2884
J. Phys. Chem. 1981, 85,2684-2686
Determination of Photoconductivity Thresholds for Trapped Electrons In Amine and Alcohol Glasses from Optical and Photoconductivity Studies Norlyuki Kato, Shlnji Takagl, and Kenji Fueki" Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya, Japan (Received March 17, 198 I)
Photoconductivity thresholds for trapped electrons in glassy amines (diisopropylamineand 1,2-propanediamine) and alcohols (methanol + 5% water, ethanol, l-propanol, 2-propanol,and 1-butanol)at 77 K have been determined from optical and photoconductivity studies. In the optical studies the wavelength dependence of the increase in the yield of pyrene anions, which were produced from pyrene present as a solute, upon photobleaching of trapped electrons was measured by optical absorption, and it was extrapolated to longer wavelengths by use of an empirical equation to obtain the photoconductivity threshold value. The same extrapolation procedure was applied to the photoconductivity data. The photoconductivity thresholds thus determined from optical and photoconductivity studies agree reasonably well with each other and are correlated with matrix polarity. These results indicate that electron transfer by photoexcitation from the trapped electron to pyrene occurs via excitation to the conduction state. It is also shown that the photoconductivity threshold is generally higher than the corresponding optical absorption threshold for the trapped electron in these matrices. Introduction Electron localization and trapping can occur in a variety of aqueous and organic glassy matrices. These trapped electrons (e;) are characterized by optical absorption spectra extending from the ultraviolet to the infrared region. Although the optical absorption spectra of e; in organic glasses at 77 K have been assigned to bound-bound and bound-free transitions, the nature of the optical transitions is still a question under debate. The optical transitions are associated with the conduction state as well as bound-excited states of e;. Photoexcitation of e; to the conduction state can be probed by photoconductivity measurements. Comparison of photoconductivity and optical spectra allows us to assign the optical spectrum to bound-bound and/or bound-free transitions.'-' In the present work we attempt to determine photoconductivity thresholds for e; in glassy amines and alcohols a t 77 K from optical and photoconductivity studies and compare them with optical absorption thresholds obtained from the optical spectra of e;. From these studies we get significant information on the nature of the optical transitions of e; in these matrices. Experimental Section Diisopropylamine (DIPA) and 1,2-propanediamine (PDA) (Tokyo Kasei research grade) were purified as previously describeda8 Methanol and ethanol (Nakarai Chemicals spectrograde) were used without further purification. Methanol-water (95/5 ~ 0 1 % )solutions were prepared by adding distilled water to methanol. 1Propanol (Kishida Chemicals research grade), 2-propanol, and l-butanol (Nakarai Chemicals research grade) were purified by distillation. Pyrene (Tokyo Kasei ultrapure) was used as received. The purified samples were distilled under vacuum into optical or photoconductivity cells. y irradiations were carried out at 77 K in a 6oCosource for 10 min a t a dose rate of 9 X lo6 rd h-l. (1) K. F. Baverstock and P. J. Dyne, Can. J. Chem., 48,2182 (1970). (2) I. Eisele and L. Kevan, J. Chem. Phys., 53, 1867 (1970). (3) T. Huann. I. Eisele, D. P. Lin, and L. Kevan, J. Chem. Phys., 56, 4702 (1972). (4) T. Huang and L. Kevan, J. Am. Chem. Soc., 95, 3122 (1973), (5) S. Noda and L. Kevan, J. Phys. Chem., 78, 2454 (1974). (6) S. A. Rice and L. Kevan, J. Phys. Chem., 81, 847 (1977). (7) S.A. Rice, G. Dolivo, and L. Kevan, J. Chem.Phys., 70,18 (1979). (8) S. Noda, K. Fueki, and Z. Kuri, Can. J. Chem., 50, 2699 (1972). 0022-365418112085-2684$01.25/0
Optical absorption measurements were made a t 77 K on a Hitachi Model 323 spectrophotometer. The optical path length of the cell was 1.5 mm. A l-kW Xebex xenon lamp was used as a photoexciting light source for the entire wavelength range studied. Ritsu Model MC-1ON (250-800 nm) and Shimadzu Bausch and Lomb (700-1600 nm) high-intensity monochromators were used, and secondorder diffracted light was reduced by appropriate glass filters. The bandwidths of photoexciting light were 20 nm for wavelengths h < 700 nm and 40 nm for h I700 nm. Light intensities were measured by a YSI Model 65A radiometer. The optical density at 492 nm was measured for determining the relative yield of pyrene anions produced. The optical density a t 492 nm was measured for a sample immediately after y irradiation and then measured for the same sample after 5-min illumination with monochromatic light (A > 550 nm). The optical density at 492 nm was also measured for the reference sample under conditions (y irradiation dose and time for optical absorption measurements) identical with those described above except that light illumination was not performed after y irradiation. The difference in optical density at 492 nm between the sample and the reference sample was taken as corresponding to the change in the yield of pyrene anions caused by light illumination. The optical density at 492 nm was read from a plateau line (A > 520 nm) in the optical spectrum for the amine samples, and it was read from a base line connecting optical-density minima in the shorter and longer wavelength tails of the 492-nm band for the alcohol samples to eliminate the possible overlap of the e; spectrum. Photocurrents were measured at 77 K with a Toa Model PM-18C dc microvolt ammeter and recorded on a Toa Model EPR-100A electronic polyrecorder. The response time of the detection system was 0.5 s. The potential applied was 800 V to electrodes separated by 1mm. The photoconductivity measurement was previously described in some detailsg Results and Discussion Optical Results. It is well-known that the e; absorption band in the visible and infrared and the pyrene anion (Py-) band peaking at 492 nm are observed for y-irradiated organic glasses containing a small amount of pyrene.'O In ~~
~
(9) N. Kato, K. Akiyama, and K. Fueki, J. Phys. Chem., in press.
0 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85, No. 18, 198 1 2665
Photoconductivity Thresholds for Trapped Electrons O.
V
450
I
1000
500
P
0
15
10
0.5
1400
I
PHOTON ENERGY, eV WAVELENGTH, nm
Flgure 3. (AAIE)2'3vs. photon energy: (0) DIPA; (0)PDA.
Figure 1. Optical absorption spectra of y-irradiated diisopropyiamine containing 0.01 mol% pyrene at 77 K before (-) and after (---) illumination with light of 1250 nm for 5 min. y Irradiation dose: 1.5 X lo5 rd. ul = 5
d
-
WAVELENGTH, nm 1600 1200 1000
-
0
1.0
1.5
2.0
2.5
PHOTON ENERGY, eV
Flgure 4. (AAlE)2'3vs. photon energy: (0) 2-propanol; (0) 1propanol: ( A ) 1-butanol. 05
10
15
PHOTON ENERGY, eV
Flgure 2. Wavelength dependence of increase in the yield of Py- by photobleaching of e{ in y-irradiated diisopropyiamine containing 0.0 1 mol % pyrene and optical absorption spectrum of e; in neat diisopropylamine at 77 K.
the present work both optical absorption bands were observed for y-irradiated glassy amines and alcohols containing 0.01 mol% pyrene. Figure 1 illustrates optical spectra for y-irradiated DIPA containing 0.01 mol % pyrene at 77 K before and after light illumination. Photobleaching of e t with monochromatic light (A > 550 nm) decreased the et- band and increased the Py- band in the optical spectrum. This indicates that electron transfer from e t to pyrene occurs by photoexcitation of e t with light of h > 550 nm. It was confirmed that the Py- band was not photobleached with light of X > 550 nm for the sample in which the e; concentration was zero. The change in optical density at 492 nm in the Py- band increased with decreasing photoexciting wavelength for h > 550 nm. Figure 2 illustrates the change in the yield of Pyupon photobleaching vs. photon energy or wavelength for y-irradiated DIPA containing 0.01 mol% pyrene, where AA corresponds to the difference in optical density at 492 nm per incident photon between the photobleached sample and the unbleached reference sample as described in the previous section and is given in arbitrary units. The optical absorption spectrum of c?- in neat DIPA glass is also shown in Figure 2. It is seen in Figure 2 that AA increases with increasing photon energy in the energy range studied. However, the threshold energy, at which AA becomes zero, cannot be directly determined from these experimental (10)A. Kira and M. Imamura, J. Phys. Chern., 82, 1966 (1978).
TABLE I: Photoconductivity and Optical Absorption Threshold Energies for Trapped Electrons in Amine and Alcohol Glasses at 7 7 K matrices diisopropylamine 1,2-propanediamine 2-propanol 1 -butanol 1-propanol ethanol methanol t 5% water
0.69 1.1, 1.1, 1.6, 1.7,
0.69 0.91 1.2, 1.6, 1.6, 1.6, 2.0,
0.46'~~ 0.7'1 (l.O)*?C
(1.3)b1c
(1.5bblc 1.5 1.7b
a Taken from ref 13. Taken from ref 14. The values in parentheses were taken from optical spectra of et- which were obtained by removing structures in the low-energy tail of the optical spectra by partial photobleaching of the 7-irradiated samples with light of ?, > 1000 nm.
data because a line through the data points is curved in the low-energy region. Therefore, we attempt to treat data by use of an empirical equation which enables us to readily extrapolate the data to the threshold. We use the following equation: AA = BE(E - EtJ3l2 (1) where E is the photon energy, Eth is the threshold energy at which AA vanishes, and B is a constant. Equation 1is rewritten as ( A A / E ) 2 / 3= B'(E - Eth) (2) where B' = B2/3.If eq 2 is applicable, a plot of ( A L ~ / E ) ~ / ~ vs. E should give a straight line, and an intercept of the straight line on the abscissa yields Eth. The equation of this type was previously used for analyzing photodetachment spectra of such molecular anions as 02-and C6F6in nonpolar liquids.11J2 Figures 3 and 4 show plots of
2686
The Journal of Physical Chemistry, Vol. 85, No. 18, 1981 1.01
/
/I
Kat0 et al.
I
10 r 3
ed
. "m -
-W
05-
v
0
0.5
0 1.0
15
vs. photon energy: (0)DIPA; (0)PDA.
( A A / E ) 2 / 3vs. E for the amine and alcohol matrices, respectively. The threshold energies thus determined are given in Table I as BnoP. The thresholds for the methanol + 5% water and ethanol matrices could not be determined by this method since it was difficult to measure AA in the low photon energy region for these matrices. Roughly speaking, the value of EnOP increases with increasing matrix polarity, ranging from 0.69 eV for DIPA to 1.7 eV for 1-propanol. In Table I is also given the optical absorption threshold, Enab, for e; which is defined as the photon energy in the long wavelength limit of the optical spectrum of et.13J4 It is seen in Table I that the value of Ethab increases with increasing matrix polarity, ranging from 0.46 eV for DIPA to 1.7 eV for methanol + 5% water. I t should be noted that EthoP is higher than Ethabfor each matrix. EthoPmay be regarded as the photoconductivity threshold because photogenerated Py- is formed by the processes involving photodetachment of the trapped electron, electron transport through the medium, and electron attachment to pyrene. Such an interpretation can be substantiated when the optical results are compared to the photoconductivity ones which are presented below. Photoconductivity Results. Photoconductivity data were obtained as initial photocurrents per incident photon vs. wavelength. The photoconductivity was measured for the PDA, 2-propanol, and ethanol matrices in the present work, and the data for other matrices were taken from ref 9. An empirical equation similar to eq 2 was used to determine the photoconductivity threshold. Here AA in eq (11)U.Sowada and R. A. Holroyd, J. Chem. Phys., 70,3586(1979). (12)U.Sowada and R. A. Holroyd, J. Phys. Chem., 84,1150 (1980). (13)T.Ito and K. Fueki, unpublished results. (14)T. Shida, S. Iwata, and T. Watanabe, J. Phys. Chem., 76, 3683 (1972).
30
PHOTON ENERGY, eV
PHOTON ENERGY, eV
Figure 5.
20
Flgure 6. (II€)2'3vs. photon energy: (0)2-propanol; (0)1-propanol; (A) 1-butanol; (0)ethanol; (A)methanol 5 % water.
+
2 was replaced by initial photocurrents per incident photon, I , in arbitrary units. Figures 5 and 6 show plots of ( I / E ) 2 / 3vs. E for the amine and alcohol matrices, respectively. Since it was difficult to measure reliably small photocurrents for low photon energies, the data points at energies higher than those involved in the determination of EthoPwere included in these plots. The photoconductivity threshold energies thus obtained are given in Table I as EthPc. It is seen in Table I that the value of E,hW increases with increasing matrix polarity, ranging from 0.69 eV for DIPA to 2.0 eV for methanol 5% water. The value of E6F agrees with that of EnoP within experimental error for each matrix. The reasonable agreement between EthoPand Ethw allows us to identify EnoP with the photoconductivity threshold. Implication for the Optical Transitions of Trapped Electrons. It is of importance to note that the values of E60P and Enp" are higher than that of Enab for each matrix. This indicates that there exist bound-excited states for e t in all of the matrices studied here and that the optical transitions of e; in these matrices consist of bound-bound and bound-free transitions. In a previous photoconductivity study15we have obtained EthPC= 0.62 eV for e; in 3-methylpentane glass 7-irradiated at 77 K. This value is higher than Ethab= 0.47 eV for e; in this matrix, indicating the existence of bound-excited states for e; in 3methylpentane glass. Thus, it appears that the optical absorption spectra of e; in organic glasses at 77 K can be generally assigned to bound-bound and bound-free transitions.
+
(15)T.Funabashi, Phys., in press.
N. Okabe, T. Kimura, and K. Fueki, J. Chem.
