1408
J . Phys. Chem. 1987, 91, 1408-1413
Spectroscopic and Kinetic Investigations of Internally Hydrogen-6onded (Hydroxyphenyl)benzotriazoles Alan L. Hustont and Gary W. Scott*% Department of Chemistry, University of California, Riverside, Riverside, California 92521 (Received: September 5, 1985; In Final Form: October 2, 1986)
The effects of solvent and temperature, from room temperature to 10 K, on the electronic spectra and excited-state lifetimes of three (hydroxypheny1)benzotriazoles are presented. Spectral effects include a dual fluorescence with the lower energy band assigned to an excited-state, intramolecular proton-transferred tautomer. The magnitude of the Stokes shift for this band is less for molecules with more than one intramolecular hydrogen-bonding site than it is for the molecule with only one H bond. Conversely,the absorption spectra of molecules with more than one hydrogen-bondingsite show a greater solvent dependence. Activation energies for nonradiative excited-state decay rates are obtained for these molecules from the temperature dependence of the measured rates.
1. Introduction There have been a number of spectroscopic and kinetic investigations of the excied states of molecules which contain an intramolecular hydrogen-bond ( H In many of these molecules, rapid excited-state decay is accompanied by fast intramolecular proton transfer. The excited states of intramolecularly H-bonded aromatic molecules have a number of common properties: (1) The excited-state lifetime of these molecules is often short due to a rapid proton transfer followed by fast internal conversion, mitigated by low-frequency molecular torsional modes. (2) Because of this rapid internal conversion, the lower energy, excited-state, proton-transferred tautomeric form of the molecule emits with a low quantum yield. This tautomeric emission is to an elevated portion of the ground-state potential energy surface and is thus significantly Stokes-shifted from emission produced by similar molecules that do not have the structural feature of an intramolecular H bond. (3) These molecules often have high photostability due to their ability to efficiently and rapidly dissipate potentially photochemically damaging ultraviolet energy. Several molecules with intramolecular hydrogen bonds are commercially employed as polymer photostabilizers due to their strong, near-UV absorption bands; short, excited-state lifetimes; and high photostability. Two widely used photostabilizing chromophores are 2-hydroxybenzophenone and 2-(2’-hydroxypheny1)benzotriazole. Recently, the latter of these two chromophores has received a lot of a t t e n t i ~ n . ~ -We ] ~ have reported the mechanism and kinetics of excited-state decay of 2-(2’-hydroxy5’-methylphenyl) benzotriazole chromophores under widely different conditions, including low-temperature g l a s s e ~room-tem,~ perature solutions,“~’copolymer films,I3and low-temperature mixed crysta1s.l’ Recently, others have also reported studies of these molecules in low-temperature g l a s s e ~ s and ~ ~ ’single ~ crystals.’* The present paper explores the effects of solvent and temperature on the electronic spectra and excited-state decay kinetics of 2-(2’-hydroxy-5’-methylphenyl)benzotriazole(MeHPB) as well as two derivatives of the parent chromophore: 1,3-(dibenzotriazole-2’-yl)-2,4-dihydroxybenzene(2,4-DBDHB) and 1,3(dibenzotriazole-2’-yl)-2,4,6-trihydroxybenzene (2,4,6-DBTHB) (see Figure 1). The photophysical properties of these latter two derivatives were investigated and compared with those of MeHPB. Solvent effects on absorption spectra were found to be more pronounced for 2,4-DBDHB and 2,4,6-DBTHB, probably due to the additional intramolecular H bonds and extra sites for external hydrogen bonding in these molecules. The excited-state lifetimes of all three of these molecules were found to depend strongly on temperature. This study extends our previous work5 and that of ‘Present address: Optical Sciences Division, Naval Research Laboratory, Washington, DC 20375. * Author for correspondence. f NASA-ASEE Summer Faculty Research Fellow, Jet Propulsion Laboratory, 1984 and 1985.
0022-3654/87/2091-1408$01.50/0
others10s12to obtain activation energies for the excited-state nonradiative decay processes in these types of molecules.
(1) Merritt, C.; Scott, G. W.; Gupta, A.; Yavrouian, A. Chem. Phys. Leff. 1980, 69, 169.
(2) Hou, S.-Y.;Hetherington 111, W. M.; Korenowski, G. M.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 68, 282. (3) Lamola, A. A,; Sharp, L. J. J . Phys. Chem. 1966, 70, 2634. (4) Huston, A. L.; Scott, G. W.; Gupta, A. J . Chem. Phys. 1982,76,4978. ( 5 ) Huston, A. L.; Scott, G. W. Proc. SOC.Photo-opt. Instrum. Eng. 1982, 322, 215. (6) Shizuka, H.; Matsui, K.; Tanaka, I. J . Phys. Chem. 1977, 81, 2243. (7) Werner, T. J . Phys. Chem. 1979, 83, 320. (8) Klopffer, W. Adu. Photochem. 1977, 10, 311. (9) Otterstedt, J. A. J . Chem. Phys. 1973, 58, 5716. (10) Flom, S.R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488. (11) Bocian, D. F.; Huston, A. L.; Scott, G. W. J . Chem. Phys. 1983, 79, 5802. (12) Wiissner, G.; Goeller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. K. A,; Kramer, H. E. A. J . Phys. Chem. 1984, 88, 5544. (13) OConnor, D. B.; Scott, G. W.; Coulter, D. R.; Gupta, A.; Webb, S. P.; Yeh, S.W.; Clark, J. H. Chem. Phys. Lett. 1985, 121, 417. (14) Marsh, J. K.; J . Chem. SOC.1924, 125, 418. (1 5) Weller, A. Z . Elecktrochem. 1956, 60, 1144. (16) Weller, A. Prog. React. Kinet. 1961, 1 , 188. (17) Heimbrook, L. A,; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J. Chem. Phys. 1981, 75, 5201. (18) Goodman, J.; Brus, L. E. J. Am. Chem. SOC.1978, 100, 7422. (19) Sandros, K. Acta Chem. Scand., Ser. A 1976, 30, 761. (20) Smith, K. K.; Kaufmann, J . Phys. Chem. 1978.82, 2286. (21) Klopffer, W.; Kaufmann, G. J. Lumin. 1979, 20, 283. (22) Acuna, A,; Amat-Guerri, F.; Catalan, J.; Gonzalez-Tablas, F. J. Phys. Chem. 1980, 84, 629. (23) Acuna, A,; Catalan, J.; Toribio, F. J . Phys. Chem. 1981, 85, 241. (24) Lopez-Delgado, R.; Lazare, S . J . Phys. Chem. 1981, 85, 763. (25) Ford, D.; Thistlethwaite, P. J.; Wwlfe, G. J. Chem. Phys. Lett. 1980, 69, 246. (26) Catalan, J.; Toribio, F.; Acuna, A. U. J . Phys. Chem. 1982, 86, 303. (27) Catalan, J.; Fernandez-Alonso, J. I. J . Mol. Struct. 1975, 27, 59. (28) Catalan, J.; Tomas, F. Adu. Mol. Relaxation Processes 1976.8, 87. (29) Smith, K. K.; Kaufmann, K. J. J. Phys. Chem. 1981, 85, 2895. (30) Rossetti, R.; Brus, L. E. J . Chem. Phys. 1980, 73, 1547. (31) Rossetti, R.; Haddon, R. C.; Brus, L. E. J . Am. Chem. SOC.1980,102, 6913. (32) Rossetti, R.; Rayford, R.; Haddon, R. C.; Brus, L. E. J . Am. Chem. SOC.1981, 103, 4303. (33) Heimbrwk, L. A,; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J . Phys. Chem. 1983, 87, 280. (34) Mordzinski, A,; Grabowska, A. Chem. Phys. Left. 1982, 90, 122. (35) Wwlfe, G. J.; Thistlethwaite, P. J. J . Am. Chem. SOC.1981, 103, 3849.
0 1987 American Chemical Society
Internally Hydrogen-Bonded (Hydroxyphenyl) benzotriazoles
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1409
P-H\
P--H
C)
-
30,000 35,000 4Kooo E / h c (cm-’) Figure 2. Room temperature absorption spectra of MeHPB in (a) methanol, (b) dioxane, and (c) chloroform. Extinction coefficients are given in Table I. 25,000
,k--ti’
6 1
Figure 1. Molecular structures of (a) 2-(2’-hydroxy-5’-methylphenyl)benzotriazole (MeHPB), (b) 1,3-(dibenzotriazole-2’-yl)-2,4-dihydroxybenzene (2,4-DBDHB), and (c) 1,3-(dibenzotriazole-2‘-yl)-2,4,6-tri-
hydroxybenzene (2,4,6-DBTHB).
2. Experimental Techniques Commercially available 2-(2’-hydroxy-S’-methylphenyl)benzotriazole (MeHPB; Ciba-Geigy, Tinuvin P) was purified by extensive zone refining. 1,3-(Dibenzotriazole-2’-yl)-2,4-dihydroxybenzene (2,4-DBDHB) and 1,3-(dibenzotriazole-2’yl)-2,4,6-trihydroxybenzene (2,4,6-DBTHB) were synthesized and subsequently purified by recrystallization.@ All solvents used in this investigation were of spectroscopic quality. They were methanol (Mallinckrodt AR), chloroform (Mallinckrodt AR), tetrahydrofuran (THF, Mallinckrodt AR), and dioxane (Aldrich Gold Label). Room-temperature absorption spectra were obtained with a Cary 17D spectrophotometer. Solution samples were prepared to have a maximum optical1 density in the near-UV of between 1.0 and 2.0 in a 1-mm quartz cell, corresponding to concentrations of -10-3 M. Low-temperature emission studies employed a closed-cycle helium refrigerator (Air Products, Displex 202E) to cool the samples. The lowest temperatures obtained were approximately 10 K. Variable temperature operation between 10 and 200 K could be controlled to about f l K with this refrigerator. Fluorescence spectra were obtained by using the 325-nm line of a HeCd laser (Liconix, Model 4050/4055) as the excitation source. Emission was collected and focussed with a 4-cm focal length, 4-cm diameter quartz lens onto the slie of a 1.O-m scanning monochromator (Hilger-Engis, Model 1000). The dispersed emission was detected with a photomultiplier tube (Hamamatsu, R1104), producing a signal current that was amplified with a (36) Mordzifiski, A.; Grabowska, A.; Kiihnle, W.; Krbwczynski, A. Chem. Phys. Lett. 1983, 101, 291. (37) Mordziiiski, A.; Grabowska, A. J . Mol. Struct. 1984, 114, 337. (38) Mordzifiski, A.; Grabowska, A. Teuchner, K. Chem. Phys. Lett 1984, 111, 383. (39) Brackmann, U.; Ernsting, N. P.; Ouw, D.; Schmitt, K. Chem. Phys. Lett. 1984, 110, 319. (40) Li, S.; Gupta, A.; Vogl, 0. Monatsh. Chem. 1983, 114, 937.
picoammeter (Keithley, Model 414A) and recorded with a x-y recorder (Hewlett-Packard, Model 7040A). Fluoiescence excitation spectra were obtained from samples cooled to 10 K. Sample excitation was accomplished as follows: Light from a 450-W xenon lamp (Osram, XBO-450) was dispersed and wavelength-scanned with the 1.O-m monochromator specified above. A portion (-10%) of this dispersed light was directed onto a relative quantum counter which consisted of a 1-cm pathlength quartz dye cell containing a concentrated solution of Rhodamine 6G (10 g/L) optically coupled to a photomultiplier tube (Hamamatsu, 1P28). The remainder of the narrow bandwidth light ( A i = 2 A) was used to excite the sample. Sample fluorescence was isolated from the excitation light with filters (Le., Schott GG400 X 6 mm, Schott BG3 X 3 mm, and Schott FGlO X 3 mm glass filters were used when monitoring emission in the wavelength region 400-500 nm; a Schott GG455 X 3 mm glass filter was used when detecting X > 455 nm; and a Schott RG610 X 3 mm filter was used to isolate emission wavelengths greater than 610 nm). The spectrally isolated fluorescence was directed onto the photocathode of a photomultipler (Hamamatsu R1104). The sample fluorescence signal from the photomultipler was amplified electronically and divided by the amplified reference signal from the quantum counter by a ratiometer (Evans Associates, Model 4122). The output from the ratiometer was recorded with the same x-y recorder. Variable-temperature kinetics experiments used a short-pulse nitrogen laser (A = 337 nm, AT 400 ps) for the excitation source.41 Samples were cooled in the closed cycle helium refrigerator described above. Fluorescence wavelength regions were isolated as described above. Broad-band emission was detected by a fast biplanar photodiode (ITT-F4000) and displayed on a Tektronix 7904 oscilloscope with a 7A19 vertical amplifier and a 7B85 time base. Oscilloscope traces were photographed, enlarged, and digitized with a microcomputer-plotter combination (HP-85 computer and HP-7470A plotter). The fluorescence lifetimes were obtained by deconvolution of the fluorescence kinetics traces from the detection system response function with simultaneous least-squares fitting to an assumed exponential
-
(41) Scott, G. W.; Shen, S. G.-2.; Cox, A. J. Reu. Sci. Instrum. 1984, 55,
358.
1410 The Journal of Physical Chemistry. Vol. 91, No. 6, 1987
Huston and Scott
TABLE I: Summary of Observed Features in the Absorption, Fluorescence, and Phosphorescence Spectra of (Hydroxypheny1)benzotriazoles wavenumber and extinction coefficients of absorption maxima wavenumber of emission maxima (room temperature) (10 K matrix), cm-l compd solvent i ) , , cm-' e,, cm-' M-' D,, cm-' e,, cm-' M-' fluorescence phosphorescence MeHPB MeOH 29850 f 100 25200 f 1000 33900 f 200 23400 f 1000 24400, 17250 21 300, 19600 dioxane 29500 f 100 17200 f 1000 33700 f 200 14800 f 1000 CHC13 29400 f 100 18700 f 1000 33300 & 200 15800 f 1000 16650 none 2,4-DBDHB
2,4,6-DBTHB
MeOH dioxane CHCl3 NaOH/MeOH THF
29850 f 29850 f 30800 f 27 000 f
100"
MeOH dioxane CHCI,
29400 & 100 29200 f 100 29 400 f 100
100
100 100
24000 f 1000 34900 f 1000 35700 f 1000
35 100 f 200 35 100 f 200
19900 f 1000 25000 f 1000
25000, 18900
21 280
18900
none
35 100 f 200 19100 f 1000 24400 f 1000 42 200 f 1000
35 100 f 200 35300 f 200
23500 f 1000 15400 f 1000
24 100, 18 900
none
24400 (19000)b 25000, 19000 26000 (19000)b
21 150, 9 690 21 150 21 040, 9 480
* A weak, lower energy peak was also observed at 25650 cm-l. bEstimated; fluorescence band is obscured by strong phosphorescence.
1.0 -
I
1
I
I
II
I
I
I
I I
0.5 w 0.0 -
W
0
5
1.0 -
I
m (L
3
cn
2
w
> -
-
2
0.0
w
1.0-
-J
a
0.5 -
W
I
I
I
-
I.0-
1-
20,000
,
I
I I
0.5 -
0.5 -
0.0
> -
!1z 0.0 -
I
I
25,000
30,000
I
0.0 -
I
35,000
40,000
20,000
I
25,000
I
30,000
-
I
I
35,000
I
40,000
E / h c (cm-llFigure 3. Room temperature absorption spectra of 2,4-DBDHB in (a) methanol, (b) dioxane, and (c) chloroform. Extinction coefficients are given in Table I.
E/hc (cm-l) Figure 4. Room temperature absorption spectra of 2,4,6-DBTHB in (a) methanol, (b) dioxane, and (c) chloroform. Extinction coefficients are given in Table I.
decay.5,42-46 The response function pulse was obtained by detecting a scattered laser pulse.
In chloroform, however, only a single absorption band is observed. As shown in Figure 3, absorbance of the 35 OOO-cm-' band decreases relative to the 30 000-cm-' band as the hydrogen-bonding strength of the solvent is decreased. An additional feature-a low intensity shoulder-is observed for 2,4-DBDHB in methanol at -25650 cm-'. Room-temperature absorption spectra of 2,4,6-DBTHB in methanol, dioxane, and chloroform are shown in Figure 4. Of the three compounds studied, the absorption spectra of 2,4,6DBTHB are the most sensitive to the hydrogen-bonding nature of the solvent used. In methanol, the absorbance of the 35 000cm-I band is greater than that of the 29000-cm-' band, and a weak shoulder on the red edge of the absorption band is observed. Conversely, in dioxane, the absorbance of the 29 000-cm-' band is greater than the 35000-cm-' band. As in the case of 2,4DBDHB, in chloroform, the absorption spectrum of 2,4,6-DBTB consists of a single strong absorption band centered at 29 400 cm-'. A summary of the absorption spectral data, including extinction coefficients, for MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB is presented in Table I. 3.2. Emission Spectra. The emission spectra of MeHPB in methanol and chloroform at 10 K are shown in Figure 5 . The emission spectrum in methanol consists of two widely separated
3. Results 3.1. Absorption Spectra. Room-temperature absorption spectra of MeHPB in methanol, dioxane, and chloroform are shown in Figure 2. These spectra each consist of two absorption bands with maxima at approximately 33 600 and 29 600 cm-'. The features of the absorption spectrum of MeHPB are relatively insensitive to the solvent used. Room temperature absorption spectra of 2,4-DBDHB in methanol, dioxane, and chloroform are presented in Figure 3. The spectral characteristics of this molecule are more sensitive to the solvent used. In both methanol and dioxane, the spectrum consists of two absorption bands similar to those observed for MeHPB. (42) Huston, A. L.Ph.D. Dissertation, University of California, Riverside, 1983. (43) Brady, S . S. Reu. Sci. Instrum. 1957, 28, 1021. (44) Hang, A.; Kohler, B. E.; Priestly, E. B.; Robinson, G. W. Rev. Sci. Instrum. 1969, 40, 1439. (45) Demas, J. N.; Crosby, G . A. Anal. Chem. 1970, 42, 1010. (46) Ware, W. R. In Creation and Detection of the Excited State l A , Lamola, A. A., Ed.; Marcel Dekker, New York, 1971; Chapter 5.
Internally Hydrogen-Bonded (Hydroxypheny1)benzotriazoles I
15,000
I
I
20,000
-
25,000
I
The Journal of Physical Chemistry, Vol. 91, No. 6,1987 1411
a
I
I
I
30,000
E / h c (cm-') Figure 5. Low-temperature (10 K) emission spectra of MeHPB in (a) methanol and (b) chloroform: R, red fluorescence; P, phosphorescence; B, blue fluorescence.
E/hc (cm-')Figure 7. Low-temperature (10 K) emission spectra of 2,4,6-DBTHB in (a) methanol, (b) dioxane, and (c) chloroform: G, green fluorescence; P, phosphorescence; B, blue fluorescence. at 21 250 cm-' (- 10-s lifetime), and an intense fluorescence band at 18900 cm-l. The spectrum is similar in T H F having fluorescence maxima at 24 100 and 18 900 cm-'; however, no phosphorescence was observed. In chloroform, the emission spectrum of 2,4-DBDHB is entirely contained in a fluorescence band having a maximum at 18 900 cm-I. The emission spectra of 2,4,6-DBTHB in methanol, dioxane, and chloroform are presented in Figure 7. In each solvent the spectrum consists of a fluorescence band in the 25 000-cm-' region, an intense phosphorescence progression ( T 0.5 s) between about 21 000 and 19 000 cm-', and a fluorescence band around 19 000 cm-*. A summary of the emission spectral data for MeHPB, 2,4DBDHB, and 2,4,6-DBTHB is presented in Table I. 3.3. Fluorescence Excitation Spectra. Fluorescence excitation spectra of MeHPB in methanol at 10 K are shown in Figure 8. Figure 8a shows the excitation spectrum resulting when the long wavelength fluorescence is monitored (A > 600 nm), the maximum occurring at 26650 cm-'. Figure 8b shows the excitation spectrum that results when the short wave length fluorescence band is monitored (400 nm < X < 500 nm), and the maximum in this case occurs at 29,850 cm-l. For comparison, Figure 8c shows the low-temperature absorption spectrum of MeHPB in an EPA glass. This spectrum consists of a shoulder at about 27 000 cm-l and two broad bands at 30000 and 33 000 cm-l. 3.4. Variable-Temperature Fluorescence Kinetics. Variabletemperature fluorescence kinetics measurements were obtained for MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB in a number of different solvent host matrices. The fluorescence lifetimes of MeHPB were determined for both the moderately Stokes-shifted and the highly Stokes-shifted flourescence bands by isolating these bands with the filter combinations as described in section 2. The fluorescence lifetimes and fluorescence quantum yields are relatively indpendent of temperature in the very low temperature range.5,42 As the temperature is increased, the fluorescence lifetimes and fluorescence quantum yields decrease. A summary of the fluorescence lifetimes as a function of temperature and solvent for MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB is presented in Table 11. The variation of the fluorescence lifetimes with temperature may be used to calculate activation energies for the nonradiative
-
15,000
20,000
25,000
E/hc (cm-')Figure 6. Low-temperature (10 K) emission spectra of 2,4-DBDHBin (a) methanol, (b) tetrahydrofuran, and (c) chloroform: G, green fluorescence; P, phosphorescence; B, blue fluorescence.
fluorescence bands having maxima at 24400 and 17 250 cm-'. Both of these bands show lifetimes on the order of a nanosecond (see Section 3.4). There are also phosphorescence bands ( r 0.5 s) in the spectrum at 21 300 and 19 600 cm-I. The emission spectrum in chloroform consists entirely of a fluorescence band with a single, highly Stokes-shifted maximum a t 16650 cm-l. The emission spectra of 2,4-DBDHB at 10 K in methanol, THF, and chloroform are shown in Figure 6. In methanol the emission consists of a low-intensity, broad fluorescence band (-nanosecond lifetime) at approximately 25 000 cm-', a phoshorescence band
-
1412 The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 I
a
-
Huston and Scott TABLE 11: Temperature Dependence of the Fluorescence Lifetimes of (Hydroxyphenyl)benzotriazoles molecule/emission" solvent T, K 7,ns MeHPB/Blue methanol IO 1.53 f 0.15 21 0.97 f 0.10 40 0.54 f 0.10
MeHPB/Red
methanol
60
0.32 f 0.10
10
0.96 f 0.10
21
0.70 f 0.10 0.46 f 0.10 0.17 f 0.10
40 60 2,4-DBDHB/green
chloroform
10
70 100 119 139
158 178
197 2,4-DBDHB/green
25,000
30,000
35,000
tetrahydrofuran
40,000
E / h c (crn-ll-
Figure 8. Low-temperature excitation spectra of MeHPB in methanol and absorption spectrum of MeHPB in EPA at I O K: (a) fluorescence excitation spectrum in methanol, monitoring emission at 17 250 cm-'; (b) fluorescence excitation spectrum in methanol, monitoring emission at 24 400 cm-l; (c) absorption spectrum in EPA
2,4-DBDHB/blue
methanol
10
40 99 2,4-DBDHB/green
methanol
= kr + knr + kn,(T) (1) The measured fluorescence lifetimes ranged from 2.2 to 0.5 ns. A summary of the activation energies for the temperaturedependent nonradiative decay of MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB is presented in Table 111. The fluorescence intensity of the moderately Stokes-shifted, higher energy emission in the cases of 2,4-DBDHB and 2,4,6-DBTHB was too weak to achieve adequate signal-to-noise for determination of the activation energies for these bands. The empirically obtained activation energies show a significant solvent dependence.
(47) Smith, K.
K.: Kaufmann, K.
J. J . Phys. Chem. 1977, 82, 2286.
0.98 f 0.10 0.87 f 0.10 0.76 f 0.10 1.67 f 0.17 1.55 f 0.16 1.21 f 0.12 1.10 f 0.11 0.94 f 0.10 0.91 f 0.10 0.52 f 0.10
196 10
70 99 129
f
0.24
1.78 f 0.18 1.20 f 0.12 1.16 f 0.12 0.81 f 0.10
'Blue emission was detected through filters isolating the 400-500-
nm region, green was detected through filters passing X > 455 nm, and red, through filters passing X > 610 nm. TABLE III: Activation Energies from Fluorescence Lifetimes for the Temperature-DependentNonradiative Decay of MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB in Several Solvents
4. Discussion
4.1. Electronic Spectra. The intensities of the electronic absorption bands of MeHPB, 2,4-DBDHB, and 2,4,6-DBTHB are sensitive to the hydrogen-bonding ability of the solvent. Of the solvents used in the investigation of these molecules, methanol forms the strongest hydrogen bonds and is capable of acting as a H-bond donor or as an acceptor. Dioxane and THF, both cyclic ethers, are capable of acting as a H-bond acceptors only, while chloroform would not be expected to form hydrogen bonds. The absorption spectra presented in Figures 2-4 are arranged for comparison of the relative intensities of the bands. The changes in the relative absorption strength as a function of the hydrogen bonding ability of the solvent are most noticeable for 2,4-DBDHB and 2,4,6-DBTHB. The relative intensity of the UV band at approximately 35 000 cm-' is greater in methanol than it is in dioxane or chloroform. This may be rationalized by assuming that the lower energy UV band is due to molecules with strong intramolecular hydrogen bonds. Intermolecular hydrogen-bonding interactions such as may occur in methanol can disrupt or at least lengthen, on the average, the intramolecular hydrogen bonds leading to the observation of a decrease in the absorbance of the lower energy band.
1.02 f 0.10 1.08 f 0.11 0.90 f 0.10 0.80 f 0.10
2.36
118 138 158
chloroform
2.00 f 0.20 1.60 f 0.16
10
99
2,4,6-DBTHB/green
1.62 f 0.16 1.12 f 0.11 1.23 f 0.12 1.06 f 0.11 0.91 f 0.10 0.71 f 0.10
40 80
decay p r o c e s ~ . ~ Such calculations assume that the total rate constant is composed of contributions from a radiative decay rate, k,, a temperature-independent nonradiative rate, k,,, and a temperature-dependent nonradiative decay rate, knr(T ) , which can be expressed as follow^:^^'^^^^^^^ k t o t a ~= 1 / 7 f
13
40 50 60 70 100
2.19 f 0.22 2.09 f 0.21
n-nonane MeOH MeOH n-octane
wavenumber of fluorescence max monitored, cm-' 16 700 17 250 24 400 16700
MeHPB-d,
n-nonane
I 6 700
214 f 34
2,4-DBDHB
CHCIj THF MeOH
18 900 18 900 18900
202 f 40
molecule MeHPB
solvent
2,4,6-DBTHB CHC13 19 000 a Estimated from relative quantum yield data.
Elhe, cm-' 289 f 80 65 f 9 43 f 7 191 f 8"
50 f 15
85 f 12 102 f 38
Fluorescence excitation spectra of MeHPB obtained by monitoring either its blue or red emission show that both these emissions emanate from excitation of the broad 29 OOO-cm-' absorption band and that this band is actually the composite of two bands (see Figure 8). Both components of this band likely result from intramolecularly hydrogen-bonded forms of the molecule, as argued above. The component of this absorption band which results in a red emission is then most likely due to absorption by a planar or nearly planar conformation of the molecule with an
Internally Hydrogen-Bonded (Hydroxypheny1)benzotriazoles
H bond which is short enough for favorable excited-state proton trnasfer. The component of this absorption band which leads to blue, moderately Stokes-shifted emission probably arises from intramolecularly hydorgen-bonded molecules in more twisted conformations leading to longer intramoleculear hydrogen bonds which are not as favorable for excited-state proton transfer. Molecular orbital calculations give added support to the idea of a conformational dependence of the excited-state proton-transfer reaction for M ~ H P B . " S ~ ~ The higher energy, 33 900-cm-' absorption band of MeHPB probably does not require the presence of an intramolecular hydrogen bond in the m o l e c ~ l e . ~For ~ ~ ~MeHPB * in methanol, absorption into this higher energy UV band does not seem lead to strong emission (see Figure 8). However, very recent results for MeHPB in a low-temperature hydrocarbon matrix show that this band is reproduced in the excitation spectrum.53 Whether this difference in these results is due to the solvent difference or due to our limited instrumental sensitivity for obtaining excitation spectra at energies above 30000 cm-' is unknown. The electronic absorption spectra of both 2,4-DBDHB and 2,4,6-DBTHB in methanol show low-intensity absorption shoulders at 25 600 cm-l (see Figures 3a and 4a). When N a O H is added to the methanol solvent, this shoulder becomes the dominant spectral feature for 2,4-DBDHB, completely replacing the 30000-cm-' band.@ Such behavior is indicative of the absorption of an ionic species such as that observed for methyl salicylate and related molecules in basic s o l ~ t i o n s . ~ ~ , ~ ~ , ~ ~ All three of the molecules studied exhibit a highly StokesShifted fluorescence emission associated with excited-state proton transfer. The Stokes shift is somewhat smaller for both 2,4DBDHB and 2,4,6-DBTHB than it is for MeHPB. The observation of this smaller Stokes shift means that the energy gap between the excited state and the ground state of the proton transferred forms must be greater for the two larger molecules than it is for MeHPB. This situation would be obtained if the excited state were less stabilized by proton transfer, the ground state less destabilized by proton transfer, or a combination of these two possibilities. The observation of ionic absorption by 2,4DBDHB and 2,4,6-DBTHB in methanol suggests that these two molecules are better able to stabilize a charged species in the ground state than is MeHPB. Thus, a proton-transferred ground-state form is probably more stable in the two larger molecules than it is in MeHPB. The emission spectrum of 2,4,6-DBTHB in all solvents exhibits a large component of phosphorescence as identified by its long lifetime. The relative intensity of the moderately Stokes-shifted emission of 2,4,6-DBTHB is less in the polar solvent, methanol, than it is in dioxane or chloroform. These changes in the emission characteristics are indicative of changes in the excited-state electronic distributions of 2,4,6-DBTHB relative to 2,4-DBDHB and MeHPB. The increase in the phosphorescence intensity suggests that intersystem crossing, probably from the bluefluorescing form of the molecule, becomes a more favorably competitive deactivation process for the trihydroxy compound. The increased rate of intersystem crossing could be the result of a shift of the energy of the excited-state mr* levels resulting from the hydroxy substitutions on the aromatic ring.50 4.2. Variable-Temperature Fluorescence Kinetics. The fluorescence lifetime of the moderately Stokes-shifted emission is measurably different from the lifetime of the highly Stokes(48) Klopffer, W.; Naundorf, G. J . Lumin. 1974, 8, 457. (49) Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Lett. 1979, 63, 401. (50) Beckett, A.; Porter, G. Trans. Faraday Soc. 1963, 59, 2051.
The Journal of Physical Chemistry, Vol. 91, No. 6, 1987 1413 shifted emission for all three of these molecules. This fact indicates that these two emissions result from two different excited-state species which are not in equilibrium. This is consistent with the observation that the excitation spectrum of MeHPB in methanol shows that the moderately and highly Stokes-shifted emissions result from excitation of different ground-state species. The activation energies for nonradiative decay of MeHPB and 2,4-DBDHB are similar; in nonhydrogen bonding solvents, they are approximately 200 cm-', whereas in hydrogen-bonding solvents the activation energies are decreased by a factor of 3 or 4 (see Table 111). The measured activation energies for the (hydroxypheny1)benzotriazoles are quite low when compared to those measured for other internally hydrogen-bonded stabilizers such as the salicylates.*o This indicates the importance of the additional competitive nonradiative decay route involving torsional motions about the C-N bond. However, activation energies for nonradiative decay of the blue- and red-emitting species of MeHPB in methanol are quite similar. The decrease in activation energy that occurs in hydrogen-bonding solvents suggests that intermolecular hydrogen-bonding interactions modify the nonradiative deactivation pathway. Intermolecular hydrogen-bonding vibrational modes have been observed for the phenol-dioxane pair.s1 These modes consist of an 0-H. -0stretching vibration at 127 cm-' and other bending and torsional modes at lower frequencies. The decrease in the activation energy observed for MeHPB and 2,4-DBDHB in hydrogen-bonding solvents may be the result of an increase in the density of low-frequency vibronic states resulting from such intermolecular hydrogen-bonding interactions. The activation energy obtained for 2,4,6-DBTHB in chloroform is about half that of 2,4-DBDHB or MeHPB in nonhydrogenbonding solvents. As mentioned above, the emission characteristics of 2,4,6-DBTHB are also different from those of 2,4-DBDHB and MeHPB. Differences in the emission characteristics may be due to an excited-state electronic distribution which does not promote excited-state proton transfer as a means of rapid excited-state deactivation. The observation of a high-intensity phosphorescence suggests that triplet states become a more important decay channel in the electronic deactivation of 2,4,6-DBTHB than in the other molecules. The evidence therefore suggests that the lowest lying electronic state of 2,4,6-DBTHB may have a significant admixture of n r * character.
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Acknowledgment. We thank Dr. Amitava Gupta of the Jet Propulsion Laboratory for helpful suggestions and encouragement on this project. We also thank Professor Otto Vogl of Polytechnic Institute of New York for supplying the 2,4-DBDHB and 2,4,6-DBTHB used in these studies. We acknowledge the assistance of Mr. Don OConnor in obtaining the room-temperature absorption spectra of the molecules. This research was supported in part by a grant from the Universitywide Energy Research Group of the University of California. Additional support was provided by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Registry No. MeHPB, 2440-22-4; 2,4-DBDHB, 885 18-29-0; 2,4,6DBTHB, 88518-31-4. (51) Abe, H.; Mikani, N.; Udagawa, Y . Chem. Phys. Lett. 1982, 93, 217. (52) Huston, A. L.; Merritt, C. D.; Scott, G . W.; Gupta, A. In Picosecond Phenomena II, Hochstrasser, R. M., Kaiser, W., Shank, C. V., Ed.; Springer-Verlag: New York, 1980; p 232. (53) Woessner, G.;Goeller, G.; Rieker, J.; Hoier, H.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Kramer, H. E. A. J . Phys. Chem. 1985, 89, 3629.
