1208
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
R. 0.Loutfy and J. H. Sharp
Electronic Properties of Furanquinone Pigments. Correlation between Photosensitivity and Emissivity Rafik 0. Loutfy” and James
H. Sharp
Xerox Research Centre of Canada, Mississauga, Ontario L5L 1J9, Canada (Received August 15, 1978; Revised Manuscript Received January 23, 1979) Publication costs assisted by the Xerox Research Centre of Canada
The spectroscopic properties of two derivatives of dinaphtho[2,1,2’,3’]furan-8,13-dione, I, the 6-(2”-pyri111,were investigated in a variety of solvents and dyl)carboxamide, 11,and 3-bromo-6-(2”-pyridyl)carboxamide, as a function of temperature. These electron-withdrawing substituents reduced the electron density on the naphthalene moiety and caused a decrease in intensity and hypsochromic shift of the So SI transition. The change in dipole moment involved in this transition, obtained from solvent-induced frequency shift of the absorption and fluorescence, was 11.4 and 12.1 D for I1 and 111, respectively, compared to 17.3 D for I which suggests significant intramolecular charge transfer character. The solid state fluorescence emission and absorption spectra of I1 and I11 were slightly red shifted with respect to solution molecular spectra. The similarity of the solid state and molecular spectra of I1 and I11 reflects the relatively weak intermolecular interactions in the crystal phase. The effect of bromine substitution consisted of (a) hypsochromic shift of both absorption and emission bands; (b) a marked decrease in fluorescence efficiency,and (c) increase in phosphorescence quantum yield. A correlation between the fluorescence efficiency and photoimaging sensitivity was found. +
Introduction Despite numerous experimental and theoretical investigations on the electrical and photoconductive properties of organic materials,l the relationship between molecular structure and semi- and photoconductivity is not well understood.2 Understanding of the molecular origin of semi- and photoconducting properties in organic materials is of great importance since, potentially, synthesis can be engineered to produce organic molecular crystals with specific electrical properties for some electronic applications. However, so far, the effect of molecular modification is best understood when a series of closely related materials is studied. This restriction is necessary, since the electronic properties of materials depend not only on the overlap between molecular or atomic orbitals of neighboring molecules but also on chemical purity and structure imperfection (molecular disorder) which is difficult to control in organic materials. A comparison, for instance, of the photocurrent for two unrelated materials is insufficient, in the absence of information relating the chemical and structural characteristics to charge carrier generation and carrier mobility, to arrive a t any relevant conclusion; an alternative approach, which is more meaningful, is to investigate a series of reasonably similar molecular materials. The optical and electrical properties of furanquinone pigments have recently received considerable a t t e n t i ~ n . ~ 8,13-Dioxadinaphtha[ 2,1,2’,3’]furan-6- (2”-pyridyl)carboxamide, 11, is of particular interest because of its pronounced photoconductive properties and its consequent application in electrophotographic imaging p r o c e ~ s e s . ~ The preparation and crystallographic characteristics of I1 have been reported by Weinberger and co-~orkers.~ The electrical and photoconductive properties were studied recently by Tutihasi? showing that the quantum efficiency of photogeneration of charge carriers increases linearly with the square of the electric field. Walker, Kuder, and Miller3 have investigated the photophysical properties of the parent compound dinaphtho[2,1,2’,3’]furan-8,13-dione (I) by studying the absorption and emission characteristics in a variety of solvents and as a function of temperature. The long wavelength absorption of I was assigned as a R R*
-
0022-3654/79/2083-1208$01 .OO/O
0
NH
ll
@
X = H
Ill X = B r
transition with significant intramolecular charge transfer character. The dipole moment change involved in this transition, obtained from solvent-induced frequency shift of the absorption and fluorescence, was 17.3 D and this is in accord with such an assignments3 Huckel molecular orbital calculations3 showed that the So SI transition involves significant R-electron density rearrangement in these molecules. Electronic charge is transferred from the naphthalene moiety to the quinone part of the molecule on excitation to the lowest excited singlet state of I. Therefore, any perturbation of the electron densities at the naphthalene ring would affect the transition energy causing a shift of the absorption and emission bands. Introducing substituents a t positions 3 and 6 is practical and is expected to influence both the optical and electrical properties of the molecule. Two derivatives of I have been synthesized, the 6(2”-pyridyl)carboxamide, 11, and the 3-bromo-642”pyridyl)carboxamide, 111, and their spectroscopic properties were examined. In this paper the results of experimental investigations of the absorption and emission properties of I1 and I11 in solution and the solid state are described. A particular +
0 1979 American Chemical Society
The Journal of Physical Chemistty, Val. 83, No. 9, 7979
Electronic Properties of Furanquinone Pigments
1209
TABLE I: Spectral Data for I, 11, and I11 absorptn max, nm (molar extinctn coeff, L mol-l cm-') no.
a
1 2 3 4 5 6 7 8 9 See ref 3.
solvent p-dioxane chlorobenzene 1,2-dichloroethane acetone acetonitrile dimethylformamide dimethyl sulfoxide 2-methyltetrahydrofuran solid state
Ia 435 447 445 435 437 441 443 455
emphasis is placed on the relationship between emissivity and photoconductivity in these molecular solids.
Experimental Section Materials. The synthesis and purification of furanquinone I1 and I11 have been described previ~usly.~ All solvents were Matheson Coleman and Bell, Spectroquality Reagent. Piccoflex-100 (copolystyrene-acrylonitrile 88:12) was supplied by ICI, Fluorescence Standard GG21 was obtained from Hellma. Spectroscopy. Ultraviolet and visible absorption spectra were recorded on a (Gary 17 spectrophotometer by using Suprasil quartz cells. Because of the photosensitivity of the materials, solutions were prepared by dissolving the furanquinone in the appropriate solvent and measurements were obtained within 20 min of preparing the solution. Emission spectra were recorded on a Perkin-Elmer MPF 4 spectrofluorimeter using suitable attachments for measuring true emission and true excitation spectra. Phosphorescence was recorded using the phosphorescence attachment equipped with a rotating chopper at 77 K. The EM1 photomultiplier used had an S20 response. Emission from solid samples was obtained using a solid sampling attachment and front surface illumination. The sample was prepared by intimately mixing 1 wt % of the pigment with Piccoflex 100 in an Auger morter. The fine powdery mixture was then placed in a die and pressed to 20000 lb/in.2 with a Carver laboratory press for 3 min. The finished pellets had a diameter of 1.3 cm and a thickness of 500 pm. Fluorescence quaintum yields were measured by comparing to rhodamine B in ethylene glycol,6or to the GG21 Hellma fluorescence standard. These have fluorescence yields of 0.65 and 10.5, respectively. Phosphorescence quantum yields were measured by comparing with the fluorescence standards. Emission and excitation spectra were all normalized with regard to spectral response of the photomultiplier and light source spectrum, respectively. Results Absorption. The ultraviolet and visible absorption spectra, recorded a t ambient temperature for quinone I1 and I11 in chlorobenzene solvent, are shown in Figure 1. In polar solvents the spectra show a general red shift of the absorption band. The absorption maximum of the long wavelength band and molar extinction coefficient of I1 and 111, along with those of I for comparison, in a variety of solvents is given in Table I. The solvent-induced shifts of the absorption band for I, 11, and I11 to longer wavelength with increased solvent polarity is consistent with the r-r" assignment for this transition having significant charge transfer ~ h a r a c t e r . ~ The So S1 transition of I is shifted by 12 nm to the blue on substitution with 2'-pyridylcarboxamide. The bromine atom caused a further hypsochromic shift of 8 nm ---t
e
4.0
I1
I11
428 (5500) 435 (5620) 433 (6250) 428 428 435 (5300) 435 (5750) 425 445
422 (6816) 428 (6949) 424 420 419 421 437
1
350
300
400
500
450
550
X (nm) Flgure 1. Absorption spectra of I1 and I11 in Chlorobenzene.
04-
03
-
400
450
500
Wavelength (nm)
Figure 2. Solid state absorption spectra of I1 and I11 along with that of I11 in chlorobenzene.
in all solvents. It is apparent that electron-withdrawing substituents decrease the electron density on the naphthalene moiety and cause a decrease in both intensity and hypsochromic shift of the So S1 transition. The radiative lifetime, T ~ of, the emitting state, S1, was calculated for I1 and I11 to be 18 and 14 ns, respectively, from the integrated absorption band using the formula
-
1 kr
3.47 x 1 0 y dr,
T O = - =
ii2
where k , is the first excited singlet state radiative rate constant and 3 is the frequency of maximum absorption. The small difference between the radiative lifetime of I1 and I11 reflects the similarity in the dipole moment change in the So Sl transition. The solid state absorption spectra of I1 and 111, along with their solution (chlorobenzene)absorption spectra, are shown in Figure 2. A slight red shift of the crystal long
-
The Journal of Physical Chemistry, Vol. 83,No. 9, 1979
1210
TABLE 11: -
R. 0. Loutfy and J. H. Sharp
Furanquinones Fluorescence Data Ib
p-dioxane chlorobenzene 1,2-dichloroethane acetone acetonitrile dime thylformamide dimethyl sulfoxide
0.04 0.3 0.49 0.65 0.71 0.67 0.65
560 578 600 620 627 640
* f ( n , D ) =[ ( D - 1 / D +
I11
0.23 0.16 0.11
2-methyltetrahydro furan
room temp 71 K solid state
I1
513 510 0.41 582 2 ) - ( n Z - l / n z t 2)]. See ref 3
.
5171 6322 6754 6721 6948
545 560 550 559 580 600
0.26 0.25
580 0.11 505 4796 572 In nm. In cm-'.
~-
_ _ ~ .
4640 5237 5183 5415 5147 6322
530 545 552 560
0.11 0.14 0.14 0.09
4551 5236 5693 6009
6288
565 49 3 545
0.06
6054
4990
4535
TABLE 111: Phosphorescence Data luminescence max, nma (#Q,)
a
Ib
I1
I11
620 (0.01)
620 (0.05)
614 (0.165)
In 2-MTHF at 77 K.
70
t
e
/I
See ref 3.
[/D-I/D+2}-
(n2-l/n2+2}]
Figure 4. Solvent-induced shifts of the absorption and fluorescence band of I, 11, and 111 according to McRae's orientation polarization formula.
11
300
1
the frequency difference, Av,between the 0-0 bands in the absorption and emission transitions of an isotropic solute molecule is
I
400
Wavelength (nrn)
Figure 3. Fluorescence emission and excitation spectra of I1 and 111 in chlorobenzene.
wavelength absorption edge is seen with respect to solution molecular spectra. Fluorescence Spectra. The wavelength and quantum yield of fluorescence of furanquinones I1 and I11 are sensitive to both solvent polarity and temperature. Therefore, fluorescence spectra and quantum yield determinations were made in a variety of solvents at ambient temperature and the results are given in Table 11, along with those of I for comparison. The fluorescence emission and excitation spectra of I1 and I11 in chlorobenzene at lo-* M are shown in Figure 3. In polar solvent the fluorescence maxima of I1 and I11 are red shifted, in agreement with that found for I. Again, the electron-withdrawing substituents (carboxamide and bromine) cause a hypsochromic shift of the emission spectra. In any given solvent the fluorescence wavelength maxima shifts are found to be ordered in series I > I1 > 111, which parallels their respective absorption shifts. Several authors have discussed solvent effects on electronic transitions and interpreted solvent-induced frequency shifts in terms of dispersive and static dipole interactions. The solvent-induced frequency shift in the absorption and emission transition ( v A - v F ) between the ground and excited state of I was previously discussed by Walker and Miller.3 The formula due to McRae7 relating
where pg and he are the dipole moments of the solute in the ground and excited states, respectively, a is the cavity radius, and D and n are the solvent dielectric constant and reflective index, respectively. The stark quadratic term is neglected here. Lippert8 and Matagag have derived similar orientation-polarization formula to that given in eq 2 and applied their formula to the derivation of the excited state dipole moments. Figure 4 shows a plot of ( v A - vF) for I, 11, and I11 against the solvent parameter term. A similar correlation was obtained using the Lippert and Mataga function. It can be seen that eq 2 is only fairly adequate in correlating the observed shift with solvent refractive index and dielectric constant. The change in solute dipole moment on excitation, Ap, ( p e - pg) may be calculated from eq 2 with the assumption that the formula is adequate in explaining the observed solvent-induced shift in (vA - v F ) . Taking a as 7.5 A3, the dipole moment change Ap for I1 and I11 was estimated to be 11.4 and 12.1 D, respectively, compared to 17.3 D found for I.3 The ground state dipole moment of I was estimated3 to be 1.4 D, while that of the excited state was found to be 18.7 D. The decrease in Ap as a result of electronwithdrawing substituents can be due to either an increase or a decrease in pe. The former is very unlikely since in l g the substituents are pulling electron charge in the opposite
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
Electronic Properties of Furanquinone Pigments
';?
1211
-II
Ir\
---.m
Phosphorescence Excitation
Phosphorescence
III
I I
\
I
Wavelength (nm)
Figure 7. Phosphorescenceemission and excitation spectra for I1 and I11 in 2-MTHF at 77 K.
Wavelength ( n m 1
Figure 5. Fluorescence emission of I1 and I11 in 2-MTHF at 77 K.
400
500 600 Wavelength ( n m )
700
Figure 6. Solid state fluorescence emission and excitation spectra of I1 and 111.
direction of the molecular dipole moment vector so, if anything, a slight decrease in pg is expected. Therefore, the change in Ap must be predominantly due to a decrease in p e . With this assumption the excited state dipole moments of I1 and 111 are 12.8 and 13.5 D, respectively. The fluorescence of I, 11, and I11 in polar media is also temperature dependent. A blue shift and an increase in intensity on lowering the temperature from room temperature to 77 K was observed. The emission maxima in 2-methyltetrahydrofuran (2MTHF) of I, 11, and I11 shift by about 70 nm going from room temperature to 77 K (see Table 11). The fluorescence of I1 and I11 is 2-MTHF at 77 K is shown in Figure 5. The fluorescence lifetime of I1 and I11 in chlorobenzene can be estimated using the radiative lifetime and quantum yield of fluorescence, +F, according to TF = TO+F (3) which gives 4.8 X and 1.6 X s, respectively. The solid state fluorescence emission and excitation of I1 and I11 are given in Figure 6. The similarity between the solid and molecular emission spectra reflect the weak intermolecular interaction in the crystal of these materials. Phosphorescence. A weak nonstructured phosphorescence was observed from I and 11, however, a much stronger emission from I11 was obtained (Figure 7) in 2-methyltetrahydrofuran glass at 77 K. The phosphorescence quantum yields were estimated to be 0.01, 0.05, and 0.16 for thie three compounds, respectively. The singlet-triplet splitting was of the order of 3700 cm-'.
Measurement of the phosphorescence decay was difficult due to the low intensity of the emission, but the triplet lifetime was of the order of 30 ms. The increase of phosphorescence quantum yield on bromination is consistent with the well-known phenomena of enhanced intersystem crossing via the heavy atom effect.
Discussion The similarity between the solid and molecular absorption spectra of the quinone I1 and I11 reflects the relatively weak intermolecular interactions in the crystal phase. The observed solid-state shift of the long wavelength absorption band was 516 and 480 cm-l for I1 and 111, respectively. The shift in the fluorescence emission bands on going from the molecular to solid phase of 11and I11 are 870 and 520 cm-l, respectively, which is slightly larger than the absorption shifts. The halogen-free compound I1 consistently exhibits greater shift than the brominated analogue 111. The magnitude of the crystal emission shift of these materials suggests that the molecules are stacked with an interplanar distance of 3.5-4.0 i% (the emission shifts being slightly less than those observed for pyrene and perylene, where the distance is close to 3.5 &.lo This interplanar distance is consistent with the X-ray data of G o l d ~ t e i n . ~ The effect of bromine atom substitution on the spectroscopic properties of the furanquinone system under study consists of (a) a hypsochromic shift of both absorption and emission bands, (b) a marked decrease in fluorescence quantum yield (a factor of 2), and (c) an increase in phosphorescence quantum yield. The hyposochromic shift of the absorption and emission caused by the electron-withdrawing (inductive effect) bromine substituent is due to lowering the energy of both the a and a* orbitals of the furanquinone. However, lowering the energy of the i7* orbital is less than the i7-bonding orbital and this increases the band gap.'l The decrease in the fluorescence yield and increase in phosphorescence yield on bromination of I1 is due to the decrease in singlet state population as a result of enhanced intersystem crossing to the triplet manifold. Enhanced intersystem crossing via the heavy atom effect between %,a* and 3a,7r* states is well documented. Although the singlet state radiative lifetime obtained from integrated absorption of both I1 and I11 is almost the same, the singlet and triplet state populations were significantly altered by bromination. This example offers a good possibility of examining the relationship between electronic relaxation processes and photoconductivity in organic compounds.12 The majority of organic photoconductors are fluorescent material^.'^ It has been observed that doping with small
The Journal of Physical Chemistry, Vol. 83, No. 9, 1979
R. 0. Loutfy and J. H. Sharp
TABLE IV: S u m m a r y of Photophysical D a t a for II a n d I11
excited state is the major precursor of charge carriers then structural modification in the molecule which leads to a reduction in the singlet population will also result in lower photosensitivity. The relative ratio of photogeneration efficiency of structurally similar molecules can be given as _q -- k(E)+ k,' + k,,' - k ( E ) + 1/7' 7' k ( E ) + k, + k,, k ( E ) 1 / ~ (6)
1212
-
Gf(ch1orobenzene) E,, e V k r , s-' knr, s " TF, n s $p
(2-MTHF, 77 K )
ET, eV
I1
I11
0.26 2.49 5.6 x io7 1.59 x l o 8 4.8 0.05 2.00
0.11 2.56 7.1 x 107 5.74 x l o 8 1.6 0.17
+
2.03
amounts of fluorescent quenchers results in the suppression of photoconductivity of the host molecule^.^^ The initiating act in the generation of photocurrent is the absorption of light quanta. Thereafter, three energy-dissipative processes occur: (1)luminescence emission, with a rate h,; (2) nonradiative decay, k,, either via internal conversion (kiJ and/or intersystem crossing (kiB);and (3) production of carriers, h ( E ) , which is usually field dependent for organic materials. Processes l and 2 are competitive with respect to process 3. In the case of a uniformly excited particle, one may write
iph = q7pEeA
(4)
where II is the quantum efficiency of carrier generation in an applied field of E , 7 is the carrier lifetime, p is their mobility, and e is the electronic charge. Very little is known regarding either 7 or p and their dependence on material structure and chemical purity. However, for a series of structurally similar molecules, it is reasonable to assume that 7 and p will be of the same order of magnitude. Therefore, the relative photocurrent is mainly determined by the quan tum efficiency of carrier generation. The quantum efficiency of carrier generation 7 can be given by14 1 =
k(E) kr + h n r + k ( E )
(5)
From eq 5, as the radiative and nonradiative decay rates for a given material increase, this will lead to a decrease in photogeneration efficiency. The radiative and nonradiative decay rate constants of I1 and I11 are given in Table IV, along with other spectroscopic data. The dramatic increase in the nonradiative rate constant of I11 over I1 is mainly due to enhanced intersystem crossing, lowering the singlet state population. However, this results in an increase in the triplet state population of 111. Thus, if the singlet excited state was the precursor of charge carriers, then I1 is expected to be more photoactive than 111. Photoconductivity of organic compounds is usually measured using either a single crystal or a thin evaporated film sandwiched between two e1e~trodes.l~ Recently, Cressman2J6described a technique for measuring the quantum efficiency charge injection from photoactive organic particles or evaporated films into insulating fluids. Cressman determined the photoelectrophoretic imaging sensitivity of I1 and I11 particles suspended in an insulating fluid, which relate to photogeneration efficiency, to be 0.025 and 0.01 electron/photon, respectively. If the singlet
when 7 < 0.10; i.e., k(E)is less than 10% of (12, + k,), then the relative photocurrent will mainly be determined by their singlet lifetime. According to eq 6 and the data in Table IV, the ratio of qn/qIII was estimated to be 2.6 which is in good agreement with Cressman's photoimaging sensitivity ratio of 2.5 for the two compounds. This suggests that the singlet state is involved in the generation act of charge carriers, however, the involvement of the triplet state cannot be totally excluded without knowledge of the triplet lifetime. In conclusion, structural modification of furanquinones which leads to enhanced nonradiative decay processeg significantly decrease the probability of photoconductivity. Full understanding of the origin and control of photo- and semiconductivity of organic materials requires systematic spectroscopic, electrical, and chemical analysis. In this work a correlation between the emissitivity of furanquinones and their photoimaging sensitivity has been found. References a n d Notes (a) F. Gutmann and L. E. Lyons, "Organic Semiconducts", Wiley, New York, 1967; (b) J. H. Sharp and M. Smith, "Physical Chemistry", Voi. 10, Academic Press, New York, 1970, p 435; (c) H. Meier, "Organic Semiconductors", H. F. Hebei, Ed., Verhg Chemle, Veinheim, 1974. (a) P. J. Cressman, G. C. Hartmann, J. E. Kuder, F. D. Saeva, and D. Wychick, J. Chem. Phys., 61, 2740 (1974); (b) E. W. Gooden, Nature(London),203,515 (1964); (c) H. Baessler and N. Riehl, Phys. Lett., 12, 101 (1964): (d) R. Loutfy and J. H. Sharp, J . Am. Chem. Soc., 99, 4049 (1977); (e) P. J. Rencroft, 0. N. Rudyj, R. E. Salomon, and M. M. Labes, J . Cbem. Pbys., 43, 767 (1965). M. S. Walker, J. E. Kuder, and R. L. Miller, J . Phys. Cbem., 75, 3257 (1971); 76, 2240 (1972); M. S. Walker, R. L. Miller, C. H. Griffiths, and P. Goidstein, Mol. Cryst. Liq. Cryst., 16, 203 (1972); G. Pfiitser and P. Nielsen, J . Appi. Pbys., 43, 3104 (1972); S. Tutihasi, ibid., 43, 3097 (1972); J. E. Kuder, D. Wychick, R. L. Miller, and M. S. Walker, J . Pbys. Cbem., 78, 1715 (1974). V. Tulagin, J . Opt. SOC. Am., 59, 328 (1969). L. Weinberger, P. Unger, and P. Cherin, J . Heterocycl. Chem., 6, 761 (1969). C. A. Parker and W. T. Rees, Analyst, 85, 584 (1960). E. G. McRae, J . Pbys. Cbem., 61, 562 (1957). E. Lippert, W. Luder, and H. Boos, Adv. Mol. Spectrosc., 1, 443 (1962). N. Mataga, V. Torihashi, and K. Ezumi, Tbeor. Cbim. Acta, 2, 158 ( 1964). B. Stevens, Spectrocbim. Acta, 18, 439 (1962). R. 0. Loutfv and R. 0. Loutfv. Can. J . Cbem.. 50. 4050 (1972). M. Kleinerman, L. Azarraga, i n d S. P. McGlynn, J . Cbem.'Pbys:, 37, 1825 (1962). D. 0. Northrop and 0. Simpson, Proc. R. SOC. London, Ser. A , 244-377 (1958). R. E. Menzei and Z. D. Popovic, Cbem. Phys. Lett., 55, 177 (1978). (a) A. Golubovic, J. Pbys. Cbem., 73, 1352 (1969); (b) D. D. Eley, H. Inokuchi, and M. R. Willis, Discuss. Faraday Soc., 28, 54 (1959); (c) T. K. Mukheiyee, J . Phys. Chem.,74, 3006 (1970); (d) Y. Okamoto and S. K. Kundu, ibid., 77, 2677 (1973); (e) G. Toiiln, D. R. Kearns, and M. Calvin, J . Cbem. Pbys., 32, 1013 (1960). P. J. Cressman, Pbotogr. Sci. Eng., 18, 284 (1974).
