18392
J. Phys. Chem. 1996, 100, 18392-18398
The TICT Mechanism in 9,9′-Biaryl Compounds: Solvatochromism of 9,9′-Bianthryl, N-(9-Anthryl)carbazole, and N,N′-Bicarbazyl J. Catala´ n,*,† C. Dı´az,† V. Lo´ pez,† P. Pe´ rez,† and R. M. Claramunt‡ Departamento de Quı´mica Fı´sica Aplicada, UniVersidad Auto´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain, and Departamento de Quı´mica Orga´ nica y Biologı´a, Facultad de Ciencias, UNED, Ciudad UniVersitaria, E-28040 Madrid, Spain ReceiVed: May 3, 1996; In Final Form: September 5, 1996X
The UV-vis absorption and fluorescence spectra of 9,9′-bianthryl (BA), N-(9-anthryl)carbazole (C9A), and N,N′-bicarbazyl (BC) in 60 different solvents were precisely recorded. An analysis of the solvatochromic behavior of the three chromophores in terms of the polarity/polarizability of the solvents (SPP) revealed (a) that BC exhibits no TICT mechanism throughout the solvent polarity range studied and (b) that TICT emission prevails in BA for solvents with SPP > 0.8 and in C9A for solvents with SPP > 0.6. One other interesting conclusion is that BA exhibits structured fluorescence at room temperature in three of the solvents studied. The presence of structure in the fluorescence of the three chromophores was studied also in DMSO solutions at 77 K.
Introduction Aromatic compounds possessing two chromophores and exhibiting dual fluorescence are of a high interest1-5 because the additional fluorescence is frequently associated with the formation of a highly polar excited electronic structure. Such polar forms usually originate in the transfer of an electron from a chromophore to another within the molecular structure, a process that is involved in some highly significant naturally occurring mechanisms such as vision and photosynthesis.6 This type of process is also of potential interest for producing laser light,7 storing solar energy,8 and developing organic conductors or even superconductors. Although every charge separation process in a chromophore is favored by a polar environment, the solvent polarity is believed to facilitate the formation of the more polar forms through increased viability of a twisted intramolecular charge transfer (TICT) mechanism. In any case, it is obvious that, if the solvent polarity induces a change in the excited electronic state leading to a highly polar form of the chromophore, then the solvatochromism of the compounds involved will be of a high potential interest. The solvatochromism of this type of system was first addressed in the pioneering work of Schneider and Lippert9 on 9,9′-bianthryl in 30 solvents. Because the two excited electronic forms of this type of system must have rather disparate dipole moments, the solvatochromism of the system must be bilinearly related to the solvent polarity. Schneider and Lippert10 showed that 9,9′-bianthryl conformed to this relation in 22 of the solvents studied; however, the segment spanned by the nine least polar solvents had a zero slope: ∆f ′ e 0.22, with ∆f ′ ) {( - 1)/ (2 +1) - 0.5[(n2 - 1)/(2n2 + 1)]}. Recently, BA was studied in the gas phase at a low temperature in the absence of solvent using free-jet spectroscopy.11-15 A plot of the Stokes shift data of Scheneider and Lippert10 against our solvent polarity/ polarizability parameter (SPP)16 exhibits a bilinear relationship; however, the segment corresponding to the least polar solvents has a non-zero intercept and tends to zero in the gas phase.17 Recently, Acree et al.18 studied the behavior of BA in 45 †
Universidad Auto´noma de Madrid. Ciudad Universitaria. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡
S0022-3654(96)01259-2 CCC: $12.00
solvents; however, in plotting the Stokes shifts in the 42 solvents where Lippert’s ∆f function19 or the Bilot-Kawski F(,n) function20 could be evaluated, they obtained such highly disperse results that the bilinear relationship suggested by Scheneider and Lippert was only clearly observed if eight data were removed. Acree et al. considered the possibility of BA photoreactions taking place in the eight solvents and of the emission of the LE and TICT states being so close in these solvents that the emission maxima did not belong exclusively to either. On the assumption of an orthogonal structure (D2d) for BA in its ground electronic state, and because the TICT mechanism breaks that symmetry, Rettig et al.21 studied the solvatochromism of an asymmetric structure, viz. that of N-(9-anthryl)carbazole; in an interesting paper, they reported the normalized emission spectra for C9A in n-hexane, ethyl ether, tetrahydrofuran, and ACN, as well as a plot of Vmax (TICT) vs ∆f ′ in ethyl ether, BuCl, THF, Cl2CH2, BuCN, and ACN on one hand, and the homologue series MeOH/n-pentanol and MeCN/BuCN. C9A was also studied in the absence of solvent (gas phase) at a low temperature using free-jet spectroscopy.22,23 Durocher et al.24,25 studied bicarbazole in the gas phase and in various matrices at a low temperature and inferred that the ground electronic state of this compound has a D2d structure. Subsequently, on the basis of photophysical data for the compound in the gas phase and at room temperature in 3-methylpentane, EtOH, and ACN, they concluded that, unlike BA, bicarbazole does not seem to exhibit a TICT mechanism. Recently, our group16 developed a pure polarity scale called the solvent polarity/polarizability (SPP) scale, based on the solvatochromism of the probe 2-(N,N-dimethylamino)-7-nitrofluorene and its homomorph 2-fluoro-7-nitrofluorene that was subsequently extended to 145 solvents and the gas phase.16,17,26,27 This SPP scale encompasses values between 1 for DMSO and 0 for the gas phase (i.e., the absence of solvent). The SPP scale has proved a powerful tool for rationalizing spectroscopic (UVvis,16,28 IR,16 fluorescence,16,17 NMR16), thermodynamic (equilibrium constants,16 differential solvation energies),16 and kinetic data.16 Also the SPP scale is a highly useful tool17 for determining whether the solvent polarity can modify the emitting electronic © 1996 American Chemical Society
TICT Mechanism in 9,9′-Biaryl Compounds
J. Phys. Chem., Vol. 100, No. 47, 1996 18393
TABLE 1: Wavelengths for the First Absorption and Fluorescence Emission Maximum for BA, C9A, and BC in the Solvents Studied BA
C9A
380.8
BA
λabs
λabs
max λem
385.6; 404.2
378.5
404.8
325.9
330.2
387.3 387.6 387.7 388.4 389.4 388.8
395.0; 411.4 411.4 411.6 414.9 413.6 414.0
383.9 383.3 383.9 384.6 385.0 385.0
413.2 413.2 414.1 415.8 415.2 415.5
328.0 327.6 327.7 328.2 328.8 328.1
389.4 391.0 388.8 391.8 389.7 392.3
414.6 416.7 414.2 419.0 415.6 418.2
385.9 386.6 385.7 388.6 385.9 387.6
416.5 417.3 420.4 432.6 418.3 436.4
391.6 391.4 389.5 391.5 392.4
419.8 408.2; 430.2 414.4 419.5 421.0
388.2 388.1 386.4 388.1 388.7
tetralin diethyl ether
393.0 388.5
421.4 415.8
1,4-dioxane
391.1
isoamyl acetate fluorobenzene
C9A
0-0 λabs
max λem
2-propanol
388.3
338.0 336.6 335.4 336.1 338.4 336.4
ethanol methanol n-propyl formate dibenzyl ether anisole chlorobenzene
328.8 329.2 328.1 329.5 328.6
337.7 339.6 338.8 341.0 342.7
434.6 434.1 425.7 435.6 436.5
327.0 329.1 328.5 329.0 329.5
339.4 339.4 339.4 339.6 344.3
389.6 385.1
432.1 432.2
330.5 327.5
342.9 340.8
423.1
388.1
441.7
328.2
344.3
389.7 391.0
420.2 417.7
386.8 387.6
435.8 438.0
328.4
345.3
chloroform
391.8
437.6
388.0
447.1
ethyl acetate 1-hexanol
390.7 389.9
433.0 430.1
386.3 386.9
443.4 441.8
327.8 328.4
344.1 346.4
1-pentanol
389.3
428.7
386.3
443.2
327.9
347.7
tert-butyl alcohol butanol 1-propanol
388.0 389.5 388.7
421.0 437.5 440.1
384.9 386.3 386.4
441.6 444.2 447.4
327.4 328.5 327.7
345.1 344.1 345.7
perfluoron-hexane 2-methylbutane n-pentane n-hexane n-heptane cyclohexane methylcyclohexane n-hexadecane cis-decalin triethylamine p-xylene tri-n-butylamine tetrachloromethane o-xylene ethylbenzene di-n-butyl ether toluene benzene
max λem
BC
max λem
solvent
0-0 λabs
state of a chromophore in such a way as to alter its dipole moment, and for that, it shows a solvatochromism of bilinear relationship with the solvent polarity. The results obtained support the assumption that the SPP values are more accurately descriptive of solvent polarity than are the dielectric constant and refractive index functions usually employed for this purpose.19,20 The purpose of this work was to analyze absorption and emission data for the three above-mentioned biaryls in a wide set of solvents in terms of the SPP values for the solvents in order to rationalize the solvatochromic behavior of this type of chromophore. Experimental Section Absorption spectra were recorded on a Shimadzu 2100 UVvis spectrophotometer. The monochromator was calibrated as regards wavelength by using the 486.0 and 656.1 lines from a deuterium lamp. A Cary 5 spectrophotometer was used in those cases where any of the chromophores was scarcely soluble in any of the solvents. Both instruments were routinely checked for wavelength accuracy by using holmium oxide and didymium filters. All spectral measurements were made at 25 °C by using a matched pair of quartz cells of 1 cm path length. The maximum wavelength in each case was determined from the corresponding derivative function.
BC
λabs
max λem
λabs
max λem
440.5
385.4
449.7
327.0
346.1
388.8 388.2 389.3 394.0 392.4 393.0
449.5 453.4 436.9 434.9 433.6 433.4
386.1 385.7 386.5 390.8 389.2 389.5
449.9 457.6 446.5 449.1 448.7 443.1
397.4 327.4 328.1
343.9 344.7 347.6
329.2 329.7
349.6 349.3
ethyl benzoate methyl benzoate 1-chlorobutane tetrahydrofuran acetonitrile propionitrile
392.3 392.8 389.9 390.7 389.2 389.2
437.9 442.9 417.6 433.0 470.6 456.2
389.1 389.7 386.8 387.2 386.0 386.3
446.0 450.9 437.5 444.6 472.5 455.4
329.2
348.9
328.3 328.4 327.8 324.3
344.1 345.4 344.3 347.2
butyronitrile valeronitrile benzonitrile dichloromethane 1,2-dichlorobenzene pyridine propylene carbonate N,N-diethylacetamide ethylene glycol N,N-diethylformamide N-methylimidazole tetramethylurea N,N-dimethylformamide N,N-dimethylacetamide γ-butyrolactone sulfolane dimethyl sulfoxide
389.5 389.7 390.6 391.5 393.2
449.4 442.2 448.6 447.9 440.6
386.7 386.8 390.6 388.3 390.3
457.7 451.5 459.5 455.3 448.8
327.6 327.8 329.6 328.8 330.1
348.9 347.4 351.2 348.5 351.9
393.6 391.1
441.9 472.4
390.4 387.7
459.5 474.6
329.8 327.3
351.3 349.8
391.2
454.3
388.9
459.2
328.4
348.0
391.0 391.3
477.6 457.8
388.8 388.7
471.1 461.9
328.6
348.0
393.6
475.1
390.6
473.1
329.3
351.9
391.6 391.8
443.4 467.6
388.8 389.3
457.1 468.8
328.1 328.5
349.9 347.6
391.2
459.4
388.8
464.2
327.4
349.8
391.6 393.2 393.6
464.0 470.5 478.2
388.6 390.2 390.6
468.7 471.4 477.9
327.9 328.7 328.7
349.9 348.9 349.9
solvent
Fluorescence spectra at 25 °C and 77 K were obtained on a Aminco Bowman Series 2 spectrofluorimeter with its excitation and emission polarizers arranged at the magic angle. The emission monochromator was calibrated in wavelength by using an Oriel 6035 Hg (Ar) spectral calibration lamp. Both emission and excitation spectra were duly corrected. The solutions used were dilute enough to exclude any significant inner-filtering effects. As with absorption spectra, the maximum wavelength was determined from the derivative function in each case. Samples were synthesized following reported procedures, viz. BA was synthesized by reduction of anthrone with zinc29 and BC was synthesized by oxidation of carbazole with potassium permanganate30 and purified by column chromatography using 90:10 n-hexane/chloroform as the eluent. CA was obtained by fusion of carbazole with 9-bromoanthracene with heating at 300 °C for 2.5 h and purified by column chromatography using 60: 40 n-hexane/chloroform as the eluent.31 All solvents used were of the highest available purity and supplied by Aldrich, Fluka, or Merck; some were redistilled in an inert atmosphere prior to use. Results and Discussion Table 1 gives the wavelengths for the first absorption and fluorescence emission maximum for BA, C9A, and BC in the solvents studied. Table 2 compares these data with previously
18394 J. Phys. Chem., Vol. 100, No. 47, 1996
Catala´n et al.
TABLE 2: Comparison of the Results Obtained in This Work with Wavelengths for the Fluorescence or First Absorption Maximum for BA Reported in the Literature em solvent n-hexane decalin benzene diethyl ether 2-propanol ethanol methanol acetonitrile dimethyl sulfoxide
abs
this Mataga Acree this Mataga Acreeb work et al.a (32) (18) work et al.a (32) (18) 411.6 416.7 421.0 415.8
413.2 416.6 421.9 418.4
411
389.1 392.1 395.2 389.1
366
419 414
387.7 391.0 392.4 388.5
440.5 449.5 453.4 470.6 478.2
438.6 442.5 454.5 469.5
435 442 448 461 464
388.7 388.8 388.2 389.2 393.6
389.1 389.1 387.6 390.6
366 366 366 366 371
370 366
a These data refer to degassed solutions. b λex values used in that work to calculate the Stokes shifts for BA in various solvents.
reported values. The position of the emission maxima for BA obtained in this work are consistent with those of Mataga et al.32 and with those of Acree et al.,18 but only at the shorter wavelengths in the latter case. Obviously, because the data of Acree et al. were not corrected for the equipment sensitivity, those obtained at the longer wavelengths (i.e., in the more polar solvents) were markedly deviant (for example, the maximum for DMSO was shifted by 14 nm relative to ours). As regards absorption, the data of Mataga et al. are bathochromically shifted by about 1 nm with respect to ours, except for benzene, which is shifted by 2.8 nm in the same direction, and MeOH, which is hypsochromically shifted by 0.6 nm. On the other hand, the λex values of Acree are shifted hypsochromically by about 22 nm relative to ours and obviously include more than absorption. The absorption spectra for each of the chromophores in the 60 solvents studied preserved their spectral envelope and the bathochromic shift they exhibited with increase in the solvent polarity is quite normal. Thus, the differences between the positions of the first absorption peak for each chromophore in a highly “inert” solvent such as perfluorohexane (SPP ) 0.214) and a highly polar solvent such as DMSO (SPP ) 1.0) are 854, 818, and 256 cm-1, respectively; the former two are very similar to those for anthracene in the two solvents (λperfluorohexane ) 368.4 nm and λDMSO ) 380.4 nm, so ∆ν˜ ) 856 cm-1), which is the chromophore representing both. Figure 1 shows the emission and excitation spectra for BA, C9A, and BC in the previous two solvents. Note that (a) the emissions of the three chromophores are structured in perfluorohexane but unstructured in DMSO; (b) the emission of C9A in DMSO is a well-defined band, whereas that of BA in the same solvent starts with a shoulder in the 400 nm region that can theoretically be ascribed to LE emission; and (c) the excitations in both solvents are structured for the three chromophores. It should be noted that the excitation bands are superimposable to the absorption spectra for the chromophores, which hinders the interpretation of the λex values of Acree et al.18 Figure 2 shows the same spectra in DMSO, recorded at 77 K. The situation is clearly different; in fact, the emissions are structured and can be ascribed to LE emissions. None of the long bathochromic shifts observed for BA and C9A in DMSO at room temperature are apparent (see Figure 1). This evidence is consistent with the findings of Schneider and Lippert9 for BA in glycerol in switching from 344 to 182 K and those of Visser et al.33 in 2-methylbutane in changing
Figure 1. Normalized emission and fluorescence excitation spectra for BA, C9A, and BC at room temperature in perfluorohexane (s) and DMSO (- -).
from 293 to 77 K. Also, Durocher et al.25 observed a dramatic change in the fluorescence of BC in EtOH between 165 and 77 K. All these observations suggest that, as the temperature is lowered, a point is reached where the chromophores emit with a molecular structure, which resembles the Franck-Condon geometry yielded by the ground electronic state, without the solvent being rearranged or a TICT mechanism coming into play. We should also note that the fluorescence of BA at room temperature was structured not only in perfluorohexane but also in 2-methylbutane and, more surprising, ethylbenzene (see Figure 3), when it was unstructured in benzene and toluene.
TICT Mechanism in 9,9′-Biaryl Compounds
J. Phys. Chem., Vol. 100, No. 47, 1996 18395
Figure 3. Emission and fluorescence excitation spectra for BA in 2-methylbutane (a) and ethylbenzene (b) at room temperature.
Figure 2. Normalized emission and fluorescence excitation spectra for BA, C9A, and BC at 77 K in DMSO.
The fluorescence spectrum for BA in 2-methylbutane (Figure 3) exhibits two well-defined maxima that are consistent with the spectrum reported by Visser et al.,33 but not with that obtained by Wortmann et al.,34 which consisted of a shoulder and a maximum. The envelope of the fluorescence spectrum for C9A warrants some comment; thus, it exhibits two maxima only in perfluorohexane, 2-methylbutane, pentane, hexane, heptane, cis-decalin, cyclohexane, methylcyclohexane, hexadecane, But3N, and Et3Nsin the last, the maximum, at 435 nm, is rather ill-defined. As can be deduced from the fluorescence spectra of BA and C9A at room temperature (see Figure 1), the contamination of the TICT band by the LE component in DMSO and the
contamination of the LE band by the TICT component in n-perfluorohexane are negligible. The simulated time-resolved fluorescence spectra for BA in propylene carbonate and acetone reported by Kang et al.,35 in Figures 10 and 11 of their paper, allow deduction of the low contamination effect of the static fluorescence maxima position by the LE component in these solvents. Because the BA fluorescence deconvolution in the LE and TICT components is a complex subject21,35-38 and in our opinion not definitively solved,39 we have not attempted the decomposition of the static fluorescences obtained at room temperature in their LE and TICT components. Table 3 lists the Stokes shifts calculated from the data in Table 1 for BA, C9A, and BC. The table also includes the Stokes shifts for BA calculated by Schneider and Lippert;9 as can be seen, they agree to a reasonable extent with ours. No data from Acree et al.18 were included as they were about 2000 cm-1 greater and hence difficult to compare. Analysis of Solvatochromism. The solvatochromism of fluorophores is usually assessed from their Stokes shifts, i.e. from the difference between the frequencies of their absorption (ν˜ a) and emission maxima (ν˜ f) in various solvents; data are processed by applying the simplified relations derived by Lippert19,40 and Mataga et al.,41-43 based on the theory of Ooshika,44 which are of the form
ν˜ a - ν˜ f )
(
)
2 -1 n2 - 1 (µ* - µ)2 + const 3 2 + 2 2 pca 2n + 1
or from the more universal relation developed by Bakshiev45
18396 J. Phys. Chem., Vol. 100, No. 47, 1996
Catala´n et al.
TABLE 3: Stokes Shifts for BA, C9A, and BC in the Solvents Studied and SPP Values for the Solvents dS solvent perfluoro-n-hexane 2-methylbutane n-pentane n-hexane n-heptane cyclohexane methylcyclohexane n-hexadecane cis-decalin triethylamine p-xylene tri-n-butylamine tetrachloromethane o-xylene ethylbenzene di-n-butyl ether toluene benzene tetralin diethyl ether 1,4-dioxane isoamyl acetate fluorobenzene chloroform ethyl acetate 1-hexanol 1-pentanol tert-butyl alcohol 1-butanol 1-propanol a
BA9
SPP 0.214 0.479 0.507 0.519 0.526 0.557 0.563 0.578 0.601 0.617 0.617 0.624 0.632 0.641 0.650 0.652 0.655 0.667 0.668 0.694 0.701 0.752 0.769 0.786 0.795 0.810 0.817 0.829 0.837 0.847
a
923 1009a [1580] 1492 1498 [1540] 1647 1502 [1510] 1566 1560 1577 [1540] 1577 1659 1601 1579 [1440] 1715 1675a 1541 [1670] 1705 1731 [1870] 1716 1690 [1780] 1932 [1990] 1863 [2040] 1636 2671 [2540] 2599 [2320] 2401 2358 2021 2816 3006
dS C9A
BC
solvent
SPP
BA9
C9A
BC
1715 1852 1889 1853 1949 1889 1910 1902 1901 2139 2615 2006 2884 2745 2734 2392 2811 2820 2524 2831 3125 2909 2969 3405 3336 3209 3319 3332 3374 3526
392 901 817 697 714 895 746 802 930 966 1025 1251
2-propanol ethanol methanol n-propyl formate dibenzyl ether anisole chlorobenzene ethyl benzoate methyl benzoate 1-chlorobutane tetrahydrofuran acetonitrile propionitrile butyronitrile valeronitrile benzonitrile dichloromethane 1,2-dichlorobenzene pyridine propylene carbonate N,N-diethylacetamide ethylene glycol N,N-diethylformamide N-methylimidazole tetramethylurea N,N-dimethylformamide N,N-dimethylacetamide γ-butyrolactone sulfolane dimethyl sulfoxide
0.848 0.853 0.857 0.815 0.819 0.823 0.824 0.835 0.836 0.837 0.838 0.895 0.875 0.915 0.900 0.960 0.876 0.911 0.922 0.930 0.930 0.932 0.939 0.950 0.952 0.954 0.970 0.987 1.003 1.000
3052 [2520] 3477 [3340] 3679 [3590] 2796 2391 2421 2372 [2270] 2655 2877 1702 2554 4389 [4040] 3776 3422 3055 3308 3215 [3020] 2732 [2520] 2778 [3500] 4401 3546 4636 [4080] 3710 4361 2985 4135 3792 3987 4178 4494
3705 3673 4076 3478 3322 3320 3105 3280 3484 2995 3334 4743 3931 4008 3702 3837 3790 3340 3850 4724 3939 4490 4077 4460 3842 4354 4175 4399 4412 4679
1690 1469 1531 1713
1115 922 976 944 1307 1091 1189 1426 1490 1448 1588 1592 1572 1406 1592
1778 1701 1712 1395 1502 1466 1752 1864 1726 1868 1716 1874 1862 1965 1714 1693 1955 1903 1672 1954 2006 1757 1844
ν˜ em was taken to be the average of the frequencies of the two emission maxima (see Table 1).
on the basis of the Onsager model:46
ν˜ a - ν˜ f )
(
)(
)
2 - 1 n2 - 1 2n2 + 1 2 2 (µ - µ*2 pca3 + 2 n2 + 2 n2 + 2 2µµ* cos R) + const
In these relations, p is the Planck constant, c the speed of light, a the radius of the Onsager cavity, n the refractive index of the solvent, the dielectric constant, µ the dipole moment for the chromophore in its ground electronic state, and µ* that in the excited electronic state involved in the transition considered. A given chromophore will have a certain µ and µ* value, so its Stokes shifts in an array of solvents will conform to linear polarity/polarizability functions of the following form:
ν˜ a - ν˜ f ) mf(polarity/polarizability) + const Therefore, if a different excited electronic state of a higher dipole moment emerges above a given solvent polarity, then the solvatochromic behavior observed will change in terms of slope, so the overall plot against solvent polarity will become bilinear. Recently, our group17 showed that substituting the polarity functions, f(,n) for the solvents by their corresponding SPP values in the solvatochromic function provides excellent results as regards both linear and bilinear relationships. In the light of this finding, the solvatochromic behavior of the three chromophores considered here (BA, C9A, and BC) against the SPP values of the solvents studied (see Table 3) was investigated. Figure 4 shows the Stokes shifts for BA in the 60 solvents studied as a function of their SPP values. Note that (a) the plot is clearly bilinear, with an interesting no-man’s land between the two trends; (b) the zone of greater slope, i.e. that resulting from the TICT mechanism, corresponds to solvents
Figure 4. Plot of the Stokes shifts for BA in different solvents against the SPP value of the solvents.
with SPP above ca. 0.8; and (c) this clear distribution into two groups suggests that the fluorescence emission maxima for BA in each of the solvents studied correspond largely to an LE or a TICT mechanism. Figure 5 shows the Stokes shifts for C9A in the 60 solvents studied, again in relation to SPP values. The plot is also clearly bilinear; however, the zone of greater slope (TICT mechanism) corresponds to solvents with an SPP value above ca. 0.62, i.e. considerably lower than that for BA. Therefore, this asymmetric chromophore is more prone to adopt the TICT mechanism than is the symmetric chromophore BA. As with BA, the solvents distributed in two groups: those that exhibit TICT and those
TICT Mechanism in 9,9′-Biaryl Compounds
J. Phys. Chem., Vol. 100, No. 47, 1996 18397 TABLE 4: Ionization Potentials and Redox Potentials of Biaryl Derivatives (All Data in eV) EoxD48 PI47
ACN
EredA ACN
DMF
-1.7048
9-cyanoanthracene 7.80 1.74 carbazole 7.57 (1.47)a -2.6249 anthracene 7.47 1.45 -1.9648 -1.9249 a
EoxD - EredA 3.44 4.15 3.41
Estimated value from PI values versus EoxD.
Figure 5. Plot of the Stokes shifts for C9A in different solvents against the SPP value of the solvents.
Figure 7. Plot of the wavenumber of the fluorescence maximum for Cu153 in various nonprotic solvents studied by Maroncelli et al.39 against the SPP values of the solvents: (b) nonprotic aliphatic solvents; (4) nonprotic aromatic solvents.
Figure 6. Plot of the Stokes shifts for BC in different solvents against the SPP value of the solvents.
that do notsthe latter coincided with those that exhibited structured emission spectra as noted above. Finally, Figure 6 shows the Stokes shifts for BC, which exhibited no bilinear behavior. In addition, the data distribution corresponds to a small slope. According to the recently reported analysis of van der Haar et al.,38 the transition energy TICT in biaryl derivatives is related to the donor and acceptor properties of the two aryl moieties, and more precisely to their redox potential difference (EoxD - EredA). From the donor and acceptor properties of 9-cyanoanthracene, anthracene, and carbazole gathered in Table 4, we can conclude that because the redox potential difference for 9-cyanoanthracene and anthracene lies on 3.4 eV and that for carbazole is higher, about 4.2 eV, in polar solvents, 9,9′-dicyanoanthracene and BA present TICT emission and BC does not. It is also interesting to note that a statistical analysis of the behavior apparent from Figures 4-6 revealed high dispersion in the data. Thus, the TICT segments for BA corresponded to n (number of solvents) ) 36, r (correlation coefficient) ) 0.75,
and sd (standard deviation) ) 469 cm-1, and those for C9A to n ) 50, r ) 0.896, and sd ) 291 cm-1. Similarly, the LE segments for BA corresponded to n ) 11, r ) 0.785, and sd ) 68 cm-1, but the situation for BC was considerably better, with n ) 54, r ) 0.946, and sd ) 135 cm-1. These results may have various origins. For example, BA exhibits an extremely high sensitivity in a homologue series of solvents. Thus, its Stokes shift decreases by about 1300 cm-1 from MeOH to n-hexanol, which in principle can be ascribed to the different acidity of the two alcohols. This is not so, however; in fact, from Table 3, it is apparent that the change in the Stokes shift between ACN and valeronitrile (two solvents of the nitrile series) is also 1300 cm-1. On the other hand, the angle between the two chromophores may be slightly different in different solvents and the absorption and emission wavelengths be shifted as a result. Also, the two emissions may coexist to a substantial extent in some solvents, thereby causing an artificial shift in the emission maximum.21,35-38 All these considerations can be extended to C9A. On the other hand, the above-described dispersion cannot be ascribed to an imprecise description of the solvents by their SPP values since these have proved accurate tools for describing the solvatochromism of various chromophores.16,17 To further support this assertion, we analyzed the data recently reported by Maroncelli et al.50 for the fluorescence maxima for Cu153, a stiff chromophore regarded as a suitable solvent probe. Figure 7 shows the data for the 26 nonprotic solvents studied (aromatics included); as can be seen, the fitting is good (n ) 26, r ) 0.942, and sd ) 0.37 kK). Notwithstanding the previous assertions, the segments corresponding to TICT processes in BA are clearly more sensitive
18398 J. Phys. Chem., Vol. 100, No. 47, 1996 to the solvent polarity than are those in C9A; the respective slopes are 8500 and 5200 and consistent with the results of Rettig et al.21 for nonprotic solvents. Therefore, the variation in the dipole moment with electronic excitation must be greater for BA than for C9A. Conclusions The solvatochromism of BA and C9A clearly exhibits a bilinear behavior: one segment that is less markedly dependent on the solvent polarity and ascribable to LE fluorescence and another whose slope can be ascribed to TICT emission. Clearly, the TICT mechanism is more feasible in C9A than in BA; however, the emission wavelength is more sensitive to the solvent polarity in the latter case. The solvatochromism of BC clearly indicates that the chromophore cannot adopt the TICT mechanism throughout the solvent polarity segment bounded by perfluorohexane and DMSO. This is the likely result, to a great extent, of the low electron affinity of the carbazole nucleus that results in a large redox potential difference. Acknowledgment. We are greatly indebted to DGICYT of Spain (Projects PB93-0280 and PB93-0197-C02-01). C.D. thanks the Ministry of Education of Spain for a FPI grant. References and Notes (1) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. New. J. Chem. 1979, 3, 443. (2) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (3) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel F.; Miehe´, J. A. AdV. Chem. Phys. 1987, 68, 1. (4) Rettig W.; Baumann W. Progress in Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1992; Chapter 3. (5) Rettig, W. Topics in Current Chemistry 169. Electron-transfer I; Mattay, J., Ed.; Springer-Verlag: Berlin, 1994, pp 254. (6) Lueck, H.; Windsor M. W.; Rettig, W. J. Phys. Chem. 1990, 94, 4550. (7) Wiessner, A.; Hu¨ttmann, G.; Ku¨hnle W.; Staerk, H. J. Phys. Chem. 1995, 99, 14923. (8) Rettig, W. Nachr. Chem. Int. Ed. Engl. 1986, 25, 971. (9) Schneider F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1155. (10) Schneider F.; Lippert, E. Ber. Bunsen-Ges. Phys Chem. 1970, 74, 624. (11) Khundkar L. R.; Zewail, A. H. J. Chem. Phys. 1986, 84, 1302. (12) Yamasoki, K.; Arita, K.; Kajimoto O.; Hara, K. Chem. Phys. Lett., 1986, 123, 277. (13) Kajimoto, O.; Yamasaki, K.; Arita, K.; Hara, K. Chem. Phys. Lett. 1986, 125, 184. (14) Subaric-Leitis, A.; Monte, Ch.; Roggan, A.; Rettig, W.; Zimmermann P.; Heinze, J. J. Chem. Phys. 1990, 93, 4543. (15) Honma, K.; Arita, K.; Yamasaki K.; Kajimoto, O. J. Chem. Phys. 1991, 94, 3496.
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