rate constant kb versus amine concentration gave the second-order triplet quenching rate constant, k,. Such a plot for the N-Ph- 1,8-NI + TEA system in acetonitrile is shown in the inset of Figure 2. The value of k, = (6.3 f 0.4) X 108 dm3 mol-' SKI is derived from the slope, while the intercept gives a triplet lifetime of 0.25 f 0.12 ms for the N-Ph-l,&NI-TEA system in acetonitrile. A similar plot of N-Ph-2,3-NI + TEA in acetonitrile yields k, = (2.6 f 0.4) X lo8 dm3 mol-' s-I. The triplet energies of the two naphthalimides are somewhat different; however, thedifference in thequenching rateconstants is probably the result of the much more negative reduction potential of the 2,3-naphthalimide (see E$ in the Experimental Section). Rate constants for triplet-state quenching by amines in various solvents aresummarized in Table I. In acetonitrile, thequenching rate constants increase from the very low value of 1.4 X lo5dm3 mol-' s-I for the primary amine up to the high value of 5.3 X lo9 dm3 mol-' s-I found for the only aromatic amine studied. The sequence of the amines corresponds to their decreasing ionization potentials (IP). The semilogarithmic plot of k, vs IP is found to be linear for the aliphatic amines in a 4 orders of magnitude
Naphthalimide Photoreduction by Aliphatic Amines
The Journal of Physical Chemistry, Vol. 97, No. 13, 1993 3219
bo1
'
/-.
'
'------I
l i io
75
80
85
90
95
IPleV
Figure 2. Plot of the N-phenyl- 1,8-naphthalimide triplet quenching rate constants in acetonitrile vs the vertical ionization potentials of theamines. Inset: plot ofthepseudo-first-order rateconstants for N-Ph-l,8-NI triplet quenching vs TEA concentration.
reactivity range (Figure 2). Strong dependence of the quenching rate constants on solvent polarity was observed in experiments made with triethylamine. The quenching rate constant increases by more than 2 orders of magnitude from n-hexane to acetonitrile. The plot of the logarithm of the rate constant versus the ET(30) empirical parameter of the solvent polarity gave a good straight line. On the basis of the observations, it can be concluded that the reactions between the triplet naphthalimides and the amines involve a large degree of charge separation or complete electron transfer. B. Transient Formation and Decay in the NaphthalimideDABCO System. Laser flash photolysis experiments carried out in the presence of DABCO revealed the formation of a second transient which decayed on a time scale longer than that of the triplet. The transient kinetic traces observed in the laser flash photolysis (70 mJ/flash) of N-Ph-l,8-NI-DABCO-acetonitrile areshownin Figure 3. Thegrowthof theabsorptionof thelongerlived transient occurs exactly in the same time interval as does the triplet-state decay (see the inset in Figure 3), which indicates that the triplet state is the precursor of the longer-lived transient. Moreover, the similarity of the transient absorption profile and the transient conductivity trace establishes that the longer-lived transient is a charged species, most likely the radical anion, NO-, formed from the naphthalimide molecule by electron transfer from the amine. The identificationof the longer-lived transient as No-is strongly supported by the ESR spectrum shown in the upper part of Figure 4 which was obtained with N-Ph-1,8-NI TEA in acetonitrile irradiated directly in the cavity of the ESR spectrometer. The good agreement between the calculated and the experimental
+
v,
I
0.1 0.2 0.3 Timelms Figure 3. Transient-time profiles following laser flash excitation of 4.7 X mol dm-3 N-Ph-1,8-NI and 1 X mol dm-3 DABCO in acetonitrile: (a) transient absorption at41 5 nm; (b) transientconductivity. Inset: Triplet decay (475 nm) and transient buildup (at 415 nm). 0
I
.
.
Figure 4. ESR spectrum of the longer-lived transient identified as N*-: (a) experimental spectrum; (b) simulated s ctrum using uN = 1.43 G, a; = u," = 5.1 3 G, u," = u," = 0.87 G,and u s uy = 5.85 G parameters.
spectra confirms the identification of the transient as a naphthalimide derived radical anion. No reactant consumption or product formation was observed in the photolysis of N-Ph-2,3-NI DABCO or N-Ph-1.8-NI DABCO solutions. Moreover, the decay of the longer-lived transients in these systems strictly followed second-orderkinetics. (Thus, the solid line in Figure 3a is calculated, taking the secondorder rate constant of k = 1.4 X 10'0 dmj mol-' s-1, in very good agreement with the experimental results.) On the basis of the fast second-order decay and from the absence of detectable
+
+
TABLE I: Naphthalimide Triplet Quenching Rate Constants and Relative Transient Yields, y = @(transient)/@(Mplet)' naphthalimide
amine
N-Ph-l,l-NI N-Ph-1,8-NI N-Ph-1,8-NI N-Ph-1.8-NI N-Ph-1,8-NI N-Ph- 1.8-NI N-Ph- 1.8-NI N-Ph-1,8-NI N-Ph-1,8-NI N-Ph-1.8-NI N-Ph-2,3-N1 N-Ph-2,3-NI N-Ph-2,3-NI N-Ph-2,3-N1
DPhEA DABCO DABCO TEA TEA TEA TEA TEA DEA PA DABCO DABCO TEA TEA
solvent
(6)
acetonitrile (37.50) acetonitrile (37.50) cyclohexane (2.02) acetonitrile (37.50) 1,2-dichloroethane (10.36) ethyl acetate (6.02) dibutyl ether (3.08) n-hexane (1.88) acetonitrile (37.50) acetonitrile (37.50) acetonitrile (37.50) cyclohexane (2.02) acetonitrile (37.50) cyclohexane (2.02)
k,, dm3 mol-' s-l
* *
(5.3 1.0) x ( 5 . 5 1.2) x
