5670
J . Phys. Chem. 1994, 98, 5670-5678
An Investigation of Triplet States, Radical Ions, and Triplet Excimers Produced by Laser Photoexcitation of Phenyldibenzophosphole in Fluid Solutions and Rigid Media Tapan Ganguly,? Richard D. Burkhart,' and John H. Nelson Department of Chemistry, University of Nevada, Reno, Reno, Nevada 89557-0020 Received: December 21, 1993; In Final Form: March 23, 1994"
Phenyldibenzophosphole (PDP), which is the phosphorus analogue of N-phenylcarbazole, yields excited singlet states, triplet states, radical cations, and anions, as well as excimeric species upon laser photoexcitation in fluid solutions or in rigid media such as polystyrene films. Delayed fluorescence occurs concurrently either by T-T annihilation or by the E-type process of thermally induced reverse intersystem crossing depending upon the solvent and the transient population of triplets. In solvents of high dielectric constant such as N,Ndimethylformamide (DMF) or ethanol/glycerol(30/70 V/V)(EtOH/glycerol), transient absorption signals due to triplet states and to radical cations and anions are observed. The purposeful introduction of electron donors or acceptors enhances the transient signals due to cations or anions, respectively, when D M F is the solvent. In EtOH/glycerol triplet excimers are produced by ion recombination, but in rigid media it appears that exciton migration and trapping a t excimer forming sites take place.
Introduction The carbazolyl chromophore has proved to be a very useful subject for photophysical studies because of its wide range of activity and the diversity of chemical environments in which it may be incorporated.l-10 In addition to carbazole itself, the N-alkyl and N-aryl derivatives have been in~estigated~-~ as well as poly(N-vinylcarbaz01e)~~~~ and a host of other polymers and copolymers containing the carbazolyl g r o ~ p . I * -Bichromophoric ~~ molecules containing the carbazolyl group have proved to be especially useful in understanding the photophysics of this species which includes not only monomeric emission but also excimer fluorescence and phosphoresence. Highly efficient exothermic electron-transfer reactions have also been observed for several bichromophoric molecules containing the carbazolyl group as electron donor and various covalently linked substituents as electron acceptors, a single methylene group being used as a spacer. Polyester oligomers containing these bichromophoric moieties as pendant groups have also shown efficient electron-transfer proper tie^.^^ These systems are potentially valuable photoconducting materials having significant commercial importance.3lJ2 In view of this widespread interest in the carbazolyl system, it is interesting to note that very little attention has been paid to the phosphorus analogue, dibenzophosphole. One might anticipate that a heteroaromatic containing the phosphorus atom should more efficiently produce triplet states than does carbazole due to the heavy atom effect. A smaller ionization potential might also be expected. Thus, an investigation of the photophysical properties of dibenzophosphole substituted at the phosphorus atom with an alkyl or aryl group not only is of interest on its own merits but also may provide further insight into the photophysical behavior of this general class of molecules. A recent photophysical investigation by Cai and Lim36 on two other molecules of the same general type, dibenzofuran and dibenzothiophene, has recently been reported. One disadvantage of dibenzophospholes as subjects for spectroscopic study is their tendency toward air sensitivity. For this reason we deliberately chose phenyldibenzophosphole (PDP) as the test species. Its air sensitivity is such that, in the pure solid state, it can be stored on the shelf without recourse to an inert On leave from the Department of Spectroscopy of the Indian Association for the Cultivation of Science, Jadavapur, Calcutta, India. e Abstract published in Advance ACS Abstracts, May 1, 1994.
0022-3654/94/2098-5670%04.50/0
atmosphere, although an inert atmosphere was used for its initial preparation and pukication. It is Gobably for this reason that the only previous investigation of the photophysical properties of this system also employed PDP. Rodionov and CO-workers33-35 in a series of three papers on the spectroscopy of certain heteroaromatic systems included PDP in their study. They used isopentane for room temperature spectra and 3: 1 methylcyclohexane:isopentane as a glass-forming solvent at 77 K. In the present work we report upon the photophysical activity of PDP in 2-methyltetrahydrofuran, N,N-dimethylformamide, and a 30/ 7 0 (v/v) mixture of ethanol/glycerol in fluid solutions at ambient temperature. In addition, low-temperature spectra and kinetics were investigated using 2-methyltetrahydrofuran and polystyrene as rigid glassy solvents. Both emission spectra and transient absorption spectra are reported here as well as the kinetics of excited-state decay. Evidence for the laser-induced formation of transient ions is also presented, and the interplay involving transient ions and excited electronic state species is discussed.
Experimental Techniques Purification of Chemicals. Absolute ethanol was distilled from P2O5 before use. Glycerol was spectroscopic grade material obtained from Aldrich. Some samples of glycerol were filtered through finely divided charcoal before use, but the treatment was discontinued since no difference in photophysical activity between treated and untreated material was noted. N,N-Dimethylformamide (DMF) was first stirred for several hours with CaH2 and then decanted away from the solid. It was then distilled under vacuum. This distillate was then distilled twice more under vacuum from anhydrous magnesium sulfate. The 2-methyltetrahydrofuran (MTHF) wasdistilled from lithium aluminum hydride before use. Polystyrene was synthesized by anionic polymerization in an oxygen-free nitrogen atmosphere using n-butyllithium as initiator. It was subsequently purified by multiple reprecipitations using benzene as solvent and methanol as nonsolvent. Phenyldibenzophosphole (PDP) was synthesized by the method described by Affandi et al.37and was finally purified by multiple recrystallizations from ethanol in a nitrogen atmosphere. Some experiments were also carried out on PDP samples purified as deicribed here but with the additional step of vacuum sublimation. The vacuum-sublimed material gave the same results as recrystallized material. Repeated crystallizations from the mixed 0 1994 American Chemical Society
Phenyldibenzophosphole in Fluid Solutions and Rigid Media solvent acetone and n-hexane3* were used to purify 1,4diazabicyclo[2.2.2]octane (DABCO). Dimethyl terephthalate (DMTP) was purified by vacuum sublimation. Sample Preparation. Solutions were prepared in 5-mL volumetric flasks and were transferred to a glass apparatus for degassing under vacuum using the freeze-pumpthaw method. The apparatus consisted of a 19/20 standard taper joint to which had been sealed a 6-in. section of 8-mm tubing a t the bottom of which was attached a 50-mL round bottom flask. The optical cell was attached, by a quartz to Pyrex graded seal, in a T-orientation to the 8-mm tubing. The sample was initially introduced into the round bottom flask for degassing. After sealing off under vacuum, the solution was tipped into the optical cell. For low-temperature experiments either 1- or 2-mm-path length cylindrical cells were used. Some samples were prepared by purging with argon rather than degassing, but their photophysical activity was essentially the same as for degassed samples. For ambient temperatures, 1-cm-path length rectangular cells were used. Spectroscopic Apparatus. For low-temperature work the samples were mounted in a cryotip system consisting of a copper ring with indium gaskets. A closed cycle liquid helium system provided temperatures down to 20 K with temperature regulation to better than 0.2 K. The major components of the optical system are the xenon probe lamp, the sample holder, and the monochromator or spectrograph which are arranged in a collinear geometry. The beam from the XeCl laser is perpendicular to this axis. When using the cryosystem, the laser beam first encounters a mirror situated directly behind the sample and coated specifically to reflect 308-nm light a t an angle of 45'. The reflected beam passes through the sample, and then it encounters a reflecting mirror placeddirectly in front of the monochromator slit to prevent entrance of this excitation beam and possible damage to the detector. For transient absorption experiments the collimated beam from the xenon lamp was arranged to pass through the 45O reflector and the sample before finally entering the slit of the monochromator. For ambient temperatures a cell holder with an offset 0.5-mm slit was used. The slit was designed so that the probe beam from the xenon lamp sampled only that layer of solution which was adjacent to the side of the sample cell from which the laser excitation entered. In this way, effects of spatial inhomogeneities are minimized and that portion of the solution having the largest population of transients is probed. For spectroscopic work a Princeton Instruments diode array system was employed, and for kinetic decays a Hamamatsu R928 photomultiplier working in conjunction with a LeCroy model 9410 digital oscilloscope was used. A mechanical chopper with an optical cutoff was used to initiate timing sequences. Further details concerning this system may be found in recent publications from this laboratory.9 The prompt fluorescence emission was recorded using a PerkinElmer Model M P F 44A fluorescence spectrometer.
"-
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5671
12000
*
-
1
1: 270 ns 2: lpm
-
10000-.
500
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lrvelength (nm)
4000
1 1: 270
1
2: 3:
nr
a #a
70
x
300
500 Wavelength (nm)
400
600
Experimental Results Emission Spectra at Ambient Temperature (296 K). Let us first examine the emission spectra of PDP in solvents of different polarity and viscosity. It may be seen from Figure 1 that at 270-ns delay, the shortest delay used in the present investigation, the maxima of the emission bands depend somewhat on the solvent used. In MTHF and D M F the maximum is a t 376 nm, but in the mixed solvent of EtOH/glycerol(30/70 v/v) the maximum is at 390 nm. Theseemission spectra might reasonably beassigned primarily as delayed fluorescence emissions as they have been observed in fluid solutions at room temperature but at a delay time too large to be considered prompt fluorescence. On the other hand, they are rather broad and structureless so they may have more than one source. The red shift and enhancement of
500 600 700 Wavelength (nm) Figure 1. Delayed luminescence spectra of PDP in EtOH/glycerol (upper), M T H F (middle), and D M F (lower) at ambient temperature at various delay times.
300
400
the emission in the EtOH/glycerol mixed solvent relative to that observed for the other two solvents suggest the presence of some
Ganguly et al. 14000
I
1
I
1
300
400 500 Wavelength (nm)
2
600
I
300
400
Comparison of delayed luminescencespectraof PDP in EtOH/ glycerol and DMF at ambient temperature and 270-11s delay. Figure 2.
500
700
600
Wavelength (nm)
3000
specific interactions such as excited-state H bonding between PDP and hydroxyl groups. It is also possible that the observed emission band consists of two or more overlapping components, for example, phosphorescence and delayed fluorescence. A reduction in emission intensity at a given delay time is observed as one moves from the large dielectric solvent D M F (eo = 36.7) to the low dielectric MTHF. In all three solvents it is observed that the emission intensity decreases with increasing delay time, but if onecarefully compares these changes it will be observed that in the mixed solvent the decrement of emission intensity within the region 300-440 nm seems to be much faster than the remaining part in the longer wavelength region down to 600 nm. In addition, a prominent tail is found to persist at the long wavelength side even at 70-ps delay time. However, tovisualize this effect quantitatively,the emission spectra of PDP in the mixed solvent and in DMF are normalized for comparison and reproduced in Figure 2. In this figure it is observed that the ratio 1460/1380 is 0.15 in the case of the mixed solvent but only 0.07 in DMF. Apparently, in the mixed solvent, at least two different species are responsible for the observed emission a t room temperature. It is reasonable to presume that, in this highly viscous environment apart from delayed fluorescence emission at 376 nm, phosphorescence emission is also present at longer wavelength. On increasing the delay time from 270 ns to 7.0 ms, a prominent band near 500 nm is seen using the mixed solvent (Figure 3a) accompanied by a significant reduction of the delayed fluorescence emission around 390 nm. Of course, it should be noted that a nearly 10-fold larger slit setting must be used for the observation of the spectrum at 500 nm with 7.0-ms delay. Figure 3b is a magnified spectrum near 500 nm showing clearly the peak position of this long wavelength emission band. To provide a more convincing identification of these luminescence bands, emission spectra of PDP were recorded at 20 K in MTHF rigid glass and in a polystyrene matrix using different delay times after the excitation pulse. These are shown in Figure 4. It was interesting to find such a dramatic difference in spectral resolution for these two different matrices. Presumably the broadening in MTHF is due to site inhomogeneities along with an average site separation distance large enough to inhibit a significant degree of energy migration. By contrast, the chromophore concentration in the polystyrene matrix is sufficiently large that energy migration is expected to be efficient. Thus, the emission spectrum observed here undoubtedly involves excited states residing at the low-energy side of the inhomogeneous band. The spectrum recorded in polystyrene is remarkably similar to the phosphorescence spectrum of various N-substituted
2500
300
400
500 Wavelength (nm)
600
Figure 3. Delayed luminescence spectra of PDP at two different delay times in EtOH/glycerol at ambient temperature (a, upper). Delayed luminescence spectrum of PDP in EtOH/glycerol at a delay time of 7.0 ms at ambient temperature (b, lower).
carbazoles, and indeed, it seems reasonable to assign this spectrum to the phosphorescence of PDP. The zero-zero band occurs at 408 nm, placing the triplet energy of PDP at 24 500 cm-I. This may be compared with a triplet energy of 24 270 cm-1 for N-isopropyl~arbazole.~~ It may be noted that this triplet energy is in considerable disagreement with the value of 22 090 cm-' reported for PDP by Rodionov and co-workers. Another important feature of the spectrum in the polystyrene matrix is the existence of the band near 540 nm, which is found to persist even at long times after the excitation pulse. It is evident, therefore, that more than one emitting species is present. An impurity emission is one possible explanation for this band, but in view of recent findings in the carbazole system: triplet excimers must also be included as a possible emitting source. In MTHF no emission feature a t 540 nm is found. Therefore, this emission band is not likely due to an impurity and is more likely due to a triplet excimer. Additional evidence for the presence of triplet excimers in this system will be presented below. Emission Decay Kinetics at Ambient Temperature. Table 1 contains a summary of the emission decay times of PDP in different solvents measured a t room temperature. Inspection of these data reveals that in nonpolar MTHF, when the emission wavelength is monitored at either 380 or 500 nm, the decay shows a
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5673 Residuals
1
.,
2450 x 2
a
0.03
1 270 nr delay 2 8 ps d e l a y 3 70 ps d e l a y
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Wavelength (nm)
1
4
4000
1 .o
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3500 0.c
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Figure 5. Luminescence decay of PDP in MTHF monitored at 380 nm and ambient temperature. The solid line represents the best fit to a double-exponentialdecay. Only one-tenth of the data points are shown
3000
0
n
for clarity.
'Z 2500
300
400
520
600
700
Wavelength (nm) Figure 4. (upper) Delayed luminescence spectra of PDP in MTHF rigid glass at 20 K and at various delay times. (lower) Delayed luminescence spectrum of PDP in polystyrene matrix at 20 K and 70-ps delay.
TABLE 1: Results of Kinetic Decays of PDP Emission at Ambient Temperature in Different Fluid Solutions emission solvent wavelength (nm) fia ha T I (ps) 7 2 (ps) MTHF
380 500
EtOH/glycerol DMF
390 500 380 500
0.89 0.11 0.77 0.23
1.5 1.9
4.2
6.5
6.1 6.4 6.1
5.0
"fi = u~T,/&u,T, where a, is the preexponential coefficient of thejth component. biexponential nature. Typical data are displayed in Figure 5 . The fractional contribution, f l , of the shorter-lived component is larger than that, f2, of the longer-lived component at both wavelengths. In EtOH/glycerol mixed solvent and in DMF, single-exponential decays provide an excellent fit to the data at all wavelengths monitored. In fact, the emission decay times are all nearly the same and fall into the time interval 5.0-6.4 ps. Transient Absorption Spectra of PDP in Fluid Solutions at Ambient Temperature (296 K). Absorption spectra of transients produced by laser excitation of PDP in different solvents are shown in the Figure 6. In MTHF a prominent peak near 360 nm
and weaker but well-resolved bands near 490 nm are observed. In this low dielectric solvent no bands were noticed at wavelengths longer than 550 nm. As expected, the absorbance of transients decreases monotonically with increasing delay time over the entire wavelength range. In the mixed solvent of EtOH/glycerol the absorption band in the range 320-440 nm is found to be much broader than in MTHF, but the absorption maximum is at the same position (360 nm). A prominent shoulder near 340 nm is also seen in the mixed solvent. The absorption around 490 nm is more diffuse than that observed in MTHF, and the rate of decay of the bands near 490 nm is seen to be much slower than at lower wavelengths. In the higher dielectric solvent, DMF, the absorption maximum at 270-ns delay occurs at about 340 nm. Also, a resolved but much weaker series of bands around 490 nm are observed similar to that found in MTHF. The time-dependent behavior of the bands near 490 nm is very interesting in this solvent in that the absorption at I-ps delay is larger than that at 270 ns. That is, a buildup period precedes the decay on this time scale at this particular wavelength. This may also have occurred in EtOH/ glycerol since the absorbance at 490 nm is the same at 270 ns and 1 I.CS in this solvent. It is noteworthy that the rise in absorbance at 490 nm is accompanied by a disappearance of the absorbance at 340 nm. At a delay time of 270 ns a small absorption band near 780 nm is also observed in DMF. By analogy with the observations made in the case of N E C by earlier authors,40 this long wavelength 780-nm band could be assigned as the absorption of the cationic species (2PDP+). To investigate this possibility further, we have recorded transient absorption spectra of PDP, at a delay time 270 ns, in the presence of DMTP as an electron acceptor. As seen in Figure 7, in the presence of DMTP this 780-nm band is enhanced and broadened. Since the 340-nm transient absorption is observed only in D M F fluid solution, it is likely that this band is also due to a transient
Ganguly et al.
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
5674
0.6 1 delay = 2 delay = 3 delay = 4 delay =
0.5
270 ns 1 ps 8 ps 70 q s
0.04
0.4
0.03
A
A
0.3 0.02
t
0.2 0.01
0.1
n no 700
600
0.0
300
600
500
400
Wavelength (nm)
Figure 7. Transient absorption spectrum of PDP in D M F at a delay time of 270 ns. Spectrum 1 was recorded in the presence of 7.8 X 10-4 M DMTP. Spectrum 0 was recorded in the absence of DMTP. Spectrum 1 has been displaced upward by 0.02 unit for clarity. 0.14 I
1 delay = 2 delay = 3 delay = 4 delay =
800
wavelength (nm)
700
270 ns 1 c(s 8 p 70 ps
I
0.12
-
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0.08 -
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-
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-
0.00
1
300
I
I
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400
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Wavelength (nm) Wavelength ( n m )
0.3
1 2 3
1
delay = 270 ns delay = 1 c(s delay = 70 ps
0.2
A
0.1
0.0 300
500 600 700 Wavelength (nm) Figure6. Transient absorptionspectra of PDPin EtOH/glycerol (upper), M T H F (middle), and D M F (lower) at ambient temperature andvarious delay times.
400
ion and, by analogy with NEC, the radical anion is a likely
Figure 8. Transient absorption spectrum of PDP in D M F at a delay time of 8 ps. Spectrum 1 was recorded in the presence of 6.9 X 10-3 M DABCO, and spectrum 0 was recorded in the absence of added DABCO.
possibility. On increasing the delay time from 270 ns to 1 bs, it is observed that the 360-nm band persists associated with a very weak shoulder around the 340-nm region as seen in Figure 6. On further increase of delay time the 340-nm band disappears. In DMF, another significant observation is that with an increase of delay time the band near 360 nm and the group of bands between 400 and 500 nm become better resolved. In addition, an intensity inversion occurs between 360 and 400 nm as the delay time increases. This provides strong evidence that there are a t least two absorbing species present in this wavelength range plus a third species near 340 nm. Using low-temperature spectra (see below), we are able to assign the band at 360 nm to the dibenzophospholyl triplet. The group of bands between 400 and 500 nm are also seen in low-temperature spectra and will be assigned to the triplet excimer as described below. To assign unambiguously the band position of * P D P , we measured the transient absorption spectra of PDP in a DMF solution containing DABCO as an electron donor. Different delay times were used, and the spectrum in Figure 8 was recorded at an 8 - ~ sdelay. Here one finds that in the absence of DABCO the transient absorption spectrum of PDP exhibits a prominent peak at around 360 nm and a shoulder near 340 nm. With addition of DABCO the 340-nm band is found to be enhanced significantly and, in fact, becomes the major low wavelength peak. Con-
Phenyldibenzophosphole in Fluid Solutions and Rigid Media comitantly, there is a reduction of the 360-nm band, and enhancement of the band near490nm is also found. The DABCO positive ion has a very small molar absorptivity in this wavelength41 range, and its contribution to the spectra here is considered to be negligible. These observations strengthen the notion that the 340-nm band is due to a radical anion and the 360-nm band is due mainly to the triplet state. In MTHF, which has a much smaller dielectric constant than DMF, it is observed that not much change in the transient absorption spectral pattern of PDP is seen with addition of DABCO. Thus, ions which are formed in D M F have lifetimes long enough to be observed in the hundreds of nanoseconds to microsecond time domain. In MTHF, ion formation may be occurring, but if so the lifetimes are apparently too short to be observed in these experiments. The behavior of the band near 490 nm is interesting. If it were part of the absorption band system of the monomeric triplet state, one would expect its absorbance to decrease upon the addition of DABCO as does the 360-nm band; instead it increases. If one reviews the emission spectra presented above, a common characteristic is that most of them contain emission signals which may reasonably be associated with more thanonechemical species. An exception is the delayed emission of PDP in MTHF at 20 K. Here it appears the emission is due simply to the monomeric triplet state of PDP, and so the corresponding transient absorption spectrum may also be assigned to that of the monomeric triplet. This spectrum is presented in the upper part of Figure 9. Clearly there is no absorption feature in this spectrum near 490 nm, and so this transient absorption band at 490 nm observed both in fluid solution and in polystyrene matrices cannot be assigned to the monomeric triplet. In the lower part of Figure 9 there is a transient absorption spectrum of a sample consisting of 12 wt % PDP in a polystyrene matrix, also at 20 K. Here a definite absorption feature is present at 490 nm; in fact, there is a very broad band spanning the region from 350 to 500 nm. The corresponding emission spectrum for this sample consists of monomeric phosphorescence and a second band which we attribute to the triplet excimer. The natural conclusion from these observations is that the transient absorption feature at 490 nm is due to the triplet excimer. Both of the spectra presented in Figure 9 contain what appear to be very narrow absorption features near 340 nm. In view of our assignment of the 340-nm absorption to the radical anion, it is tempting to suggest the same sort of assignment for these bands in these low-temperature spectra. Unfortunately, the intensity of the probe beam begins to decline rapidly in this wavelength range. This fact, combined with the rather small absorbances obtained for these optically thin films, makes a definitive assignment for these narrow bands impossible at this time. Even if one accepts the reasoning presented here that the triplet excimer has a transient absorption in the spectral region near 490 nm, one still may ask why the introduction of the electron donor DABCO results in an enhancement of this absorption in fluid solutions. This is an interesting question and leads to the supposition that radical ions are in some way involved with the production of the triplet excimer. Recently completed work in these laboratories on N-ethylcarbazole has indicated that ion recombination in fluid solutions can lead to the formation of triplet excimers.9 It is possible that in the case of PDP a more vivid demonstration of the operation of this mechanism is occurring. We will discuss this point in more detail below. Transient Absorption Decay Kinetics. The kinetics of transient absorption decay have been evaluated at certain selected wavelengths in each of the three solvents. As the data in Table 2 illustrate, lifetimes measured in this way depend upon the wavelength selected and upon the solvent in which the measurement was made. In MTHF, for example, a fast decaying component having a lifetime slightly larger than 4 ps was found at both 360 and 490 nm. At the longer wavelength, however, the
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 0.025
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0.020
0.015
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Wavelength (nm) 0.020
0.016
-
0.012
-
I
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1
I
A
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700
Wavelength (nm) Figure9. Transient absorption spectrumof PDP in glassy MTHF (upper) and in a polystyrene matrix (lower) both at 20 K. Delay time was 270 ns for both.
TABLE 2: Results of Kinetic Decays of PDP Transient Absorption at Ambient Temperature in Different Fluid Solutions emission solvent wavelength (nm) flu ff 71 (rs) 7 2 (ps) MTHF 360 4.1 EtOH/gl ycerol DMF
aJ
490 360 500 340 360 490
0.07 0.96 0.25 0.28 0.25
0.93 0.04 0.75 0.72 0.75
4.4 762 4.0 5.4 7.6 100
87 121 124 41 112
= CI,~~/&CI,T~where uj is the preexponential coefficient of the j t h
component.
decay was best fit by a double exponential, and the second component had a significantly longer lifetime of 87 ps. In EtOH/ glycerol, on the other hand, double-exponential decays provided the best fit at both 360 and 500 nm. The very long-lived component of 762 ps at 360 nm is especially interesting as well as those having lifetimes near 120 ps at both 360 and 500 nm. In DMF, double-exponential decays provided the best fit to the data a t both 340 and 360 nm. Short-lived components of 5.4 and 7.6 ws were found at 340 and 360 nm along with longer-lived
5676
Ganguly et al.
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 0.006
1
0.004 . ..
2
0.002
0.000
B
%
I
2oo
,
3
2!
I
150
l----l
t
-0.002 -0.004
-0.006 I 0
I
50
100
150
I 200 -50
0.0
0.2
0.1
0.3
2
(absorbance) x l 0
I
0'04
0.03 *0° 150
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0.01
0.00
-50 I
0.0
-0.01
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0
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I
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50
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Time (psec)
Figure 10. Kinetic analysis of a transient absorption decay of PDP in DMFat ambient temperaturemonitored at 340 nm. Solid line is the best fit to a double-exponentialdecay.
components. At 490 nm only one component is observed having a lifetime of 100 ps. Representative decay data are presented in Figure 10. Kinetics and Mechanism of Delayed Fluorescence Emission. Delayed fluorescence emission was certainly expected from these solutions, and the mechanismof its formation is of definite interest. A rather direct way to assess the importance of triplet-triplet annihilation as a mechanism is to prepare a graph of delayed fluorescence intensity versus (triplet concentration)" or (triplet absorbance)". If T-T annihilation is the source of delayed fluorescence, one expects to find n = 2. In Figure 1 1 are presented two graphs of this sort, using D M F as solvent, one with n = 2 and one with n = 1. Probably the most instructive results are those for n = 1 where linearity is found over approximately 75% of the absorbance range. The marked positive deviation from linearity at the largest absorbances suggests that a higher-order process is taking place under these conditions. It appears, therefore, that delayed fluorescence is occurring primarily by a first-order E-type process42 (reverse intersystem crossing) but that at the highest triplet concentrations second-order triplettriplet annihilation makes a contribution. As an additional test of this mechanism, we also measured the delayed fluorescence intensity as a function of laser fluence and found a linear relation between the two. Under the same conditions the absorbance due to triplet states was measured as a function of laser fluence. Again a linear relationship was found. These results reinforce our conclusion that delayed fluorescence for PDP occurs primarily by an E-type process at ambient temperature.
I
0.2
absorbance
Figure 11. Graphs of delayed fluorescence intensity versus (triplet absorbance)*andversus (triplet absorbance)for PDP in DMFat ambient
temperature. In this section of the paper we will construct a more comprehensive and critical view of the photophysical processes involved. Monomeric/Excimeric Phosphorescence. At low temperature in a polystyrene glass, the phosphorescence spectrum of PDP has been obtained showing that the zero-zero band of the emission is at 408 nm. Vibronic components of 1253 and 1422 cm-l may be identified from this spectrum, and corresponding infrared absorptions, evidently due to ring breathing modes, are found for PDP at 1258 and 1432 cm-l. From this information therefore, and by analogy with the carbazole system, we can assign the ground state to triplet transition as being a,a* in character. The lifetime of this phosphorescent emission at 20 K proved to be very difficult to measure using the photomultiplier system. At this time we are only able to estimate the lifetime of the monomeric emission as being near 100ps from time-resolved emission spectra. Furthermore, if one carefully examines time-resolved spectra taken in a polystyrene glass, it is clear that the emission at 510 nm is longer-lived than that at shorter wavelengths. Thus, we postulate the existence of not only the monomeric triplet state but also at least one other species having a lifetime longer than the monomeric species. This may be a triplet excimer; however, the possibility that it is due to impurities in the system must not be overlooked. We believe it is not likely due to impurities because of the method of synthesis used and because repeated additional purification measures did not significantly alter the emission or transient absorption characteristics. The triplet photophysical processes in low-temperature glasses do not appear to have any ionic component and so may be represented by the following equations assuming monomeric triplet states T, have been initially formed by intersystem crossing from the excited singlet state produced by the laser pulse
T, Discussion Several different emission and transient absorption bands have been identified in the preceding section, and in some cases, suggestions about the origin of these species have been presented.
I
0.1
T,
-
'So
+ hv/heat
(1)
+ E F! 3E*+ ' S o
In these equations 'SOis the ground-state molecule, E represents
Phenyldibenzophosphole in Fluid Solutions and Rigid Media an excimer-forming site in the matrix, and 3E* is thecorresponding triplet excimer. At ambient temperatureevidence for phosphorescence emission is limited since the bulk of the emission is due to delayed fluorescence. Careful inspection of Figure 1 with EtOH/glycerol as solvent indicates that, at longer delay times, there is a weakly emitting component near 500 nm as well as a significantly broadened delayed fluorescence band which may include monomeric phosphorescence. The band near 500 nm is most likely an excimeric phosphorescence from 3E*. Even in M T H F fluid solutions a small residual emission in the 400-500-nm range is found at the longest delay time used. In the wavelength range between 340 and 400 nm, emission decay times between 4.0 and 6.4 ps are found for all solvents (Table 1). Since this emission signal has been characterized as being primarily E-type delayed fluorescence, we may conclude that this range of decay times are actually those of the transient monomeric triplet state. Transient absorption decay times reinforce this conclusion. Time-resolved spectra show that longer-lived species are present and have been attributed to triplet excimers. The emission signals from these species were too weak to detect in our kinetics experiments. On theother hand, longer-lived transient absorptions were found in all solvents between 87 and 124 ps. In view of the spectral assignments, these are attributed to the excimeric triplet state. Theoriginof theverylong-lived transient (762psobserved in EtOH/glycerol is not certain. It may be due to a specifically solvated radical cation, but additional work, will be required to provide a definite assignment. Delayed Fluorescence/Delayed Excimer Fluorescence. The emission band which has been observed in room temperature spectra in the wavelength range from 370 to 390 nm has been characterized as delayed fluorescence. Its lack of structure suggests that it may be excimeric in origin or may simply be a result of inhomogeneous broadening. The delayed emission spectrum in Figure 1 for D M F solvent is particularly useful to characterize the delayed fluorescence since this band does not shift with a change in delay time and the lifetime is essentially the same (6.1 to 5.0 ps) over the entire band envelope. It seems likely, therefore, that this spectrum arises from a single excited state species. Clearly, the lifetimeof this emission must correspond to whatever species is the precursor to the excited singlet state because the singlet state itself would likely have a lifetime in the nanosecond range. The evidence suggests that conventional T-T annihilation does not occur in MTHF or EtOH/glycerol but cannot be ruled out in the case of D M F at short delay times when the transient triplet population is relatively large. The possibility that this emission results directly from ion recombination is not likely since its lifetime is the same in MTHF, where ion recombination is too fast to be measured by our equipment, and in DMF where transient ion signals decay much more slowly. On the other hand, it is likely that ion recombination in D M F can produce a significant population of triplet states which subsequently become involved in delayed fluorescence production. The essence of this hypothesis is that triplet states may be formed by two distinct and concurrent processes, the conventionally recognized intersystem crossing on the one hand and ion recombination on the other. In solvents of low dielectricconstant, ion recombination is relatively fast and excited singlet states are the primary products. In solvents of high dielectric constant, recombination is relatively slower and the net yield of triplet states is larger because their probability of formation by the ion route is enhanced and because they are also formed in intersystem crossing. If we represent an ion pair of the type [zM+-.2M-] by IP, then the formation of triplet states may be characterized by the reactions
IP ---* T,
+ IM,
(3)
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5677 and
T, s 'M*
(4)
The reversibility arrows in eq 4 are used advisedly since thesinglettriplet energy gap for PDP is only 2100 cm-I, smaller than that of eosin which is the classical prototype for E-type delayed fluores~ence.~~ If the transient triplet population were sufficiently large, then T-T annihilation could compete with the reverse of eq 4 to produce delayed fluorescence,
T,
+ T,
+
+ ls,
ls*
(5)
It is likely that this is occurring in D M F a t delay times less than 1 ps where the triplet population is relatively large and the solvent is sufficiently fluid so that a diffusion-controlled reaction, such as eq 5, can occur with sufficient speed. In the mixed solvent, EtOH/glycerol, the transient triplet population is also expected to be relatively large, but in this case solvent viscosity works against eq 5 and the E-type production of delayed fluorescence is favored. Transient Absorption Bands. The simplest transient absorption spectrum was found for an M T H F solution of PDP at 20 K. The single band observed in this case is the one at 360 nm, and it has been assigned to the monomeric triplet of PDP. In addition, transient absorption bands have been observed at 340 and 780 nm in fluid solutions of DMF. These bands are consistent with the presence of transient ions, cations for the 780-nm component and anions for the 340-nm component. Finally, transient absorption features have been observed in the 450-500-nm region which are attributable to the triplet excimer of PDP. As discussed above, the appearance of the triplet excimer transient absorption band coincides with the appearance of excimeric phosphorescence in polystyrene films doped with PDP. It is particularly noteworthy that a buildup of the excimeric triplet absorbance is found in D M F between 270 ns and 1 ps. The fact that this buildup occurs in thesame time interval during which the radical aniondisappears is the strongest evidence we have found to date for an ion recombination mechanism of triplet excimer formation. In EtOH/glycerol the appearance of emission attributable to excimeric phosphorescence could be ionic in origin. It has been noted before9 that ion recombination in fluid solution has the effect of placing a triplet-state molecule in the immediatevicinity of a ground-state molecule so that the possibility of excimer formation is enhanced. Since the triplet lifetime is on the order of hundreds of microseconds, there is sufficient time for the reaction
T,
+ 'M,
-
'E*
to take place before the triplet relaxes to the ground state. That such a sequence could take place in the EtOH/glycerol medium is particularly likely for two reasons. First of all, the dielectric constant is large enough so that ion recombination is slowed to the microsecond time domain. Additionally, the viscosity of the medium slows the occurrence of cation dimer formation, which would compete with eq 3. Finally, it is recognized that the probability of triplet-state formation upon ion recombination increases with increasing lifetime of the ion pair.43 Thus, the yield of triplets is expected to increase as the dielectric constant of the medium increases. It is probably for this reason that the relative intensity of the delayed fluorescence signal in the three different solvents increases in the order EtOH/glycerol > D M F > MTHF. Finally, some comments are necessary concerning the lack of agreement between the results found here and those previously observed by Rodionov and cc-workers for the triplet photophysical parameters of PDP. We note, in particular, that they find the origin of the phosphorescence signal to be 453 nm compared with
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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
our value of 408 nm. Furthermore, the lifetime of the triplet state is quoted in their work as 1.4 s whereas our work indicates that lifetimes are in the range of several hundred microseconds. Such a significant incompatibility between the two sets of results suggests that impurities may be affecting one or the other set of results. The mode of synthesis and purification procedures used in the present work have been described in some detail in the experimental section. No similar information is available in the work of Rodionov and co-workers.
Conclusions Laser photoexcitation of PDP leads to the production of both singlet and triplet excited states and, in high dielectric media, to radical cations and anions. Delayed emissions include delayed fluorescence as well as monomeric and excimeric phosphorescence. Delayed fluorescence is produced primarily by the E-type process in all solvents, but in the case of D M F it appears that triplettriplet annihilation takes place at the largest triplet concentrations. The phosphorescence spectrum of PDP is very similar to that of N-alkyl- or N-arylcarbazoles but shifted slightly to higher energy. This is an especially interesting result since it implies that the electronic transition involved is closely associated with the carbon skeleton of the heteroaromatic system and very little dependent upon the nature of the heteroatom. Even though the spectral properties of these nitrogen and phosphorous analogues are similar, the triplet-state lifetimes differ considerably. Typical tripletstate lifetimes for the carbazole chromophore are on the order of 6-7 s. For the larger phosphorus atom one expects to find enhanced spin-orbit coupling and a corresponding reduction in the triplet-state lifetime. This certainly is the case; the triplet lifetime of PDP is 4 orders of magnitude smaller than for carbazolyl species. Initially, we could only speculate about a possible interaction of the phenyl group of PDP with the benzophospholyl aromatic system. The results suggest that there is no interaction judging from the similarity of carbazolyl and benzophospholyl phosphorescence spectra.
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