J. Phys. Chem. 1991, 95,971-975
97 1
Intramolecular Naphthalene Triplet Excimers in Solutions of Phosphazene Copolymers Lucia Flamigni,* Nadia Camaioni, Istituto F.R.A.E.-C.N.R., Via de' Castagnoli, 1. 40126 Bologna, Italy
Pietro Bortolus, Francesco Minto, and Mario Gleria Istituto F.R.A.E.X.N.R.,Serione di Legnaro, Via Romea 4, 35020 Legnaro, Padova, Italy (Received: April 23, 1990; In Final Form: July 25, 1990)
Phosphorescence and transient absorption spectra of poly[bis(j3-naphthoxy)phosphazene] and of its model compound, hexakis(0-naphthoxy)cyclophosphazene, have been found to be significantly different in both 2-methyltetrahydrofuran/methylene chloride fluid solutions and rigid matrices. The spectroscopic characteristics of the model compound closely resemble those of the naphthalene chromophore while, on the contrary, no naphthalenic triplet-triplet absorption or phosphorescence appears in the polymer. Population of the naphthalene triplet was achieved by energy transfer in a series of phosphazene copolymers containing benzophenone and naphthalene in variable amounts. Copolymer samples containing high naphthalene percentages show a peculiar red-shifted and broadened phosphorescence with respect to copolymers containing low naphthalene content, which exhibit the typical naphthalene bands. Transient absorption experiments have shown that a broad absorption band around 500 nm is due to the same state responsible for this anomalous phosphorescence. On the basis of close similarities with literature data and of the effect of naphthalene crowding on the formation of the species, this is identified as a naphthalenic triplet excimeric state.
Introduction While the existence of singlet excimers is a well-established phenomenon for many aromatic compounds in solution, relatively little is known about triplet excimers. Nevertheless, the existence of phosphorescent excimers of aromatic hydrocarbons for both intramolecular and intermolecular systems in solution has been reported since the 1 9 7 0 ~ . ' - ~In some cases such triplet excimeric states have also been characterized by their absorption A review on the theoretical and experimental work done to characterize triplet molecular excimers in solution, with particular emphasis on naphthalene, has been published by Lim.* One of the main conclusions of this work is that the geometry of the naphthalene triplet excimer is completely different from that of the corresponding singlet excimer in which the two aromatic rings lie parallel to each other. The preferred geometry is a butterflyshaped arrangement of the two naphthalenes in which the long axes of the molecules are parallel and the short axes make an angle of about 109O. These conclusions have been confirmed by more recent work of the same author9 and another group.1° However, an energy level diagram for singlet ahd triplet excimer forming systems has been published which assumes a similar geometry for the two excimers." Such a diagram was used to derive an approximate zero binding energy for the naphthalene triplet excimer, raising doubts on the existence of this species. Moreover, the failure to detect excimeric bands in the phosphorescence spectra of solutions containing thoroughly purified naphthalene and naphthalene derivatives led Nickel and Rodriguez Prieto to conclude that the emission was due to a sensitized phosphorescenceof a biacetyl-like impurity of the solvent.'* Locke and Lim were able to answer this criticism on the basis of ad~~
( I ) Takemura, T.; Baba, H.; Shindo, Y. Chem. Lett. 1974, 1091. (2) Subudhi, P. C.; Lim, E. C. J . Chem. Phys. 1975.63, 5491. (3) Takemura, T.; Aikawa, M.; Baba, H.; Shindo, Y. J . Am. Chem. Soc. 1976, 98, 2205. (4) Subudhi, P. C.; Lim. E. C. Chem. Phys. Leu. 1976,44, 479. (5) Okajima, S.; Subudhi, P. C.; Lim, E. C. J. Chem. Phys. 1977,67,461 I . (6) Subudhi, P. C.; Lim, E. C. Chem. Phys. Lerr. 1978, 56, 59. (7) Zachariaue, K. A.; Busse, R.; Schracder, V.; Kuhnle, W. Chem. Phys. Lctr. 1982, 89, 303. (8) Lim, E. C. Acc. Chem. Res. 1987, 20, 8. (9) Locke, R. J.; Lim, E. C. J . Phys. Chem. 1989, 93, 6017. (10) Tung, C.-H.; Wang, Y.-M. J . Chem. SOC.,Chem. Commun. 1989, 1891. (11) Klopffer, W. EPA Newsl. 1987, No. 29, 15. (12) Nickel, B.; Rodriguez Prieto, M. F. 2.Phys. Chem. (Munich) 1985, 150, 31. Nickel, 8.;Rodriguez Prieto, M. F. Chem. Phys. Lett. 1988, 146, 125.
ditional results13 and reasserted the assignment of the emission and transient absorption to the naphthalene triplet excimer. Recently new negative results have been reported on solutions of 1-brom~naphthalene,~~ confirming that the topic is still far from being settled. The existence of naphthalene triplet excimers in polymeric systems has been reported in several papers. Films of poly(2vinylnaphthalene) display low-temperature emission spectra, attributed to the phosphorescenceof triplet excimers, similar to those reported for free ~ ~ a p h t h a l e n e . 'Solutions ~ * ~ ~ of the above polymers, at room temperature, showed a transient absorption around 480 nm attributed to the triplet excimer." Naphthalene endlabeled poly(ethy1ene glycol) emits excimer phosphorescence in isopentane glass at 77 K.Io The existence of naphthalene triplet excimers in polymeric films is accepted since it is believed that the chromophores are pressed together "by some local strain induced during the formation of the film"." On the contrary, the existence of triplet excimers in polymer solutions is still doubtful because chromophores are free to rotate and assume the most stable configuration. In this respect, the situation could be similar to the case of solutions of dinaphthylalkanes, the difference being a much higher local concentration of chromophores in polymers, favoring excimeric interactions. In conclusion, we believe that detection of naphthalene triplet excimers in polymer solutions could be a positive contribution to the dispute about the existence of such species in dinaphthylalkane solutions. In this work we examined the photophysical behavior of solutions of poly[bis(P-naphthoxy)phosphazene], a polymer with a phosphorus-nitrogen skeleton bearing two naphthoxy groups on each phosphorus, and of its low molecular weight cyclic trimer, the hexakis(j3-naphthoxy)cyclophosphazene. A series of phosphazene copolymers containing both naphthalene and benzophenone groups were also studied to rationalize the phenomena observed in the homopolymer. The aim of thin work was to verify in solution the existence of naphthalenic triplet species different from the monomeric naphthalene triplet, to derive its spectroscopic and kinetic properties, and to compare them with previously reported data. (13) Locke, R. J.; Lim, E. C. Chem. Phys. Leu. 1987, 138,489. Locke, R. J.; Lim, E. C. Chem. Phys. Lerr. 1989, 160,96. (14) Beckhman, S.; Wright, T. M.; Schuh, M. D.J . Phys. Chem. 1988, 92, 7057. (15) Kim, N.; Webber, S . E. Macromolecules 1980, 13, 1233. (16) Chakraborty, D. K.; Burkhart, R. D. J . Phys. Chem. 1989,93,4797. (17) Pratte, J. F.; Noyes, W. A., Jr.; Webbcr, S.E. Polym. Phorochem. 1981, 1, 3.
0022-3654/91/2095-097 1 $02.50/0 0 1991 American Chemical Society
Flamigni et al.
972 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 SCHEME I
0, N4
9
P
'N
x=
POPN TRIN
x
COPOPX%N
=--@ c
II
0
Experimental Section Marerials. Poly [bis(8-napht hoxy) phosphazene] (POPN) and the trimer hexakis(&naphthoxy)cyclophosphazene (TRIN) were prepared according to the procedures described in previous w o r k ~ . I * * Particular '~ care was taken in the purification of the &naphthol used for the synthesis to eliminate emitting impurities. To this aim it was sublimed several times under vacuum immediately before use. The polymer was purified by several dissolutions in tetrahydrofuran and precipitations in methanol. The trimer was purified by crystallization from n-heptane. The copolymers containing both benzophenone and naphthalene groups (COPOPX%N) were synthesized with the same procedure adopted for POPN, Le., by substitution in poly(dich1orophosphazene) of the chlorines with proper amounts of &naphthol and p hydroxybenzophenone.20 The copolymers were purified by multiple dissolutions in T H F and precipitations in water (four times), n-hexane (two times), and ethyl ether (two times). The synthesized copolymers contained percentages of naphthalene ranging from 20% to 90%. The molecular weight of the polymers, M, (weight average), was in the range (1-2) X IO6. 2-Methyltetrahydrofuran (MeTHF) was first distilled over KOH and, subsequently, from LiAlH4. Methylene chloride (CH2CIz) was twice distilled from LiAIH4. Most of the polymers were hardly soluble in MeTHF so the solutions were prepared by diluting stock CH2CIzsolutions with MeTHF: the optical density at the excitation wavelength was in the range 0.5-1.1. The experiments were carried out in solvent mixtures of MeTHF/CH2CI2 (%lo v/v): this methylene chloride percentage does not significantly alter the quality of glasses at 77 K. The solutions, contained in specially designed vacuum-tight cells (I-cm path), were bubbled with nitrogen for 5 min and placed in the modified holder of a liquid nitrogen cryostat (Thor) for experiments ranging from room temperature to 90 K. Quartz tubes (0.4-cm i.d.) dipped in liquid nitrogen were used for experiments at 77 K. Apparurus. Uncorrected emission spectra were recorded on an MPF 44 Perkin-Elmer spectrofluorimeter. Phosphorescence spectra were measured with the pertinent phosphorescence accessory. The chopper speed was calibrated by placing a small dc lamp in the sample position and directly detecting the frequency of the photomultiplier signal on an oscilloscope. Time-resolved experiments were performed with a Nd:YAG laser (JK Lasers) with 15-ns pulse duration. The fourth (A = 266 (18) Glcria, M.; Barigelletti, F.; Dellonte, S.;Lora, S.;Minto, F.; Bortolus, P. Chem. Phys. Lett. 1981, 83, 559. (19) Gleria, M.;Minto, F.; Lora, S.; Bortolus, P.Eur. folym. J. 1979, IS,
671.
(20) Minto, F.; Flamigni, meral. folym., in press.
L.;Bortolus, P.; Gleria, M. J . Inorg. Organo-
a
4
m C
.-0u)
.-
u)
E
yi
h,nm
Figure 1. Emission spectra of POPN (- - - ) and TRIN (-) solutions in MeTHF/CH2CI2 at room temperature (a) and 77 K (b). Excitation at 280 nm.
nm) and third (A = 355 nm) harmonics were used for excitation. The beam was focused to an area 10 mm wide X 3 mm high. A xenon lamp, either continuous or pulsed, was used as the analyzing source at 90° to the excitation source. A mask was placed on the cell holder of the cryostat,to analyze a small volume (2 mm deep X 3 mm high) in the first slice of the irradiated sample. The light transmitted by the sample was focused on a monochromator and detected with a resolution of 4 nm by a R936 photomultiplier. Sequence generation, lamp pulser, and automatic back-off were all homemade. The energy at 266 nm was 25 mJ/cm2, and a t 355 nm was 28 mJ/cm2. The triplet-triplet absorption bands of = 24 500 M-' cm-I) and bennaphthalene in cyclohexane zophenone in benzene (esjo = 7220 M-' cm-I) were used as actinometers for the 266- and 355-nm pulses, respectively. A similar setup was used to detect the emission lifetimes. The mask on the cell holder was removed, the xenon lamp was turned off, and a combination of broad-band interference filters and cutoff filters replaced the monochromator. A digital storage oscilloscope (Tektronix 468) or a transient digitizer (Tektronix R7912) in conjunction with a microprocessor (Digital PDP 11/23) was used to acquire and process the signals by standard iterative nonlinear procedures, Results The room-temperature emission spectra of the homopolymer, POPN, and of the low molecular weight analogue, the cyclic trimer TRIN, in MeTHF/CH2C12 solutions excited at 280 nm are shown in Figure 1 a. The trimer solutions have two, clearly distinct, fluorescencebands attributed to the emission of a monomeric (L = 335 nm) and excimeric (A, = 390 nm) species.I8 In the polymer, the more prominent emission is due to the excimer fluorescence, while the monomer emission can be hardly detected
The Journal of Physical Chemistry, Vol. 95, No.2, 1991 973
Naphthalene Triplet Excimers
A,nm
h,nm Figure 2. Emission spectra detected with IO-ms delay of TRIN (a) and POPN (b) solutions in MeTHF/CH2CI2at 77 K. Excitation at 280 nm, OD280 = 0.6.
Figure 5. Emission spectra detected with IO-ms delay of COPOPZON (a) and COPOP77N (b) in MeTHF/CH2CI2 at 77 K. Excitation at 355 nm, OD351= 0.6. For comparison is shown the phosphorescence spec-
trum of the polyphosphazene fully substituted with benzophenone chromophores in MeTHF/CH2CI2at 77 K (-- -).
0.05
m 0.10-
O.D.
d
d Q 0.05-
aoo
A, nm Figure 3. Transient absorption spectra upon 266-nm laser excitation of a TRIN solution in MeTHF/CH2C12(ODm = 1.1). Room temperature: ( 0 ) t = 0 IS, (M) t = 80 ps. 90 K: (0) t = 0 ps. (0) t = 800 ms.
X,nm Figure 6. Transient absorption spectra upon 355-nm laser excitation of
COPOPZON solution in MeTHF/CH2Clz (OD,,,= 0.6). Room temperature: ( 0 )r = 0 fis, (M) t = 45 ps. 90 K: ( 0 )t = 0 I S , (0)t = 1.9
a
S.
a
350
400
*
500 I
450
h,nm Figure 4. Transient absorption spectra upon 266-nm laser excitation of a POPN solution in MeTHF/CH,CI, (OD,, = 1.1). Room temperat = 0 ps, (0) t = 1.8 ms. ture: ( 0 )t = 0 ps. 90 K: (0) as a shoulder on the onset of the emission. At 77 K,the emission spectra of both POPN and T R I N are dominated by a monomer-like naphthalene fluorescence which, in the polymer, appears quite broadened and red-shifted compared to the cyclic trimer (Figure lb). The delayed emission spectra of the previous solutions, detected 10 ms after the excitation, are shown in Figure 2. The trimer bands are those typical of naphthalene phosphorescence while, in the same spectral region, only a broad, very weak emission appears in the POPN sample. At 90 K, the transient absorption spectra following laser excitation with 266-nm light of a T R I N solution show the typical triplet-triplet absorption bands of naphthalene (Figure 3). The excited species decays with mixed first- and second-order kinetics. The polymer sample (Figure 4) shows a very weak absorption (OD < 0.01) with no pronounced features, which evolves into an even broader, longer lived, weaker absorption. Figures 3 and 4 also depict the transient spectra at room temperature. These experiments were performed with matched optical densities and at the same laser energy. From the data reported above, it clearly follows that in POPN no naphthalenic triplet can be observed with our time resolution, in contrast to what has been found for poly(2-
~inylnaphthalene).”*~’The observation of a naphthalenic triplet in the low molecular trimer indicates that the lack of detection in the heavy polymer is caused by intramolecular phenomena rather than by a specific phosphorus-naphthalene interaction. In order to get more information on the observed phenomena, experiments were performed to achieve sensitization of the triplet naphthalene appended to the polyphosphazene backbone.22 To this aim, several copolymers containing different percentages of naphthalene and benzophenone linked to the phosphazene chain were synthesized.20 Their spectroscopic characteristics upon excitation with 355-nm light, which is absorbed by the benzophenone group, are reported below. The phosphorescence spectra at 77 K recorded with a delay of 10 ms after 355-nm excitation of a MeTHF/CH2CI2 solution of COPOPZON, a polyphosphazene containing 20% naphthalenic and 80% benzophenonic chromophore, is reported in Figure 5a. In Figure 5b the phosphorescence spectrum of a solution of COPOP77N (the sample containing 77% naphthalenic and 23% benzophenonic chromophores) in MeTHF/CH2C12registered at 77 K under comparable conditions is presented. The experimental conditions, including OD a t the exciting wavelength, are similar to those of Figure 2. For the sake of comparison, the phosphorescence spectrum of the polyphosphazene fully substituted with benzophenone is also shown in Figure 5. It is clear that in the copolymers the energy absorbed by benzophenone is transferred to naphthalene. The phosphorescence spectrum of the polymer at low naphthalene content (COPOP20N). where the naphthalenic chromophores can be considered virtually “isolated”, (21) Bensasson, R. V.;Ronfard-Haret, J. C.; Land, E. J.; Webber, S.E.
Chem. Phys. Len. 1979, 68, 438.
(22) Triplet sensitization by excitation of free benzophenone addd to the solution of POPN gave results not easy to handle. In fact, owing to the scarce solubilityof POPN in the MeTHF/CH2Cl2mixture, the hydrogen abtraction of the excited carbonyl from the solvent very efficiently competeswith energy transfer to the polymer, and a low yield of naphthalene triplet was observed.
974
Flamigni et al.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
A
t
I
d
d
a
400
4so
500
X ,nm Figure 7. Transient absorption spectra upon 355-nm laser excitation of a COPOP77N solution in MeTHF/CH2C12(OD355 0.6). Room temt = 300 perature: ( 0 )r = 0 ps, (m) r = 45 p. 90 K: (0)r = 0 p, (0) W. I
O.1°
" " " " '
t
0.00
350
400
4so
500
A , nm
Figure 8. Transient absorption spectra upon 355-nm laser excitation of a COPOP90N solution in MeTHF/CH2C12(OD355= 0.5). Room temperature: ( 0 )t = 0 ps, (m) I = 45 p. 90 K: (0)t = 0 c(s, (0)r = 100 ps. The insert shows the decay at room temperature detected at 490 nm.
shows intense bands identical with the ones of the trimer, which are assignable to the naphthalenic triplet moiety. The polymer at high naphthalene content, COPOP77N, where an appreciable fraction of the excited naphthalene is likely to be surrounded by the same group presents a phosphorence spectrum with shifted maxima (A,, = 495, 530, and 570 nm) and broader bands, identical with the one reported for naphthalene triplet excimer at 77 detected in solid samples of poly(2-~inylnaphthalene)~~+~~ K. The transient absorption spectrum immediately after excitation with 355-nm light of MeTHF/CH2CI2 solutions of COPOPZON at 90 K, shown in Figure 6, is very similar, though less structured = 425 nm) to the and with a few nanometers red shift (A, spectrum of the trimer sample. The decay is exponential with 7 = I .5 s, and no residual absorption is left. On melting of the glass, above 105-1 10 K, the decay no longer remained exponential, probably because of second-order annihilation reactions. Spectra at room temperature are also shown in this figure. The transient absorptions immediately following excitation with 3554111 light of COPOP77N and COPOP90N in MeTHF/ CH2CI2 solutions at 90 K and at room temperature again show a naphthalenic triplet, as can be seen in Figures 7 and 8. In both samples at 90 K the decay is no longer exponential, and it is faster the higher the naphthalene content in the sample. After the complex decay of the triplet, a residual absorption extending from 400 to 600 nm is left, with intensity increasing with the naphthalene content of the sample. Part of the naphthalene triplet
Figure 9. Temperature dependence of the rate constant of the absorption decay measured at 500 nm (A)and of the emission decay measured in the region 560-650 nm (0)in MeTHF/CH2CI2solutions of COPOP77N and COPOP90N upon laser excitation at 355 nm.
band remains (see Discussion); depurated from this contribution, the residual absorption appears to be broad, with a maximum around 500 nm, similar to the one assigned by Lim to the naphthalene triplet excimer.6*13The decay of the 500-nm species is exponential with 7 = 0.5 s at 90 K. The exponential character is also retained upon melting of the glass, and the temperature dependence of the rate constants is reported in Figure 9. In the same graph are reported the rates of phosphorescence decay measured in the region 560-650 nm upon laser excitation at 355 nm of the COPOP77N and COPOP90N samples. Clearly the emitting species of Figure 5 is also responsible for the absorption band around 500 nm. The curve in Figure 9 is drawn with a least-squares fitting procedure according to the Arrhenius equation k = ko + Ae-hE/RT.The resulting parameters are ko = 2 s-I, A = 4.5 X lo3 s-I and, AE = 5.8 kJ/mol.
Discussion The excited states of naphthalene appended via an oxygen bond to a phosphazenic skeleton are very similar to that of free naphthalene in the same solvent. This can be deduced from the spectroscopic properties of the trimer where 77 K emission and triplet-triplet absorption spectra exhibit only a few nanometers red shift with respect to naphthalene in the same solvent. The different features in the spectra of the polymers must be ascribed to intramolecular interactions taking place in the polymer. Phenomena associated with the singlet excited state and its excimeric interaction will be extensively discussed elsewhere.23 With regards to the POPN, neither at room temperature nor a t 77 K is the naphthalene triplet detectable in polymer solutions. The lack of detection of the triplet states in the polymer could be explained by a greatly reduced excited singlet to triplet intersystem crossing efficiency of naphthalene when appended to polymers. A similar behavior has been reported for poly(2-vinylnaphthalene) where the 41schas been shown to decrease by increasing the molecular weight of the macromolecule.2' Nevertheless, the characteristic of the naphthalene triplet linked to the polyphosphazene chain can be studied both by sensitization of POPN by benzophenone dissolved in the solution and by "internal" sensitization by benzophenone appended to the same backbone in a series of copolymers containing both benzophenone and naphthalene groups. The latter method is by far more effective and operates also in glassy solutions. The advantage in the sensitization approach consists in the observation of mere triplet interactions without complications arising from singlet-state phenomena. The naphthalene triplet produced by internal sensitization in the polymers shows an absorption spectrum very similar to the free-naphthalene triplet in the same solvent except for a IO-nm red shift in the peaks maximum and a loss in the structure. At 90 K, in polymers with high naphthalene content, this triplet has a relatively short lifetime, because of triplet-triplet annihilation (23) Flamigni, L.; et al. Manuscript in preparation.
Naphthalene Triplet Excimers processes. The short triplet lifetime allows the observation of an underlying band, appearing as a shoulder of the intense triplettriplet absorption, with maximum at -500 nm. The increasing intensity of this absorption with the naphthalene content of the copolymer indicates a correlation with the crowding of naphthalene groups. The transient absorption is very similar to that found by Lim in some dinaphthylalkanes and attributed to triplet-triplet absorption of excimem6 The 77 K emission of the copolymers changes from the structured naphthalene phosphorescence in copolymers with low naphthalene content to a broader, red-shifted, long-lived emission in copolymers containing a high naphthalene percentage. Lifetime determinations over a wide temperature range of both the transient absorbing a t 500 nm and the above-described phosphorescence emitter showed that a single species is responsible for such spectral properties. The value of the temperature-independent term of the Arrhenius plot (ko = 2 PI) indicates that the associated process is a spin-forbidden radiative transition of the intermediate to the ground state. The spin-forbidden nature of the deactivation is confirmed by the low value of the frequency factor A in the temperature-dependent part of the Arrhenius plot. The value of the energy barrier, 5.8 kJ/mol, indicates a shallow minimum in the excited-state energy surface and could be associated with an internal rotation. For the sake of comparison, the energy barrier for the formation of the singlet excimer in the low molecular weight analogue, TRIN, is about 10 kJ/mol.'s The spectral and kinetic evidence reported above indicate the presence of a naphthalenic triplet excimeric species at low temperature in naphthalene-benzophenone phosphazene copolymers with high naphthalene content. At room temperature, 355-nm laser excitation of a copolymer solution containing 90% or 77% naphthalene shows, after the decay of the triplet, a very low residual absorption (OD < 2 X around 500 nm) whose lifetime is of the order 200-400 ps (see insert in Figure 8). The lifetime of this species decreases with increasing the laser intensity, indicating a contribution by a second-order component. Reasonably, this species should be identified with that detected at low temperatures, Le., a triplet e x ~ i m e r . In ~ ~agreement with this, the lifetime of this transient at room temperature is of the same order of magnitude of that previously reported3vs for naphthalene triplet excimers at room temperature in isooctane. (24) From the Arrhenius parameters a lifetime of a few milliseconds, 1 order of magnitute higher than that experimentally observed, can be calculated for the transient at room temperature. The disagreement between the calculated and the found lifetime is not unexpected given the severe uncertainty associated with the Arrhenius parameters and the contribution of a different, sccond-order component to the deactivation of the state.
The Journal of Physical Chemistry, Vol. 95, NO. 2, 1991 975 Contrary to what has been reported by Lim, no indication of a measurable risetime for the formation of the triplet excimer was found in neither absorption nor emission spectra of the examined copolymers. Together with the observation of the excimeric species at 77 K in rigid solution, this would imply that the two interacting ground-state chromophores are in the orientation favorable for excimer formation. However, in contrast to what was observed for poly( 1-vinylnaphthalene) and poly(2-~inylnaphthalene),~~ no indication of a ground-state interaction could be obtained from the absorption spectrum. In copolymers with high naphthalene content, some triplet monomer coexists with the excimer, raising doubts on the existence of an equilibrium between triplet monomer and excimer. A comparison of Figures 7 and 8 shows a 2-fold increase of the excimer optical density (A = 500 nm) in contrast to a nearly unaffected monomer absorption in the region 400-420 nm when a correction is made for the underlying excimer. This would exclude the equilibrium in favor of "isolated" naphthalene chromophores which retain their monomeric behavior. The fact that the excimeric species is not detected in T R I N may be explained by the constraining cyclic geometry of the molecule which could prevent a favorable orientation of the chromophores.z6
Conclusions The existence of a naphthalenic triplet species different from the monomer triplet naphthalene has been proved in solid and fluid solutions of polyphosphazenes containing naphthalene as pendant. On the basis of the close similarities with previously reported optical properties and of the effect of naphthalene crowding found in the present study, the triplet species is identified as a naphthalene triplet excimer. It is very unlikely that the same type of impurities is responsible for absorption and emission spectra of systems whose only common characteristic is the naphthalene group. The statement on the existence of triplet excimers in solutions of molecules containing the naphthalene group seems confirmed by this work. Acknowledgment. Technical assistance by Learco Minghetti and Maurizio Minghetti is acknowledged. This work was supported in part by Progetto Finalizzato Chimica Fine e Secondaria 11. Registry No. POPN, 72664-32-5; TRIN, 2202-50-8. (25) Irk, M.; Kamijo. T.; Aikawa, M.: Takemura, T.; Hayashi, K.; Baba,
H.J . Phys. Chem. 1977.81, 1571. (26) Bandoli, G.;Casellato, U.;Gleria, M.; Grassi, A.; Montoneri, E.; Pappalardo, G. C. Z. Narurforsch. 1989,448, 575.
