J. Phys. Chem. 1995, 99, 9818-9824
9818
Squaraine Chemistry. Absorption, Fluorescence Emission, and Photophysics of Unsymmetrical Squaraines Kock-Yee Law Joseph C. Wilson Center for Research and Technology, Xerox Corporation, 800 Phillips Road, 0114-390, Webster, New York 14580 Received: December 19, 1994; In Final Form: February 17, 1995@
The absorption, fluorescence emission, and photophysics of 4-(methoxyphenyl)-4’-[(dimethylamino)phenyl]squaraine and its derivatives (USq1 -USql4), a class of unsymmetrical donor-acceptor-donor (D-A-D) compounds, have been investigated. Similar to symmetrical and pseudo-unsymmetrical (with two different aniline rings) squaraines, USql -USql4 also exhibit intense absorption bands in the visible region. Their absorption maxima (Amax) range from 562 to 593 nm and are blue-shifted relative to those of symmetrical and pseudo-unsymmetrical squaraines. The blue-shift is attributed to the decrease in the D-A-D CT character, arising from the introduction of a less electron-donating anisole ring. As it turned out, the introduction of asymmetry through the anisole ring has a significant impact on the electronic spectra. The asymmetry in USql -USq 14 enhances vibronic coupling during electronic transition, producing vibrational fine structures in both absorption and fluorescence spectra. The vibronic coupling is particularly pronounced when the C - 0 group in the central four-membered ring is H-bonded, either intramolecularIy or intermolecularly with solvent molecules. USql -USql4 are shown to form solvent complexes with solvent molecules. The multiple fluorescences observed for these compounds, although quite complex, are shown to be the sum of the vibronic bands of the unsymmetrical squaraine and its complex. At room temperature in solution, the q+ values for USql -USql4 are a factor of 130 lower than those of symmetrical and pseudo-unsymmetrical squaraines. Their lifetimes are 10.25 ns. There is a large temperature effect on both ~$f and lifetime. At 77 K in a 2-methyltetrahydrofuran matrix, the @f values approach 10.5 and the fluorescence lifetimes increase to a constant value, -2.4 ns. Hydroxy unsymmetrical squaraines are an exception; they exhibit biexponential decays (-2.4 ns and a subnanosecond decay) at 77 K in 2-methyltetrahydrofuran. The observation may be attributable to a conformational effect. The large temperature effect on the photophysical processes indicates that there exists an efficient radiationless decay process for these compounds. Evidence is provided that rotation of the C-C bond between the anisole ring and the four-membered ring is responsible for the radiationless decay. The fast decay is attributable to the single-bond character of the rotating C-C bond in these compounds. Substituents, both in the aniline ring and the anisole ring, are shown to have effects on the electronic spectra. These substituent effects are discussed in terms of their effects on the D-A-D CT character of the squaraine chromophore.
Introduction The squaraines are a class of organic photoconductors that are shown to be useful in xerographic photoreceptors.’ These compounds are traditionally synthesized by condensation of 1 equiv of squaric acid with 2 equivs of N,N-dialkylanilines.2 MNDO and CNDO semiempirical molecular-orbital calculations showed that both the ground and excited states of squaraine are intramolecular donor-acceptor-donor charge-transfer states, with the anilino moieties and the oxygen atoms being electron donors (D) and the central four-membered ring being an electron acceptor (A).3 As a monomer in solution, these compounds absorb strongly at 620-670 nm with very high molar extinction coefficients (- lo5 cm-I M-I) and exhibit intense fluorescence emissions with small Stokes ~ h i f t s . ~In - ~the microcrystalline solid state, due to the strong intermolecular D-A interaction, the solid state absorption is very broad, from 550 to 900 nm, and is red-shifted from the monomer abs~rption.’,~The solid state absorption at 450-550 nm is relatively weak, and this low absorbance has hindered the use of squaraine-based ‘Abstract published in Advance ACS Abstracts, May 15, 1995.
0022-365419512099-98 18$09.00/0
photoreceptors in copiers, where a flat response across the visible region (450-650 nm) is desirable. In an attempt to improve the spectral property, 4-(methoxyphenyl)-4’-[(dimethylamino)phenyllsquaraine (USq1) and derivatives (USq2-USql4, structures in Table 1) were designed and ~ynthesized.~ Expectedly, the introduction of a less powerful electron-donating group (the anisole group) reduces the charge-transfer interaction, producing a spectral blue-shift in the monomer absorption. The blue-shift has translated into an enhanced absorptivity between 450 and 550 nm. Recent xerographic data indicate that the spectral sensitivity is indeed improved from 450 to 650 nm.loxll Here we report an investigation on the absorption, fluorescence emission, and photophysics of USql -USq14.12 Results show that these compounds absorb at shorter wavelengths relative to squaraines synthesized from squaric acid. Vibronic fine structures are discernible in both absorption and fluorescence spectra. Multiple fluorescence emission bands with significantly lower quantum efficiencies and shorter lifetimes, relative to squaraines synthesized from squaric acid, are observed. The impact of introducing asymmetry in the squaraine structure through the anisole ring on the spectroscopic and 1995 American Chemical Society
Fluorescence of Unsymmetrical Squaraines
J. Phys. Chem., Vol. 99, No. 24, 1995 9819
TABLE 1: Absorption, Fluorescence Emission, and Fluorescence Lifetime Data for Unsymmetrical Squaraines 0-
aC
H
P
e
'CH3
p-
578.8
5.37
598, 606(s)
0.005
2.3
usq2
581.1
5.40
597, 609(s)
0.0014
2.4
usq3
583.5
5.32
599, 609(s)
0.0001 1
2.3
usq4
583.6
5.23
597, 607(s)
0.00092
2.4
usqs
563.6
5.20
585,
593
0.016
2.3(54%) 1.0(46%)
USqC
587.0
5.34
601,
606
0.01 1
2.5
usq7
590.6
5.32
602,
607
0.0059
2.3
USql
F.
CHsO
USq8
592.4
5.35
602,609
0.0019
2.5
usqs
582.4
5.08
599, 605(s)
0.0033
2.4
USqlO
572.1
5.20
_-____ , 598
0.030
2.2 (73%) 0.4 (27%)
USqll
583.1
5.39
602,
610
0.0077
2.3
USql2
562.4
5.12
601,
608
0.022
2.3 (21%) 0.79 (79%)
usq13
585.6
5.26
601, 609 (5)
0.005
2.4
USql4
569.1
5.06
______ ,
0.019
2.5 (33%) 1.6 (67%)
595
Absorption maximum, in nm iz 0.5 nm. Taken from ref 9. Molar extinction coefficient, in cm-I M-I. Fluorescence maximum, in nm, i.l nm. e Fluorescence quantum yield, estimated error +lo%. /Fluorescence lifefime, in ns, in 2-methyltetrahydrofuran at 77 K. g This work. photophysical properties is summarized and discussed. Substituents, both in the anisole ring and the aniline ring, are shown to affect the spectroscopic properties, and these effects are highlighted.
Experimental Section Materials. Unsymmetrical squaraines, USql -USql4, were synthesized by condensing an equivalent amount of a l-aryl-
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9820 J. Phys. Chem., Vol. 99, No. 24, 1995
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Figure 1. Absorption spectra of USql (-), USqS (- - -), and HSq (--) in chloroform.
Figure 2. Fluorescence excitation and emission spectra (corrected) of USql in chloroform.
-2-hydroxycyclobutene-3,4-dionewith an appropriate N,N-dialkylaniline in 2-propanol at reflux. Details of the synthesis and characterization data have been given earlier.9$'3 The solvents for spectroscopic measurements were either of spectro grade from Fisher or of analytical grade from Baker; they were routinely stored over 3 8, molecular sieves before use. General Technique. Absorption spectra were recorded with either a Cary 17 spectrophotometer or a Hewlett-Packard 8451A diode array spectrophotometer. Fluorescence spectra were taken on a Perkin-Elmer MPF-66 fluorescence spectrophotometer. A 1,l,2-trichloroethane solution of tetra-tert-butyl metal-free phthalocyanine (-1.2 x M) was used to correct for the spectral responses from 510 to 750 nm.I4 Fluorescence quantum yields were determined (in a corrected mode) by comparing with the emission of bis[2-methyl-4-(dimethylamino)phenyl]squaraine (& = 0.023 in CH2C12),4 and a refractive index correction was made.I5 Fluorescence lifetimes were determined using the time-correlated single-photon-counting technique on a PTI LS-100-04 Fluorescence Lifetime System. The fluorescence decay data were analyzed on an NEC personal computer using 'the deconvolution software program from PTI.
with a 2-hydroxy group, such as in USqS (Figure 1). The observation of absorption fine structures is in contrast to results obtained from symmetrical squaraines whose spectra are similar to that of HSq.4 The molar extinction coefficients for USql-USql4 are in the range (1.2-2.5) x lo5 cm-' M-I. These values, although considered high as far as absorption chromophores are concemed, are generally lower than those of symmetrical squaraines ((1.3-3.6) x lo5 cm-' M-').4-6 We suggest that the slightly lower values may be due to the unsymmetrical structure. Substituents, both in the aniline and the anisole rings, have profound effects on the Amax and emax. These effects will be discussed further with the fluorescence emission data. Fluorescence Emission. ( 1 ) Emission Spectra. Figure 2 shows the fluorescence excitation and emission spectra of USql in chloroform. The excitation spectrum is similar to the absorption spectrum and is independent of the monitoring wavelengths. In the emission spectrum, an emission maximum at 598 nm, a shoulder at -606 nm, and a number of weaker emission bands from 620 to 740 nm are observed. The Stokes shifts for the first two emission bands, designated a and p, respectively, are 555 and 715 cm-I from the A,,. As discussed in the spectral assignment section, these two bands are emissions from the excited squaraine and the excited squaraine-solvent complex, respectively. Substituents at the C-2 position of the aniline ring are found to have a profound effect on the relative intensity of the a- and p-emission. While very little effect is observed for the F substituent (Figure 3a), CH3, CH30, and OH groups seem to increase the relative intensity of the p-emission (Figure 3b-d). Very similar substituent effects are also observed for other unsymmetrical squaraines, e.g., USq6-USqlO (Figure 4a-e). The spectral data of all the compounds investigated are tabulated in Table 1. The fluorescence quantum yields for USql -USql4 (Table 1) are in the range and are significantly lower to 3 x than those of HSq and its derivatives, which vary from 0.1 to 0.8 in c h l ~ r o f o r m . ~ - The ~ , ' ~low & values for USql-USql4 are attributable to the efficient radiationless decay in these compounds because their fluorescence intensities increase drastically (& 2 0.5) at low temperature, such as at 77 K in toluene and 2-methyltetrahydrofuran matrices. (2) Fluorescence Lifetimes. The fluorescence lifetimes for USql-USql4 were found to be very short (50.25 ns) in chloroform, toluene, or 2-methyltetrahydrofuran at room temperature. Similar to the steady state results, there is a very large temperature effect on the lifetime at 77 K in 2-methyltetrahydrofuran. They increase from subnanosecond to about a
Results Absorption Spectra. USql -USql4 exhibit intense absorption bands with Amax ranging from 562 to 593 nm in the visible region in chloroform; the spectral data are summarized in Table 1. Typical absorption spectra using USql and USqS as examples are given in Fgure 1. Figure 1 also depicts the absorption spectrum of a typical squaraine synthesized from squaric acid and an N,N-dialkylaniline, such as bis[4-(dimethylamino)phenyl]squaraine (HSq). The comparison shows that the absorption maxima of USql -USql4 are blue-shifted relative to that of HSq. Since both the ground and excited states of squaraine are intramolecular charge-transfer state^,^ the blueshift is readily attributable to the substitution of one of the aniline rings by an anisole ring in USql-USql4. As a result of the structural change, the intramolecular charge-transfer interaction in USq-USql4 decreases, leading to a higher energy electronic transition or a blue-shift in the absorption spectrum. The observation is consistent with the general behavior of molecules having charge-transfer tarnsitions.I6 Another notable distinction between the absorption spectra of USq1-USql4 and that of HSq is the absorption shoulders in the blue edge of the main absorption bands. As shown later in this work, these are vibronic fine structures. In the case of USql in chloroform, these bands are at 580.1 (O,O), 546.3 (O,l), and 517.3 nm (0,2) (Figure 1). The fine structures are found to be particularly pronounced when the aniline ring is substituted
J. Phys. Chem., Vol. 99, No. 24, 1995 9821
Fluorescence of Unsymmetrical Squaraines ,
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650 700 Wavelength (nm)
--
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WAVELENGTH (nm)
Figure 5. Observed and calculated emission spectra of USql in chloroform at -25 'C: (-) experimental data; (.* *) calculated emission spectrum for the squaraine; (-+-) calculated emission spectrum for the squaraine-solvent complex. 550
600
650 700 Wavelength (nm)
750
550
600
650 700 Wavelength (nm)
750
Figure 3. Fluorescence emission spectra of (a) USq2, (b) USq3, (c) USq4, and (d) USq5 in chloroform.
cence of USql.'* In toluene at room temperature, the emission spectrum is very similar to that in Figure 2, with AF at 600 nm, a shoulder at 609 nm, and a number of weaker emission bands around 620-740 nm. At 77 K, the complex emission spectrum changes to three emission bands, centered around 609,663, and 727 nm. Variable-temperature and low-temperature experiments showed that USql forms a solute-solvent complex in toluene at 77 K. From the spacing of these three bands (1332 f 5 cm-I) in the low-temperature spectrum, it was concluded that these bands are (O,O), (O,l), and (0,2) vibronic bands of the S I SOfluorescence for the USql-toluene complex. Since both the emission wavelength and the bandwidth of the squaraine emission are insensitive to the ambient temperature? we have been able to deconvolute the complex emission observed at room temperature to six bands, attributed to the vibronic bands of the SI SOfluorescence of USql (597.5, 635.6, and 683.4 nm) and its solvent complex (609, 663, and 727 nm). For simplicity we designate the squaraine and the complex emissions to the a-and /?-band, respectively. The fluorescence emission of USql in chloroform is similar to that in toluene. Here we attempt to deconvolute the spectrum in Figure 2 using the low-temperature (77 K) spectral parameters (bandwidth, spacing, and shape) obtained in toluene as the starting parameters. The procedure of the deconvolution has been described earlier.I2 The calculated emission spectra for the SI states of USql and its solvent complex in chloroform are shown in Figure 5, along with the observed spectrum. The calculations show that (1) the emission bands from excited USql are at 597, 636, and 685 nm and (2) the emission bands from the excited complex are at 609, 663, and 706 nm, respectively. These bands are attributable to the (O,O), (O,l), and (0,2) vibronic bands in their respective fluorescence spectrum. It is important to note that the Z(O,I~Z(O,O~ ratio for the (calculated) room temperature spectrum of the USql -solvent complex is significantly higher than that at 77 K. The result is consistent with intuition simply because one would expect to have a higher population of the zero vibrational level at 77 K.I7 The sum of the six vibronic bands in Figure 5 is in -98% agreement with the observed spectrum. The major disagreement is in the blue edge of the emission spectrum, presumably arising from the different band shapes observed for the room-temperature and the low-temperature spectra. In addition, we have also studied and deconvolted the roomtemperature emission spectra of USq5 and USqlO in a similar detailed fashion. In each case, the multiple emission can be resolved into vibronic bands originating from the squaraine and the solvent complex. We hence conclude that the multiple emissions observed in unsymmetrical squaraines USql -USql4
-
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650 700 Wavelength (nm)
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550
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650 700 Wwclcnglh (nm)
750
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Wavelength (nm)
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750
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Figure 4. Fluorescence emission spectra of (a) USq6, (b) USq7, (c) USq8, (d) USq9, and (e) USqlO in chloroform.
constant value at 2.3-2.5 ns, confirming the existence of an efficient radiationless decay at room temperature. Slightly different results are obtained for hydroxyl-substituted unsymmetrical squaraines (USq5, USqlO, USql2, and USql4). Their decays are biexponential, one at 2.4 ns and the other below 1 ns. The probable cause for the biexponential decay will be discussed below.
Discussion Spectral Assignments for the Multiple Fluorescence Emission. In 1992, we reported preliminary results on the fluores-
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9822 J. Phys. Chem., Vol. 99, No. 24, 1995
are vibronic fine structures of the emissions of the squaraine and the squaraine-solvent complex. Origin of the Vibronic Fine Structures. One of the special features of the absorption and fluorescence of Usql-USql4 is the vibrational fine structures, which are absent from the spectra of symmetrical squaraines, such as HSq. Unsymmetrical squaraines having two different aniline rings in the squaraine structure, which is designated as pseudo-unsymmetrical squaraine in this work, have been synthesized, and their spectroscopic properties have been studied In solution, these
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eBw 2
0400
550 600 Wavelength (nm)
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Figure 6. Absorption spectra of USql (-) and USq5 (- - -) in ethanol.
pseudo unsymmetrical squaraines, X = F, CH3,OCH3, OH compounds exhibit only a single sharp absorption band at 625633 nm. The spectral characteristics (shape and spectral shift) are identical to those of symmetrical squaraines. Although multiple emissions are observed from these compounds, they can be explained by the solute-solvent complex model developed for symmetrical s q ~ a r a i n e s . ~Since . ~ MO calculations show that the SO SI electronic transition is localized in the central C402 unit,3 we suggest that due to the similarity in electron-donicity between the two different aniline rings in these compounds, the perturbation to the electronic transition is small. On the other hand, the electronic transitions in USql-USql4 are perturbed significantly owing to the relatively large difference in electron-donicitybetween the aniline ring and the anisole ring. The perturbation enhances the vibronic coupling, which leads to the fine structures in both the absorption and fluorescence spectra. Aromatic molecules, such as benzene, naphthalene, anthracene, and pyrene, exhibit fine vibrational structures in their absorption as well as fluorescence emission spectra.20-28 Solvents are known to affect the relative intensity of these vibronic bands. Notable examples are the effect of solvent on the absorption fine structure of benzene (Ham effect) and the effect of solvent on the relative intensity of the fine structure in the fluorescence emission of pyrene. In both. cases, interactions of the aromatic molecule with solvent molecules were reported to reduce the molecular symmetry, which decreases the forbiddance of certain vibronic coupling. We thus believe that the origin for the occurrence of vibronic fine structures in the electronic spectra of USql-USql4 is similar. Owing to the asymmetrical eletronic distribution in USq-USql4, the symmetry in the electronic transition is broken, which enhances vibronic coupling and results in the fine structures in the electronic spectra. The vibronic fine structures are quite pronounced for.hydroxysubstituted unsymmetrical squaraines, e.g., US@, USqlO, USql2, and USql4. As discussed in the Substituent Effects section below, one of the predominating resonance structures for hydroxy-substituted squaraines is the tautomeric structure formed by transferring the proton in the OH group to the central C - 0 group. This contributed resonance structure enhances the asymmetry of the electronic transition, which is centralized in the cental C402 unit. To test the impact of H-bonding to the central C - 0 group on the fine structures, we study the absorption spectra of USql and USq5 in ethanol. The spectral results (Figure 6) show that the vibronic structures are indeed enhanced in both cases.
-
Photophysics of Unsymmetrical Squaraines. The fluorescence quantum yields of USq1-USq14 range from 1 x to 3 x in chloroform (Table 1) and are a factor of >30 lower than those of symmetrical and pseudo-unsymmetrical squaraines, whose yields vary from 0.08 to 1.0 with the same set of substituent^.^-^*'^'^ The measured lifetimes for these compounds are 10.25 ns in fluid solution at room temperature. At 77 K in 2-methyltetrahydrofuran glass, drastic increases in fluorescence intensities and lifetimes are observed. With the exception of hydroxyl-substituted compounds, which show biexponential decays, all the decays can be fitted by a monoexponential decay function with x2 values better than 1.4. The most surprising observation is their almost constant lifetimes at 2.4 f0.1 ns. The large temperature effect in the photophysics of USql-USql4 suggests that there exists a very efficient radiationless decay process at room temperature. Since the fluorescence of pseudo-unsymmetrical squaraines is very similar to those of the symmetrical analogs, we propose that the efficient radiationless decay originates from the anisole ring. This proposition is not unprecedented based on earlier radiationless decay data of benzylidenemalononitrile dyes. For example, Loutfy and Law reported that the fluorescence quantum yield of p-(N,N-dimethy1amino)benzylidenemalononitrile(DMBM) is -lop3 in fluid solution at room temperature, and the value increases to 20.5 at 77 K in 2-methyltetrahydrofruan matrix.I6 Inoues and co-workers showed that p-methoxybenzylidenemalononitrile (MBM) is nonfluorescent (< lov4)at room temperature and the quantum yield of MBM increases when the chromophore is rigidized or when it is recorded at low t e m p e r a t ~ r e .These ~ ~ reports suggest that q5f can decrease by a factor >10 when the donor group in the CT molecule changes from an N,N-dimethylaniline group to an anisole group. The smaller q5f values observed for Usql -USql4 relative to other squaraines are then not unreasonable.
Probable mechanisms for the fast radiationless decay in USql-USql4 that is associated with the anisole ring are (1) rotation of the C-C bond between the anisole ring and the central four-membered ring and ( 2 ) rotation of the C-OCH3 bond in the anisole ring. The contribution from the later rotation can be excluded because there is practically no increase in q5f for USql3 and USql4 when the C-OCH3 bond is rigidized (Table 1). The rapid radiationless decay, on the other hand,
Fluorescence of Unsymmetrical Squaraines
J. Phys. Chem., Vol. 99, No. 24, 1995 9823
SCHEME 1
SCHEME 2 0
CH3> ;;:
