701
J. Phys. Chem. 1992,96,701-710
electronic absorption spectra of the products, and this would account for the &crease in magnitudeof the product absorption cross-sections from reactions 5 and 6. These results suggest that, in an atmospheric context, CH2SH will be removed by reaction with 02.This is in contrast to the low reactivity of CH3Swith respect to oxygen where the possibility of significant reaction with N O and NO2 must be considered. Similar arguments apply in the case of hydroxymethyl and methoxy radicals; hydroxymethyl radicals are removed exclusively by reaction with oxygen, whereas reaction of CH30 with NO and NO2 may be significant, particularly in areas of high NO, concentrations. However, the detailed chemistry of CH2SH and CH20H appear to be very different. Hydroxymethyl radicals will yield H 0 2 and formaldehyde via reaction 15b, whereas CH2SH will yield a variety of oxidized products, following initial addition of oxygen:
CHzSH
+0 2
-
+ NO HSCH20 + 02
HSCHzOz
HSCH20
+
+
HSCH202
HSCH2O
HSCHO
HS
+ NO2
+ HOz
+ CHI0
(4a) (17) (18) (19)
A likely intermediate is the HS radical which will be oxidized to SO2;however, other oxygenated sulfur species may be formed. Acknowledgment. We would like to thank Mrs. Jette Munk of Rista National Laboratories for her valuable assistance in conducting the experiments. We gratefully acknowledge the support of the SERC and National Power PLC for the award of a CASE studentship for M.B. Registry NO. CHISH, 17032-46-1; 02,7782-44-7; NO, 10102-43-9; N02, 10102-44-0.
Photophysical Properties of Styryl Derivatives of Aminobenzoxazinones S. Fery-Forgues: M. T. Le Bris,? J.-C. Mialocq,' J. Pouget,+W. Rettig,o and B. Valeur*vt Laboratoire de Chimie GEngrale (CNRS URA 1103). Conservatoire National des Arts et Mgtiers, 292 rue Saint-Martin, F-75003 Paris, France, CEAICE-Saclay/SCM/CNRS URA 331, F-91191 Gif-sur Yvette, France, and Iwan-N.-Stranski-Institut f i r Physikalische und Theoretische Chemie, Strasse des I7 Juni 112, 0-1000 Berlin 12, Germany (Received: February 25, 1991; In Final Form: August 6, 1991)
The spectra, fluorescence quantum yields, and lifetimes of styryl derivatves of (dimethy1amino)benzoxazinone in various solvents are reported. The results show that the main chromophoreis the aminobenzoxazinone moiety and the para-substituted styryl moiety is a substituent as a whole. Therefore, these compounds do not behave as stilbene derivatives in spite of analogy in chemical structure. The effect of the substituent in the para position of the styryl moiety has been carefully examined with the help of quantum mechanical calculations by PPP and CNDO/S methods. The possibilities of internal rotation leading in particular to twisted intramolecular charge transfer (TICT) states are discussed.
SCHEME I
Introduction
Photophysics of fluorescent dyes is a field of constant interest because a better understanding of the excited-state properties helps
in the design of new molecules offering the best performances for a given application: laser dyes, probes for polymeric, micellar and biological systems, molecules for nonlinear optics, for molecular devices, etc. Generally, each class of dyes has its own characteristics, e.g., ranges of absorption and emission wavelengths. Coumarins, rhodamines, and oxazines, are well-known families. Aminobenzoxazinone (1) derivatives, a class of dyes mainly developed by some of us,14 exhibit very different properties ac-
R.
--cHo
DFSBO
R-
-H
802-H
1
cording to the nature of the substituent R2, but the common feature of these dyes is the presence of electron donor (amino group) and electron acceptor (carbonyl group and heterocyclic nitrogen of oxazinone) moieties which leads to an intramolecular charge transfer upon excitation that results in a large increase of the dipole moment and hence a strong solvatochromiceffect. Rullitre et al.'** investigated the photophysics of some of these compounds on the picosecond scale to study solvation dynamics at a molecular level. The laser properties of these compounds were also ~ t u d i e d . ~ . ~ *Towhom correspondence should be addressed. 'rCEA/CE-Saclay. Conservatoire National des Arts et MQiers. Iwan-NStranski-Institut.
0022-365419212096-701$03.00/0
U
Among aminobenzoxazinones, the styryl derivatives (Scheme I) are of particular interest. Special attention was first paid to ( 1 ) (a) Le Bris, M. T.J . Hererocycl. Chem. 1984, 21, 551. (b) Le Bris, M.T.Ibid. 1985, 22, 1275. (c) Le Bris, M. T.Ibid. 1989, 26, 429. (2) Le Bris, M. T.;Mugnicr, J.; Bourson, J.; Valeur, E. Chem. Phys. Le??. 1984,106, 124.
0 1992 American Chemical Society
Fery-Forgues et al.
702 The Journal of Physical Chemisfry, Vol. 96, No. 2, 1992 TABLE I: Photophysical Characteristics of BOZ-NMe2 no. 1
2 3 4 5 6 7 8 9 10 11
12 13 14
solvent n-heptane cvclohexane toluene dioxane chloroform ethyl acetate methylene chloride dimethyl sulfoxide dimethylformamide acetone acetonitrile glycerol propylene glycol ethanol
xR
Af
T*
50.90 50.00 47.20 48.40 44.20 47.20 44.90 42.00 43.70 45.70 45.70
0.000 0.000 0.0 16 0.020 0.148 0.200 0.219 0.267 0.275 0.285 0.304 0.264 0.270 0.288
-0.081 0.000 0.535 0.553 0.760 0.545 0.802 1.ooo 0.875 0.683 0.713 0.730 0.930 0.540
nm 465.5 467.5 485.5 478.5 497 482.5 495 503 498 488 489 512.5 502 494
A,,
AF1(l nm
519 521 567 581 627 587 619 666 650 626 650 659 649.5 666
Stokes shift, cm-' 2215 2195 2960 3685 4170 3690 4050 4865 4695 4515 5065 4335 4525 5230
kr,
knr,
P
(T), ns
lo8 s-'
lo8 s-'
0.56 0.57 0.57 0.59 0.52 0.52 0.55 0.04 0.22 0.36 0.21 0.019 0.024 0.016
1.95b 2.04b 2.35b 2.75b 2.78b 2.74b 3.056 0.62b 1.51b 2.40b 1.60b 0.45E 0.13' 0.08c
2.9 2.8 2.4 2.1, 1.9 1.9 1.8 0.65 1.45 1.5 1.3 0.4 1.8 2.0
2.3 2.3 1.8 1.5 1.7 I .75 1.5 16 5.2 2.7 4.9 22 77 125
a&xc = 480 nm. bMeasured by multifrequency phase fluorometry (A,, = 488 nm). CMeasured by time-correlated single photon counting technique (&, = 500 nm): the decay is biexponential with a predominant short component r1 5 0.2 ns and a weak second component ( T =~= 3-4 ns); the mean lifetime is calculated according to ( 7 ) = ( A 1 r l+ &*)/(AI + A*).
DFSBO (7-dimethylamino)-3-(p-formylstyryl)1,4-benzoxazin2-one) because of its interesting spectral characteristics,2and a study of this compound in parallel with the well-known laser dye DCM has been recently carried out.I0 Successful applications of DFSBO, BVC (p-[~-(7-(dimethylamino)-l,4-benzoxazin-2one-3-yl)vinyl]phenylpropenoicacid), and its kanamycin derivatives as fluorescent probes in molecular and cellular biology have been d e ~ c r i b e d . ~ , ~ Recently, a new compound, BOZ-crown, in which the substituent R is monoazaz-15-crown-5 ether, was prepared and the effects of cation binding on the photophysical properties were studied.]] In addition to the capabilities of recognition of alkaline-earth-metal ions, this compound offers the possibility of tuning the electron-donor character of the crown nitrogen atom which is involved in the conjugation of the molecule. In our investigation, comparison was made with the "uncrowned" compound BOZ-H, and the compound in which the azacrown is replaced by a dimethylamino group (BOZ-NMe2). The aim of the present paper is to further examine the photophysical properties of styryl derivatives of aminobenzoxazinones. Spectroscopic data on DFSBO are already available.2.10Attention will thus be focused on BOZ-NMe,, BOZ-H, and BOZ-Jul. The latter is a new compound in which the nitrogen atom is blocked in a julolidyl ring thus preventing the rotation of the terminal amino group. This rotation was indeed anticipated to be partly responsible of the observed properties of BOZ-crown and BOZNMe2.Ilb More generally, in styryl compounds and in particular in stilbene-like molecules, photoinduced internal rotations (leading for instance to twisted internal charge-transfer states (TICT)) are known to play an important role.I2 Experimental Section Materials. Solvents from Merck, Prolabo, and Janssen (spectrometric grade) were used as received for absorption and fluorescence measurements. Tripropionin (glycerol tripropionate) was purchased from T.C.I. and redistilled prior to use. (3) Dupuy, F.; Rullitre, C.; Le Bris, M. T.; Valeur, B. Opt. Commun. 1984,
51, 36.
(4) Mugnier, J.; Dordet, Y.; Pouget, J.; Le Bris, M. T.; Valeur, B. Solar Energy Mater. 1987, 15, 65. ( 5 ) Monsigny, M.; Midoux, P.; Le Bris, M. T.; Roche, A. C.; Valeur, B. Biol. Cell. 1989, 67. (6) Depierreux, C.; Le Bris, M. T.; Michel, M. F.; Valeur, B.; Monsigny, M.; Delmotte, F. FEMS Microbiol. Lett. 1990, 67, 237. (7) (a) Decltmy, A,; RulliEre, C.; Kottis, P. Chem. Phys. Letr. 1983, 101, 401. ( b ) Decltmy, A.; RulliEre, C.; Kottis, P. Ibid. 1987, 133, 448. (8) DeclEmy, A.; RuMre, C. Chem. Phys. Lett. 1988, 146, 1. (9) Khochkina, 0. I.; Sokolova, I. V.; Loboda, L. I. fzu. Vyssh. Uchebny. Zaved., Fis. 1988, 6 , 98. (10) Mialocq, J. C.; Meyer, M. Laser Chem. 1990, 10, 277. ( 1 1 ) Fery-Forgues, S.;Le Bris, M. T.; GuettC, J. P.; Valeur, B. (a) J. Chem. Soc. Chem. Commun. 1985, 5, 384. (b) J. Phys. Chem. 1988, 92, 6233. (12) Rettig. W.;Majenz, W. Chem. Phys. Lett. 1989, 154, 335.
The syntheses of BOZ-NMe2 and BOZ-H were previously described.Ib BOZ-Jul (3-(9-julolidylvinylene)-8H,12H6,7,10,1l-tetrahydroquinolizino[g,h]-l,4-benzoxazin-2-one) was prepared in the following way. 7-(N,N-Dimethylamino)-3methyl- 1,4-benzoxazin-2-0ne~~ (2.0 g, 10 mmol), was suspended in acetic anhydride (2 mL) and reacted with 9-formyljuloiidine (2.0 g, 10 mmol) prepared according to the procedure described by Benington et al.I3 The mixture was heated for 5 h at 140 OC in an oil bath and, after cooling, stirred with ethanol for 1 h, and then filtered. Dark green needles (OS g, yield 13%) were recrystallized from diluted pyridine (2/ 1) to yield dark reflecting crystals with bronze colour: IR (KBr disk) v (an-I): 2920,2840, 2810 (CH,), 1720 (C=O), 1620-1580 (C=N, C=C); RMN (CDClJ: 7.81 (d, H 9, Jg,lo = 16 Hz), 7.51 (d, H 5, 55.6 = 9 Hz), 7.28(d,H10),7.11(~,Hll+H18),6.67(d-dH6,J6,8=2.7 Hz), 6.44 (d, H 8), 3.21 (t, CH2 14 15, J13.14 = 6 Hz), 3.07 (s, N(CH3),), 2.76 (t, CH2 12 17, J12,13 = 6 Hz), 1.99 (quintuplet, CHI 13 16). Anal. Calcd for C24HZSN302: C, 74.42; H, 6.46; N, 10.85. Found C, 74.19; H, 6.39; N, 10.93. Apparatus and Methods. Absorption spectra were recorded on a Uvikon 820 spectrophotometer. The emission spectra were obtained on an Aminco SPF 500 spectrofluorimeter interfaced to a Kontron PSI 80 microcomputer for storage, correction, and data analysis. Standard procedures (calibrated tungsten lamp) were used for correction of the emission spectra. The fluorescence quantum yields were measured by using coumarin 6 in ethanol (aF= 0.78)14 as a standard. The experimental error is 5-10%. Most of the fluorescence lifetimes were measured on our multifrequency phase modulation fluorometer equipped with an argon ion laser and a Pockels cell operating at frequencies ranging from 0.1 to 200 MHz.IS In the case of low fluorescence quantum yields, the fluorescence lifetimes were determined with the timecorrelated single photon counting technique, the source of light pulses being the synchrotron radiation from the Berlin electron storage ring BESSY operating in the single bunch mode.I6 The nanosecond laser absorption spectroscopy setup has already been described.I0J7 The styryl derivatives were excited at 532 nm with a 3-11s pulse in a 1 cm thick cell. The transient absorption variations were analyzed perpendicularly to the excitation and the optical pathlength was 1 cm. Freshly prepared solutions in methanol (Merck Uvasol for spectroscopy) or chloroform (Merck pro analysi) were deaerated by bubbling with argon. The optical density of the solutions at the excitation wavelength was 0.3. Combinations of band-pass filters were inserted between the xenon
+
+
+
(13) Benington, F.; Morin, R. D.; Clark, L. C. J . Org. Chem. 1956, 21,
2470.
(14) Reynolds, G. A,; Drexhage, K. H. Opt. Commun. 1975, 13, 222. (1 5 ) Pouget. J.; Mugnier, J.; Valeur, B. J. Phys. E Sei. fnstrum. 1989,
22, 855.
(16) Vogel, M.; Rettig, W .Ber. Bunsen-Ges.Phys. Chem. 1987, 91, 1241. (17) Mialocq, J. C. Chem. Phys. 1982, 73, 107.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 703
Photophysical Properties of Benzoxazinones TABLE 11: Photophysical Characteristics of BOZ-Jul
no.
solvent n-heptane
1 2 3 4 5 6 7 8 9 10 11 14
cyclohexane toluene dioxane
chloroform ethyl acetate methylene chloride dimethyl sulfoxide dimethylformamide acetone
acetonitrile ethanol
A,, nm 483 487 505.5 495.5 517 501 514.5 522.5 516 507 507.5 513
XF,II nm 5 40 543 596 61 1 67 1 628 678 715 704 679 709 705
Stokes
shift, cm-l 2185 2115 3005 3815 4440 4035 4685 5150 5175 4995 5600 5310
0.32 0.37 0.55 0.56 0.20 0.39 0.19 0.030 0.036 0.08 0.023 0.008
kr,
ns
knr,
2.02b
108 s-' 1.5
108 s-1 3.35
2.60b 2.90b 1.35b 2.40b 1.35b 0.32c 0.18c O.8Ob 0.22c CO.1
2.1 1.9 1.5 1.65 1.4 0.9 2.0 1 .o 1 .o >0.8
1.75 1.5 5.9 2.55 6.0 31 56 11.5 45 >loo
(T),
@a
a&xc = 480 nm. bMeasured by multifrequency phase fluorometry (hac = 488 nm). cMea~uredby time-correlated single photon counting technique (hac = 510 nm): the decay is biexponential with a predominant short component T , I0.3 ns and a weak second component ( T = ~ 2-4 ns).
arc lamp and the cuvette to avoid photolysis of the sample prior to the laser pulse excitation. Unless otherwise stated, the experiments were performed at 25 "C with freshly prepared dilute solutions to 5 X 10" M). The solutions were not degassed. The styryl compounds under study are in the trans conformation, as confirmed by NMR spectra. It should be noted that the julolidyl derivative was found to be more sensitive to light than the other compounds; therefore, the samples were handled in inactive glassware to prevent photoreaction. ReSultS Absorption and Fluorescence Spectra. The absorption spectra of BOZ-NMe2 undergo a significant red shift when increasing the polarity of the solvent (Table I). This shift can be analyzed in relation to various solvatochromic scales. We have chosen the scale introduced by Kamlet et al.18919 As compared to other empirical scales such as ET:o that scale offers the advantage of being established with many different chromophores which enabled the separation of hydrogen-bonding effects. The benzoxazinone derivatives considered in this work are not hydrogen-bond donors but only hydrogen-bond acceptors, mainly on the carbonyl groups. Thus, considering only solvents which are not hydrogen-bond donors (solvents 1-1 1 in Table I), the wavenumber (in cm-I) of the maximum absorption can be plotted as a function of the solvent polarity-polarizability parameter ?r* (the parameters a and /3 are not to be considered in this case). A least-squares analysis of the data leads to the following relation: pA X
= 21.4 - 1 . 4 9 ~ * (correlation coefficient r = 0.985)
An equally satisfactory linear correlation was also found when the scale based on the solvent parameter xR propounded by Brooker et a1.21for merocyanine VI1 was used. DA X
= 11.8 + 0.19OxR ( r = 0.986)
This scale has been recently used with success in the case of DCMZ2and in the case of the other benzoxazinone derivative DFSBO.'O The polarity of the solvent has a larger effect on the fluorescence spectrum of BOZNMe, than on the absorption spectrum (Figure 1). The red shift is quite large in polar solvents: the position of the emission maximum ranges from 519 nm in n-heptane to 666 nm in dimethyl sulfoxide and ethanol (Table I). It should be noted that the position of the emission maximum may depend on the excitation wavelength (see below); the values reported in (18) Taft, R. W.; Kamlet, M. J. J. Am. Chem. SOC.1976, 98, 2886. (19) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog.Phys. Org. Chem. 1981, 13, 485. (20) Reichardt, C. In Molecular Inreracrion; Ratajczak, H., OrvilleThomas, W. J., Eds.; Wiley: New York, 1981; Vol. 3, p 241. (21) Brooker, L. G. S.;Craig, A. C.; Heseltine, D. W.; Jenkins, P. W.; Lincoln, L. L. J. Am. Chem. Soc. 1965, 87, 2443. (22) Meyer, M.; Mialocq, J. C. Opt. Commun. 1987, 64, 264.
1
2
3
4
6
7
5
U
Figure 1. Corrected fluorescence spectra of BOZ-NMe2. The numbers indicate the solvents as in Table I. The spectra are normalized according to the quantum yield.
Table I were obtained with excitation at 480 nm. The correlation with the ?r* parameter is satisfactory: = 19.2 - 4 . 1 8 ~ * (r = 0.981) The red shift can be interpreted in terms of an increased dipole moment of the dye upon excitation, and solvent relaxation in the excited state. According to the theories of L i ~ p e rand t ~ ~Mataga,24 the Stokes shift (ijA - PF) (in cm-') should be linearly correlated with Af defined as Af=--- t - 1 n2-1 26 + 1 2n2+ 1 The slope of the straight line is given byz3 IF X
2(P, - pg)* hca3 where u is the radius (in A) of the spherical cavity in Onsager's theory of reaction field, h is Planck's constant, and c is the speed of light. From the absorption and fluorescence spectra recorded in different solvents it is then possible to calculate the difference between the dipole moment in the excited state and the ground state (re - P*). When the data of Table I for the nonprotic solvents (1 to 1 1) is used, a linear least-squares analysis of pA - BF (in cm-*)versus Af (Figure 2B) yields 10-3(D,4- DF) = 7.24Af 2.69 ( r = 0.91) With u r 9.5 A, the difference (p, - p,) is found to be 25 D. However, the emission spectra in n-heptane, cyclohexane, and to a less extent toluene are structured and are thus not likely to S =
+
(23) Lippert, E. Z . Naturforsch. 1955, loa, 541: Z. Elekrrochem. 1957, 61, 962. (24) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956,29, 465.
IO4 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
Fery-Forgues et al.
TABLE Uk Photophysical Characteristicsof BOZ-H no.
solvent n-heptane cyclohexane toluene dioxane chloroform ethyl acetate methylene chloride dimethyl sulfoxide dimethylformamide acetone acetonitrile ethanol
1 2
3 4 5 6 7
8 9 10 11 14 a&xc
= 450 nm.
b&xc
= 460 nm.
c&xc
XA, nm
XF. nm
Stokes shift, cm-I
@
436 438.5 459.5 453 467 457 467.5 475 469 462 463 466
485' 488' 523' 533' 542b 541b 547b 581b 571b 5576 5676 5746
2315 2315 2640 3315 2965 3395 3110 3840 3810 3690 3960 4035
0.74' 0.77' 0.77' 0.79' 0.8 1 0.756 0.81b 0.6 1 0.666 0.71b 0.54b 0.55b
T,C
2.80'' 2.82d 3 .w 3.52' 3.74e 3.58C 3.83' 3.58' 3.65' 3.82e 3.9Y 3.77'
108 s-I 0.95
0.80 0.75 0.60
0.50 0.70
0.50 1.10 0.95 0.75 1.15 1.20
1.8 1.85 1.35 1.45
TABLE IV Correlation with the Solvent Polarity Parameters r* a d
shift (ad) I
I
Af
or
UA
4000 0
3500
1
absorption
= SX*, cm-' UF
BOZ-NMe2 BOZ-Julolidyl BOZ-H DFSBO"
10% -1.49 -1.41 -1.74 -1.20
21.4 20.6 22.8 21.4
emission r 0.985 0.978 0.994 0.90
~o-~sr 19.2 18.4 20.4 20.9
i, 10-3(u. - uF) = a A f + 8
I
'From ref
2.
i
-E 80.
-4.18 -4.64 -3.18 -4.49
0.981 0.977 0.981 0.95 r 0.91 0.93 0.89 0.93
From ref 10.
I
0
8 2.61 2.70 2.54 1.56
a 7.33 8.96 4.39 7.43
BOZ-NMe2 BOZ-Julolidyl BOZ-H DFSBOb
I
4
2500
knr,
108 s-I 2.65 2.75 2.6 2.2 2.15 2.1 2.1 1.7
= 488 nm. dEmission filter OG-515. eEmission filter OG-590.
Stokes
3000 2500
kr,
ns
BO2
-
NMe2I
cyclohexane
1 0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 Af
Figure 2. Stokes shift of BOZ-NMe2as a function of the polarity parameter AJ (A) solvents 4-1 l of Table I; (B) solvents 1-1 l of Table I.
20. v)
1
a
F(MHz)
be charge transfer in nature. After removal of the corresponding three points, a linear least-squares analysis (Figure 2A) leads to 10-3(S, - SF) = 4.4lAf
+ 3.39
( r = 0.79)
The difference (p, - pg) is then about 20 D. It is worthwhile to compare the properties of BOZ-NMe2 with those of BOZ-Jul (Table 11) and BOZ-H (Table 111). The red shift of the emission spectra observed when the polarity of the solvent is increased is larger in BOZ-Jul than in BOZNMe2, and in both cases much larger than in BOZ-H. The presence of a donor substituent, especially in a rigid position, favors a larger charge delocalization over the molecule. It should be noted that, for both BOZ-NMe2 and BOZ-Jul, the Stokes shifts are exceptionally large (e.g.. 5600 cm-' for BOZ-Jul in acetonitrile). The parameters involved in the correlations with the solvent polarity parameters ?r* and Af are given in Table IV where the differences between BOZ-H and the other two compounds, especially regarding the emission properties, are also noted. A comparison with DFSBO is also of interest because the difference between this compound and BOZ-NMe, is the terminal group which is CHO instead of NMe2. Therefore, information on the influence of this substituent in terms of electrondonating or -withdrawing character can be obtained. The emission spectra of both compounds undergo a large red shift when the solvent polarity increases, the effect for BOZ-NMe2 being larger. Moreover, Mialocq and Meyer have recently found the following correlation for DFSBO'O 10-3(S, - SF) = 7.43Af 1.56 ( r = 0.93)
+
Eu
$3 g:
2 0
.2
O
n
o
0 O
~ P O X " " "
0
0 IJ
0
" 2EXP
Figure 3. Data of multifrequency phase-modulation fluorometry for BOZ-NMe, in cyclohexane at 25 O C . The solid line corresponh to curve fitting with two exponentials. See Table V for the results of data analysis.
which shows that the slope and, consequently, the difference (p, These fmdings are quite surprising because CHO is an electron-withdrawing substituent whereas NMe2 is a strong electron-donating substituent. This point will be examined in the Discussion. Fluorescewe Q u a o h Yield d Lifetime. As shown in Table I, the fluorescence quantum yield of BOZ-NMe, strongly depends on the solvent polarity. In low polarity solvents (1-5) or chlorinated ones, the quantum yield @ lies between 0.59 and 0.50. It decreases in highly polar solvents and collapses in DMSO and in protic solvents. In some cases,the fluorescence decay of BOZ-NMezwas found to be not strictly monoexponential (for example, the data obtained in cyclohexane are shown in Figure 3,and the results of some data analyses are reported in Table V). This point will be discussed later. Therefore, the lifetime values reported in Table I are average values in these cases. It is noted that the mean lifetime is far - p,J are similar to those obtained for BOZ-NMq.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 705
Photophysical Properties of Benzoxazinones
TABLE V Decay Panmeters of BOZ-NMe2 in Tbree Solvents (kc = 488 am)"
solvent (temp) cyclohexane (25 "C)
rI, ns
1 exp 2 exp
ethyl acetate (25 "C)
1 exp 2 exp
tripropionin ( 5 "C)
1 exp 2 exp
2.04 fO.01 2.02 f0.02 2.74 fO.01 2.72 f0.02 3.15 f0.04 3.36 fO.01
72,
ns
0.66 10.7, 1.o 10.7
fi
Q1
1.02b
1.01b 10.01
1.O2b
1.01b 10.01
XR2
0.88
0.97 10.01
%[-2,+2] 82
5.39
1.24
0.90
96
1.53
2.7
85
0.75
-0.94
100
95 0.80 f0.06
zx2
2.41
0.64
297
15
-1.14
100
'The data obtained by multifrequency phase fluorometry were analyzed by a nonlinear least-squares method. The estimated uncertainties are 0.lo for phase and 0.002 for modulation. ai are the normalized preexponential factors (Eai = l),J are the fractional intensities Cr, = air,/xa,r,),xR2 is the reduced chi-square (xR2= x2/v, where v is the total degree of freedom). Zxz = [ ( 1 / 2 ) ~ ] ' /(x2 ~ - 1). %[-2,+2] is the percentage of weighted residuals in the interval 1-2,+2]. bThese values, which should not be greater than 1, are the results of the nonlinear least squares analysis of the data. The decay is so close to a single exponential that the program fails to evaluate the fraction of the second component. Nevertheless, a significant improvement of the statistical criteria is observed with two components instead of one component.
shorter in highly polar media (0.62 ns in DMSO) and especially in protic solvents. The changes in quantum yield and lifetime of BOZ-Jul (Table 11) are comparable to those of BOZ-NMe,. On the other hand, the quantum yield of BOZ-H (Table 111) remains high whatever the nature of the solvent; only a slight decrease is observed in highly polar solvents and there is no quenching in ethanol. Moreover, the lifetime is only slightly affected by the nature of the solvent. The rate constants of radiative (k,) and nonradiative (k,,) deactivations were calculated from the quantum yield and lifetime values according to classical relations:
90-
80-
E
70-
3
5
-ID
'
60-
I-
k, = @ / T
k,, = (1 - @ ) / T
The k, and k,, values calculated show considerable scatter. Nevertheless, some clear trends can be extracted. For BOZ-NMe, (Table I) a slight and regular decrease of k, is observed on increasing the polarity of the solvent. In contrast, the nonradiative rate constant which remains steady in the first group of solvents (1-7) increases sharply with polarity. BOZ-Jul (Table 11) exhibits the same trend, whereas the values of k , for BOZ-H (Table 111) are lower and almost independent of the nature of the solvent. Therefore, the presence of a donor group in the para position o€ the styryl group is in fact responsible for an efficient nonradiative deactivation pathway in polar solvents. Moreover, a strong quenching effect is observed in alcohols for BOZNMe2and BOZJul, but not for BOZ-H or DFSBO.Ia This means that an additional pathway for deactivation may arise from hydrogen bonds formed between alcohols and the nitrogen atom of the terminal amino group of BOZ-NMe2 and BOZ-Jul. Effect of Viscosity. The viscosity dependence of the quantum yield of BOZ-NMe2 was examined in mixtures of ethyl acetate and tripropionin (glycerol tripropionate). Since these solvents are of similar chemical nature, their combination allows the viscosity to vary at constant temperature and p0larity.2~ At 25 OC,when the viscosity increases from 0.48 CP (pure ethyl acetate) to 6.9 CP(pure tripropionin), neither the quantum yield nor the average lifetime shows any significant variation. Moreover, the quantum yield obtained in glycerol (vZsoc = 964 cP) is of the same magnitude as the quantum yield determined in other protic solvents. On the other hand, in a frozen medium, a large increase in quantum yield is observed: the quantum yield of an ethanolic solution at 77 K (Le., in a glass) is about 50 times larger than at room temperature (*= 0.016); moreover, the emission spectrum is blue-shifted by 96 nm and resembles in shape the spectrum obtained in cyclohexane at room temperature. Effect of Excitation Wavelength on the Emission Spectra. Solutions of BOZ-NMe2in polar solvents like acetonitrile, DMF, and DMSO show no variation of the emission spectrum (for this (25) Valeur, B.;Monnerie, L.J . Polym. Sci., Polym. Phys. Ed. 1976, 14, 11.
E
5040-
z r
5 30. W
z
::
i2
20. 10-
500
540
5bO
650
A,nm
Figure 4. Effect of the excitation wavelength on the emission spectrum of BOZ-NMe2 in cyclohexane. TABLE VI: Variation in Emission Wavelength (am)Due to Change in Excitation Wavelength ethyl acetate tripropionin BOZ-NMe2# 8 36 BOZ-Hb 7 8 BOZ-Julolidyl" 4 17
'hXc, from 450 to 540 nm. h,,, from 400 to 520 nm. test, the excitation wavelength was varied from 450 to 540 nm for acetonitrile and DMF, and from 490 to 580 for DMSO). In contrast, significant changes are observed in cyclohexane (Figure 4): the emission band at 521 nm narrows as the excitation wavelength is increased from 450 to 510 nm, whereas the intensity of the emission band at 556 nm increases markedly. In toluene, spectra are not so well resolved, so that a broadening of the right shoulder is observed instead of the appearance of a separate band. Frozen solutions of BOZ-NMe2 in ethanol exhibit a behavior similar to that found in cyclohexane: the band at 570 nm decreases relative to the band at 615 nm, as the excitation wavelength increases from 460 to 550 nm. In addition, experiments carried out with BOZ-NMe2in ethyl acetate and in tripropionin show that viscosity has a drastic effect
706 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
Fery-Forgues et al.
BO2 - NMe2 I tripropionin
Lu
a
FIMHz)
a 0 20
7
60
40
80
100
120
140
160
200
180
400
500
600
700
nm
800
Figure 7. Differential absorption spectra obtained in a BOZ-NMe2 solution in chloroform.
glu
2
$ 2 0
3'
R
R
8 "
b Q
R
-
0
-2
"
PEXP
0
Figure 5. Data of multifrequency phase-modulation fluorometry for BOZ-NMe2 in tripropionin at 5 O C . The solid line corresponds to curve fitting with two exponentials. See Table V for the results of data analysis.
0.03c 0.021
. 'lf o
10 14
z!
3 ma
,,
- 0.01
f
-0.02
-0.03 -
-
I
I
I
s@
arb.scale
1 I 400 500 600 700 800 Figure 8. Differential absorption spectra obtained in a BOZ-Jul deaerated solution in chloroform.
I
I
-
on the change of the emission spectrum upon variation of the excitation wavelength (Table VI). In ethyl acetate, a red shift of 8 nm is observed when the excitation wavelength increases from 450 to 540 nm, whereas the emission spectrum in tripropionin undergoes a red shift of 36 nm under identical conditions. Along this line, the fluorescence decay in tripropionin is found to be more different from a single exponential (Figure 5 and Table V) than in ethyl acetate. Furthermore, in both solvents, the excitation spectrum is red-shifted when the emission wavelength monitored is increased. BOZ-Jul and BOZ-H exhibit the same general behavior, the effects being less marked in the latter. Because of these wavelength dependencies, the values of k, and k, reported in Tables 1-111 (calculated from fluorescencequantum yield and lifetime) must be considered as approximate values; nevertheless, the general trends of these values as a function of solvent polarity may be useful (see Discussion). Nanosecond Laser Absorption Studies. When excited at 532 nm, deaerated methanolic solutions of BOZ-H, BOZ-NMe,, or BOZ-Jul do not show any transient absorption or bleaching attributable to the depletion of their ground state and formation of a metastable triplet state or photoisomer. It was thus decided to undertake the study of the behavior of these compounds in chloroform solution since the styrenic DCM laser dye undergoes a very efficient trans cis photoisomerization in this solvent.26 Laser excitation at 532 nm of a BOZ-H solution in chloroform flushed with argon gave a differential absorption spectrum which presents a large absorption band between 520 and 650 nm with a maximum around 575 nm, a less intense one around 480 nm, and a small photobleaching around 505 nm (Figure 6). It must
-
(26) Meyer, M.; Mialocq, J. C.; Perly, B. J . Phys. Chem. 1990, 94, 98.
be emphasized that the BOZ-H absorption maximum in chloroform is located at 470 nm. The absorption band at 470-480 nm of the differential absorption spectrum thus indicates that the metastable species formed after laser excitation has a greater oscillator strength than the So ground state. At low laser intensity, the transient absorption disappears within 5 ms, with first-order kinetics, k = 2.9 X lo3 s-', i.e., r = 340 ps. The same behavior was found in aerated solution. The decay rate measured at 580 nm increases with the laser intensity. At very high laser intensity, excellent second-order decay kinetics was found giving a slope 2k/eSsM = (1.3 f 0.2) X IO5 cm s-I and the residual absorption was very small. Laser excitation of an aerated BOZ-NMe2 solution in chloroform gave the differential absorption spectrum presented in Figure 7. A large absorption band in the 600-800-nm region with a maximum'at 740 nm corresponds to the photobleaching observed in the 440-570-nm region with a maximum at 510 nm, Le., slightly displaced with respect to the Soground-state maximum (A, = 500 nm). Part of the absorption band and the photobleaching disappear with the same decay kinetics, independently of the presence of oxygen. At low laser intensity, first-order decay analysis gives a rate constant k = 0.78 X lo3s-', i.e., 7 = 1.3 ms. At high laser intensity, part of the 740-nm absorbance disappears with second-order decay kinetics, 2k/e740nm = 3.8 X lo4 cm s-l. After 4 ms, a great part of the red absorption band and blue-green photobleaching remain constant but the maximum of the latter is shifted from 510 to 520 nm. Moreover, a slight absorption is observed below 440 nm. It is found that excitation of the solution by a great number of laser pulses or by the continuous light of a xenon arc lamp above 490 nm (optical filter Schott 00495) induces a blue shift by ca. 5 nm of the final absorption spectrum after 2 h illumination. Laser excitation of a deaerated BOZ-Jul solution in chloroform is followed by the immediate appearance (in less than 10 ns) of a bleaching between 450 and 590 nm. Its maximum located at 525 nm at t = 10 ws, close to the absorption maximum of the So ground state (A- = 520 nm), is shifted to 530 nm at t = 2.0 ms, as shown in the differential absorption spectrum presented in Figure 8. The final bleaching remains constant for ca. 50 ms. A large absorption band between 600 and 800 nm, with a maximum a t 680 nm, was also observed. It results in fact from the
Photophysical Properties of Benzoxazinones contribution of two absorption bands whose ratio depends on the laser intensity. At low laser intensity, a precise analysis is difficult but the 660-nm absorbance decays slowly over 2 ms. At intermediate laser intensity, below 730 nm, about one-third of the absorption decays within 2.0 ms, and above 730 nm, the fast decaying component has a much smaller contribution. The same spectral features and decay kinetics were observed in aerated solution. At high laser intensity, the contribution of the fast decaying component of the 760-nm absorbance increases more than that of the longer lived component. A tentative explanation is that the short-lived species is formed via a mechanism involving an excitation by two photons probably absorbed in two steps. Anyway, the short-lived component decays with second-order kinetics, 2k/t660nm = 5.3 X lo4 cm s-' and 2k/t7" = 1.5 X lo5 cm s-l, in agreement with the ratio of the optical density variations Ad660nm/Ad?60nm = 3.6 of the fast decaying transient. Again the final absorption spectrum of the solution after excitation by a great number of laser pulses is significantly shifted to the blue. Continuous photolysis by the light of a xenon arc lamp above 500 nm (optical filter Schott OG515) or in the blue (optical filter MTO A440b) gave also a blue shift of the absorption maximum. After 2 h, for a decrease of the maximum absorbance by a factor of 2, the absorption maximum is shifted from 520 to 485 nm.
Discussion Styryl derivatives of aminobenzoxazinones can be considered either as substituted aminobenzoxazinones or as substituted stilbene-like chromophores. To clear up this important point, it is first interesting to compare the emission maximum in dimethylformamide for BOZ-NMe, (650 nm), or BOZ-Jul (704 nm), BOZ-H (571 nm), and DFSBO (594 nm). If the formyl group of DFSBO were the main electron-withdrawing group involved in the photoinduced charge transfer from the dimethylamino group, its emission maximum would be at higher wavelength than BOZ-NMe, or BOZ-Jul, and the emission maximum of BOZ-H would be intermediate. The fact that the observations are contrary to these expectations means that, in all derivatives, the charge transfer is likely to involve the carbonyl group and the heterocyclic nitrogen of the oxazinone moiety as the main electron-withdrawing groups, and the dimethylamino group substituted on the benzo ring of benzoxazinone as the main electron-donating group. It should also be noted that no photoisomerization was detected in DFSB0,'O whatever the nature of the solvent, in contrast to stilbene-like molecules. Moreover, the nonradiative rate constant k,,, for BOZ-NMe, and BOZ-Jul increases as the solvent polarity increases, whereas the reverse trend was observed for donoracceptor-type stilbene compounds like DCS (4-(dimethylamino)-4'-cyanostilbene) and the laser dye DCM.', Furthermore, the changes in photophysical properties of BOZ-crown'lband D C M - ~ r o w nupon ~ ~ cation complexation are completely different. All these observations lead us to consider the whole para-substituted styryl moiety as a substituent of the aminobenzoxazinone moiety. Along these lines, it is worthwhile to examine the changes in charge distribution upon excitation and the consequent change in dipole moments. The two extreme cases,Le., when the terminal group is electron donating (NMe, in BOZ-NMe,) or electron withdrawing (CHO in DFSBO), will be considered. A PPP (Pariser, Parr, Pople) calculation leads to the results reported in Scheme 11. Regarding DFSBO, the terminal CHO group does not undergo a significant change in charge, and the intramolecular charge transfer upon excitation mainly occurs from the dimethylamino group to the heterocyclic nitrogen atom, whereas the carbonyl group plays a minor role. The predicted increase in dipole moment upon excitation is 1 1.8 D, which is much lower than the value of 21.8 D obtained from the solvatochromicshifts and Lippert and Mataga theories.I0 In BOZ-NMq, both dimethylaminogroups act, upon excitation, as electron donors toward the heterocyclic nitrogen atom and to a lesser extent toward the carbonyl group. These crossed charge (27) Bourson, J.; Valeur, B. J . Phys. Chem. 1989, 93, 3871.
The Journal of Physical Chemistry, VoI. 96, No. 2, 1992 707
SCHEME 11: Variations in Charges upon Excitation (9, - ql) According to PPP Calculations CH,
BOZ-NMe,
0.03
0.
n .n 06
! J g = l . lD
Ie=2.4D
CH,
* o o 7 v L / A022 C H =+002 C H
005
transfers explain the small overall increase in dipole moment upon excitation ( p e - pg = 1.3 D). This value is completely different from the value of 25 D deduced from the solvatochromic shifts and Lippert and Mataga theories (see above). The discrepancies regarding (p, - pg) values mean either that the PPP calculations are not appropriate or that the observed solvatochromicshifts are due to changes of local dipole moments and not of the overall dipole moment. The latter concept is new and deserves further attention. Considering the size of the solvent molecules with respect to the quite large solute, it is indeed conceivable that the reorientation of the solvent dipoles around the excited molecule occurs locally according to the local changes in charge within the solute. An alternative explanation for the observed large excited-state dipole moment for BOZ-NMe2, in contrast to the much smaller value expected from the calculations, is to invoke adiabatic photochemistry. There is a wealth of experimental and theoretical data regarding flexible donor-acceptor molecules which support the idea that in favorable cases, the molecule can relax from a planar conformation (delocalized excited state, DE) to a twisted one with perpendicular chromophores (twisted intramolecular charge transfer state, TICT) and full charge ~eparation.~*-~l In an extension of the TICT this behavior is linked to the properties of biradicals and biradi~aloids.~~ This model explains photoinduced twisting of both single and double bonds'2*31J2s3"36 and charge separation even in planar molecules.37 If a TICT state is reached, and the energetic separation is not too small, back charge transfer emission can in principle occur, which usually shows up as a structureless red-shifted fluorescence band. Because of the high polarity of the TICT state, this band shows a pronounced solvatochromicredshift. If the transition to the TICT state is very fast, virtually all the DE fluorescence is quenched by this adiabatic photoreaction and a single fluorescence (28) Grabowski, 2. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. Nouv. J. Chim. 1979, 3, 443. (29) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (30) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel, F.; Miehl, J. A. Adv. Chem. Phys. 1987,68, 1 . (31) Rettig, W. In Modern Models of Bonding and Delocalization; Molecular Structure and Energetics, Vol. 6; Liebman, J. F., Greenberg, A., Eds.; VCH Publishers: New York, 1988; Chapter 5, p 229. (32) Bonacic-Koutecky, V.; Koutecky, J.; Michl, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 170. (33) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972,II. 92. (34) Bonacic-Koutecky, V.; Kohler, J.; Michl, J. Chem. Phys. Len. 1984, 104, 440. (35) Bonacic-Koutecky, V.; Michl, J. J . Am. Chem. Soc. 1985,107, 1765. (36) Klessinger, M.; Michl, J. Lichtabsorption und Photochemie organischer Moleknle; VCH: Weinheim, 1989. (37) Bonacic-Koutecky, V.; Schoffel, K.; Michl, J. J . Am. Chem. Soc. 1989, 111, 6140.
Fery-Forgues et al.
708 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 SCHEME Ill: Variations in Charges upon Excitation (9c- 9 J According to CNDO/S Calculations CH,
BOZ-NMe,
0 00
pg = 3.6D
402
Ke = 6.0 D
CH;
Vinyldimethylaniline to aminobenzoxazinone charge transfer; the second TICT state at 5.27 eV possesses reverse charge transfer.
DFSBO
1
TABLE VII: Energy Differences to the Ground State (eV) and Dipole Momenta (D) (in P a r e n t h ) of S, and the Lowest zr*, nr*, and TICT Excited S i t State of Planar and Perpendicularly Twisted Confowrtioas of BOZ-NMe2 (See Scheme IV) As Calculated by CNDO/S conformation S, UT* nr* TICT all planar (3.6) 2.95 (6.0) 3.91 (2.7) twisted A (4.5) 3.01 (6.4) 3.55 (5.6) 5.16 (32.1) twisted B (4.7) 3.25 (9.3) 3.60 (4.2) 4.27 (27.1) twisted C (13.8) 0.32 (10.8) twisted D (4.3) 3.55 (11.8) 3.83 (6.2) 3.28O (22.0) twisted E (4.7) 3.05 (8.9) 3.50 (3.9) 5.04 (26.0)
SCHEME IV +006/-;
‘CH=C Ool
014
1003
0, 000
Kg = 8.0 D
000
‘H
pe= 14.6 D
-
band is observed. For slower TICT state formation, or if the reverse reaction TICT DE also takes place, dual fluorescence consisting of DE and TICT band is possible. In the case of 4-(dimethylamino)-4’-cyanostilbene(DCS), the observed kinetics data are consistent with extremely rapid formation of a fluorescent TICT state through rotation around one of the single bonds connecting ethylene to dimethylaniline or benzonitrileI2whereas rotation of the terminal dimethylamino group is not important in this respect.38 In competition to this single bond twist channel, the molecule also undergoes double bond twisting leading to the biradicaloid “phantom singlet state” P* which leads to fluorescence quenching and trans/& isomerization. For DCS and similar donoracceptor stilbenoid dyes, P* is of low polarity nature, contrary to the case of unsubstituted stilbene, and the different solvent response to the P* and the TICT state determine the strong solvent polarity dependence of the fluorescence quantum yield of these dyes.I2 In selectively bridged DCS derivatives, these competing channels can be independently controlled,and tailor-made fluorescence dyes of enhanced fluorescence quantum yield can thus be developed.39 The large excited-state dipole moment measured for BOZNMe2 and BOZ-Jul finds a natural explanation if, after photoexcitation to the planar DE state, single bond twisting would lead to a highly,polar fluorescent TICT state. For this to occur, the TICT state should be low lying, comparable to or lower than the DE state. In order to get some indication of the TICT energies, quantum chemical calculations within the CNDO/S f0rmalism~9~’ were performed. In this way both u and A orbitals are included, and thus the a-a-coupling present in nonplanar molecules is treated properly. Scheme I11 contains the variations of charges upon excitation, for planar BOZ-NMe2 and DFSBO. These results are similar to the PPP results contained in Scheme I1 and substantiate the conclusion that, for DFSBO, the benzaldehyde group is very little affected and the main charge transfer occurs from the dimethylamino group to the benzoxazinone nucleus, whereas for BOZ-NMe, both dimethylamino and dimethylanilino groups on opposite ends of the molecule act as donors, and therefore the charge-transfer components upon excitation partly cancel each other. This shows up in the different dipole moment values. The trends of PPP and CNDO/S are quite similar, namely a small increase of the dipole moment for BOZ-NMe2and a large ~~
~
(38) Gruen, H.; Garner, H. 2.Naturforsch. 1983, 38a. 928. (39) Lapouyade, R.; Czeschka, K.; Majenz, W.; Rettig, W.; Gilabert, E.; Rullitre, C. To be published. (40) Del Bene, J.; Jam, H. H. J . Chem. Phys. 1968,48,1907,4050; 1968, 49, 1221; 1969.50, 1126. (41) Program no. 333 from Quantum Chemistry Program Exchange, Bloomington, IN, was used with the original parameters but in an updated version providing excited-statedipole moment. Forty-nine singly excited states were used for configuration interaction (cutoff energy 10.0 eV).
increase for DFSBO, whereas the actual values somewhat differ, mainly due to the inclusion of the local dipoles of u-type into the CNDO/S calculation. Table VI1 contains the energetic and dipole moment results for the planar and the different singly twisted conformations of BOZ-NMe, (Scheme IV). In each case, the excited states were analyzed as to their character (AT*, nr*, and TICT), and the energetically lowest one of each group is given. TICT and nu* states possess oscillator strengths which approach zero and can be distinguished by the orbitals involved in the various transitions. The oscillator strengths of the delocalized excited (DE) or the various locally excited (LE, for twisted compounds), AT* states vary between 0.4and 0.7. The distinction between AA*-DEand -LE states is normally not made (LE for both types of states) but is appropriate to indicate that the excitation is delocalized over the whole molecule for planar conformations but is localized to the subunits for twisted ones. Whereas for all cases the AT* states are situated around 3.S3.5 eV and the nr* states around 3.5-3.9 eV, the energy of the lowest TICT state strongly varies, depending on the bond twisted. The lowest energy difference between S1 and So is found for the double-bond twist (phantom-singlet state P*), but its absolute energy (with respect to the DE state) depends on the ground-state energy for this conformation (i.e., the activation energy for ground state trans/cis isomerization) which is not available with the program ~ s e d . 4 But ~ the small energy gap for this conformation is consistent with this state being a nonradiative one (photochemical funnel to the ground state). Interestingly, similar to the case of DCS,12 the dipole moment of P* is smaller than that of the ground state, and thus, within valence bond l a n g ~ a g e , ~ ~ , ~ ~ , ~ ~ P* is of dot-dot nature and Soof hole pair, contrary to the case of stilbene. For conformation D, the lowest excited state is of TICT character (vinyldimethylanilineto aminobenzoxazinonecharge transfer), with a large dipole moment in this case. Its energy is not far above that of the DE state (A** state of the planar (42) PPP and CNDO/S calculations are made in vacuum state and the energies derived are thus not appropriate for systems driven by solvent polarity. However, these calculations provide interestingtrends for the change in charge distribution upon excitation of molecules and the magnitude and direction of the subsequent solvent relaxation. (43) It is well known that m a t aemiempiricalmethods like CNDO, INDO, MINDO, or MNDO provide an incorrect dewription of ground-state twisting of single bonds. In many cases, e.g., biphenyl, the most stable conformation turns out to be the perpendicular one. This difficulty seems to be overcome with the recent CSINDO formalism,which provides a more correct description even for twisted stilbenes and cyanines (see ref 44).
Photophysical Properties of Benzoxazinones molecules), such that it can be lowered below DE in polar solvents. The TICT state of conformation B (twisted dimethylaniline group) is calculated to be 1 eV higher in energy, and the TICT states for the twisted terminal dimethylamino groups are still higher in energy. This leads to the following conclusions: (i) In addition to the phantom singlet state P* (twisted double bond C), BOZ-NMe2 possesses a low-lying TICT state (twisted single bond D connecting bemxazhone and ethylene) which could explain the large solvatochromic redshift observed. (ii) The TICT states for twisted terminal groups are higher in energy than if the twist occurs towards the center of the molecule. This is in line with the predictions for twisted cyanines4 and with qualitative consideration^'^ and substantiates the experimental fmdings for model compounds with bridged dimethylaminogroups: a bridged DCS DFSBO and other derivatives,ICand BOZ-Jul (this work) where in all cases the photophysical behavior was found not to be markedly affected with respect to the unbridged compound. (iii) In some cases,TICT states are low lying for both directions of charge transfer (conformation D), consistent with a good electron donor and acceptor property of the molecular moieties involved. The prototype case of such a behavior is 9,9'-bianthryl, where both TICT states are isoenergetic and interconvert within a few picosecond^.^^ (iv) nr* states are higher lying and do not seem to be important for fluorescence quenching. The photophysical data in Tables I and I1 are consistent with this picture: (i) The radiative rate constant k, decreases with increasing solvent polarity from around 3 X 108 s-' for BOZ-NMe, in alkanes to around 2 X lo8 s-' or less in alcohols. Normally k,, which is given by eq 1 depends on the solvent properties only indirectly, through the change of the refractive index n of the solvent and the decrease of the fluorescence energy pF with increasing solvent polarity.46 64r4 k, = -n3pF31q2 3hc3 In the solvents considered here, these factors alone would decrease
k, to about 43% for the solvent change cyclohexaneto acetonitrile, rather close to the experimentally observed value. The intrinsic transition moment M is therefore approximately solvent independent. This could indicate either that transition moments of DE and TICT states are similar or that a TICT state is populated already in cyclohexane. (ii) The nonradiative decay rate constant k,incrmarkedly with solvent polarity. Of course, the DE TICT channel is also accelerated with increasing solvent polarity, and for a proper discussion, further experiments are necessary. For instance, low-temperature lifetime measurements and picosecond timeresolved spectra are helpful, as previously demonstrated in the case of DCS.'2v39*47 Let us examine now the possible origins of the excitation wavelength dependence of the emission spectra. Similar observations have been previously reported with DFSBO'O whereas we have found no significant effect with the compound in which the styryl substituent is replaced by a methyl group (7-(dimethylamino)-fmethyl- 1,4-benzoxazinone). These observations are to be related to the fact that the fluorescence decay of B O Z N M q in cyclohexane and ethyl acetate is not strictly exponential (see above), and that the decay in tripropionin is even more different from a single exponential, consistent with a stronger dependence of the emission spectrum
-
(44) Momicchioli, F.; Baraldi, I. Chem. Phys. 1988, 123, 103. (45) Toublanc, T. B.; Fessenden, R. W.; Hitachi, A. J . Phys. Chem. 1989, 93, 2893. (46) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; pp 51, 86, 87. (47) Gilabert, E.; Lapouyade, R.; RulliZre, C. Chem.Phys. Lett. 1988, 145,
262.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 709 SCHEME V
?-to
"'"q--+ It
*I
R 0"'H
-
on the excitation wavelength. Analysis of the data in tripropionin leads to a satisfactory fit to a double-exponential decay (Figure 6 and Table V). All these observations are consistent with the presence of the two rotamers shown in Scheme V, as already observed in dia r y l e t h y l e n e ~ . The ~ ~ ~appearance of a second rotamer is favored in nonpolar solvents or in viscous solvents. This seems to indicate that one of the rotamers is stabilized by a hydrogen bond, as previously suggested for DFSBO.'O Finally, from the nanosecond laser experiments, it can be concluded that 532-nm excitation of BOZ-H, BOZ-NMe2, or BOZ-Jul in methanolic solution does not give any transient absorption attributable to a triplet state or a cis isomer, even under high laser intensity. A correct actinometry is made difficult when the exciting pulse intensity is so high that the ground state of the compound under study is significantly depleted. By comparing our present results with those obtained previously for DCMZ6and DFSB0,'O we conclude that the quantum yields of intersystem crossing to the triplet state and trans-cis photoisomerization are negligible in methanolic solution. In chloroform, laser excitation of BOZ-H, BOZ-NMe? or BOZ-Jul gives rise to the formation of two transient absorptions. Their respective contribution and the decay of the short-lived species depend on the intensity of the exciting laser pulse. Second-order decay kinetics is indeed found at high laser intensity. Moreover, dissolved oxygen in the solution plays no role. It is thus difficult to attribute the fast-decaying component of the absorption to a triplet state unless its energy level is too low for an efficient energy transfer to oxygen as for example the energy level of ,%carotene (for a review, see ref 5 5 ) . In the case of carotenoids, the quenching of triplet states by oxygen however occurs but it does not involve an energy-transfer mechanism. Alternatively, the formation of a radical may result from a fast reaction of the excited singlet state SIor from the photoexcitation of SIby a second photon leading to a highly reactive excited singlet state S,. Further experiments are planned to elucidate the reaction mechanism but the products are formed with a low quantum efficiency.
Conclusions Styryl derivatives of aminobenzoxazinones exhibit a complex photophysical behavior. From the comparison between derivatives with electron-donating or electron-withdrawing substituents in para position in the styryl group, or without substituent in this (48) Birh, J. B.; Bartocci, G.;Aloisi, G.G.;Dellonte, S.; Barigelletti, F. Chem. Phys. 1980,51, 113. (49) Fischer, E. J. Phorochem. 1981, 17, 331. (50) Fischer, E. J. Mol. Struct. 1982, 84, 219. (51) Masetti, F.; Bartocci, G.;Mazzucato, U.; Galiazzo, G. Gazz. Chim. Ital. 1982, 112, 255. (52) Baraldi, I.; Momicchioli, F.; Ponterini, G.J . Mol. Srrucr. ( T H E 0 CHEM) 1984,110, 187. (53) Bartocci, G.;Mazzucato, U.; Masetti, F.; Aloisi, G.G.Chem. Phys. 1986, 101, 461. (54) (a) Saltiel, J.; Sears,D. F., Jr.; Mallory, F. B.; Mallory, C. W.; Buser, C. A. J . Am. Chem. Soc. 1986,108, 1688. (b) Sun, Y . P.; Sears, D. F., Jr.; Saltiel, J.; Mallory, F. B.; Mallory, C. W.; Buser, C. A. J . Am. Chem. Soc. 1988, 1 IO, 6974. (55) Bensasson, R. V.; Land, E. J.; Truscott, T. G.Flash Photolysis and Pulse Radiolysis; Pergamon Press: Oxford, U.K., 1983; p 70.
710
J . Phys. Chem. 1992, 96, 710-715
position, it can be concluded that these compounds are not stilbene-like but that the para-substituted styryl moiety acts as an integral substituent. The observed solvatochromic shift is very large for BOZ-NMe2 and BOZ-Jul, although the increases in dipole moments upon excitation predicted by PPP and CNDO/S calculations are small. This can be explained in terms of changes in local dipole moments inducing local relaxation of solvent molecules, instead of changes in overall dipole moment. A more likely alternative explanation is the existence of a highly polar fluorescent TICT state formed by single bond twisting upon excitation. In fact, CNDO/S calculations show that BOZ-NMe2 possesses a low-lying TICT state (twisted single bond connecting benzoxazinone and ethylene). The existence of two rotamers is likely to be responsible for the excitation wavelength dependence of the emission spectra in some solvents, consistent with a more or less pronounced departure of the fluorescence decay from a single exponential. If these two
rotamers lead to the same TICT state predicted by the theory, no excitation wavelength dependence of the emission spectra would be observed. Therefore, additional radiative deactivation channels (different for the two rotamers) may be the origin of the wavelength dependence. It is interesting to note that the fluorescence quantum yield of BOZNMez is high in nonpolar solvents and low in polar solvents. Therefore, this molecule appears to be of potential use as a probe of hydrophobic regions of biological samples of surfactant assemblies. Acknowledgment. We thank Dr. C. Rulliere for helpful discussions and for performing the PPP calculations. Registry No. DFSBO, 90422-12-1;BVC, 126959-78-2;BOZ-crown, 114880-42-1;BOZ-H, 137334-95-3; BOZ-NMe2, 113501-50-1;BOZJul, 137334-96-4;7-(N,N-dimethylamino)-3-methyl1,4-benzoxazin-2one, 925 10-33-3; 9-formyljulolidine, 33985-71-6.
Electron Transfer in Linked Vlologen-Quinone Molecules: Rate Constant Enhancement with Increased Chain Length A. M. Brun,? S . M. Hubig,*.+M. A. J. Rodgem,*.*and W. H. Wades Chemistry Department, University of Texas at Austin, Austin, Texas 78712, Center for Fast Kinetics Research, University of Texas at Austin, Austin, Texas 78712, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 (Received: February 28, 1991; In Final Form: September 25, 1991)
We synthesized a homologous series of molecules (MVnnQ) where a methylviologen (MV) and a aminochloronaphthoquinone (Q) are linked to each other via a flexiblechain. Using the electron pulse radiolysis technique, we have measured timeresolved spectra and rate constants for intra- and intermolecular electron transfer between donor and acceptor site of the MVnnQ molecules in water and in SDS micellar solution. For comparison, we also irradiated a solution containing a 1:l mixture of methylviologen and aminochloronaphthoquinoneand measured spectra and intermolecular ET reactions between the separated electron donor and acceptor molecules. The intramolecular electron transfer rate constants of all MVnnQ molecules were surprisingly low both in water and in aqueous SDS micellar suspensions. The intramolecular rate constants measured in water increase with increasing number of intervening bonds, leading to the conclusion that electron transfer occurs by a through-space rather than through-bond mechanism. The intramolecular rate constants virtually lose their chain length dependence in SDS suspensions where because of an extended codiguration of the micellii MVnnQ molecules through-space interaction is not favored.
Introduction The movement of an electron from one molecule (or part of one molecule) to another has been the subject of intense experimental and theoretical study in recent years.' On the experimental side important advances were made by groups who synthetically prepared new molecular systems in which donor and acceptor residues were separated from each other by rigid spacer groups, thereby removing the distractions created by diffusional uncertainties for intermolecular electron transfer (ET) reactions. Strategies such as this have enabled the characterization of long distance (i.e., noncollisional) ET reactions in liquid-phase systems. Synthetic approaches have been to covalently link the electron donor-acceptor couples at opposite ends of molecular frameworks provided by steroids,2 cyclic alkanes,3 polybicyclic alkanes? oligopeptides of p r ~ l i n ea, ~cyclic amino acid, and covalently-linked porphyrin/quinone molecules.6 Another approach has been to covalently modify a redox protein at a peripheral site with an electron-donating residue. Here, the heme moiety in, e.g., cyto*Authors to whom correspondence should be addressed. 'Center for Fast Kinetics Research, University of Texas. *Bowling Green State University. Chemistry Department, University of Texas.
chrome c becomes the acceptor entity.' Others have used electrostatically driven self-association to form complexes between (1) Several excellent reviews exist; some of these are: (a) Marcus, R. A,; Sutin, N. Biochim. Biophy?ys.Acta 1985, 811, 265-322. (b) Closs, G. L.; Miller, J. R. Science 1988,240, -7. (c) McLendon,G. Acc. Chem. Res. 1988, 21, 160-167. (d) Cusanovich, M. A.; Hazzard, J. T.; Meyer, T. E.; Tollin, G. J. Macrobiol. Sci.-Chem. 1989, A26(203),433-443. (e) Newton,
M. D.; Sutin, N. Annu. Reu. Phys. Chem. 1984, 35,437-480. (f) Khairutdinov, R. F.;Brickenstein, E . U . Photochem. Photobiol. 1986, 43, 339-356. (g) Sykes, A. G . Chem. Soc. Rev. 1985, 283. (2) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. SOC. 1983,105,670. (b) Clw, G. L.; Piotrowiak, P.;Maclnnis, J. M.; Fleming, G. R. J. Am. Chem. Soc. 1988, 110, 2652-2653. (c) Olivier, A. M.; Craig, D. C.; Paddon-Row, M. N.; Krwn, J.; Verhoeven, J. W. Chem. Phys. Lett. 1988, 150, 366-373. (d) Johnson, M. D.; Miller, J. R.; Green, N. S.;Closs, G. L. J. Phys. Chem. 1989, 93, 1173-1 176. (3) Warman, J. M.; De Haas, M. P.; Oevering, H.; Verhoeven, J. W.; Paddon-Row, M. N.; Olivier, A. M.; Hush, N. S . Chem. Phys. Lett. 1986, 128, 95. (4) (a) Ashikaga, K.; Ito, S.;Yamamoto, M.; Nishijima, Y .J. Am. Chem. SOC.1988, 110, 198-204. (b) Verhoeven, J. W. Appl. Chem. 1986, 58, 1285-1 290. (5) (a) Isied, S.S.;Vassilian,A.; Magnuson, R. A.; Schwarz, H. A. J. Am. Chem. Soc. 1985,107, 7432. (b) Schanze, K. S.;Sauer, K. J. Am. Chem. Soc. 1988, 110, 1180-1186. (c) Faraggi, M.; DeFelippis, M. R.; Klapper, M. H. J. Am. Chem. SOC.1989, 1 1 1 , 5141-5145.
0022-3654/92/2096-710%03.00/00 1992 American Chemical Society
