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Spectroscopic properties of organic photoconductors
JOURNAL
OF
MOLECULAR
SPECTROSCOPY
Spectroscopic
216-222
1,
Properties
(1957)
of Organic
Photoconductors
Part IV. TetraphenyIporphine* JOHX Department
of Physics
w.
and Astronomy,
WEIGLt Ohio State University,
Columbus,
Ohio
Absorption, specular reflection, and action spectra for photoconduction of solid films of tetraphenylporphine are presented. The strong, sharp Soret band is greatly broadened in the crystal by resonance force interactions, while weak transitions are not affected. Light absorbed in any of these bands can serve to excite photoconduction.
The absorption spectrum of metal-free cr/3y&tetraphenylporphine (“TPP”) in neutral solut,ion (1) consists of five weak bands in the visible region, which are ascribed to a pair of orthogonally polarized, momentum-forbidden transistions and their vibrational satellites, and an intense, so called “Soret” band in the violet, which is due to two nearly degenerate, polarized, strongly allowed transitions (2, 3). This dye differs in two important respects from those whose spectra and photoconductivity have been previously discussed in this series of papers (4) : (a) The strongest singlet transition, by far, is not the one to the lowest excited state, and (b) the compound is nonionic. The second of these factors places TPP in a position intermediate between the cationic dye photoconductors, studied extensively by Vartanian (5) and in this laboratory (4, 6), on the one hand, and the various polynuclear hydrocarbons, investigated by Lyons, Akamatu, and others (7-10). Except for recent, as yet unpublished, observations of methyl chlorophyllide a (11) and chlorophyll a (1 I, Ii?), no photoconductivity has as yet been reported for porphyrins or other nonionic dyes. The related phthalocyanines appear to be semiconductive, but without appreciable photoresponse (13, 14a, b, c). The new theory of Simpson and Peterson’ for resonance-force transfer of * This work was supported by the Charles F. Kettering Foundation. t Present address: Research Department, Ozalid, Johnson City, New York. 1Simpson and Peterson’s paper (16) cites additional pertinent, references, and other applications of the theory. 216
ORGANIC
PHOTOCONDUCTORS
217
electronic energy in van der Waals solids suggested that the spectroscopic properties of solid films of TPP might be very interesting. The theory predicts that, this compound should exhibit both “strong” and “weak” coupling in the solid state: the forbidden visible bands of the isolated molecules should be practically unaltered by resonance-force interaction, whereas the intense, narrow Soret band should be strongly perturbed and split (or at least broadened) in the solid state. TPP should therefore provide an excellent dual Oest for the theory. EXPERIMENTAL
A. ABSORPTION
AND
REFLECTION
RESULTS
SPECTRA
TPP deposits from organic solvents were found to be crystalline; therefore all samples discussed here were prepared by vacuum sublimation and measured in vacuum2. This technique produced uniform light brown films with blue-purple reflection colors. The solution spectra of material recovered from the films indicated t,hat decomposition was negligible. The methods of measurement and calculation have been described previously (,Gj.Extinction coefficients, E, are reported, as customary, in (liters/mole cm) to the log base 10, in order to make solution and film values commensurable. The oscillator strength, f, was measured by graphical integrabion and comput,ed from the well-known formula, f
=a.32
x 10-g
s
ytdi;,
with y set) equal to unity (16, 17). [A-] Y is a measure of effective band width. Figure 1 and Table I present the absorption spectrum of solid TPP, compared to that of “isolated” TPP molecules in dilute benzene solut’ion. It is clear that.: (a) The weak bands are essentially unchanged by the solid state int’eraction. (b) The intense, narrow Soret band (at X 4190 a.u. in solution) is dramatically broadened, while its peak extinction coefficient is reduced nearly fourfold in the film3. The peak of the major component is, in addition, shifted to t,he red by about 800 cm-‘. (c) The tot’al oscillator strength of each transition is little altered. (d) Lowering the film temperature to 77°K tends to sharpen the weak bands somewhat, as it does in solution (18), but has little effect on the interactionbroadened Soret peak. Figure 1 also shows a typical specular reflection spectrum of a thick film of * We are greatly indebt,ed to Dr. G. D. Dorough of the University of California for a fen milligrams of pure TPP. 3 This effect is IL& due to the photometric errors which can arise from nonhomogeneous deposits or stray light at. high extinctions. The measurements were taken under conditions designed to avoid these errors; furthermore, the observed approximate constancy of the oscillator strength confirms the significance of the results.
218
WEIGL
7300
”
1 I4
6000
5000
4000
I
16
18
20
22
16
I8
20
22
AU,
24xlO%~-’
FIG. 1. Q: steady-state photoconductivity, arbitrary units. %R: specular reflectivity of a thick film (corrected for absorption and multiple reflections). er : log base 10 extinction coefficients of TPP film (liters/mole cm) at 25°C (solid line) and 77°K (dotted line). Soret band ordinate reduced tenfold. B*: log base 10 extinction coefficient of benzene solut.ion of TPP (liters/mole cm) at 25°C. Soret band ordinate reduced tenfold.
25°C 77°K
Film, Film,
a Band width lAC1 =.f/~
25°C
25°C 77°K
Film, Film,
Sol’n,
25°C:
Sol’n,
f
[A?]’
25°C 77°K
Film, Film,
X 10-d
25°C 77°K
25°C
Sol’n,
e max
Mfs
25°C 77°K
Film, Film,
(cm-l)
f 25°C’J
25°C 77°K
Film, Film,
Sol’n
25°C
Sol’n,
Band
r
(a.u.)
x
--___
max
505 355
440 -
8.6 X 1O-3 9.0 x 10-a
6.9 X IO-3
0.39 0.58
0.366
15,450 15,450
15.440
6475 6475
6480
1
EXTINCTION
1.07 0.94
2
17.2 X 1O-3 13.6 X 1O-3
800 560
745 -
I
785 600
660 -
25.5 X lo-’ 19.0 x 10-S
22.5 X 10-Z
0.73 0.72
0.788
18,100 18,100
18,220
5525 5525
5490
OSCILLATOR STRENGTH,
TABLE
17.3 X 10-a
0.49 0.56
0.537
16,880 16,950
16.920
5925 5900
5910
COEFFICIENTS,
WIDTH
(670)
990 840
10-q 10-S)
(1000)
(11 x (1.5 x
880
0.95 0.71
69 X 1OV 59 x 10-a
10-Z)
(0.38) (0.14)
1.60 1.63
(15 x
(0.35)
1.94
74 X 10-a
20,600 20,600
20,7oo
4850 4850
4830
19,250 19,250
19,450
5200 5200
5150
BAND
2600 2550
785 -
0.93 0.89
1.47 1.41
1.58
13.0 12.9
46.8
23,040 23,100
23,830
4340 4330
4190
220
WEIGL
TPP. This consists, as expected, of a complex series of anomalous-dispersion curves, one for each separate absorption band. The most intense reflectivity is associated with the strongest absorption peak. It is not generally true, as has sometimes been assumed (19), that reflection minima are always to be found at approximately the same frequencies as absorption maxima: the Soret band here is an obvious exception. B. PHOTOCONDUCTIVITY The question now arose, whether the strong resonance-force coupling between neighboring molecules, which prevails for the Soret band transition but not for the others, is required for the excitation of charge carriers to a conduction band. If it were, one would expect to find no electrical response to illuminat8ion at wavelengths in excess of 4500 a.u. The answer was provided by a set of action spectra for photoconduction, two of which are combined in the top section of Fig. 1. Clearly here, as in all other dyes and similar compounds studied, excitation to any of the singlet states can lead to carrier production, and with roughly comparable efficiency. The following figures will give an idea of the magnitude of CT:at X 6450 a.u. (i; = 15,500 cm-‘), 2 X lOI quanta/set absorbed fairly uniformly in a film over an electrode gap roughly 10 mm long and 0.3 mm across, with a thickness of about 0.4 micron (optical density 0.29), caused a steady-state photoconductance at 27°C of 0.21 X lo-l3 mho-corresponding to a conductivity of about 1.5 X lo-” mho/cm. (The dark conductance due to the glass blank, for which correction was made, was about 6 % of this value.) Measurement of the temperature dependence of steady state photoconduction at four points between 27” and 77°C showed an activation energy for conduction (trap depth) of 0.16 f 0.01 ev. The decay kinetics were complex, consisting of a fast and a slow process (the latter, at least, of the second order); typical time scales were 1 and lo-20 minutes, respectively, for 90% decay. Another indication of the complexity of the kinetics came from the apparent (illumination)1’3 dependence of the steady state conductance, observed in one sample. The faster of two dark processes may be associated with surface recombination: in one experiment, the response at the Soret peak (which is so strong as to cause absorption in the extreme surface layers) was much lower on the vacuum side of the film than on the glass side, while response to other wavelengths was normal on both sides. Slight surface contamination could have been responsible for this effect. Admission of traces of air to the evacuated cells quickly rendered them noisy and insensitive. DISCUSSION Simpson and Peterson (15) discuss resonance-force crystal interaction energy, V, which varies w&h
coupling f/R3 (in
in terms of the which f is the
ORGANIC
221
PHOTOCONDUCTORS
oscillator strength, and R is the mean distance between neighbors), and (vibrational) band width, A: for the strong coupling with the “molecular” 2V/A >> 1, and for the weak coupling, 2V/A << 1. In TPP, the strong coupling condition holds for the Soret band: in striking agreement with the theory, this band is broadened (or possibly split) by about 1500 cm-‘. This effect may be understood most simply in terms of a “time-dependent” argument, used previously in discussions of anthracene (SO), carotenoid pigments (dl), cationic dyes (4)) and a number of cases involving weaker interactions [see (Is)]. Radiant energy absorbed in t’he Soret band t’ransition at one site in the crystal migrates from molecule to molecule, with a time constant of 7 E h/ Ii = 4 X lo-l5 see per transfer. Judging by the fluorescence of solid TPP (2,9), the lifetime of this state must be about as small as it is in solution-of the order of lo-l3 second. Therefore the excitation may, on the average, migrat’e through about 25 lattice sites before it is internally converted to the longer-lived lowest excited singlet state, in which it is essentially immobile. The Simpson criterion predicts weak coupling for the two other transitions. Here, too, the theory is confirmed by the data: the five weak absorption bands are not appreciably changed by solid state interactions. Energy absorbed in these bands by a given molecule remains localized until it, is degraded by one of the available mechanisms. Sonetheless, carriers can be excited as efficiently by these as by the Soret transition. It, follows that carriers can be formed anywhere in the lattice with equal ease, and it is not necessary that t,he energy be funneled to special excitation sites by rapid migration. The spectroscopic and photoconductive properties of TPP resemble t.hose of anthracene in several ways: anthracene, too, exhibits bot’h strong and weak coupling in different bands (20); it’s photoconductivit,y may be excited in any of the singlet-singlet absorption bands (8, 10) ; surface impurities can exert large effects on t,he conduction (8, 20) and “ex&on motion” of a special sort, has been described (2.3). On the other hand, Tl’l’ is also closely related to the cationic dye photoconductors: evidently their ionic character, per se, is not essential for the formation or mobility of carriers. TPP may therefore be considered t.o close the gap between t,he photoconductive hydrorarbolls and the photoconduct’ive dyes.
The author is indebted to Dr. William T. Simpson of the University of Washington for a pre-publication copy of his interesting paper, and to him and to Dr. J. Korringa and Dr. R. C. Nelson of this Department’ for some stimulating discussions. I~ECEIVED:
May 31, 1957 REFERENCES
1. S. ARONOFF AND M. CALVIN, J. Org. Chem. 8, 205 (1943). 2. J. R. PLATT, in “Radiation Biology,” Vol. III: Visible and Near-Visible der Hollaender, ed.), Chap. 3. McGraw-Hill, New York, 1956.
Light,
(Alesan-
222 3. J. W. WEIGL, J. Mol. Spectroscopy 1, 133 (1957). Q. J. W. WEIGL, J. Chem. Phys. 24, 364, 577, 883 (1956). 6. A. T. VARTANIAN, J. Phys. Chem. (U.S.S.R.) 20, 1065 (1946), 22, 769 (1948), 27, 272 (1953); Acta Physiochim. U.R.S.S. 22, 201 (1947); Bull. acad. sci. U.R.S.S. Ser. phys. 16, 169 (1952). 6. R. C. NELSON, J. Chem. Phys. 19, 798 (1951), 20, 1327 (1952), 22, 885, 890, 892 (1954); J. Opt. Sot. Amer. 46, 10 (1956). 7. N. S. BAYLISS AND J. C. RIVIERE, Nature 163, 765 (1949). 8. D. J. CARSWELL, J. Chem. Phys. 21, 1890 (1953); D. J. CARSWELL, J. FERGUSON, AND L. E. LYONS, Nature 173, 736 (1954); A. BREE AND L. E. LYONS, J. Chem. Phys. 22, 1630 (1954), A. BREE, D. J. CARSWELL, AND L. E. LYONS, J. Chem. Sot. 1728 (1955); D. J. CARSWELL AND L. E. LYONS, ibid. 1734 (1955), L. E. LYONS, J. Chem. Phys. 23, 220 (1955). 9. H. AKAMATU AND H. INOKUCHI, J. Chem. Phys. 20, 1481 (1952); H. INOKUCHI, Bull. Chem. Sot. Japan 27, 22 (1954); 29, 131 (1956). 10. A. G. CHYNOWETH AND W. G. SCHNEIDER, J. Chem. Phys. 22, 1021 (1954); A. G. CHYNOWETH, ibid. 22, 1029 (1954); W. G. SCHNEIDER AND T. C. WADDINGTON, ibid. 26, 358 (1956); 27, 160 (1957). 11. R. C. NELSON, unpublished results. II. M. CALVIN, unpublished results [quoted by W. Arnold and H. K. Sherwood, Proc. Nat. Acad. Sci. 43, 105 (1957)l. IS. A. T. VARTANIAN, J. Phys. Chem. (U.S.S.R.) 22, 769 (1948). 14~. D. D. ELEY, G. D. PARFITT, M. J. PERRY, AND D. H. TAYSUM, Trans. Faraday
SIX. 49,
79 (1953). 14b. Note added in proof: Very recently, P. E. FIELDING and F. GUTMAN, [J. Chem. Phys. 26, 411 (1957)] have demonstrated photoconductivity in a sample of metal-free phthalocyanin. 1.4~. YE. K. PUTSEIKO AND A. N. TERENIN, Zhur. Fiz. Khim. 23, 676 (1947); Doklady Akad. Naulc S.S.S.R. 70, 401 (1950); 20, 1005 (1953). 16. W. T. SIMPSON AND D. L. PETERSON, J. Chem. Phys. 26, 588 (1957). 16. R. S. MULLIKEN AND C. A. RIEKE, Repts. Progr. Phys. 8, 231 (1941). 17. J. R. PLATT AND H. B. KLEVENS, Revs. Modern Phys. 16, 182 (1944). 18. G. D. DOROUGH AND K. T. SHEN, J. Amer. Chem. Sot. 72, 3939 (1950). 19. For example, W. WEST AND B. H. CARROLL, J. Chem. Phys. 19, 417 (1951). RO. D. P. CRAIG AND P. C. HOBBINS, J. Chem. Sot. 539,2302,2X)9 (1955).
21. W. T. SIMPSON, J. Amer. Chem. Sot. 23, 6164 (1955). 92. J. W. WEIGL, to be published. 85. D. C. NORTHROP AND 0. SIMPSON, Proc. Roy. Sot. Aa34, ibid. 233, 402 (1957).
124, 136 (1956);
0.
SIMPSON,
OF
MOLECULAR
SPECTROSCOPY
Spectroscopic
216-222
1,
Properties
(1957)
of Organic
Photoconductors
Part IV. TetraphenyIporphine* JOHX Department
of Physics
w.
and Astronomy,
WEIGLt Ohio State University,
Columbus,
Ohio
Absorption, specular reflection, and action spectra for photoconduction of solid films of tetraphenylporphine are presented. The strong, sharp Soret band is greatly broadened in the crystal by resonance force interactions, while weak transitions are not affected. Light absorbed in any of these bands can serve to excite photoconduction.
The absorption spectrum of metal-free cr/3y&tetraphenylporphine (“TPP”) in neutral solut,ion (1) consists of five weak bands in the visible region, which are ascribed to a pair of orthogonally polarized, momentum-forbidden transistions and their vibrational satellites, and an intense, so called “Soret” band in the violet, which is due to two nearly degenerate, polarized, strongly allowed transitions (2, 3). This dye differs in two important respects from those whose spectra and photoconductivity have been previously discussed in this series of papers (4) : (a) The strongest singlet transition, by far, is not the one to the lowest excited state, and (b) the compound is nonionic. The second of these factors places TPP in a position intermediate between the cationic dye photoconductors, studied extensively by Vartanian (5) and in this laboratory (4, 6), on the one hand, and the various polynuclear hydrocarbons, investigated by Lyons, Akamatu, and others (7-10). Except for recent, as yet unpublished, observations of methyl chlorophyllide a (11) and chlorophyll a (1 I, Ii?), no photoconductivity has as yet been reported for porphyrins or other nonionic dyes. The related phthalocyanines appear to be semiconductive, but without appreciable photoresponse (13, 14a, b, c). The new theory of Simpson and Peterson’ for resonance-force transfer of * This work was supported by the Charles F. Kettering Foundation. t Present address: Research Department, Ozalid, Johnson City, New York. 1Simpson and Peterson’s paper (16) cites additional pertinent, references, and other applications of the theory. 216
ORGANIC
PHOTOCONDUCTORS
217
electronic energy in van der Waals solids suggested that the spectroscopic properties of solid films of TPP might be very interesting. The theory predicts that, this compound should exhibit both “strong” and “weak” coupling in the solid state: the forbidden visible bands of the isolated molecules should be practically unaltered by resonance-force interaction, whereas the intense, narrow Soret band should be strongly perturbed and split (or at least broadened) in the solid state. TPP should therefore provide an excellent dual Oest for the theory. EXPERIMENTAL
A. ABSORPTION
AND
REFLECTION
RESULTS
SPECTRA
TPP deposits from organic solvents were found to be crystalline; therefore all samples discussed here were prepared by vacuum sublimation and measured in vacuum2. This technique produced uniform light brown films with blue-purple reflection colors. The solution spectra of material recovered from the films indicated t,hat decomposition was negligible. The methods of measurement and calculation have been described previously (,Gj.Extinction coefficients, E, are reported, as customary, in (liters/mole cm) to the log base 10, in order to make solution and film values commensurable. The oscillator strength, f, was measured by graphical integrabion and comput,ed from the well-known formula, f
=a.32
x 10-g
s
ytdi;,
with y set) equal to unity (16, 17). [A-] Y is a measure of effective band width. Figure 1 and Table I present the absorption spectrum of solid TPP, compared to that of “isolated” TPP molecules in dilute benzene solut’ion. It is clear that.: (a) The weak bands are essentially unchanged by the solid state int’eraction. (b) The intense, narrow Soret band (at X 4190 a.u. in solution) is dramatically broadened, while its peak extinction coefficient is reduced nearly fourfold in the film3. The peak of the major component is, in addition, shifted to t,he red by about 800 cm-‘. (c) The tot’al oscillator strength of each transition is little altered. (d) Lowering the film temperature to 77°K tends to sharpen the weak bands somewhat, as it does in solution (18), but has little effect on the interactionbroadened Soret peak. Figure 1 also shows a typical specular reflection spectrum of a thick film of * We are greatly indebt,ed to Dr. G. D. Dorough of the University of California for a fen milligrams of pure TPP. 3 This effect is IL& due to the photometric errors which can arise from nonhomogeneous deposits or stray light at. high extinctions. The measurements were taken under conditions designed to avoid these errors; furthermore, the observed approximate constancy of the oscillator strength confirms the significance of the results.
218
WEIGL
7300
”
1 I4
6000
5000
4000
I
16
18
20
22
16
I8
20
22
AU,
24xlO%~-’
FIG. 1. Q: steady-state photoconductivity, arbitrary units. %R: specular reflectivity of a thick film (corrected for absorption and multiple reflections). er : log base 10 extinction coefficients of TPP film (liters/mole cm) at 25°C (solid line) and 77°K (dotted line). Soret band ordinate reduced tenfold. B*: log base 10 extinction coefficient of benzene solut.ion of TPP (liters/mole cm) at 25°C. Soret band ordinate reduced tenfold.
25°C 77°K
Film, Film,
a Band width lAC1 =.f/~
25°C
25°C 77°K
Film, Film,
Sol’n,
25°C:
Sol’n,
f
[A?]’
25°C 77°K
Film, Film,
X 10-d
25°C 77°K
25°C
Sol’n,
e max
Mfs
25°C 77°K
Film, Film,
(cm-l)
f 25°C’J
25°C 77°K
Film, Film,
Sol’n
25°C
Sol’n,
Band
r
(a.u.)
x
--___
max
505 355
440 -
8.6 X 1O-3 9.0 x 10-a
6.9 X IO-3
0.39 0.58
0.366
15,450 15,450
15.440
6475 6475
6480
1
EXTINCTION
1.07 0.94
2
17.2 X 1O-3 13.6 X 1O-3
800 560
745 -
I
785 600
660 -
25.5 X lo-’ 19.0 x 10-S
22.5 X 10-Z
0.73 0.72
0.788
18,100 18,100
18,220
5525 5525
5490
OSCILLATOR STRENGTH,
TABLE
17.3 X 10-a
0.49 0.56
0.537
16,880 16,950
16.920
5925 5900
5910
COEFFICIENTS,
WIDTH
(670)
990 840
10-q 10-S)
(1000)
(11 x (1.5 x
880
0.95 0.71
69 X 1OV 59 x 10-a
10-Z)
(0.38) (0.14)
1.60 1.63
(15 x
(0.35)
1.94
74 X 10-a
20,600 20,600
20,7oo
4850 4850
4830
19,250 19,250
19,450
5200 5200
5150
BAND
2600 2550
785 -
0.93 0.89
1.47 1.41
1.58
13.0 12.9
46.8
23,040 23,100
23,830
4340 4330
4190
220
WEIGL
TPP. This consists, as expected, of a complex series of anomalous-dispersion curves, one for each separate absorption band. The most intense reflectivity is associated with the strongest absorption peak. It is not generally true, as has sometimes been assumed (19), that reflection minima are always to be found at approximately the same frequencies as absorption maxima: the Soret band here is an obvious exception. B. PHOTOCONDUCTIVITY The question now arose, whether the strong resonance-force coupling between neighboring molecules, which prevails for the Soret band transition but not for the others, is required for the excitation of charge carriers to a conduction band. If it were, one would expect to find no electrical response to illuminat8ion at wavelengths in excess of 4500 a.u. The answer was provided by a set of action spectra for photoconduction, two of which are combined in the top section of Fig. 1. Clearly here, as in all other dyes and similar compounds studied, excitation to any of the singlet states can lead to carrier production, and with roughly comparable efficiency. The following figures will give an idea of the magnitude of CT:at X 6450 a.u. (i; = 15,500 cm-‘), 2 X lOI quanta/set absorbed fairly uniformly in a film over an electrode gap roughly 10 mm long and 0.3 mm across, with a thickness of about 0.4 micron (optical density 0.29), caused a steady-state photoconductance at 27°C of 0.21 X lo-l3 mho-corresponding to a conductivity of about 1.5 X lo-” mho/cm. (The dark conductance due to the glass blank, for which correction was made, was about 6 % of this value.) Measurement of the temperature dependence of steady state photoconduction at four points between 27” and 77°C showed an activation energy for conduction (trap depth) of 0.16 f 0.01 ev. The decay kinetics were complex, consisting of a fast and a slow process (the latter, at least, of the second order); typical time scales were 1 and lo-20 minutes, respectively, for 90% decay. Another indication of the complexity of the kinetics came from the apparent (illumination)1’3 dependence of the steady state conductance, observed in one sample. The faster of two dark processes may be associated with surface recombination: in one experiment, the response at the Soret peak (which is so strong as to cause absorption in the extreme surface layers) was much lower on the vacuum side of the film than on the glass side, while response to other wavelengths was normal on both sides. Slight surface contamination could have been responsible for this effect. Admission of traces of air to the evacuated cells quickly rendered them noisy and insensitive. DISCUSSION Simpson and Peterson (15) discuss resonance-force crystal interaction energy, V, which varies w&h
coupling f/R3 (in
in terms of the which f is the
ORGANIC
221
PHOTOCONDUCTORS
oscillator strength, and R is the mean distance between neighbors), and (vibrational) band width, A: for the strong coupling with the “molecular” 2V/A >> 1, and for the weak coupling, 2V/A << 1. In TPP, the strong coupling condition holds for the Soret band: in striking agreement with the theory, this band is broadened (or possibly split) by about 1500 cm-‘. This effect may be understood most simply in terms of a “time-dependent” argument, used previously in discussions of anthracene (SO), carotenoid pigments (dl), cationic dyes (4)) and a number of cases involving weaker interactions [see (Is)]. Radiant energy absorbed in t’he Soret band t’ransition at one site in the crystal migrates from molecule to molecule, with a time constant of 7 E h/ Ii = 4 X lo-l5 see per transfer. Judging by the fluorescence of solid TPP (2,9), the lifetime of this state must be about as small as it is in solution-of the order of lo-l3 second. Therefore the excitation may, on the average, migrat’e through about 25 lattice sites before it is internally converted to the longer-lived lowest excited singlet state, in which it is essentially immobile. The Simpson criterion predicts weak coupling for the two other transitions. Here, too, the theory is confirmed by the data: the five weak absorption bands are not appreciably changed by solid state interactions. Energy absorbed in these bands by a given molecule remains localized until it, is degraded by one of the available mechanisms. Sonetheless, carriers can be excited as efficiently by these as by the Soret transition. It, follows that carriers can be formed anywhere in the lattice with equal ease, and it is not necessary that t,he energy be funneled to special excitation sites by rapid migration. The spectroscopic and photoconductive properties of TPP resemble t.hose of anthracene in several ways: anthracene, too, exhibits bot’h strong and weak coupling in different bands (20); it’s photoconductivit,y may be excited in any of the singlet-singlet absorption bands (8, 10) ; surface impurities can exert large effects on t,he conduction (8, 20) and “ex&on motion” of a special sort, has been described (2.3). On the other hand, Tl’l’ is also closely related to the cationic dye photoconductors: evidently their ionic character, per se, is not essential for the formation or mobility of carriers. TPP may therefore be considered t.o close the gap between t,he photoconductive hydrorarbolls and the photoconduct’ive dyes.
The author is indebted to Dr. William T. Simpson of the University of Washington for a pre-publication copy of his interesting paper, and to him and to Dr. J. Korringa and Dr. R. C. Nelson of this Department’ for some stimulating discussions. I~ECEIVED:
May 31, 1957 REFERENCES
1. S. ARONOFF AND M. CALVIN, J. Org. Chem. 8, 205 (1943). 2. J. R. PLATT, in “Radiation Biology,” Vol. III: Visible and Near-Visible der Hollaender, ed.), Chap. 3. McGraw-Hill, New York, 1956.
Light,
(Alesan-
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