J. Phys. Chem. 1993, 97, 1583-1588
1583
Carbocationic Fluorescence and Its Efficient Electron-Transfer Quenching A. Samanta,+** K. R. Gopidas,+*gand P. K. Das*J*l Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, and School of Chemistry, University of Hyderabad, Hyderabad 500 134, India Received: August 27, 1992; In Final Form: November 23, 1992
The absorption and emission spectral properties of several carbocations have been examined in acidified acetonitrile and Nafion matrices. Xanthenium and thioxanthenium carbocations are characterized by moderately strong yellow-to-orange fluorescence ( T F = 1-30 ns). In contrast, triphenylmethyl carbocation is very weakly emitting. The fluorescence of xanthenium and thioxanthenium carbocations is readily quenched by aromatics by a predominantly charge-transfer mechanism; the quenching rate constants are in the limit of diffusion control. Electron-transfer-derived radical cations (from aromaticquenchers) and radicals (from carbocations) are observed in the course of nanosecond laser flash photolysis. The yields of net electron transfer are, however, low (10.02), suggesting efficient back electron transfer in the radical cation/radical pairs in singlet configuration.
Introduction Carbocationsare well-defined intermediates in organic reaction mechanisms.' Although many of these species can be easily stabilized in appropriate acidic environments,' very little work has been done on their excited-state properties. In view of the fact that ground-state reactivity is generally augmented in the excited state as a result of enhanced exothermity from electronic excitation,it is of interest to explore the propertiesand interactions of singlet and triplet states of carbocations and related intermediates. Not surprisingly, several reportsz-' in this area have appeared recently. In this paper we have examined the fluorescence behavior of a number of relatively stable carbocations. Because of their relatively high ~ K Rvalues + (e&, -0.84 for xanthenium cation in aqueous HzSO~),'these carbocations could be easily produced in mildly acidic media, namely, in the presence of trifluoroacetic acid. In particular, we have noted that some of thesecarbocations possess quite long fluorescence lifetimes, and hence the lowest excited-state singlets are easily subject to bimolecular reactions, e.g., nucleophilicand electron-transferprocesses. We have studied the steady-state quenching of some of the carbocationic singlets by a number of aromatic electron donors and have shown, by laser flash photolysis, that electron transfer actually occurs in the course of the quenching leading to long-lived radicals and radical cations. In this connection, we note that in a recent study,8Kochi and co-workers have observed very short lived ion radical pairs in the course of picosecond laser excitation of charge-transfer complexes of tropylium ions with various aromatic donors; these ion-radical pairs decay on a picosecond time scale predominantly via back electron transfer.
Experimental Section Triphenylmethanol (TPM), 9-phenylxanthen-9-01(PX), and 9-hydroxyxanthene(X) were purchased from Aldrich and purified by recrystallization from toluene. 9-Phenylthioxanthen-9-01 (PTX)was prepared by adaptation of reported re procedure^.^ 9-Hydroxythioxanthene(TX) wasa gift from Dr. D. Weir. Nafion films (Aldrich) were used as received. The solvents and reagents used in this work were either of spectral grades or purified by distillation or recrystallization.
' University of Notre Dame.
University of Hyderabad. Present address: CSIR, Regional Research Laboratory,Trivandrum695 019, India. 1 Present address: 327 PL, Phillip Research Center, Bartlesville, OK f
8
74004.
The absorption spectra were measured in Perkin-Elmer 3840 and HP8452 diode-array spectrophotometers (2-nm bandpass). For steady-state fluorescence measurements, an SLM spectrofluorimeter'O was used in a right-angle configurationfor excitation and emission. For emission quantum yields, quinine sulfate in I N HzSO4 was used as the reference (& = 0.55)." The stcadystate quenching of fluorescence by oxygen and HTEMPO was studied using a Perkin-Elmer LS-3 spectrofluorimeter. The fluorescence lifetimeswere determined either by nanosecond laser flash photolysis (see below) or time-correlated single-photon counting. The apparatus for the latter method, described elsewhere,l2made use of a picosecond laser source (Mode-locked, Q-switched Quantronix 416 Nd:YAG, third harmonic at 355 nm, -80 ps, with a frequency of 5 kHz and integrated power of 10 mW/pulse) for excitation of the fluorescence. The nanosecond laser flash photolysis experiments were performed using a Quanta-Ray DCR-1 Nd:YAG coupled with Quanta-Ray PDL-1 dye laser (355,425, and 532 nm, 6 ns, 5-50 mJ/pulse). Both right-angle and front-face configurations were used for laser excitation and for transient absorption/emission monitoring. The laser intensities were appropriately attenuated to suit the absorptionof thesolutions at theexcitation wavelength. The kinetic spectrophotometers, data collection systems and experimentalprocedures aredescribed in recent publicationsfrom the Radiation L a b o r a t ~ r y . l ~ ~ ' ~ - I ~ For picosecond laser flash photolysis in the absorption mode, use was made of a system from Quantel. The laser excitation wavelength was 355 nm (Nd:YAG, third harmonic, -18 ps). The equipment has been described elsewhere.I6 The steady-state photolysis experiments were done inside the sample compartment of Perkin-Elmer diode-array 3840 sptctrometer; use was made of a specially designed sample holder and the filtered light (A > 400 nm) from a fiber optic illuminator (Fiberlite Model 190). Results
Absorption-Emission Spectraand FluorescenceLifetimes. The carbocationsunder study were generated by acidifying acetonitrile solutions of the corresponding alcohols with trifluoroacetic acid, TFA (6.5M) or HzS04(10% v/v). The spectral measurements were performed within half an hour of the acidification; during this time there was no apparent deterioration of the solutions (as indicated by a lack of change in the absorption spectra). Figures 1-4show theabsorptionspectra of xanthenyl (X+),thioxanthenyl (TX+), 9-phenylthioxanthenyl (PTX+), and triphenylmethyl (TPM+) carbocations as produced from the respective alcohol
0 1993 American Chemical Society 0022-3654/93/2097-1583~04.00/0
1584 The Journal of Physical Chemistry, Vol. 97, No. 8, 199I3
Samanta et al.
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Figure 1. Absorption (A) and corrected fluorescence (B) spectra of X+ in TFA:acetonitrilemixtures. The volume ratios of TFA and acetonitrile were 1:l for absorption and 1:40 for fluorescence. &,,, for fluorescence was 440 nm.
:
2SO
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450
SO0
S50
600
Wavelength, nm
Figure 4. Absorption spectrum of TPM+ in 1:l TFA:acetonitrile (v/v).
Wavelength, nm
Figure 2. Absorption (A) and corrected fluorescence (B) spectra of TX+ in 1:l and 1:40 (both v/v) TFA:acetonitrile mixtures, respectively. &,,, for fluorescence was 440 nm.
W 0)
r" L O
350
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Wavelength, nm
Figure 3. Absorption (A) and corrected fluorescence (B) spectra of PTX+ in 1:l and 1:40 (both v/v) TFA:acetonitrile mixtures, respectively. &,,, for fluorescence was 440 nm.
substrates in acetonitrile containing 6.5 M TFA. The spectrum of 9-phenylxanthenium cation (PX+) under these conditions is very similar to that in acetonitrile acidified with 8-108 H2SO4, as reported earlier.2 The data regarding absorption maxima and molar extinction coefficientsare given in Table I. The extinction coefficients were calculated on the basis of an assumption that the parent alcohol was quantitatively converted into the carbocation in the presence of the high concentration of the acid. Absorption spectra in 10% HzS04 in acetonitrile are very similar to those in acetonitrile containing 6.5 M TFA. Also shown in Figures 1-3 are the fluorescencespectra obtained by photoexcitationof X+, TX+, and PTX+ at 36CL500nm. These are broad and structureless and display a mirror-image relationship with respect to the corresponding lowest-energy absorption band systems. While the emission intensity is high in the case of X+,PX+,and TX+, it is moderately low for PTX+ and very weak for TPM+. This is evident from the measured quantum yields. of emission (&) given in Table I. The observed fluorescence lifetimes ( T F ) in the case of X+, PX+,and TX+ were long enough to be measurable by nanosecond flash photolysis using 6-11s laser pulses at 355 and 425 nm (see
Figure 5 ) . This was, however, not the case with PTX+; for this system, a much shorter T F necessitated the use of the timecorrelated single-photon-countingtechnique based on picosecond laser pulse excitation. A typical decay profile for PTX+ fluorescence is shown in Figure 6. No attempt was made to measure T Fin the case of TPM+ by single-photoncounting because of the very weak nature of the emission; the fluorescence decay profiles of TPM+ observed upon photoexcitation by 355-nm laser pulses (6 ns) were found to follow the laser pulseclosely,suggesting that T F for this carbocation is very short ( 400 nm) in Perkin-Elmer 3840 spectrometer and the absorption-spectral
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1585
Carbocationic Fluorescence
TABLE I: Ahrptioa md Emissioo Spectral Data of Carboations in 1:l at 22 O C
carbocation
abs maxima,” nm
ext coeff! lo3 M-I cm-l
254 372 434 260 375 450 277 380 477 280 384 495 248 292 405 430
35.1 32.7 1.75 33.1 28.7 4.75 30.2 9.05 1.02 60.7 16.1 5.20 5.01 1.52 37.6 36.7
X+ PX+ TX+ PTX+ TPM+
(v/v) AcetoniMkTrifluororcetic Acid
Mixture
emission quantum yieldd
radiative rate constant: 1O’s-I
emission maximum,
