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Characterization of bisazo compounds employing ultrafast spectroscopy
Chemical Physics 269 (2001) 357±366
www.elsevier.nl/locate/chemphys
Characterization of bisazo compounds employing ultrafast spectroscopy Renata Karpicz a, Vidmantas Gulbinas a,*, Albina Stanishauskaite b, Algimantas Undzenas a b
a Institute of Physics, Gostauto St. 12, Vilnius, Lithuania LT-2600 Kaunas University of Technology, Radvilenu Plentas 19, Kaunas, Lithuania LT-3028
Received 11 January 2001
Abstract Fluorenone-based bisazo compounds with dierent substituents have been investigated in various organic solvents and in a solid state by means of absorption, ¯uorescence and picosecond time-resolved absorption pump±probe spectroscopy. The bisazo compounds were found to coexist in azo±enol and hydrazone±quinone (HQ) tautomeric forms. Absorption spectra of the two forms were determined and excited-state properties were characterized. The azo± enol tautomeric form has a short, of tens of ps, excited-state lifetime and shows strong excited-state absorption, whereas stimulated emission is weak or absent. The HQ tautomeric form has a longer excited-state lifetime and shows sizeable stimulated emission. The main spectroscopic features of bisazo molecules in a solid state are similar to those in solutions, however, the excited-state relaxation is in¯uenced by the speci®c solid phase eects. The lifetimes of the HQ form molecules in solutions dier by more than 10 times for bisazo compounds with dierent substituents and correlate with the photosensitivity of xerographic layers based on these compounds. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bisazo compound; Absorption; Fluorescence; Tautomer; Picosecond absorption spectroscopy
1. Introduction Azo pigments, bis- and trisazo compounds are currently the dominant organic photoconductive materials used for fabrication of photogenerating layers in commercial copiers. The sensitizing mechanism is attributed to the photoinduced electron transfer between the photoexcited azo pigment and the hole transporting material in the groundstate [1,2]. Since the charge carrier generation in
*
Corresponding author. Fax: +370-2-61-7070. E-mail address: [email protected] (V. Gulbinas).
organic photoconductors proceeds from the excited electronic state, the excited-state properties are also very important for their photoelectrical features. The azo±enol (AE) and hydrazone±quinone (HQ) tautomerizm (Fig. 1) of bisazo pigments containing 2-hydroxy-3-naphthanilides is well documented, both in solutions [3±5] and in the solid phase [5±7]. The equilibrium between these two forms depends on the molecular structure [8,9], temperature [10,11] and on the solvent polarity and/or basicity [12,13]. The shift of the tautomeric equilibrium was observed for dierent azo compounds by adding acids or alkalis into solvents
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Fig. 1. Azo-enol and hydrazone±quinone tautomerizm.
[3,12,14]. It has been determined that the HQ form absorbs light at longer wavelengths, and these tautomeric species provide higher photosensitivity of dual electrophotographic photoreceptors [3,5, 15]. The aim of this work was to study spectroscopic characteristics of dierent tautomeric forms of 2,7diamino¯uorenone-based bisazo (F-bisazo) compounds with various substituents in the anilide fragment in order to establish the relationship between their excited-state properties and photosensitivity of xerographic layers based on these compounds. Absorption and ¯uorescence spectra of F-bisazo compounds in dierent solvents and in a solid state were analyzed in order to monitor their tautomeric modi®cations. Transient dierential absorption spectra and kinetics have been studied by means of picosecond pump±probe absorption spectroscopy. 2. Experiment Four dierent F-bisazo compounds were investigated. Their chemical structures are presented in Fig. 2. The F-bisazo compounds were synthe-
sized according to the conventional procedure [7,16]. The general preparation procedure is as follows: the ®rst stage ± diazotization of 2,7-diamino-9-¯uorenone (sodium nitrite, 6 N hydrochloric acid) at 0°C with subsequent pouring of the tetrazonium solution into hydrogen tetra¯uoroborate; the second stage ± the coupling reaction of 9-¯uorenone-2,7-bisdiazonium bistetra¯uoroborate with respective azotols in N,N-dimethylformamide. The crude products were ®ltered out, several times washed with water and then with N ,N -dimethylformamide, stirred in acetone and ether to remove residual high-boiling organic solvents. The synthesized products were ®nally dried at 65°C overnight to yield the corresponding azo compounds. The F-bisazo compounds were investigated in a-chloronaphthalene (ACN), tetrahydrofuran (THF) and 1,2-dichloroethane (DCE) solutions. Only in ACN all compounds have sucient solubility, therefore, only some of them were investigated in THF and DCE. The ®lms were prepared as follows. The pigment and polyvinylbutyral dispersion was carried out in a ball mill. The used solvent was tetrahydrofuran. The resulting dispersion contained 75% by weight of a bisazo pigment. Films were prepared by applying the dispersion onto a glass substrate with subsequent drying at 60°C. The thickness of fabricated ®lms was in the range of 0.3±0.5 lm. The absorption spectra were recorded by a ``Beckman'' spectrophotometer UV5270. The ¯uorescence was investigated by means of Fluorolog 2 from SPEX. The transient absorption study was performed using a pump±probe spectrometer
Fig. 2. Structural formula of the F-bisazo compounds.
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equipped with a home-made low repetition rate (1 Hz) Nd3 : glass laser generating pulses of the 2-ps duration. The second harmonic of the fundamental radiation (527 nm) and the Raman scattered (624 nm) light pulses were used for the sample excitation, and the white-light continuum generated in a water cell was used to probe the samples. Transient dierential absorption spectra at selected delay times were measured by scanning the monochromator wavelength, and kinetics at a selected wavelength was measured by changing the delay time between the excitation and probe pulses. Solutions used for picosecond absorption pump± probe experiments exhibit absorbance between 0.15 and 0.45 at 527 nm in a 5-mm cuvette. The corresponding concentrations were about 10 5 mol/l. 3. Results and discussion 3.1. Solutions Absorption spectra of all investigated solutions are presented in Fig. 3, where they are arranged according to the increasing intensity of the longwavelength absorption component. Absorption spectra of bisazo compounds have three clear absorption bands with slightly dierent positions and dierent relative intensities. Second derivatives of the absorption spectra (not presented) have revealed that the absorption band maxima of all compounds in THF and in DCE solutions are located at ca. 520, 562, 612 nm. In the ACN solution all bands are shifted to the longer wavelength side by 10±15 nm. The intensity of the long-wavelength component depends on the substituents and the solvent. This band is absent in the FBA-1 solution in DCE, whereas it is the strongest one in the FBA-4 solution in ACN. It should be noted, that relative intensities of dierent absorption components also depend on other factors, such as temperature, the solution preparation procedure and storage time. However, there is no clear dependence on the solution concentration. Therefore, dierent absorption bands should be assigned to dierent tautomeric forms, rather than
Fig. 3. Absorption spectra of all investigated solutions and zero time dierential absorption spectra of these solutions measured under the 527 nm excitation. The dierential absorption spectra in ACN are corrected by subtracting two-photon absorption spectrum of ACN.
to the formation of dimers or larger aggregates. Analysis of the literature data allows us to assign the two shorter-wavelength absorption bands to the AE tautomeric form and the long-wavelength band to the HQ tautomeric form [3,6,7,12]. It leads to the conclusion that two dierent molecular forms coexist in solutions. According to the band intensities, the concentration of the HQ form in the ACN solution is always higher than that in THF or DCE solutions. This form is almost absent in FBA-1 in DCE, where the compound is predominantly in the AE form.
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Fig. 4. Absorption (1), ¯uorescence (2) and ¯uorescence excitation (3) spectra of FBA-2 in ACN. Fluorescence measured under 550 nm (±±) and 570 nm ( ). Fluorescence excitation spectra detected at 650 nm (- - -) and at 700 nm (---).
Bisazo compounds in solutions have a low ¯uorescence quantum yield. It depends both on the substituents and the solvent. For example, the quantum yield of FBA-1 in ACN is about 10 2 . In other solvents it is even lower. Fluorescence spectra of FBA-2 in the ACN solution measured for two dierent excitation wavelengths are shown in Fig. 4. Two ¯uorescence bands are observed: the low intensity one with its maximum at 585 nm and the high intensity one at 640 nm. The relative intensity of the 585 nm band for excitation at 550 nm is slightly higher. The short-wavelength band may be assigned to the AE form ¯uorescence and the long-wavelength one to the HQ form [4,17]. The ¯uorescence excitation spectra con®rm this assignment. The spectra measured by detection at 650 and 700 nm are almost identical, but dierent from the absorption spectrum (Fig. 4). The identical spectra measured at dierent detection wavelengths indicate that they should be related only to one molecular species, namely to the HQ tautomeric form. The excitation spectrum should correspond to the absorption spectrum of this form. The absorption spectrum of the AE form may be obtained by normalizing the ¯uorescence excitation spectrum to the absorption spectrum in the 625±675 nm region, where absorption of the AE form is evidently very weak, and by subtracting the normalized excitation spectrum from the absorption spectrum. Fig. 5
Fig. 5. Absorption spectra of both tautomeric forms evaluated from analysis of the absorption and ¯uorescence excitation spectra of FBA-2 in ACN.
shows the isolated absorption spectra of both tautomeric forms. It should be pointed out that the almost identical spectrum of the AE form may be also obtained by normalizing in the long-wavelength region the absorption spectra of the dierent compounds in ACN and by the analogous subtracting procedure. This con®rms the validity of the applied procedure. The obtained spectrum of the AE form is very similar to that of BFA-1 in DCE, only it is bathochromically shifted by about 10 nm. The excited-state properties of dierent tautomeric forms of F-bisazo compounds were investigated by means of the picosecond transient absorption spectroscopy. Fig. 3 shows the dierential absorption spectra of all studied solutions measured at zero delay time between excitation (kex 527 nm) and probe pulses. The dierential absorption spectrum of FBA-1 in DCE, where only the AE form of molecules is present, shows bleaching of the absorption spectrum and a strong induced absorption with a maximum at 670 nm. Weak induced absorption is also observed in the 450±475 nm region. In other solutions, where the concentration of the HQ form is higher, the relative intensity of the induced long-wavelength absorption decreases, and bleaching of the HQ form absorption band at 620 nm appears. The negative transient absorption in the 675±750 nm region is observed in FBA-2, FBA-3 and FBA-4 solutions in ACN, where the HQ form is dominating. This
R. Karpicz et al. / Chemical Physics 269 (2001) 357±366
Fig. 6. Transient dierential absorption spectra at zero delaytime of dierent F-bisazo compounds in various solvents (kex 624 nm).
signal should be assigned to the stimulated emission of the HQ form molecules. It should be noted that a two-photon absorption of ACN gives an additional induced absorption in the 450±530 nm region. This absorption component was eliminated from the spectra presented in Fig. 3 by subtracting the induced absorption spectrum measured in pure ACN. Fig. 6 shows dierential absorption spectra at zero delay time of bisazo solutions measured upon 624 nm excitation. According to the absorption spectra of the two tautomeric forms, we assume that the HQ form is selectively excited. The spectrum of FBA-4 in the ACN solution is very similar to that obtained upon 527 nm excitation. This indicates that concentration of the AE form molecules in this solution is very low. Dierential absorption spectra of the other compounds in ACN are qualitatively similar. The spectra of FBA-2 and FBA-3 in THF have features of both HQ and AE tautomeric forms, indicating that the AE form was also excited, probably by a two-photon absorption. The spectra of the other compounds due to
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the low concentration of the HQ form or its low solubility could not be detected. According to the above discussion, the dierential absorption spectrum of FBA-1 in DCE measured upon 527 nm excitation should be attributed to the AE form molecules, and the spectra of various bisazo solutions in ACN obtained upon 624 nm excitation to the HQ form. The spectra of both forms show the ground-state absorption bleaching, but their excited-state properties are evidently different. The AE form may be characterized by strong excited-state absorption in the 625±800 nm region, whereas the stimulated emission band is absent. The HQ form molecules, vice versa, have a sizeable stimulated emission band, whereas the excited-state absorption is not apparent. The excited-state relaxation kinetics of the two forms are also dierent. We will discuss the relaxation kinetics of FBA-1 in DCE upon 527 nm excitation and that of FBA-4 in ACN upon 624 nm excitation. In both cases one of excited tautomeric forms is clearly dominating. Fig. 7 shows dierential absorption spectra of the two samples measured at dierent delay times, and Fig. 8 shows absorption kinetics of these compounds measured at different wavelengths. The spectrum of FBA-1 in DCE at 50 ps delay time (see Fig. 7a) shows only weak absorption bleaching, whereas the induced absorption band is already absent. The absorption dynamics measured at 670 nm shows that the induced absorption decays exponentially with a time constant of 21 5 ps. The main part of the absorption bleaching decays with the same 21 5 ps time constant, but another decay component, with the time constant much longer than 1 ns, is also present. The dierential absorption spectrum of FBA-4 in ACN related to the HQ form molecules decays without changing its shape. The decay kinetics at dierent detection wavelengths is identical, and, however, nonexponential. It may be characterized by 7 2 and 40 5 ps time constants. Excited-state relaxation of the other solutions, where both tautomeric forms were excited, may be viewed as a superposition of the two processes, where each may be characterized by two decay times. However, a reliable determination of both time constants of one or another tautomeric form was possible only when the excited molecules of
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Fig. 8. Absorption kinetics measured at dierent wavelengths of: (a) FBA-1 in DCE under 527 nm excitation, (b) FBA-4 in ACN under 624 nm excitation.
Fig. 7. Dierential absorption spectra at dierent delay times of: (a) FBA-1 in DCE under 527 nm excitation, (b) FBA-4 in ACN under 624 nm excitation.
this tautomeric form were clearly dominating. In the other cases, only one decay component could be determined, or the determination was not possible at all if the concentration of this tautomeric form was too low. The decay times of the two tautomeric forms are presented in Table 1. Though the stationary absorption and dierential absorption spectra of one or another tautomeric form are almost identical for all molecules and only slightly depend on the solvent, the decay times depend signi®cantly both on the substitutients and the solvent. The transient absorption spectra of AE form molecules, that have dierent shape at zero and at
long-delay times, show that the absorption spectra of the excited molecules at short and long delays are dierent. The long-delay time spectrum should be attributed to some intermediate state populated during the excited-state relaxation. The intermediate state more likely should be attributed to the electronically relaxed molecules, since the long lifetime species show no excited-state absorption. Fast excited-state relaxation of the AE form molecules, which is slightly slower in the higher viscosity solvent (ACN), implies that conformational changes of molecules may be responsible for the internal conversion. This process is very typical for molecules composed of several fragments connected by ¯exible chains [18]. The molecules probably change their conformation in the excitedstate, what causes their fast relaxation to the electronic ground-state with changed quasi-stable conformation. In this conformation the molecules may have lower extinction coecient in the visible spectral region causing the absorption bleaching. The dierential absorption spectra of the HQ form molecules, showing absorption bleaching and
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Table 1 Characteristic times (ps) of azo-enol and hydrazone±quinone tautomeric forms obtained by ®tting with mono- or bi-exponential functions of transient absorption signals recorded at dierent wavelengths for the four studied F-bisazo compounds DCE, FBA-1 AE form HQ form
23 3 2500 500
THF
ACN
FBA-1
FBA-2
FBA-3
FBA-1
FBA-2
FBA-3
82 2500 500
82 2000 500
72 2000 500
50 5
40 5
10 2
90 10
70 10
400 50
380 20
78 5 525 25
stimulated emission, are quite typical for organic dyes. The nonexponential excited-state relaxation of this form hardly can be attributed to the formation of some intermediate state, since dierential absorption spectra do not change their shape with time. The attribution of dierent relaxation components to dierent molecular conformations is more plausible. The bisazo molecule has several single bonds connecting large molecule fragments and several sites where H-bonds could be formed, therefore, they probably may coexist in two or more dierent conformational states with similar spectroscopic properties. However, the excitedstate relaxation rates may be dierent if the relaxation involves conformational changes in the excited state. This is supported by the observation of a relatively short excited-state lifetime, which depends on the solvent and substituents. The red-shifted absorption spectrum of the HQ form molecules indicates that the lowest excited state is situated at lower energy than that of the AE form. Therefore, one can expect the transformation of the AE form molecules into the HQ form in the excited state. However, our data show no evidences of such transformation. The two tautomeric forms relax independently of each other. 3.2. Solid state Fig. 9 shows the stationary absorption spectra and the zero delay time dierential absorption spectra of ®lms prepared from small solid particles of various bisazo compounds dispersed into a polyvinylbutyral matrix. The spectra are qualitatively similar to those obtained for solutions. As well as in the case of solutions, the absorption
FBA-4
72 40 5
Fig. 9. Absorption and zero delay time dierential absorption spectra of solid particles of various bisazo compounds dispersed in a polyvinylbutyral matrix.
spectra are composed of three main bands situated in similar positions for all four compounds, but with dierent relative intensities. The band positions are shifted to the long-wavelength side by 20±30 nm in comparison with those in the ACN solution. The relatively large intensity of the longwavelength absorption component of FBA-4 shows that this compound in ®lms also predominantly exists in the HQ form, whereas for the other compounds, the concentrations of both forms in ®lms are quite similar. The similar absorption spectra of ®lms and solutions indicate that the
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main optical properties of individual molecules are preserved in the solid state. No new absorption bands, which could be attributed to collective excitonic states or charge transfer states typical for molecular solids, were observed. Dierential absorption spectra of solid ®lms also qualitatively resemble those of the respective solutions. FBA-1, FBA-2 and FBA-3 show bleaching of the ground-state absorption and an induced absorption in the 650±800 nm region. Dierential absorption spectra of these compounds are very similar to those of the FBA-2 in ACN, having similar ratio of AE and HQ molecules. The spectrum of FBA-4, similarly to those of solutions with high concentration of HQ form molecules, shows negative dierential absorption in a long-wavelength region. The characteristic feature of the dierential absorption spectra of solid ®lms is a more pronounced structure of the absorption bleaching bands compared to that of absorption spectra. Fig. 10 shows that the structure of the dierential absorption spectrum of the FBA-1 ®lm becomes even more pronounced at longer delay times. At the 1000 ps delay the spectrum clearly shows bleaching
maxima at the maxima of the steady-state absorption bands and the bleaching minima at the edges of bands. Similar features of the spectral changes were also observed when the sample was stationary heated. This form of the spectral changes allows attributing them to the broadening of the ground-state absorption bands. Thus, heating the samples by the laser beam evidently also contributes to the transient absorption spectrum observed at long-delay times. Thus, the differential absorption spectra are evidently composed of two parts: of the dierential absorption caused by the electronically excited molecules and of the broadening of the ground-state absorption bands due to the local heating. The heating contribution increases with time when the electronic excitations relax and their energy is converted into heat. Fig. 11 shows the transient absorption kinetics at 580 nm measured in the FBA-1 ®lm at dierent excitation intensities. The kinetics shows the initial fast partial relaxation and a slow component with the relaxation time much longer than 1 ns. The initial relaxation becomes slightly faster at the highest excitation intensity. This is an indication that the nonlinear eects, like exciton±exciton annihilation [19,20], or the relaxation enhancement by ampli®ed spontaneous emission [21] start to play a role at high excitation intensities. However, the intensity dependence of the relaxation kinetics is too weak to explain the fast relaxation component solely by the nonlinear eects. The fast component may naturally be related to the intrinsic
Fig. 10. Dierential absorption spectra of BFA-1 in a polyvinylbutyral matrix at dierent delay times.
Fig. 11. Transient absorption relaxation kinetics at 580 nm of FBA-1 ®lm at dierent excitation intensities.
R. Karpicz et al. / Chemical Physics 269 (2001) 357±366
excited-state relaxation of molecules observed in solutions. On the other hand, the nonexponential excitation relaxation was often observed in molecular solids due to excitation random walk and trapping in relaxation centers formed by lattice defects or crystallite surfaces [22]. The slow relaxation component may be attributed to the excitations localized in shallow traps, where they have long lifetime. Namely this process seems to be more plausible, since relaxation rate in a solid state is even faster, whereas the excitation relaxation involving conformational changes should be slowed down in a solid phase. However these two linear relaxational processes hardly can be separated. The local heating may also contribute to the slow kinetics since the cooling of micrometer-size particles should take place on a nanosecond time scale. However, the heating-induced broadening of the absorption spectrum should not change the oscillator strength of the electronic transitions associated with the absorption and stimulated emission, whereas electronic excitations cause negative dierential absorption. This dierence allows separating of the two dierential absorption parts. According to this consideration, the dierential absorption spectrum of the FBA-1 ®lm indicates that electronic excitations are still present even at the 1000 ps delay time. A similar conclusion may be done regarding other bisazo compounds. 4. Concluding remarks Both the steady-state and transient dierential absorption properties of the bisazo solutions may be explained by the superposition of spectroscopic properties of two tautomeric forms. The preference of bisazo molecules to exist in the HQ form increases on going from FBA-1 to FBA-4. This phenomenon has also been observed for the solid phase as well. The two tautomeric forms of the bisazo compound have dierent absorption and ¯uorescence spectra and dierent ¯uorescence quantum yields. Dierences in the excited-state properties are even more pronounced. The HQ form shows excited-state properties typical for
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large organic molecules. The dierential absorption spectrum of this form clearly shows absorption bleaching and stimulated emission contributions. The AE form molecules also show absorption bleaching, whereas instead of stimulated emission strong induced absorption has been observed. Though the absorption spectra of both AE and HQ forms are almost independent of the substituents, however, the excited-state lifetimes vary signi®cantly with substitution. They also depend on the solvent. The excited-state relaxation mechanisms of the two forms are also dierent. An intermediate state is formed during the excited-state relaxation of the AE form. This state was attributed to the electronically relaxed molecules with altered conformation. Nonexponential relaxation of the HQ form molecules indicates to the coexistence of molecules with dierent conformations as well. Thus, conformational changes probably play an important role in the excited-state relaxation. Bisazo compounds in the solid phase resemble the basic spectroscopic features observed for solutions. However, the mechanism of the excitedstate relaxation is evidently dierent. Ultrafast fractional relaxation is related to the speci®c solid state eects, such as an exciton±exciton annihilation, or exciton relaxation via structural traps and also to the local heating. The latter leads to the broadening of the absorption bands and modi®cation of the dierential absorption spectra. Additional solid phase eects burden detailed investigation of the molecular relaxation, however, there are indications that the intrinsic excited-state lifetime in solids is much longer than in solutions. This conclusion is in agreement with the proposed mechanism of the excited-state relaxation involving conformational changes of molecules in the excited state. The conformational changes in a solid state are hindered and this relaxation mechanism is slowed down. How spectroscopic properties of bisazo pigments are related to the photosensitivity of photogenerating layers fabricated from these materials? According to Pacansky and Waltman [3], the HQ form of pigments is more photosensitive, thus relative concentration and properties of the HQ form should determine the photosensitivity of
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the charge-generating layer. As follows from our data, the preference for the HQ form increases in the series FBA-1, -2, -3, -4. However, Law and Tarnawskyj [7] measured photosensitivity of three of the investigated compounds and found that it increased in the opposite order FBA-4, -3, -1. Our preliminary investigations showed the same tendency. Thus, the properties of the HQ form, rather than its relative concentration, seem to play the major role. The photosensitivity correlates with the lifetimes of the bisazo compounds in the ACN solution. The photosensitivity of compounds having longer lifetime is higher. Though, the lifetimes depend signi®cantly on the solvent and their detailed investigation in a solid state was not possible, however, since in the ACN solution they vary more than by two orders of magnitude, it is reasonable to expect that this tendency may remain in a solid state as well. Since the charge carrier generation in molecular solids proceeds from the molecular excited state, the excited-state lifetime may be important for the photosensitivity of bisazo compounds with dierent substituents in charge generating layers.
References [1] M. Umeda, T. Niimi, M. Hashimoto, Jpn. J. Appl. Phys. 29 (1990) 2746. [2] M. Umeda, M. Hashimoto, J. Appl. Phys. 72 (1992) 112.
[3] J. Pacansky, R.J. Waltman, J. Am. Chem. Soc. 114 (1992) 5813. [4] R. Karpicz, V. Gulbinas, A. Ruseckas, A. Undzenas, Environ. Chem. Phys. 21 (1999) 43. [5] T. Niimi, M. Umeda, Jpn. J. Appl. Phys. 36 (1997) 3522. [6] K.-Y. Law, Chem. Rev. 93 (1993) 449. [7] K.-Y. Law, I.W. Tarnawskyj, J. Imaging Sci. Technol. 37 (1993) 22. [8] K.-Y. Law, I.W. Tarnawskyj, J. Imaging Sci. Technol. 39 (1995) 1. [9] M. Hashimoto, Electrophotography 25 (1986) 230. [10] A. Takase, S. Sakagami, K. Nonaka, T. Koga, J. Raman Spectrosc. 24 (1993) 447. [11] R. Potashnik, M. Ottolenghi, J. Chem. Phys. 51 (1969) 3671. [12] B.E. Zaitsev, E.S. Lisitsyna, E.D. Sycheva, T.A. Mikhailova, T.A. Dyumaev, Zh. Org. Khim. (rus.) 19 (1983) 1281. [13] W.F. Traven, E.E. Kostiuchenko, R.A. Mhitarow, N.F. Menshykowa, M.M. Oreshyn, B.I. Stepanow, J. Org. Chem. (rus.) 16 (1980) 1047. [14] L.A. Gribow, Ju.M. Dedkow, A.W. Kotow, Theor. Exp. Chem. (rus.) 4 (1968) 316. [15] P.J. Cressman, G.C. Hartmann, J.E. Kuder, F.D. Saeva, D. Wychick, J. Chem. Phys. 61 (1974) 2740. [16] Japan Patent 45664 C 09 B 35/34 (1985). [17] R. Karpicz, V. Gulbinas, A. Undzenas, J. Chin. Chem. Soc. 47 (2000) 589. [18] E. Lippert, W. Rettig, V. Bonacic-Kautecky, F. Heisel, J.A. Miehe, Adv. Chem. Phys. 68 (1987) 1. [19] R. Kopelman, S. Parus, J. Prasad, J. Chem. Phys. 128 (1988) 209. [20] V. Gulbinas, M. Chachisvilis, A. Persson, S. Svanberg, V. Sundstr om, J. Phys. Chem. 98 (1994) 8118. [21] G. Cerullo, S. Stagira, M. Nisoli, S. De Silvestri, G. Lanzani, G. Kranzelbinder, W. Graupner, G. Leising, Phys. Rev. B 57 (1998) 12806. [22] R. Kersting, U. Lemmer, R.F. Mahrt, K. Leo, H. Kurz, H. Bassler, E.O. G obel, Phys. Rev. Lett. 70 (1993) 3820.
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Characterization of bisazo compounds employing ultrafast spectroscopy Renata Karpicz a, Vidmantas Gulbinas a,*, Albina Stanishauskaite b, Algimantas Undzenas a b
a Institute of Physics, Gostauto St. 12, Vilnius, Lithuania LT-2600 Kaunas University of Technology, Radvilenu Plentas 19, Kaunas, Lithuania LT-3028
Received 11 January 2001
Abstract Fluorenone-based bisazo compounds with dierent substituents have been investigated in various organic solvents and in a solid state by means of absorption, ¯uorescence and picosecond time-resolved absorption pump±probe spectroscopy. The bisazo compounds were found to coexist in azo±enol and hydrazone±quinone (HQ) tautomeric forms. Absorption spectra of the two forms were determined and excited-state properties were characterized. The azo± enol tautomeric form has a short, of tens of ps, excited-state lifetime and shows strong excited-state absorption, whereas stimulated emission is weak or absent. The HQ tautomeric form has a longer excited-state lifetime and shows sizeable stimulated emission. The main spectroscopic features of bisazo molecules in a solid state are similar to those in solutions, however, the excited-state relaxation is in¯uenced by the speci®c solid phase eects. The lifetimes of the HQ form molecules in solutions dier by more than 10 times for bisazo compounds with dierent substituents and correlate with the photosensitivity of xerographic layers based on these compounds. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bisazo compound; Absorption; Fluorescence; Tautomer; Picosecond absorption spectroscopy
1. Introduction Azo pigments, bis- and trisazo compounds are currently the dominant organic photoconductive materials used for fabrication of photogenerating layers in commercial copiers. The sensitizing mechanism is attributed to the photoinduced electron transfer between the photoexcited azo pigment and the hole transporting material in the groundstate [1,2]. Since the charge carrier generation in
*
Corresponding author. Fax: +370-2-61-7070. E-mail address: [email protected] (V. Gulbinas).
organic photoconductors proceeds from the excited electronic state, the excited-state properties are also very important for their photoelectrical features. The azo±enol (AE) and hydrazone±quinone (HQ) tautomerizm (Fig. 1) of bisazo pigments containing 2-hydroxy-3-naphthanilides is well documented, both in solutions [3±5] and in the solid phase [5±7]. The equilibrium between these two forms depends on the molecular structure [8,9], temperature [10,11] and on the solvent polarity and/or basicity [12,13]. The shift of the tautomeric equilibrium was observed for dierent azo compounds by adding acids or alkalis into solvents
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Fig. 1. Azo-enol and hydrazone±quinone tautomerizm.
[3,12,14]. It has been determined that the HQ form absorbs light at longer wavelengths, and these tautomeric species provide higher photosensitivity of dual electrophotographic photoreceptors [3,5, 15]. The aim of this work was to study spectroscopic characteristics of dierent tautomeric forms of 2,7diamino¯uorenone-based bisazo (F-bisazo) compounds with various substituents in the anilide fragment in order to establish the relationship between their excited-state properties and photosensitivity of xerographic layers based on these compounds. Absorption and ¯uorescence spectra of F-bisazo compounds in dierent solvents and in a solid state were analyzed in order to monitor their tautomeric modi®cations. Transient dierential absorption spectra and kinetics have been studied by means of picosecond pump±probe absorption spectroscopy. 2. Experiment Four dierent F-bisazo compounds were investigated. Their chemical structures are presented in Fig. 2. The F-bisazo compounds were synthe-
sized according to the conventional procedure [7,16]. The general preparation procedure is as follows: the ®rst stage ± diazotization of 2,7-diamino-9-¯uorenone (sodium nitrite, 6 N hydrochloric acid) at 0°C with subsequent pouring of the tetrazonium solution into hydrogen tetra¯uoroborate; the second stage ± the coupling reaction of 9-¯uorenone-2,7-bisdiazonium bistetra¯uoroborate with respective azotols in N,N-dimethylformamide. The crude products were ®ltered out, several times washed with water and then with N ,N -dimethylformamide, stirred in acetone and ether to remove residual high-boiling organic solvents. The synthesized products were ®nally dried at 65°C overnight to yield the corresponding azo compounds. The F-bisazo compounds were investigated in a-chloronaphthalene (ACN), tetrahydrofuran (THF) and 1,2-dichloroethane (DCE) solutions. Only in ACN all compounds have sucient solubility, therefore, only some of them were investigated in THF and DCE. The ®lms were prepared as follows. The pigment and polyvinylbutyral dispersion was carried out in a ball mill. The used solvent was tetrahydrofuran. The resulting dispersion contained 75% by weight of a bisazo pigment. Films were prepared by applying the dispersion onto a glass substrate with subsequent drying at 60°C. The thickness of fabricated ®lms was in the range of 0.3±0.5 lm. The absorption spectra were recorded by a ``Beckman'' spectrophotometer UV5270. The ¯uorescence was investigated by means of Fluorolog 2 from SPEX. The transient absorption study was performed using a pump±probe spectrometer
Fig. 2. Structural formula of the F-bisazo compounds.
R. Karpicz et al. / Chemical Physics 269 (2001) 357±366
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equipped with a home-made low repetition rate (1 Hz) Nd3 : glass laser generating pulses of the 2-ps duration. The second harmonic of the fundamental radiation (527 nm) and the Raman scattered (624 nm) light pulses were used for the sample excitation, and the white-light continuum generated in a water cell was used to probe the samples. Transient dierential absorption spectra at selected delay times were measured by scanning the monochromator wavelength, and kinetics at a selected wavelength was measured by changing the delay time between the excitation and probe pulses. Solutions used for picosecond absorption pump± probe experiments exhibit absorbance between 0.15 and 0.45 at 527 nm in a 5-mm cuvette. The corresponding concentrations were about 10 5 mol/l. 3. Results and discussion 3.1. Solutions Absorption spectra of all investigated solutions are presented in Fig. 3, where they are arranged according to the increasing intensity of the longwavelength absorption component. Absorption spectra of bisazo compounds have three clear absorption bands with slightly dierent positions and dierent relative intensities. Second derivatives of the absorption spectra (not presented) have revealed that the absorption band maxima of all compounds in THF and in DCE solutions are located at ca. 520, 562, 612 nm. In the ACN solution all bands are shifted to the longer wavelength side by 10±15 nm. The intensity of the long-wavelength component depends on the substituents and the solvent. This band is absent in the FBA-1 solution in DCE, whereas it is the strongest one in the FBA-4 solution in ACN. It should be noted, that relative intensities of dierent absorption components also depend on other factors, such as temperature, the solution preparation procedure and storage time. However, there is no clear dependence on the solution concentration. Therefore, dierent absorption bands should be assigned to dierent tautomeric forms, rather than
Fig. 3. Absorption spectra of all investigated solutions and zero time dierential absorption spectra of these solutions measured under the 527 nm excitation. The dierential absorption spectra in ACN are corrected by subtracting two-photon absorption spectrum of ACN.
to the formation of dimers or larger aggregates. Analysis of the literature data allows us to assign the two shorter-wavelength absorption bands to the AE tautomeric form and the long-wavelength band to the HQ tautomeric form [3,6,7,12]. It leads to the conclusion that two dierent molecular forms coexist in solutions. According to the band intensities, the concentration of the HQ form in the ACN solution is always higher than that in THF or DCE solutions. This form is almost absent in FBA-1 in DCE, where the compound is predominantly in the AE form.
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Fig. 4. Absorption (1), ¯uorescence (2) and ¯uorescence excitation (3) spectra of FBA-2 in ACN. Fluorescence measured under 550 nm (±±) and 570 nm ( ). Fluorescence excitation spectra detected at 650 nm (- - -) and at 700 nm (---).
Bisazo compounds in solutions have a low ¯uorescence quantum yield. It depends both on the substituents and the solvent. For example, the quantum yield of FBA-1 in ACN is about 10 2 . In other solvents it is even lower. Fluorescence spectra of FBA-2 in the ACN solution measured for two dierent excitation wavelengths are shown in Fig. 4. Two ¯uorescence bands are observed: the low intensity one with its maximum at 585 nm and the high intensity one at 640 nm. The relative intensity of the 585 nm band for excitation at 550 nm is slightly higher. The short-wavelength band may be assigned to the AE form ¯uorescence and the long-wavelength one to the HQ form [4,17]. The ¯uorescence excitation spectra con®rm this assignment. The spectra measured by detection at 650 and 700 nm are almost identical, but dierent from the absorption spectrum (Fig. 4). The identical spectra measured at dierent detection wavelengths indicate that they should be related only to one molecular species, namely to the HQ tautomeric form. The excitation spectrum should correspond to the absorption spectrum of this form. The absorption spectrum of the AE form may be obtained by normalizing the ¯uorescence excitation spectrum to the absorption spectrum in the 625±675 nm region, where absorption of the AE form is evidently very weak, and by subtracting the normalized excitation spectrum from the absorption spectrum. Fig. 5
Fig. 5. Absorption spectra of both tautomeric forms evaluated from analysis of the absorption and ¯uorescence excitation spectra of FBA-2 in ACN.
shows the isolated absorption spectra of both tautomeric forms. It should be pointed out that the almost identical spectrum of the AE form may be also obtained by normalizing in the long-wavelength region the absorption spectra of the dierent compounds in ACN and by the analogous subtracting procedure. This con®rms the validity of the applied procedure. The obtained spectrum of the AE form is very similar to that of BFA-1 in DCE, only it is bathochromically shifted by about 10 nm. The excited-state properties of dierent tautomeric forms of F-bisazo compounds were investigated by means of the picosecond transient absorption spectroscopy. Fig. 3 shows the dierential absorption spectra of all studied solutions measured at zero delay time between excitation (kex 527 nm) and probe pulses. The dierential absorption spectrum of FBA-1 in DCE, where only the AE form of molecules is present, shows bleaching of the absorption spectrum and a strong induced absorption with a maximum at 670 nm. Weak induced absorption is also observed in the 450±475 nm region. In other solutions, where the concentration of the HQ form is higher, the relative intensity of the induced long-wavelength absorption decreases, and bleaching of the HQ form absorption band at 620 nm appears. The negative transient absorption in the 675±750 nm region is observed in FBA-2, FBA-3 and FBA-4 solutions in ACN, where the HQ form is dominating. This
R. Karpicz et al. / Chemical Physics 269 (2001) 357±366
Fig. 6. Transient dierential absorption spectra at zero delaytime of dierent F-bisazo compounds in various solvents (kex 624 nm).
signal should be assigned to the stimulated emission of the HQ form molecules. It should be noted that a two-photon absorption of ACN gives an additional induced absorption in the 450±530 nm region. This absorption component was eliminated from the spectra presented in Fig. 3 by subtracting the induced absorption spectrum measured in pure ACN. Fig. 6 shows dierential absorption spectra at zero delay time of bisazo solutions measured upon 624 nm excitation. According to the absorption spectra of the two tautomeric forms, we assume that the HQ form is selectively excited. The spectrum of FBA-4 in the ACN solution is very similar to that obtained upon 527 nm excitation. This indicates that concentration of the AE form molecules in this solution is very low. Dierential absorption spectra of the other compounds in ACN are qualitatively similar. The spectra of FBA-2 and FBA-3 in THF have features of both HQ and AE tautomeric forms, indicating that the AE form was also excited, probably by a two-photon absorption. The spectra of the other compounds due to
361
the low concentration of the HQ form or its low solubility could not be detected. According to the above discussion, the dierential absorption spectrum of FBA-1 in DCE measured upon 527 nm excitation should be attributed to the AE form molecules, and the spectra of various bisazo solutions in ACN obtained upon 624 nm excitation to the HQ form. The spectra of both forms show the ground-state absorption bleaching, but their excited-state properties are evidently different. The AE form may be characterized by strong excited-state absorption in the 625±800 nm region, whereas the stimulated emission band is absent. The HQ form molecules, vice versa, have a sizeable stimulated emission band, whereas the excited-state absorption is not apparent. The excited-state relaxation kinetics of the two forms are also dierent. We will discuss the relaxation kinetics of FBA-1 in DCE upon 527 nm excitation and that of FBA-4 in ACN upon 624 nm excitation. In both cases one of excited tautomeric forms is clearly dominating. Fig. 7 shows dierential absorption spectra of the two samples measured at dierent delay times, and Fig. 8 shows absorption kinetics of these compounds measured at different wavelengths. The spectrum of FBA-1 in DCE at 50 ps delay time (see Fig. 7a) shows only weak absorption bleaching, whereas the induced absorption band is already absent. The absorption dynamics measured at 670 nm shows that the induced absorption decays exponentially with a time constant of 21 5 ps. The main part of the absorption bleaching decays with the same 21 5 ps time constant, but another decay component, with the time constant much longer than 1 ns, is also present. The dierential absorption spectrum of FBA-4 in ACN related to the HQ form molecules decays without changing its shape. The decay kinetics at dierent detection wavelengths is identical, and, however, nonexponential. It may be characterized by 7 2 and 40 5 ps time constants. Excited-state relaxation of the other solutions, where both tautomeric forms were excited, may be viewed as a superposition of the two processes, where each may be characterized by two decay times. However, a reliable determination of both time constants of one or another tautomeric form was possible only when the excited molecules of
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Fig. 8. Absorption kinetics measured at dierent wavelengths of: (a) FBA-1 in DCE under 527 nm excitation, (b) FBA-4 in ACN under 624 nm excitation.
Fig. 7. Dierential absorption spectra at dierent delay times of: (a) FBA-1 in DCE under 527 nm excitation, (b) FBA-4 in ACN under 624 nm excitation.
this tautomeric form were clearly dominating. In the other cases, only one decay component could be determined, or the determination was not possible at all if the concentration of this tautomeric form was too low. The decay times of the two tautomeric forms are presented in Table 1. Though the stationary absorption and dierential absorption spectra of one or another tautomeric form are almost identical for all molecules and only slightly depend on the solvent, the decay times depend signi®cantly both on the substitutients and the solvent. The transient absorption spectra of AE form molecules, that have dierent shape at zero and at
long-delay times, show that the absorption spectra of the excited molecules at short and long delays are dierent. The long-delay time spectrum should be attributed to some intermediate state populated during the excited-state relaxation. The intermediate state more likely should be attributed to the electronically relaxed molecules, since the long lifetime species show no excited-state absorption. Fast excited-state relaxation of the AE form molecules, which is slightly slower in the higher viscosity solvent (ACN), implies that conformational changes of molecules may be responsible for the internal conversion. This process is very typical for molecules composed of several fragments connected by ¯exible chains [18]. The molecules probably change their conformation in the excitedstate, what causes their fast relaxation to the electronic ground-state with changed quasi-stable conformation. In this conformation the molecules may have lower extinction coecient in the visible spectral region causing the absorption bleaching. The dierential absorption spectra of the HQ form molecules, showing absorption bleaching and
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363
Table 1 Characteristic times (ps) of azo-enol and hydrazone±quinone tautomeric forms obtained by ®tting with mono- or bi-exponential functions of transient absorption signals recorded at dierent wavelengths for the four studied F-bisazo compounds DCE, FBA-1 AE form HQ form
23 3 2500 500
THF
ACN
FBA-1
FBA-2
FBA-3
FBA-1
FBA-2
FBA-3
82 2500 500
82 2000 500
72 2000 500
50 5
40 5
10 2
90 10
70 10
400 50
380 20
78 5 525 25
stimulated emission, are quite typical for organic dyes. The nonexponential excited-state relaxation of this form hardly can be attributed to the formation of some intermediate state, since dierential absorption spectra do not change their shape with time. The attribution of dierent relaxation components to dierent molecular conformations is more plausible. The bisazo molecule has several single bonds connecting large molecule fragments and several sites where H-bonds could be formed, therefore, they probably may coexist in two or more dierent conformational states with similar spectroscopic properties. However, the excitedstate relaxation rates may be dierent if the relaxation involves conformational changes in the excited state. This is supported by the observation of a relatively short excited-state lifetime, which depends on the solvent and substituents. The red-shifted absorption spectrum of the HQ form molecules indicates that the lowest excited state is situated at lower energy than that of the AE form. Therefore, one can expect the transformation of the AE form molecules into the HQ form in the excited state. However, our data show no evidences of such transformation. The two tautomeric forms relax independently of each other. 3.2. Solid state Fig. 9 shows the stationary absorption spectra and the zero delay time dierential absorption spectra of ®lms prepared from small solid particles of various bisazo compounds dispersed into a polyvinylbutyral matrix. The spectra are qualitatively similar to those obtained for solutions. As well as in the case of solutions, the absorption
FBA-4
72 40 5
Fig. 9. Absorption and zero delay time dierential absorption spectra of solid particles of various bisazo compounds dispersed in a polyvinylbutyral matrix.
spectra are composed of three main bands situated in similar positions for all four compounds, but with dierent relative intensities. The band positions are shifted to the long-wavelength side by 20±30 nm in comparison with those in the ACN solution. The relatively large intensity of the longwavelength absorption component of FBA-4 shows that this compound in ®lms also predominantly exists in the HQ form, whereas for the other compounds, the concentrations of both forms in ®lms are quite similar. The similar absorption spectra of ®lms and solutions indicate that the
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main optical properties of individual molecules are preserved in the solid state. No new absorption bands, which could be attributed to collective excitonic states or charge transfer states typical for molecular solids, were observed. Dierential absorption spectra of solid ®lms also qualitatively resemble those of the respective solutions. FBA-1, FBA-2 and FBA-3 show bleaching of the ground-state absorption and an induced absorption in the 650±800 nm region. Dierential absorption spectra of these compounds are very similar to those of the FBA-2 in ACN, having similar ratio of AE and HQ molecules. The spectrum of FBA-4, similarly to those of solutions with high concentration of HQ form molecules, shows negative dierential absorption in a long-wavelength region. The characteristic feature of the dierential absorption spectra of solid ®lms is a more pronounced structure of the absorption bleaching bands compared to that of absorption spectra. Fig. 10 shows that the structure of the dierential absorption spectrum of the FBA-1 ®lm becomes even more pronounced at longer delay times. At the 1000 ps delay the spectrum clearly shows bleaching
maxima at the maxima of the steady-state absorption bands and the bleaching minima at the edges of bands. Similar features of the spectral changes were also observed when the sample was stationary heated. This form of the spectral changes allows attributing them to the broadening of the ground-state absorption bands. Thus, heating the samples by the laser beam evidently also contributes to the transient absorption spectrum observed at long-delay times. Thus, the differential absorption spectra are evidently composed of two parts: of the dierential absorption caused by the electronically excited molecules and of the broadening of the ground-state absorption bands due to the local heating. The heating contribution increases with time when the electronic excitations relax and their energy is converted into heat. Fig. 11 shows the transient absorption kinetics at 580 nm measured in the FBA-1 ®lm at dierent excitation intensities. The kinetics shows the initial fast partial relaxation and a slow component with the relaxation time much longer than 1 ns. The initial relaxation becomes slightly faster at the highest excitation intensity. This is an indication that the nonlinear eects, like exciton±exciton annihilation [19,20], or the relaxation enhancement by ampli®ed spontaneous emission [21] start to play a role at high excitation intensities. However, the intensity dependence of the relaxation kinetics is too weak to explain the fast relaxation component solely by the nonlinear eects. The fast component may naturally be related to the intrinsic
Fig. 10. Dierential absorption spectra of BFA-1 in a polyvinylbutyral matrix at dierent delay times.
Fig. 11. Transient absorption relaxation kinetics at 580 nm of FBA-1 ®lm at dierent excitation intensities.
R. Karpicz et al. / Chemical Physics 269 (2001) 357±366
excited-state relaxation of molecules observed in solutions. On the other hand, the nonexponential excitation relaxation was often observed in molecular solids due to excitation random walk and trapping in relaxation centers formed by lattice defects or crystallite surfaces [22]. The slow relaxation component may be attributed to the excitations localized in shallow traps, where they have long lifetime. Namely this process seems to be more plausible, since relaxation rate in a solid state is even faster, whereas the excitation relaxation involving conformational changes should be slowed down in a solid phase. However these two linear relaxational processes hardly can be separated. The local heating may also contribute to the slow kinetics since the cooling of micrometer-size particles should take place on a nanosecond time scale. However, the heating-induced broadening of the absorption spectrum should not change the oscillator strength of the electronic transitions associated with the absorption and stimulated emission, whereas electronic excitations cause negative dierential absorption. This dierence allows separating of the two dierential absorption parts. According to this consideration, the dierential absorption spectrum of the FBA-1 ®lm indicates that electronic excitations are still present even at the 1000 ps delay time. A similar conclusion may be done regarding other bisazo compounds. 4. Concluding remarks Both the steady-state and transient dierential absorption properties of the bisazo solutions may be explained by the superposition of spectroscopic properties of two tautomeric forms. The preference of bisazo molecules to exist in the HQ form increases on going from FBA-1 to FBA-4. This phenomenon has also been observed for the solid phase as well. The two tautomeric forms of the bisazo compound have dierent absorption and ¯uorescence spectra and dierent ¯uorescence quantum yields. Dierences in the excited-state properties are even more pronounced. The HQ form shows excited-state properties typical for
365
large organic molecules. The dierential absorption spectrum of this form clearly shows absorption bleaching and stimulated emission contributions. The AE form molecules also show absorption bleaching, whereas instead of stimulated emission strong induced absorption has been observed. Though the absorption spectra of both AE and HQ forms are almost independent of the substituents, however, the excited-state lifetimes vary signi®cantly with substitution. They also depend on the solvent. The excited-state relaxation mechanisms of the two forms are also dierent. An intermediate state is formed during the excited-state relaxation of the AE form. This state was attributed to the electronically relaxed molecules with altered conformation. Nonexponential relaxation of the HQ form molecules indicates to the coexistence of molecules with dierent conformations as well. Thus, conformational changes probably play an important role in the excited-state relaxation. Bisazo compounds in the solid phase resemble the basic spectroscopic features observed for solutions. However, the mechanism of the excitedstate relaxation is evidently dierent. Ultrafast fractional relaxation is related to the speci®c solid state eects, such as an exciton±exciton annihilation, or exciton relaxation via structural traps and also to the local heating. The latter leads to the broadening of the absorption bands and modi®cation of the dierential absorption spectra. Additional solid phase eects burden detailed investigation of the molecular relaxation, however, there are indications that the intrinsic excited-state lifetime in solids is much longer than in solutions. This conclusion is in agreement with the proposed mechanism of the excited-state relaxation involving conformational changes of molecules in the excited state. The conformational changes in a solid state are hindered and this relaxation mechanism is slowed down. How spectroscopic properties of bisazo pigments are related to the photosensitivity of photogenerating layers fabricated from these materials? According to Pacansky and Waltman [3], the HQ form of pigments is more photosensitive, thus relative concentration and properties of the HQ form should determine the photosensitivity of
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the charge-generating layer. As follows from our data, the preference for the HQ form increases in the series FBA-1, -2, -3, -4. However, Law and Tarnawskyj [7] measured photosensitivity of three of the investigated compounds and found that it increased in the opposite order FBA-4, -3, -1. Our preliminary investigations showed the same tendency. Thus, the properties of the HQ form, rather than its relative concentration, seem to play the major role. The photosensitivity correlates with the lifetimes of the bisazo compounds in the ACN solution. The photosensitivity of compounds having longer lifetime is higher. Though, the lifetimes depend signi®cantly on the solvent and their detailed investigation in a solid state was not possible, however, since in the ACN solution they vary more than by two orders of magnitude, it is reasonable to expect that this tendency may remain in a solid state as well. Since the charge carrier generation in molecular solids proceeds from the molecular excited state, the excited-state lifetime may be important for the photosensitivity of bisazo compounds with dierent substituents in charge generating layers.
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