J. Phys. Chem. 1995,99, 10097-10109
10097
Ultraviolet Stabilizers of the 2-(2’-Hydroxyphenyl)-1,3,5-triazineClass: Structural and Spectroscopic Characterization Guido J. Stueber? Martina Kieninger,’ Heike Schettler,’ Wolfgang Busch? Bernd Goeller,? Juergen Franke,? Horst E. A. Kramer,**tHelga Hoier? Sonja Henke1,g Peter Fischer? Helmut Port,” Thomas Hirsch,” Gerhard Rytz,l and Jean-Luc Birbauml Institute f i r Physikalische Chemie und jZr Organische Chemie und Isotopenforschung, Universitat Stuttgart, Pfaffenwaldring 55, 0-70569 Stuttgart, Germany, 3. Physikalisches Institut, Universitat Stuttgart, Pfaffenwaldring 57, 0-70569 Stuttgart, Germany, and Ciba-Geigy AG., Additives Division (AD 2.1), Research Center Fribourg, CH-I 723 Marly 1, Switzerland Received: February 7, 1995; In Final Form: April 13, 1995@
X-ray crystal structure determination of 2,4-diphenyl-6-(2’-hydroxy-4’-methoxyphenyl)1,3,5-triazine (MOH-P, Chart 2) reveals a stronger intramolecular hydrogen bond than that in 2-(2’-hydroxy-5’-methylphenyl)benzotriazole (Tinuvin P). The 0 - H distance appears elongated in M-OH-P (0.898 A), and the H***N distance shortened (1.733 A), with a definitely more linear hydrogen bond bridge ( L O - H * * N = 159.6’). IR and ‘H NMR data confirm this finding (BoH.. .N = 13.5 ppm). Both ‘H and I3C NMR spectra demonstrate the OH bridge to switch from N1 to N5 within the NMR time scale, via rotation around the resorcinyltriazine bond. Derivatives with one aryl group only, e.g. 2-(2’-hydroxyphenyl)-4,6-dimethoxy-1,3,5-triazine (DMH, Chart l), exhibit a proton-transferred fluorescence with both large Stokes shift (9890 cm-’) and high = 510 nm, 77 K, methylcyclohexane/2-methylbutane).Introduction of another quantum yield (7; = 0.24, A,, for aryl substituent into the triazine system weakens the proton-transferred fluorescence (7; FZ 9 x 2-(2’-hydroxyphenyl)-4-(dimethylamino)-6-(4’’-methylphenyl)-1,3,5-triazine(TN, Chart l), 77 K). For compounds with three aryl groups, this fluorescence is quenched completely. M-OH-P and 2-(2’,4’dimethoxyphenyl)-4,6-diphenyl1,3,5-triazine (M-MeO-P, Chart 2; with the bridging hydrogen substituted by a methyl group) display phosphorescence behavior (Table 3) analogous to that of 2,4,6-triphenyl-1,3,5triazine (TPT, Chart 2). Additionally, a new fluorescence with an exceptionally large and strongly solventdependent Stokes shift is observed when the hydrogen of the intramolecular hydrogen bond is replaced by CH3 (e.g. 2,4-diphenyl-6-(4’-(dodecyloxy)-2’-methoxyphenyl)-1,3,5-triazine(D-MeO-P, Chart 2; AFstokes = 9340 cm-I, 7~ x 2.5 x AcN, 293 K). For the six OCH3 derivatives included in this study, the change of dipole moment Ap upon excitation has been determined from Lippert-Mataga plots where (Vabs Vflu) is plotted vs the solvent orientation parameter Af. Time-resolved emission and viscosity-dependent fluorescence data may be interpreted in terms of a twist between the subunits of D-MeO-P upon excitation.
1. Introduction
CHART 1
W absorbers with an intramolecular hydrogen bond such as methyl salicylate, o-hydroxybenzophenones, 2-(2‘-hydroxy5’-methylpheny1)benzotriazole(trade name: Tinuvin P/TIN P), and derivatives of these basic structures are widely employed to inhibit photodegradation of The excited state intramolecular proton transfer (ESIPT),in accordance with Forster’s t h e ~ r y , ~and ~ 9 have determined the dihedral angles as 10.9", 7.6", (-)6.9" and 10.4", 7.5", (-)6.6", respectively. The bonding geometry (N5-08) = 2.594(4) A, 08-H8 = 0.898(46) A, N5-H8 = 1.733(38) A) is typical for a strong intramolecular hydrogen bond. The 08-H8 distance appears elongated, and the NS-HS distance shortened in M-OH-P, relative to what was found in the X-ray structure determination of 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (Tinuvin P). I6 Also, the hydrogen bond bridge is more linear. Bond distances and bond angles of Tinuvin P and M-OH-P are compared in parts a and b of Figure 4, respectively. The molecular geometry, determined experimentally for M-OH-P, clearly indicates a much stronger hydrogen bond bridge for this molecule. As a result of the strong, perhaps even chelate type hydrogen bond between 08-H8 and N5, the electron density at 0 8 is enhanced, with a concomitant increase in double-bond character between 08-C8 and C7-C6 bonds which is equivalent to a higher "keto
TABLE 1: Fractional Atomic Coordinates with Standard Deviations for 2 42'-Hydroxy-4'-methoxyphenyl)-4,6-diphenyl-l,3,5-triazine (M-OH-P) atom XIA YIB ZIC N1 0.422 66(9) 0.598 lO(10) -0.253 55(53) c2 0.435 70( 11) 0.638 95(13) -0.4 14 66(62) 0.413 95(10) 0.681 58(10) N3 -0.421 44(54) 0.376 80(11) c4 0.681 36(12) -0.253 42(64) 0.360 64(9) N5 0.642 29(9) -0.084 11(50) 0.384 89(11) -0.091 73(60) C6 0.601 49( 1 1) 0.558 49(11) 0.369 42( 11) c7 -0.091 57(62) 0.263 54(62) 0.556 94(11) 0.329 45( 12) C8 0.280 40(58) 0.596 18(11) 0.302 16(11) 08 0.437 69(69) -0.049 84(351) 0.513 33(60) 08' 0.31491(13) 0.433 69(66) 0.51474(13) c9 0.341 49( 13) 0.436 9 l(63) 0.474 22( 12) c10 0.330 28(10) 0.594 79(46) 0.431 43(8) 010 0.287 37(19) 0.760 38(88) 0.426 59(18) ClOl 0.38 1 79( 14) 0.271 95(76) c11 0.475 42( 14) c12 0.5 16 4 1(12) 0.395 69( 13) 0.103 59(66) 0.477 08( 12) C13 0.638 02(13) -0.597 72(66) 0.504 86(15) C14 0.598 18(16) -0.59 1 48(80) 0.543 58(16) -0.767 18(94) C15 0.598 05(20) 0.555 29( 17) 0.638 23(20) -0.942 87(85) C16 0.528 16(16) -0.952 08(82) C17 0.677 79( 19) 0.489 57( 14) -0.779 02(74) C18 0.677 79( 15) 0.352 03(11) C19 0.727 91(11) -0.253 84(62) 0.365 Ol(14) -0.441 08(69) c20 0.766 82(13) 0.342 94( 14) 0.810 48(14) -0.443 48(73) c 21 c22 0.308 4 1(14) -0.260 14(74) 0.816 89(13) C23 0.295 27(15) 0.778 87( 14) -0.072 95(72) 0.317 02(14) C24 -0.070 92(69) 0.734 88(14) 0.172 97(941) H8 0.321 82(187) 0.619 18(181) 0.539 3l(537) 0.286 62(102) 0.5 15 80(98) H9 0.667 13(727) 0.426 56( 144) HlOll 0.253 64(149) 0.283 34( 146) 0.844 41(770) H1012 0.393 20( 157) 0.293 19(124) 0.890 29(655) 0.458 15(126) H1013 H11 0.40 1 49( 113) 0.450 71(114) 0.283 44(601) H12 -0.0 13 90(0) 0.426 50(0) 0.517 lO(0) 0.3 10 60(0) H12' 0.268 OO(0) 0.585 80(0) 0.495 52(122) H14 -0.479 26(660) 0.568 13(126) 0.562 00(133) H15 0.57 1 43( 132) -0.743 06(728) 0.584 00(143) H16 - 1.052 19(754) 0.636 73( 137) 0.536 51(165) H17 -1.075 65(872) 0.707 38(160) 0.467 97( 123) -0.789 58(648) 0.701 73(123) H18 -0.55 105(603) 0.390 92(114) 0.762 13(115) H20 0.356 70(123) H2 1 -0.563 73(690) 0.838 30( 125) H22 0.294 01(108) -0.264 97(593) 0.846 88(113) H23 0.036 80(684) 0.267 98(125) 0.781 51(128) 0.055 99(675) H24 0.309 40( 123) 0.708 90(122) character" of the resorcinyl moiety. This becomes immediately apparent if one compares the three triazine aryl C-C bonds (C6-C7, C2-Cl3, and C4-Cl9) in M-OH-P (1.459 vs 1.470 and 1.486 A; see Figure 3). On the basis of these distinct structural differences, the improved capacity for radiationless
J. Phys. Chem., Vol. 99, No. 25, 1995 10101
Ultraviolet Stabilizers
I
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Figure 4. Comparison of (a) bond distances (A) and (b) bond angles (deg) of Tinuvin PI6 (2-(2’-hydroxy-5’-methylphenyl)benzotriazole)and M-OH-P relevant to their intramolecular hydrogen bonds.
deactivation of M-OH-P, compared to that of Tinuvin P, must be attributed to its stronger hydrogen bond. 3.2. NMR Spectroscopic Investigations. The crystal structure clearly demonstrates that the two aryl rings in M-OH-P (Chart 2) are held in an almost coplanar orientation by the 0 - H . - N hydrogen bond between one of the aza nitrogens of the triazine and the phenolic OH group of the resorcinyl moiety. The chelate character of this hydrogen bond, i.e. its special strength, is clearly indicated by the extremely low chemical shift value (13.5 ppm) for the bridging hydrogen. The absolute shift value, though, does not depend directly upon the effective strength of the individual hydrogen bond; it is determined, rather, by the individual shift contributions of the respective molecular environment. The effect of the intramolecular 0-H 0.N bond on the NMR spectra becomes immediately apparent if one compares individual resonances of the OH compound M-OH-P and its 0-methyl derivative M-MeO-P (Chart 2). The 6’-H doublet of the free ortho resorcinyl proton is shifted upfield from 8.6 in M-OH-P to 8.1 ppm in M-MeO-P. In M-OH-P, the hydrogen bridge keeps the resorcinyl and the triazine rings in coplanar orientation, thus allowing for an effective charge transfer from the electron-rich resorcinyl to the electron-deficient triazine ring. At the same time, the ortho 6’-H proton is held close to the aza nitrogen lone pair. Upon removal of the hydrogen bridge in M-MeO-P, steric interaction between the OCH3 group and the triazine ring causes a substantial twist around the aryl-triazinyl bond. Thus, electronic interaction is severely attenuated, and at the same time, 6’-H is moved out of the sphere of influence of the triazine ring. Both facts cause the substantial 0.5 ppm upfield shift for 6’-H. On the other hand, the 3‘3’-H resonances, i.e. the signals of the two meta resorcinyl protons, appear almost unchanged between M-OH-P and M-MeO-P. The ortho resonances of the two xylyl moieties likewise display virtually identical shifts in both the
OH and the OCH3 compounds. The coalescence behavior for the switching of the OH bridge from N1 to N5, however, is no longer visible in the OCH3 derivative (see below). As long as the hydrogen bond remains intact, i.e. as long as the reorientation of the H bridge is slow on the NMR time scale, the 2,4,6-triaryl1,3,5-triazine molecules appear differentiated, structurally, into two hemispheres (Scheme 1). Depending on whether reorientation of the hydrogen bond is slow, intermediate, or fast on the NMR time scale, the two sets of IH and I3C resonances for the two structurally unequal hemispheres will appear as separate signals, with well-differentiated chemical shifts, more or less exchange-broadened, or at an intermediate value if the ambient temperature of the NMR experiment is well above the coalescence temperature. Figure 5 shows the ambient-temperature ’H NMR spectra of two bis[4’-( (3-butoxy-2-hydroxypropyl)oxy)-2’-hydroxyphenyl]substituted aza heterocycles (6PDH and 6PDH-Py, Chart 2). In the case of the pyrimidine derivative 6PDH-Py, the two chelate 0-H. N bridges hold both substituted phenyl moieties in a fixed orientation. Thus, a clear ABX spectrum results for the three resorcinyl protons (e, f, and g in Figure 5a). In the triazine derivative 6PDH, on the other hand, the two hydrogen bridges wander around the heterocyclic core. The two meta protons (f and g in Figure 5b) appear well-resolved; the ortho proton (e), however, displays the chemical exchange broadening typical for coalescence. The I3C NMR spectra for this pair of compounds (Figure 6) mirror the exchange phenomenon even more strikingly. For the pyrimidine derivative 6PDH-Py, all I3C resonances are sharp and well-defined (Figure 6a). The triazine compound 6PDH has broad resonances for all carbon atoms directly involved in hydrogen bond cleavage and reclosure, Le. C2,4 and C6 in the core and C14 in the resorcinyl moieties (Figure 6b). Both the ‘H and I3C NMR spectra of the triazine derivative 6PDH display the exchange situation slightly above the respective coalescence temperatures. Generally, the fact whether the ambient-temperature spectrum displays two distinctly different sets of signals or broadened lines or finally only one set of averaged resonances directly mirrors the AG* value for the rotation around the resorcinyl-triazine bond. A detailed analysis of the NMR spectra will be published separately. 3.3. Absorption Spectra. The absorption bands of the parent compound 1,3,5-triazinehave been attributed to x x * (222 nm, E = 145 M-’ cm-’) and nn* t r a n s i t i o r ~ s ~(272 ~ - ~nm, ~ 6= 881 M-’ cm-l, both determined in cyclohexane). The intense 268 nm absorption band of 2,4,6-triphenyl- 1,3,5-triazine (TI”) ( E = 51 300 M-I cm-l, in acetonitrile) also has x x * character. According to phosphorescence polarization measurements (PhP and APPh), however, a nx* transition may also contribute to this band5’ (see section 3.4.1). Derivatives with OR (R = alkyl) and/or OH groups in one or more of the phenyl substituents
10102 J. Phys. Chem., Vol. 99, No. 25, 1995
Stueber et al.
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Figure 5. ' H NMR spectra (nominal frequency 250.133 MHz, solvent CDC13, 25 "C) of (a) the pyrimidine 6PDH-Py and (b) the triazine analogue 6PDH (* indicates signals of residual solvent protons or impurities).
Ultraviolet Stabilizers
J. Phys. Chem., Vol. 99, No. 25, 1995 10103 C 23 C-25
-CHC13 C-24
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Figure 6. I3C NMR spectra (nominal frequency 62.896 MHz, solvent CDC13,25 "C) of (a) the pyrimidine 6PDH-Py and (b) the triazine analogue 6PDH.
display an additional longer-wavelengthabsorption band (2295 nm, see Figure 7 and Table 2) which is due to an intramolecular charge transfer (nn*-CT). This CT absorption band is more intense if e.g. a methoxy substituent is in the 4' position of one phenyl ring than if the OCH3 group is in the 2' position. This
is in good agreement with the 'H NMR results (see section 3.2) from which a slight torsion of the 2'-OCH3-substituted phenyl ring with respect to the triazine plane is inferred, resulting in reduced CT. Due to the planar structure of M-OH-P (see section 3.1), its CT band appears red-shifted and intensified with
Stueber et al.
10104 J. Pkys. Ckem., Vol. 99, No. 25, 1995 Absorbance 1
Wavelengrh in
nm
Figure 8. Fluorescence spectra of DMM (-) in CyH at T = 293 K (L.,= 303 nm, c M), DMH (- - -) in CyH at T = 293 K (&, = 313 nm, c = 2.5 x M), and DMH ( - * - * - ) in MCW2-MB at T = 77 K, = 313 nm, c = M.
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Figure 7. Absorption spectra of (a) M-MeO-P (-) and M-OH-P (- - -) in n-pentane and (b) M-MeO-P (-) and M-OH-P (- - -) in M. DMSO at T = 293 K. c
-
TABLE 2: Room-Temperature Absorption Data of 1,3,5-Triazines maxima” (nm) compound M-MeO-P M-MeO-T M-MeO-X M-OH-P M-OH-T M-OH-X
extinction coefficients (L/(mol.cm))
Wavelength in run
solvent
3.1
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CyH DMF CyH DMF CyH DMF n-P DMSO n-P DMSO n-P DMSO
270 272 279 284 278 280 271 278 287 293 287 290
320 327 320’ 327’ 327’ 328’ 343 340 341 338 339 333
48400 53700 47700 31 700 46900 27 800 45900 45 600 58200 56400 41 500 24200
17000 14600 15900 10900 18 100 11 400 20900 21 200 20900 23400 19700 11 900
2.85 3.68 3.00 2.91 2.59 2.44 2.20 2.15 2.79 2.41 2.11 2.04
S = shoulder.
respect to M-MeO-P. Further absorption bands are observed at -245 and -200 nm in the appropriate solvents which have to be attributed to locally excited states of the aryl rings. The absorption spectra of DMM and of DMH, where the methoxy groups are linked directly to the triazine nucleus, are very similar to those of M-MeO-P and of M-OH-P, respectively. 3.4. Emission Spectra. 3.4.1. Emission of (2’-Hydroxyphenyl)-l,3,5-triazines. Compounds with only one aryl substituent such as DMH (see Figure 8) show a proton-transferred fluorescence (SI’ SO’,jlmax.flu= 510 nm, methylcyclohexanel 2-methylbutane, 77 K) with large Stokes shift (9890 cm-I) and high quantum yield (VF’ = 0.24). Similar results were obtained by Shizuka et al.39-42for slightly modified compounds of the DMH type where the 5’ positions are substituted by Ri = CH3 (see Chart 1). 2-(2’-Hydroxyphenyl)- 1,3,5-triazines with one additional aryl group (TN, see Chart 1) still display a weak proton-transferred fluorescence (?IF‘ 9 x lo-‘) which appears quenched completely for compounds with three aryl groups (see Chart 2 , R2 = OH). Compounds of the TPT type, e.g. M-OH-X and H-OH-X (see Chart 2), on the other hand, show very weak
-
-
Figure 9. Phosphorescence spectra of M-OH-P and 3-MP (-) at T = 77 K, A,,, = 313 nm. c
in MTHF (-) M.
fluorescence (VF < lo-’; estimated with respect to the registration sensitivity of the built-in-house instrument) with normal Stokes shift (M-OH-X, A,, = 367 nm, cyclohexane/airsaturated, 293 K; H-OH-X, = 375 nm, DMSO/airsaturated). This fluorescence, as well as the phosphorescence observed for these compounds (Figure 9, M-OH-P in MTHF and 3-MP, deaerated, 77 K), has to be attributed to the excited molecule in its non-proton-transferred S I and Ti states, respectively. Whereas no excitation spectra of the weak fluorescence could be registered, Figure 10 demonstrates the agreement between the corrected phosphorescence excitation spectrum, measured for Aem = 449 nm at 77 K, and the singlet absorption intensity (labs) spectrum at room temperature (M-OH-P). (The slight difference of both spectra in Figure 10 may be due to different temperatures.) Identical corrected phosphorescence excitation spectra of M-OH-P were also obtained for &, = 477 and 423 nm (see Figures 9 and 10); this clearly demonstrates that there is only one absorbing ground-state form whose excitation finally exhibits the observed phosphorescence. It should be pointed out in this context that the phosphorescence spectra of M-OH-P and of M-MeO-P (where the hydrogen of the intramolecular hydrogen bond is replaced by a CH3 group) are very similar (Figure 11). They are directly comparable with that of TPT; the respective triplet energies (ET),phosphorescence quantum yields ( ~ p ) and , phosphorescence lifetimes (zp) are listed in Table 3. It is worth noting also that l;lp of M-OH-P, with an intramolecular hydrogen bond, is smaller than that of M-MeO-P (Table 3) by up to 2 orders of magnitude. The phosphorescence polarization degree is slightly positive for both M-MeO-P and M-OH-P (0.04, MTHF, 77 K, i,,,, = 313 and 333 nm, Aem = 410-540 nm), in accord with the
J. Phys. Chem., Vol. 99, No. 25, 1995 10105
Ultraviolet Stabilizers
TABLE 3: Phosphorescence Quantum Yield pp, Phosphorescence Lifetimes zp, and Triplet Energies ET; T = 77 K compound solvent VP ZP (ms) ET (cm-I) M - M e 0 -P M-MeO-P M-MeO-P M-OH-P M-OH-P M-OH-P 1,3,5-tria~ine~~~ 2-phenyl- 1 , 3 , 5 - t r i a ~ i n e ~ ~ ~ 2,4-diphenyl- 1 , 3 , 5 - t r i a ~ i n e ~ ~ ~ 2,4,6-triphenyl- 1,3,5-triazine (TPT) 2-pheny1-4,6-dimethyl-1,3,5-tria~ine’~~ 2,4-diphenyl-6-methyl- 1 , 3 , 5 - t r i a ~ i n e ~ ~ ~
3-MP EEP MTHF 3-MP EEP MTHF EPA 3-MP 3-MP 3-MP 3-MP 3-MP
4.4 x 10-2 1.3 x lo-* 1.8 x 10-2 2.6 x 10-4 7.1 x 10-3 3.7 x 10-3
1000 660 570 980 720 770 400 1400 1200 1 10074b 1900 1200
-I
I
23 310 23 260 23 260 23 365 23 640 23 585 26 400 24 500 24 400 24 500 24 700 24 500
1 2 3 4 5
1-
and labs
Ire1
300
Wavelength in nm
-
Figure 10. Absorption spectrum (- - * -) (293 K) and phosphorescence excitation spectrum (-) of M-OH-P: solvent EEP (ethanoydiethyl ethedpyridine mixture 25:25:1 v/v/v), A,,,, = 449 nm, at T = 77 K, c 10-5 M.
-
350
450
400
500
550
E
0
Wavelength in nm
Figure 12. Fluorescence spectra of D-MeO-P in n-P ( l ) , MC (2), AME (31, DMF (4), and MeOH (5) at T = 293 K, A,, = 313 nm, c M (all fluorescence spectra normalized to I,,, = 1).
-
CHART 3
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300
350
400
450
500
550
600
650
Wavelength in nm
Figure 11. Phosphorescence spectra of M-MeO-P
(-) and M-OH-P in EEP (ethanoydiethyl ethedpyridine mixture 25:25: 1 v/v/v) at T = 77 K, A,,, = 313 nm, c M.
(-)
-
results of Gallivan et aL5’ for e.g. TPT. Negative polarization of phosphorescence is expected for excitation of pure nn* transitions while nn* transitions provide positive polarization degrees (see D01-r~~).The 0.04 value obtained for M-OH-P hints at an nn* transition contributing to the long-wavelength absorption region. Bigger et al.43report analogous results for 2-(2‘-hydroxy-4’methoxypheny1)-1,3,5-triazines and the respective sulphonated derivatives. S-PhOT has a structure comparable to M-OHP, with phenoxy (O-C6H5) substituents in positions 4 and 6 of the triazine ring which correspond to the OCH3 substituents in DMH. S-PhOT shows weak proton-transferred fluorescence ( 7 ~ ’= 1.87 x room temperature, methanol); no such fluorescence is observed with phenyl or p-tolyl substituents in the respective positions.43 In addition to the proton-transferred fluorescence, S-PhOT also displays a short-wavelength fluorescence. The ratio of both emissions depends on the solvent polarity. The excitation spectrum of the proton-transferred fluorescence extends to longer wavelengths than that of the short-wavelength fluore~cence.~~
S-PhOT
3.4.2. Emission of (2’-Methoxyphenyl)-l,3,5-triazines.A new fluorescence with an unusually large and strongly solventdependent Stokes shift is observedu when the hydrogen atom of the intramolecular hydrogen bond is replaced by CH3 (e.g. D-MeO-P, Stokes shift 9340 cm-I, l ; l ~ 2.5 x AcN, room temperature; see Figure 12). It is possibly a twisted intramolecular charge transfer (TICT)61,62fluorescence. The fluorescence maxima (Amax,flu, AcN) of MOTT-2 (415 nm), MOTT-4 (440 nm), and M-MeO-P (467 nm) demonstrate the increasing donor capacity of the substituted phenyl ring. Slight steric hindrance for MOTT-2 to a planar conformation can be inferred from ‘H NMR measurements (see section 3.2, chemical shift of the o-resorcinyl protons), from the viscosity dependence of both fluorescence maximum and quantum yield (see section 3.6), and also from the fact that the position of the fluorescence maximum of MOTT-2 is nearly independent of solvent polarity. A strong red shift of the fluorescence of 2-(2’-methoxy-4’alkoxyphenyl)-4,6-diaryl1,3,5-triazines is observed with higher
-
10106 J. Phys. Chem., Vol. 99, No. 25, 1995
Stueber et al.
TABLE 4: Fluorescence Quantum Yield 4~and Fluorescence Maximum of o-Methoxyphenyl-1,3,5-triahes; Acetonitrile, A,,, = 313 nm, T = 293 K chain length R1
emission data
A,, , (nm)
OCHi
1o.'VF OC~HI? 1O"VF O C I Z H ? ~A,, (nm) 103VF
substituents on the 2- and 4-phenyl groups" Ri = R, = H, R3 = Rq=H &=CH3 &=CHj 468 18.5 468 21.0 467 24.94
448 10.0 448 14.0 447 16.23
432 1.3 432 1.7 432 2.25
For formulas see Chart 2 (R2 = OCH3). 10000
9000
BOO0
~~
'cm-: 7000
6000
5000
l " " " " ' l " " " ' ~ ~ I " " " " '
0.1
0.2
0.3
0
Af
Figure 13. Lippert-Mataga plot for D-MeO-P with Af according to eq 2 (D and n from refs 70 and 81-85). Solvents: n-P (1). CyH (2). Dxn (3), 0-Xyl (41, Tph (5), TCTFE ( 6 ) , DnBE (7), DiPE (8), Tchl (9), Et (IO), DEDME ( I I ) , EEE (12), 2-MTHF (13), THF (14), MC ( I S ) , AEE (16). AME (17). DMSO (18), DMF (19), EtOH (20), AcN (21), MeOH (22).
TABLE 5: Results of Lippert-Mataga Plots
M-MeO-P M-MeO-T M-MeO-X D-MeO-P D-MeO-T D-MeO-X
8578 55.50 2371 9200 6261 3170
6338 6560 6873 6151 6327 6652
17.1 15.0 9.8 17.7 1.5.9 11.4
2.1 1.8 2.1 2.1 2.0 2.2
19.2 16.8 11.8 19.8 17.9 13.6
In benzene.
-
-
solvent polarity, e.g. in going from n-pentane (always Amax.nu 420 nm and l ; l ~ 5 x to acetonitrile. Table 4 shows the influence of substituents R1, R3, and R4 on the fluorescence characteristics (Rz is always OCH3: see Chart 2): (i) E.max.jiu and VF are nearly independent of RI (RI = OC12H25, 0CsH13, OCH3) within each series of aryl substitutents (phenyl, p-tolyl, 0-xylyl) in pentane: (ii) VF exhibits two distinct dependencies in acetonitrile [RI, V F ( O C I ~ H>~ ~q~(oC6H13) ) > V~(ocH3): R3 and R4, VF(pheny1) > VF(p-toly1) > ?,'F(o-xylyl)]; (iii) the ~ N with R3 and R4 but is red shift AF = Fflu:pentane - F ~ ~ , ; Avaries independent of the R I chain length [AijPhenyl(2400 cm-I) > AFp.to~k~ (-1500 cm-I) > AF'o.xy~y~ (-600 cm-I)]. Lippert-Mataga plots.44 As shown above, the energy of the fluorescence maximum strongly depends on solvent polarity (Figure 12, D-MeO-P). According to the Lippert-Mataga t h e ~ r y , ~ ~the- ~wavenumber j difference between the longwavelength absorption maximum and the fluorescence maximum (AF = Vabs - gnu: Stokes shift) is plotted vs the orientation parameter Afof various solvents (Figure 13, see eqs 1 and F3).
Af=--- D - 1 20+1
n2-1 2n2+1
In this context, pg and pe are the dipole moments in the ground and excited (fluorescent) state of the molecule, respectively, a is the radius of the cavity according to Onsager's theory66 of reaction field, D and n are the dielectric constant and refractive index of the solvent, respectively, h is Planck's constant, and c is the light velocity. Ap = pe - puscan be obtained from the slope in Figure 13 by assuming reasonable values for a. With experimental values of pn,the dipole moments p, in the excited state can then be calculated (see Table 5 ) . The high dipole moment in the excited state p, thus obtained is probably due to a charge-transfer transition from the OCH3 and 0-alkyl groups of the resorcinyl part into the triazine ring. Table 5 presents the results of the Lippert-Mataga plots according to eqs 1 and 2. If R2 = OCH3 is held constant and the length of the alkyl chain (RI = O-CH3 = M; O - C ~ H I= ~ H: O-CI2H2j = D) as well as the two aryl substituents is varied (phenyl = P: p-tolyl = T: 0-xylyl = X) and the radii a of the cavity according to Onsager's theory66-68are calculated using the crystal structure data of M-OH-P (see section 3.1), the following gradation of Ap (=pe - p n ) is found: Ap(pheny1) L Ap(p-tolyl) > Ap(o-xylyl) It is tempting to assume a coplanar orientation of the aryl substituents and the 1,3,5-triazine ring in the case of the phenyl and p-tolyl substituents. In contrast, a slight torsion may occur in the case of the 0-xylyl substituent, due to steric interaction between the o-methyl group of the xylyl and the triazine ring. The concomitant decrease in the nn interaction between triazine and 0-xylyl rings supplies a good rationale for the smaller Ap value of all 0-xylyl derivatives studied here. While the fluorescence (A", * 420-470 nm) with a large solvent-polarity-depending Stokes shift is observed in fluid solution at room temperature, a very weak fluorescence with small Stokes shift (A", 380 nm) is observed only in lowtemperature glasses (77 K). It seemed desirable, therefore, to also study the influence of solvent viscosity (see section 3.6). 3.5. Time-Resolved Emission Spectroscopy. In Figure 14, the time-resolved emission spectra of D-MeO-P at 0-0.4 and 0.6-1.0 ns are compared with the quasi-CW spectra. Neither the spectral shape nor the spectral dynamics display any marked differences. Fluorescence decay curves have been recorded at 18 200, 19 200, 23 300, and 25 000 cm-I (Figure 15). The curves are well-reproduced by a sum of two exponential functions. The slower component has a time constant 52 = 3.9 ns (f8%) at all detection energies. For the faster component, 51 varies from 1.3 ns (48%) at 25 000 cm-I to 1.8 ns (f8%) at 18 200 cm-'. The amplitude ratio a,/a2 decreases from 4.2 at 25 000 cm-' to 1.5 at 18 200 cm-'. Since no spectral dynamics and no slow fluorescence rise component at 18 200 cm-l, correlated to the decay, have been detected, it is concluded that there is only one excited state observed. The decrease of the ratio allaz with decreasing energy can be rationalized by a distribution of decay times.80 On the time scale of the experiment, the influence of solvent dynamics (AcN, 300 K) on a picosecond time scale can be neglected.8' Presumably, the distribution of decay times originates from a distribution of intramolecular conformations, due to twist angles between the subunits of D-MeO-P (see sections 3.2, 3.4, and 3.6).
-
Ultraviolet Stabilizers
25
J. Phys. Chem., Vol. 99, No. 25, 1995 10107
i
t
20
3
30
20
15
0
1.
10
10 0
350
5
400
450
500
550
600
650
Wavelength in nm
Figure 16. Fluorescence spectra of D-MeO-X
0 20000
15000
25000
30000
E/cm-’ Figure 14. Time-resolved spectra of D-MeO-P
in AcN at T = 300 K; time intervals of 0-0.4 and 0.6- 1.O ns and CW (the arrows marking the energies of the fluorescence decay measurements).
..-
1001 0
,
, , ,
, 5
, , , ,
I
, , , ,
10
, 15
-..---
, , , ,
,t
20
t/n s Figure 15. Logarithmic plot of the fluorescence decay of D-MeO-P (in AcN, T = 300 K) at (a) 18 200 cm-’, (b) 19 200 cm-], (c) 23 300 cm-I, and (d) 25 000 cm-I.
3.6. Influence of Solvent Viscosity on the Fluorescence. In mixtures of methanol and cyclohexanol at 20 “C, the solvent viscosity r can be adjusted between 0.597 CP (methanol) and 68.0 CP (cyclohexanol).70 Viscosity-dependent measurements were carried out for a series of 2-(2’-methoxy-4’-alkoxyphenyl)4,6-diaryl-l,3,5triazines(see Chart 2). The alkoxy substituents were RI = OCH3 and OC12H25, and the aryl substituents were phenyl (R3 = & = H), p-tolyl (R3 = H, & = CH3), and 0-xylyl (R3 = & = CH3), respectively. The 2’-substituent was always kept constant (R2 = OCH3). The fluorescence spectra of D-MeO-X in methanol, in cyclohexanol, and in nine different methanollcyclohexanol mixtures are presented in Figure 16. The fluorescence maximum is shifted to shorter wavelengths, and the fluorescence quantum yield decreases with increasing viscosity while the position of the absorption maximum remains unchanged. The corresponding phenyl and p-tolyl derivatives display similar shifts of the fluorescence maxima whereas their fluorescence quantum yields decrease but slightly with higher viscosity. The length of the alkyl chain, OCH3 vs OC12H25, has no significant influence.
in MeOH (-), CyHol (- - -), and MeOWCyHol mixtures (-) at T = 293 K, A,,, = 313 nm, c 3 x M.
-
Varying the composition of the solvent mixtures changes not only the viscosity but, to some extent, also the polarity of the medium. The blue shift observed with increasing viscosity further supports the hypothesis that the observed fluorescence originates from a TICT state: (i) ‘H NMR data have demonstrated a certain twist between the resorcinyl (R2 = OCH3, Chart 2) and triazine rings already in the electronic ground state (see section 3.2). Increasing viscosity will impede any twist of the involved groups after excitation. Emission from the originally excited twisted conformation will occur at shorter wavelengths (blue shift) in more viscous solutions. In solutions of lesser viscosity, an energetically more favorable conformation can be reached before emission, which therefore appears at longer wavelengths (red shift). The time-resolved emission spectroscopic results (see section 3.5) probably apply to an intermediate case. (ii) Relaxation of the solvent with respect to the excited (polar) molecule will also become less effective with increasing viscosity, thus providing a second contribution to the blue shift of the fluorescence.82 It is interesting to note the similarity between the fluorescence spectra of D-MeO-P in cyclohexanol and in a polymer matrix, poly(methy1 metha~rylate).’~
4. Conclusions The intramolecular hydrogen bond in 2-(2‘-hydroxypheny1)1,3,5-triazines has been established, for the solid state, as definitely stronger than that in the (2‘-hydroxypheny1)benzotriazoles (see section 3.1). The crystal structures reveal enhanced “keto character” in the electronic ground state of M-OH-P (Chart 2), as compared to TIN Pel6 This conclusion is confirmed by a detailed analysis of the IR and NMR spectral data,23$44 e.g. for 2-(2’-hydroxy-5’-methylphenyl)benzotriazole (Tinuvin P) [BOH = 11.1 ppm (in CDClj), ?OH = 3080 cm-’I and for M-OH-X and for D-OH-X (Chart 2) OH = 13.42 ppm (in CDCl3), ?OH = 2800 cm-’1, as well as by the solvent influence on the UV absorption maxima.45 A more stable intramolecular hydrogen bond is less likely to be broken by polar polymers or solvent molecules to form e.g. the distorted TIN(inter) (with intermolecular hydrogen bonds to polar groups of the matrix), which will no longer be an effective UV stabilizer. Compounds with only one aryl group, e.g. 2-(2‘-hydroxyphenyl)-4,6-dimethoxy- 1,3,5-triazine (Chart l), show proton-transferred fluorescence with high quantum yield (methylcyclohexane/2-methylbutane, 77 K, A,,, = 510 nm, VF’ = 0.24)44,7’(for earlier results on similar compounds see Shizuka et aL4’). Introduction of one additional aryl group weakens the proton732
10108 J. Phys. Chem., Vol. 99, No. 25, I995
Stueber et al.
(8) Huston. A. L.: Scott, G. W.: Gupta, A. J . Chem. Phys. 1982, 76. transferred fluorescence (e.g. 11; x 9 x low4for TN (Chart l), 4978-4985. same solvent mixture, 77 K),44,71which appears completely (9) O’Connor, D. B.; Scott, G. W.: Coulter, D. R.: Gupta, A,: Webb. quenched for compounds with three aryl groups (see section S . P.: Yeh, S. W.: Clark. J. H. Chem. Phys. Lett. 1985, 121, 417-422. 3.4.1). ( I O ) Flom. S . R.; Barbara, P. F. Chem. Phys. Len. 1983, 94, 488-493. (11) Werner, T.: Kramer, H. E. A.: Kuester, B.: Herlinger. H. Angew. In this context, two points have to be considered: In terms Makromol. Chem. 1976. 54, 15-29. of the promoting and accepting mode concept,19-21.23.59.60.72 (12) Kuester, B.: Tschang, Ch.-J.: Herlinger. H. Angew. Makromol. additional aryl or alkyl groups may be assumed to accelerate Chem. 1976, 54, 55-70. (13) Werner. T.; Kramer, H. E. A. Eur. Polym. J . 1977, 13, 501-503. the radiationless deactivation, since the additional vibration (14) Werner, T. J . Phys. Chem. 1979, 83. 320-325. modes facilitate conversion of the electronic excitation energy (15) Werner. T.: Woessner, G.; Kramer. H. E. A. In Phorodegradarion into vibrational energy of the molecule (kinetic aspect). A and Photosrahilizarion of Coarings: Pappas, S . P.. Winslow, F. H.. Eds.: ACS Symposium Series 151: American Chemical Society: Washington, second aspect concerns the proton transfer in the excited state. DC, 1981; pp 1-18. This may occur only if the acidity of the OH group and, (16) Woessner, G.; Goeller, G.: Kollat, P.; Stezowski. J. J.: Hauser, M.: concomitantly, the basicity of the N atom participating in the Klein, U. K. A.: Kramer. H. E. A. J . Phys. Chem. 1984, 88, 5544-5550. intramolecular hydrogen bond are increased to render the (17) Woessner, G.; Goeller, G.: Rieker, J.: Hoier. H.: Stezowski, J . J.: Daltrozzo. E.: Neureiter, M.: Kramer. H. E. A. J . Phys. Chem. 1985, 89, N-protonated “keto form” more stable than the phenolic form 3629-3636. (electron density/energetic aspect). (18) Kramer, H. E. A. Farbe + Lack 1986. 92, 919-924. Semiempirical electron density calculations for the excited (19) Goeller, G.: Rieker, J.; Maier. A,; Stezowski, J. J.; Daltrozzo. E.; Neureiter, M.; Port, H.: Wiechmann, M.: Kramer. H. E. A. J. Phy.s. Chem. singlet state of TIN P show a decrease at the oxygen and an 1988, 92, 1452-1458. increase at the nitrogen atomI4 relative to the ground state. The (20) Kramer. H. E. A. In Photochromism-Molecu1e.s and Sysrems: Durr. same holds for compounds 00, ON, and NN (Chart 1) of H., Bouas-Laurent. H., Eds.: Elsevier: Amsterdam, 1990: pp 654-684 and Shizuka et al.42and probably also for DMH (Chart 1) (which references therein. (21) Kramer, H. E. A. Book ofAbsfracrs. 13th International Conference has a methyl group less than compound 00). Proton-transferred on Advances in the Stabilization and Degradation of Polymers. Luceme. fluorescence likewise is observed for S-PhOT (Chart 3)j3 (see Switzerland, May 22-24, 1991: Patsis. A. V., Ed.; pp 59-78. section 3.4.1) and M-OH-R (Chart 2 ) (in PMMA),73where a (22) (a) CatalAn. J.; Fabero. F.; Guijarro. M. S.; Claramunt, R. M.: Santa Maria, M. D.: de la Concepcion Foces-Foces, M.: Cano, F. H.; Elguero. J.; phenoxy group and five methoxy groups increase the electron Sastre, R. J . Am. Chem. Soc. 1990, 112, 747-759 and references therein. donor capacity of the respective aryl substituents. (b) Catalhn, J . ; Perez, P.: Fabero, F.; Wilshire, J. F. K.: Claramunt, R. M.; The following relation holds for the overall quantum yield Elguero, J. J. Am. Chem. Soc. 1992, 114, 964-966. (c) Catalhn, J.; Fabero. F.; Guijarro, M. S.: Claramunt, R. M.; Santa Maria. M. D.: de la Concepcion VF of the SI’ fluorescence:I9
(3) [ V T ~is the proton transfer yield from SI to SI’, VF‘ is the intrinsic fluorescence quantum yield of SI’, and k ~ and ’ ZF‘ designate the respective emission rate and decay time.] For an effective proton-transferred fluorescence, several conditions must be met: The proton transfer in the excited singlet state must be favored in terms of the electron density/ energetic aspect; also, the radiationless deactivation of S 1 has to be decidedly less effective than the proton transfer (kinetic aspect, transfer yield 1 1 ~ ~ The ) . smaller energy gap (SI’ SO’) will accelerate the radiationless deactivation of S 1’ (energy gap law) and thus lower .7; The effective overall quantum yield VF, which is determined experimentally, is limited by the product VT~VF’ (see equation 3).
-
Acknowledgment. We thank Dr. V. V. Toan, Ciba-Geigy AG., Marly, Switzerland, for the synthesis of 6PDH-Py (Chart 2 ) . Further thanks are due to G. Huebner, C. Schroeder, and F. Elbe for experimental assistance. Supplementary Material Available: Complete tables of isotropic and anisotropic thermal displacement factors and bond distances and angles (12 pages); complete tables of structure factors (27 pages). Ordering information is given on any current masthead page. References and Notes (1) Heller, H. J. Eur. Polym. J . Suppl. 1969, 105-132. ( 2 ) Heller, H. J.: Blattmann, H. R. Pure Appl. Chem. (1972) 30, 145165. ( 3 ) Heller, H. J.; Blattmann, H. R. Pure Appl. Chem. 1974. 36, 141161. (4) Williams, D. L.: Heller, A. J . Phys. Chem. 1970, 74, 4473-4480. (5) Otterstedt, J.-E. A. J . Chem. Phys. 1973, 58, 5716-5725. (6) Klopffer, W. Adv. Photochem. 1977, 10. 311-358. (7) Merrit. C.: Scott. G. W.: Gupta, A,: Yavrouian, A. Chem. Phys. Lert. 1980, 69. 169-173.
Foces-Foces. M.: Cano. F. H.: Elguero. J.: Sastre. R. J . Am. Chem. Soc. 1991, 113, 4046. (23) Rieker. J.; Lemmert-Schmitt. E.: Goeller. G.: Roessler, M.: Stueber. G. J.: Schettler. H.: Kramer. H. E. A,; Stezowski. J. J.: Hoier. H.: Henkel. S.; Schmidt, A.: Port, H.: Wiechmann, M.: Rod), J.: Rytz. G.; Slongo, M.; Birbaum, J.-L. J . Phys. Chem. 1992, 96, 10225-10234. (24) Barbara, P. F.: Rentzepis. P. M.: Brus, L. E. J . Am. Chem. Soc. 1980, 102, 2786-2791, 5631-5635. (25) Barbara, P. F.. Trommsdorf, H. P.. Eds. Special issue on proton transfer. Chem. Phys. 1989. 136. 153-360. (26) (a) Wiechmann, M.: Port, H.; Laermer, F.: Frey, W.; Elsaesser, T. Chem. Phys. Lett. 1990. 165. 28-34. (b) Wiechmann, M.; Port, H.: Frey. W.: Laermer. F.: Elsaesser. T. J. Phys. Chem. 1991, 95, 1918-1923. (27) Allan, M.; Asmis, K.; Bulliard, C.: Haselbach. E.; Suppan. P. Helv. Chim. Acta 1993. 76. 993-994. (28) Catalan. J.: del Valle. J . C. J . A m Chem. Soc. 1993. 115. 43214325. (29) Plotnikov, V. G.; Efimov, A. A. Usp. Khim. 1990,59, 1362-1385 (English translation in: Russ. Chem. Rev. 1990, 59, 792-806). (30) Arnaut, L. G.: Formoshino. S . J. J. Phorochem. Photobiol., A: Chem. 1993, 75, 1-20. (31) Formoshino. S . J.: Amaut, L. G. J . Phufochem. Photobiol., A: Chem. 1993, 75, 21-48. (32) Lavtchieva, L.; Enchev. V.; Smedarchina. Z. J . Phys. Chem. 1993. 97, 306-310. (33) Huston. A. L.; Scott. G. W. J . Phys. Chem. 1987. 91, 1408-1413. (34) Noukakis. D.: Suppan, P. J . Phorochetn. Photohid. 1991. 58. 393396. (35) Allan, M.: Asmis. K.: Bulliard. C.: Haselbach. E.: Suppan. P. Helv. Chim. Acta 1993, 76. 993-994. (36) Ormson, S. M.; Brown. R. G. Prog. React. Kinet. 1994, 19, 4591. (37) Forster, Th. Z. Elekrrochem. 1950, 54. 42-46. 531-535. (38) Weller. A. In Progress of Reacrion Kinerics: Porter, G., Ed.: Pereamon: London. 1961: Vol. I. LID 187-214. (139) Shizuka. H.: Matsui, K.: Oiamura. T.: Tanaka, I. J . Phys. Chem. 1975, 79, 2731-2734. (40) Shizuka, H.: Matsui, K.: Hirata, Y. K.: Tanaka. I. J . Phys. Chem. 1976, 80, 2070-2072. (41) Shizuka, H.: Matsui. K.; Hirata. Y.: Tanaka. I. J . Phys. Chem. 1977, 81, 2243-2246. (42) Shizuka, H.; Machii, M . ; Higaki, Y.: Tanaka, M.; Tanaka, I. J . Phys. Chem. 1985, 89. 320-326. (43) Bigger, S . W.; Ghiggino, K. P.: Leaver, I. H.: Scully, A. D. J . Phorochem. Photobiol.. A: Chem. 1987, 40, 391-399. (44) Kieninger, M.; Scherf, H.; Stueber. G. J.: Roessler, M.: Fischer, P.; Schmidt, A.: Kramer. H. E. A. Book of Ahsfracrs. 34th International IUPAC Symposium on Macromolecules. Prague. July 13- 18, 1992: Contribution No. 8. 4.
Ultraviolet Stabilizers (45) Stueber, G. J. Doctoral thesis, University of Stuttgart, 1994. (46) Stewart, J. M.; Dickinson, C.; Ammon, H. L.; Heck, H. S.; Flack, H. X-RAY System-Version of 1976. Technical Report TR-446, Computer Science Center, University of Maryland, College Park, MD, 1976. (47) Sheldrick, G. Program SHELXS-86; Institut fur Anorganische Chemie der Universitat Gottingen: Gottingen, Germany, 1986. (48) Damiani, A.; Giglio, E.; Ripamonti, A. Acta Crystallogr. 1965, 19, 161-168. (49) Lindeman, S. V.; Shklover, V. E.; Strutzkov, J. T.; Mitina, L. M.; Pankratov, V. A. Zh. Strukt. Khim. 1984, 25, 180-191. (50) Johnson, C. K. ORTEP-11. A Fortran Thermal Ellipsoid Plot Program for Crystal Structure Illustrations, Technical Report ORNL-5 138, Oak Ridge National Laboratory, Oak Ridge, TN, 1971. (51) Pemn, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals; Pergamon Press: Oxford, U.K., 1988. (52) Pestemer, M. Anleitung zum Messen von Absorptionsspektren im Ultraviolett und Sichtbaren; George Thieme Verlag: Stuttgart, Germany, 1964. (53) Halverson, F.; Hirt, R. C. J . Chem. Phys. 1951, 19, 711-718. (54) Hirt, R. C.; Halverson. F.; Schmitt, R. G. J. Chem. Phys. 1954,22, 1148-1149. (55) Mason, S. F. J . Chem. SOC. 1959, 1240-1246. (56) Brinen, J. S.; Goodman, L. J . Chem. Phys. 1959, 31, 482-487; 1961, 35, 1219-1225. (57) Gallivan, J. B.; Brinen, J. S.; Koren, J. G. J . Mol. Spectrosc. 1968, 26, 24-35. ( 5 8 ) Dorr, F. In Creation and Detection of the Excited State; Lamola, A. A,, Ed.; Marcel Dekker Inc.: New York, 1971; Vol. I, Part A, pp 53122 and references cited therein. (59) Englman, E.; Jortner, J. Molecular Physics; 1970, 18, 145-164. (60) Gustav, K.; Colditz, R. Z. Chem. 1988, 28, 309-315. (61) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczyk, A,; Cowley, D. J.; Baumann, W. Nouv. J . Chim. 1979, 3, 443-454. (62) Rettig, W. In Modern Models of Bonding and Delocalization; Molecular Structure and Energetics; Liebman, J. F., Greenberg, A,, Eds.; Verlag Chemie: New York and Weinheim, 1988; Vol. 6, pp 229-282 and references cited therein. (63) (a) Liuuert. E. Z. Elektrochem. 1957. 61. 962-975. (b) Limen, .. E. Ace. Chem: Res. 1970, 3, 74-80, (64) Mataea, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. SOC.Jon. 1955, 28, 690-691-1956, 29, 465-470.
J. Phys. Chem., Vol. 99, No. 25, 1995 10109 (65) Liptay, W. Angew. Chem. 1969, 81, 195-232. (66) Onsager, L. J . Am. Chem. SOC.1936, 58, 1486-1493. (67) The value for the OCH3 substituent was taken also for larger alkyl chains, since the reaction field of the cavity is practically not influenced by the nonpolar alkyl substituents. (68) For nonsymmetrical or structured bands, the wavenumber of the center of gravity of the band has to be taken instead of the maximum.69 The absorption maxima of all p-tolyl and 0-xylyl derivatives appear as shoulders only; consequently, the experimental uncertainty for ijmax,abs is higher. A more detailed band analysis reveals, however, that the experimental uncertainty amounts only to few nanometers. This does not influence slope and thus Ap in a AiVAf plot but only the intercepts which are not considered here. (69) Kelemen, J.; Siegrist, A. E. Chimia 1973, 27, 645. (70) Handbook of Chemistry and Physics; Weast, R. C., Ed.; The Chemical Rubber Co.: Cleveland, OH, 1971. (71) Kieninger, M. Diplomarbeit, Universitat Stuttgart, 1991. (72) Gustav, K. Private communication. (73) Keck, J. Doctoral thesis, University of Stuttgart, 1995. (74) (a) Paris, J. P.; Hirt, R. C.; Schmitt, R. G. J . Chem. Phys. 1961, 34, 1851-1852. (bj Brinen, J. S.; Koren, J. G.;Hodgson, W. G. J . Chem. Phys. 1966, 44 (8), 3095-3099. (75) Handbook of Chemistry; Lange, N. A., Forker, G. M., Eds.; McGraw-Hill Book Company, Inc.: New York, Toronto, and London, 1961. (76) Handbuch des Chemikers; Nikolski, B. P., Ed.; Verlag Technik: Berlin, 1956; Band I. (77) Reichhardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Verlagsgesellschaft: Weinheim, Germany, 1988. (78) Tabeilenbuch Chemie; Kaltofen, R., Pagels, I., Schumann, K., Ziemann, J., Eds.; Volk und Wissen: Volkseigener Verlag: Berlin, 1957. (79) Handbuch des Chemikers; Nikolski, B. P., Ed.; Verlag Technik: Berlin, 1957; Band 11. (80) Ware, W. R. In Photochemistry in Organized & Constrained Media; Ramamurthy, V., Ed.; VCH Verlagsgesellschaft: Weinheim, Germany, 1991; Chapter XIII, pp 563-602. (81) Simon, J. D. Acc. Chem. Res. 1988, 21, 128-134. (82) Nagarajan, V.; Brearly, A. M.; Kang, T.-J.; Barbara, P. F. J . Chem. Phys. 1987, 86, 3183-3196. JP950361E
