J . Phys. Chem. 1993,97, 3551-3554
3551
Photophysics of Lithium Phthalocyanines and Their Radicals Sylvain L. M a t + and Thomas W. Ebbesen' Fundamental Research Loboratories, NEC Corporation, Miyukigaoka 34, Tsukuba 305, Japan Received: November 4, 1992
The photophysical properties of dilithium phthalocyanine (LizPc), dilithium naphthalocyanine (Li2NPc). and their corresponding stable radicals have been measured in both their ground and excited states. The SIlifetimes of Li2Pc and LizNPc are 5.3 and 4.4 ns, respectively. The transient absorption spectra of SIshow maxima a t 478 nm for LizPc and 520 nm for Li2NPc with differential molar absorption coefficients of 14 200 and 28 900 M-I cm-I, respectively. The fluorescence quantum yield is the same (0.5 f 0.05) for both molecules. The TI lifetimes are >60 ps for Li2Pc and > 100 FS for Li2NPc. The triplet quantum yield times the differential triplet molar absorption coefficients are 3800 M-I cm-I a t 469 nm for Li2Pc and 9500 M-' cm-I a t 603 nm for LiZNPc. Li2Pc and LizNPc will photoionize biphotonically upon excitation at 354 nm to give the corresponding stable radical phthalocyanines. The excited state of LiPc' is also reported and it has a 130-ps lifetime.
Introduction
Dilithium phthalocyanine, Li2Pc,was first synthesized in 1938 and since then it has been used in a variety of ways.' Due to its good solubility, it has been a useful precursor for the preparation of many metallophthalocyanines. Li2Pc can also be easily oxidized to give the stable radical, LiPc*,*q3which in the solid state was found to be an intrinsic molecular semicondu~tor.~.s The spectral properties of these specieshave been analyzed in quite some detail, specially in the case of L ~ ~ P C . ~However, -' the excited-state properties do not appear to have been studied so far, although LilPc should be a good example of group Ia metal phthalocyanines that can be compared to other phthalocyanines belonging to group Ia (e.g., HzPc), group IIa (e& MgPc), and group IIIa (e.g., AIPcCI).*-" H2Pc and MgPc fluoresce strongly and the main radiationless deexcitation path is internal conversion.8.9 This is in contrast to the group IIIa phthalocyanines, where the main nonradiative deactivation pathway is intersystem crossing due to the metal-induced spin-orbit coupling.'O-l' We report here that in the case of the Li2Pc and Li2NPc(dilithium naphthalocyanine) the major nonradiative deactivation pathway appears to be intersystem crossing, although spin-orbit coupling must be weak. The Li2Pc and Li2NPc are easily photoionized (biphotonically) to yield the corresponding radicals, whose spectral properties are also reported. Finally the excited-state properties of monomeric LiPc'could be measured and this should be of interest since there are very few such studies of phthalocyanine radicals.
Experimental Section Dilithium phthalocyanine was purchased from Midcentury. LiPc' was prepared electrochemically as described in the literature.24 The preparation of di-lithium naphthalocyanine and its radical LiNPc' is reported elsewhere.I2 LizPc and LiPc' are very sensitive to protic environments in which they yield H2PC. The sensitivity of LizNPc to proticity is more subtle and depends very much on the solvent. However, it will degrade upon exposure to light. Its radical products LiNPc' and aggregate [Li2NPc(LiNPc),J are, however, very stable in solution.'* For continuous irradiations, a Wacom BMO-1OOD super high pressure mercury lamp was used together with appropriate combinations of filters. The fluorescence was measured on a SPEX Fluorolog 2 spectrofluorophotometer.
' On leave from E.S.P.C.I.,Paris, France, on a NEC traineeship. Present address: Laboratory de Chimie des Interactions Moleculaires, College de France, 75005 Paris, France. 0022-365419312097-355 1$04.00/0
For the excited-state transient absorption studies, both a picosecond and a nanosecond laser flash photolysis were used. The picosecond system is an upgraded version of one that has been described in detail e1~ewhere.l~Basically it consists of a Quantel YG501-10Dp YAG laser, which gives 20-ps pulses at 10 Hz with a choice of wavelengths generated from the fundamental output at 1064 nm: 266 nm, 354 nm, 532 nm. One part of the 1064-nm pulse is used to generate a pulse of white light by passing the beam through a D20/H20 solution. This serves as a probing beam for measuring the change in absorbance at a given delay after excitation of the sample. The delay is obtained by passing the probing pulse through optical fibers of different lengths. The energy of the 354-nm excitation pulse used in these experiments was typically between 65 pJ and 2 mJ/pulse focused on a 20-mm2 area in the sample. After the sample, the probe light passes through a McPherson 2035 spectrometer and then is collected by a Princeton Instruments DDA-512 array. The DDA-512 is connected to a Princeton Instrument Controller ST- 120, which is controlled and operated by a NEC Power Mate 1 Plus computer. About 200-400 shots are averaged, which gives a noise level of ca. f 0.001 absorbance units. A spectral window of about 240 nm is recorded each time. The nanosecond laser flash photolysis system consists of a Quantel660-10 YAG laser yielding 5-ns pulses. A pulsed (1 ms) xenon lamp (Tokyo Instruments XF80S) is used to probe the change in absorbance of the sample after excitation (354 nm in these experiments). The probing light passes through an Oriel 77200 monochromator and is detected by a Hamamatsu R666 photomultiplier. The time-resolved signal, after being offset with a backoff device, is captured with HP 541 11D digitizer. The whole system is controlled and the data analyzed by a NEC PC9801RX computer. Typically 10 shots are averaged. For the laser experiments, a flowing solution was always used in case of any photodegradation. Ultrapure argon was used to degas the solutions. The reference standard was benzophenone in benzene (7220 M-' cm-I 14). The actinometry was carried down to 40 pJ/pulse (ca. 6 X 1O-Io einsteins cm2 per pulse) to be able to measure the initial slope of the transient absorbance dependenceon the laser intensity. Theabsorbanceof the reference and the sample was 0.06 in the excitation depth for accuracy in the actinometric measurements. For reasons of solubility and stability, the solvents for Li2Pc and Li2NPc were acetonitrile and acetone, respectively. The solvents (Merck, Uvasol grade) were dried over molecular sieves (Merck, 0.4 nm). 0 1993 American Chemical Society
Gilat and Ebbesen
3552 The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
-- 400 600 800 1000 1200 nm Figure 3. Absorption spectra of the LiNPc radical (- -) in acetone and a radical aggregate [LizNPc(LiNPc),] (-) in pyridine. 300 400 500 600 (nm) 801 Figure 1. Ground-state absorption spectra of Li2Pc in acetonitrile (-)
and Li2NPc in acetone (- -),and their absolute fluorescencespectra(LilPc, Li2NPc, - -).
.
- e ;
-0.11 400
O246ns
5 00 600 nm Figure 4. Differential excited singlet and triplet spectra of LizPc in acetonitrile recorded at 30 ps and 50 ns, respectively. Insert: In of the
absorbances change versus time due to decay of S I .
400
600
8 00
1000
nm
'
I
Li2NPC
Figure 2. Absorption spectra of the LiPc radical (- -) and its aggregate
(LiPc), (-) in chloronaphthalene.
Results and Discussion (1) Ground-StateAbsorptionSpectra. Theground-state spectra of Li2Pc and Li2NPc are shown in Figure 1. They are very similar in shape except that of the Li2NPc is shifted about 80 nm toward longer wavelengths. The peak absorption coefficient of LizNPc is much larger (375 000 M-' cm-I, fwhm 16 nm, acetonei2)than that of Li2Pc (Ca. 150 000 M-I cm-I, fwhm 18 nm, acetone5). This large difference in the absorption cross section is also found for the excited states as will be shown below. This can probably in part be accounted for by the much larger surface cross section of the Li2NPc. While Li2NPc obeys Beers law when the concentration is varied, it is clear from various experiments that it forms small loosely bound aggregates at concentration >cas Mal2Thiswill beillustratedin thisstudy by theconcentration dependence of the products of photoionization. The spectra of the corresponding radicals of these species are shown in Figures 2 and 3. Figure 2 gives the spectra of LiPc' in both its monomeric and aggregate forms. Contrary to what has been reported beforeS not only the peak at 945 nm but also that at 693 nm can be assigned to the aggregate form since both peaksdisappear at low concentration. Figure 3 shows the spectra of two distinct radical forms generated from Li2NPc by electrochemical oxidation, depending on the concentration of Li2NPc and the applied voltage. This is presented in detail elsewhere12and is confirmed by the photoionization studies below where again the concentration of Li2NPc affects the products that are formed. At low concentrations of LizNPc, LiNPc' is formed, while at high concentration, a radical aggregate is formed with stoichiometry [LizNPc(LiNPc),] where n is between 1 and 2 as indicated by coulometric studies.12
500 600 nm Figure 5. Differential excited singlet and triplet spectra of Li?NPc in acetone recorded at 30 ps and 50 ns, respectively. Insert: In of the absorbance change versus time due to the decay of SI.
(2) Excited-State Properties. (a) LilPc and Li2NPC. The absolute fluorescence spectra of Li2Pcand LizNPc are shown in Figure 1. The fluorescence intensities are strong with maxima a t 665 and 738 nm for LizPc and LizNPc, respectively. Excitation spectra confirm that the observed fluorescence are due to these compounds. The fluorescence bands are narrow and their Stokes shifts are extremely small (a couple of nanometers in both molecules). This must be due to the rigidity of these molecules. The fluorescence quantum yields were measured by using tetraphenylporphyrin in benzene as reference [*.F = 0.13 (ref 14)] and found to be 0.50 f 0.05 for both molecules. From the excited singlet lifetime given below, we calculate that the radiative IO8 S K I for Li2Pc and constants are 0.94 X 108 SKIand 1.14 LilNPc, respectively. After excitation at 354 nm with 20-ps laser pulses, both Li2Pc and Li2NPc yield very similar Si S,absorption spectra (Figures 4 and 5), except that in the latter case the peaks are shifted toward the red as in the ground-state spectra. The lifetime of SImeasured from the time evolutions are 5.3 and 4.4 ns for LizPc and LilNPc, respectively, as shown by the inserts. These lifetimes and absorption spectra were unaffected by the ground-state
-
-
Photophysics of Lithium Phthalocyanines concentration in the range studied (5 X 10-6-5 X 1 0 M). The differential molar absorption coefficients measured by using benzophenone triplet as reference gives 28 900 M-I cm-l at 520nm maximum for Li2NPc and 14 200 M-I cm-I for LizPc a t 478 nm. From these values and the TI to SI ratio in Figure 4 and 5, the triplet quantum yield $T times the differential molar absorption coefficients ACT can be calculated. At the broad maxima, $JTACTvalues are 9500 M-I cm-I (603 nm) for LizNPc and 3800 M-' cm-' (469 nm) for Li2Pc. The wavelength of the maximum absorption of the LizPc triplet state is typical of metallophthalocyanines. II The triplet-state lifetimes were found to be > 100and >60 us for LizNPc and Li2Pc,respectively. These lifetimes are given as lower limits since, for such long lifetimes, bimolecular reactions with residual impurities will easily shorten the true lifetime. We also tried to estimate the triplet quantum yields @T from the ratio of depopulation of the ground state at time zero right after the picosecond pulse and that after thedecay of the excited singlet state. The measurement of the depopulation was made in the Q band, where the extinction coefficients of the excited states (both singlet and triplet) might be assumed to be negligeable compared to the ground state. This method was used successfully by Brannon and Magde for group IIIa phthalocyanines.I0 For LizPc and LizNPc, the triplet quantum yields were estimated to be ca. 0.6 f 0.1. Considering the fluorescence quantum yields, this value overestimates the triplet quantum yields. This must be due to the fact that the SI extinction coefficients are not negligible in the Q band. However, if we accept these triplet quantum yields as close to the true values, then it appears that @F + @T = 1. Such a relation was observed for group IIIa metal phthalocyanines'O and indicates that the dominant nonradiative decay path is intersystem crossing. This is in contrast with phthalocyanines such as MgPc and H ~ P cwhere , the main nonradiative decay pathway is internal conversion." The difference in the nonradiativedecay paths of group IIIa phthalocyanines and MgPc or H2Pc is normally explained by the difference in spin-orbit coupling. However, although LizPcand Li2NPc have weakspinorbit coupling, intersystem crossing is a significant pathway, indicating that other factors also influence the deexcitation modes of phthalocyanines. Perhaps by studying other group Ia phthalocyanines, such factors might be better understood. (b) PbotoionizationofLilPcandLiINPc. With theabsorption spectra of excited states and the radicals of LizPc and LizNPc well defined, the possibility of photoionization of the latter compounds was explored. The transient spectra was measured 144 ns after excitation with the 20 ps pulse and the solutions of the two compounds were saturated with oxygen to trap the ejected electrons and to quench nonionized triplet states. Such experimental conditions assure that only the transient spectra of the radicals (which are stable to oxygen) are observed. The intensity was then varied over one order of magnitude between 0.5 and 5 mJ per pulse and the spectra were recorded. The transient spectra recorded for both compounds are shown in Figures 6 and 7. For the spectral window analyzed, the maxima correspond well to those of the radicals shown in Figures 2 and 3. With regards to Li2NPc, the maximum shifted to longer wavelengths with increasing ground-state concentration of LizNPc as shown in Figure 7. As can be seen in Figure 7, on going from ca. 5 X to 1.9 X to 5 X 10-5 M, the maxima shifted from 581 to 591 to 600 nm. In view that LiNPc' has a maxima at 575 nm while the aggregate radical [Li2NPc(LiNPc*),] has a maxima a t 660 nm (Figure 3), upon increasing the groundstate concentration the transient spectra reflect the increasing proportion of aggregates in solution. Within 144 ns, bimolecular reactions involvingthe naphthalocyanines can be excluded because of their low concentration, therefore the aggregates must be preexisting in solutions. In other words, Li2NPc aggregates are present in acetone already at ca. 10-5 M, even if their existence
The Journal of Physical Chemistry, Vol. 97, No. 14, I993 3553
I
600 nm
5 00
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I
Figure 6. Differential spectrum of the photoionization product recorded 144 ns after excitation of LiZPc in oxygenated acetonitrile. Insert: observed yield dependence on laser intensity (-) and pure square dependence on laser intensity (- -).
AA a.u
5 00
600
nm
Fipre7. Differentialspectraofphotoionizationproductsat threedifferent initial ground-state concentrations of Li2NPc (see text) recorded 144 ns after excitation in oxygenated acetone (the scales have been normalized for comparison). Insert: correpsonding yield dependencies on laser intensity (+, 0 , A) and pure square dependence fit (- -). The expected monophotonic fit estimated from the laser intensities and ground-state extinction coefficient is also given. goes undetected when Beers law is checked.I2 The radical aggregatecan be further photoionized to give LiNPc'. Therefore it is very important that in the above experiments the solution is flowing not to distort the experimental conclusions. The above radical species accumulate in the irradiated solution and their entire spectra recorded after the lasers experiments yield the same spectra as those of the radicals in Figures 2 and 3. This further confirmstheinterpretation of the transient spectra. Efforts were made to measure the transient species due to the photoejected electron and its reaction with the solvent; however, they could not be observed. This is perhaps not surprising considering the work by Rodgers and his colleagues showing that the pulse radiolysis of acetone gives an absorption assigned to the acetone cation and that that of acetonitrile gives a main peak a t 1450 nm tentatively assigned to acetonitrile anion.I6-l7 The latter absorption is far beyond our spectral range. These photoionization reactions are clearly biphotonic, as can be seen from the intensely dependencies shown in the inserts in Figures 6 and 7. In Figure 7, the approximate monophotonic intensity dependence of the SI yield (based on the experimentally observed dependence) and a purely square dependence on the laser intensity are given for the purpose of comparison. (c) LiPc'. Efforts were made to observe the excited state of both LiPc' and LiNPc', but only that of LiPc' could be measured. Even this proved to be difficult as only in the monomeric form could reliable data be obtained. The absorbance in the 1-mm excitation depth was only 0.002 at the 532-nm excitation wavelength. The resulting transient is shown in Figure 8, showing both the depopulation of the ground state and a absorption peak at 654 nm due to the excited state. This doublet DIexcited state has a lifetime of 130 ps as can be seen from the decay given in the insert (Figure 8). This corresponds to 7.7 X IO9 SKI nonradiative rateconstant. At higher concentration where LiPc'
3554 The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
Gilat and Ebbesen these important molecules.~.4 Finally the excited state of one of the simplest possible phthalocyanine radicals, LiPc', was measured.
Acknowledgment. We are very grateful to K. Tanigaki, H. Hiura, and S.Kuroshima for their support in the course of this work. References and Notes 500
600
nm
Figure 8. Differential excited doublet spectrum of LiPc' in chloronaphthalene and its decay kinetics (insert).
is no longer in the monomeric form, the decay is no longer a simple exponential as might be expected. The short lifetime of the LiPc' DI is similar to the lifetime we recently measured for excited state of LuPc2' (60 ps).'* However, in that case, it was clear that the short lifetime was due to deactivation via low-lying intraring charge-transfer states. Such a path is not available in LiPc'. More examples of such radical phthalocyanine photophysics will be needed before we obtain better insight into their properties.
Conclusion The photophysics of Li2Pc and Li2NPcare very similar despite the differencein degree of conjugation,and their excited properties are determined mainly by the central ions. The dominant nonradiative pathway is intersystem crossing, despite the obviously weak spin-orbit coupling. The photoionzationof these molecules lead to the quantitative formation of the corresponding stable LiPc and LiNPc radicals, providing another method for generating
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