4310
J . Phys. Chem. 1992, 96, 4310-4321
kJ/m01.~~Recent theoretical calculations are in fair agreement with this value. Using empirical corrections to a b initio atomization energies, Ho et al. obtained AH?(SiC13) = -318 f 7 k J / m 0 1 . ~ ~A- ~recent ~ semiempirical calculation yielded -485 kJ/mol,’* but this should probably be disregarded because of grass (30) Steele, W. C.; Nichols, L. D.; Stone, F. G . A. J . Am. Chem. SOC. 1962. 84. 4441-4445. (31) Jonathan, N.; Melliar-Smith, C. M.; Timlin, D.; Slater, D. H. Appl. Opt. 1971, 10, 1821. (32) Wang, J. L.-F.; Margrave, J. L.; Franklin, J. L. J . Chem. Phys. 1974, 61, 1357-1360. (33) Walsh, R. J. Chem. SOC.,Faraday Trans. 1 1983, 79, 2233-2248. (34) Ho, P.; Melius, C. F. J. Phys. Chem. 1990, 94, 512C-5127.
disagreement with other experimental and theoretical results. In the present work we obtain AHfo298(SiC13)= -353 i 12 kJ/mol by combining our theoretical IP,(SiC13) with the experimental recently reported by Fisher and A r m e n t r ~ u t . ~ ~ Our calculation of LWf0(SiCl3)is based on ion thermochemistry and differs from calculations by previous workers, who used only neutral thermochemistry. The good agreement between Walsh’s recommended value and our value indicates that the neutral and ion thermochemical data are mutually consistent. Registry No. SKI3, 19165-34-5. (35) Fisher, E. R.; Armentrout, P. B. J. Phys. Chem. 1991,95,4765-4772.
Intramolecular Photoassociation and Photoinduced Charge Transfer in Bridged Diaryi Compounds. 2. Charge-Transfer Interactions in the Lowest Excited Singlet State of Dinaphthylamines Jozef Dresner,+Steven H. Modiano, and Edward C. Lim*,l Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 (Received: December 17, 1991, I n Final Form: February 21, 1992)
The formation of an intramolecular charge-transfer (CT) exciplex is demonstrated for 1,l’-dinaphthylamine (1,l’-DNA) and 2,2’-dinaphthylamine (2,2’-DNA) in the lowest excited singlet state using steady-state and picosecond time-resolved fluorescence spectroscopy. The exciplex is formed through a mutual reorientation of the two naphthalene rings. Differences in the rate of formation and relaxation of the CT state for 1,l’-DNA and 2,2’-DNA indicate the importance of the bridge position in this process. The comparison of the steady-state fluorescence of 2,2’-DNA with that of its protonated form, as well as the fluorescence of 2,2’-dinaphthyl ether and 2,2’-dinaphthylmethane, show the role of the lone-pair electrons of the nitrogen atom in the exciplex formation.
Introduction Interchromophore interactions between an electronically excited moiety and a ground-state moiety in bridged diaryl compounds, leading to the formation of intramolecular excimers and intramolecular exciplexes, are of fundamental importance to the understanding of intermolecular forces. The primary objective of the research described in this series’ is to study intramolecular photoassociation and photoinduced charge transfer (CT) in bichromophoric systems of general structure M-X-M, where two identical aromatic hydrocarbons (M) are joined to each other by a single bridging group (X = CH2, 0, NH, etc.). The intermoiety interactions in the excited electronic state of the bridged diaryls, following the monophotonic excitation of the molecule, may be classified into three categories, depending upon the strength of the interactions. The weakest of these are the van der Waals interactions, leading to the formation of the intramolecular triplet excimem2 The strongest of these interactions are the CT interactions which lead to the formation of the excited, intramolecular CT complexes (exciplexes). The best known examples of such associations in symmetric bichromophoric systems are the twisted intramolecular charge-transfer (TICT) states of 9,9’-bianthry13and 4,4’-dimethylaminophenylsulfone (DMAPS).4 The common feature of this type of interaction is an attainment of an orthogonal conformation of the aromatic moieties, leading to a minimum donor-acceptor overlap, which is essential for the stabilization of TICT complexes.5 The dynamics of the TICT process have been studied in recent years using pico- and subpicosecond techniques to probe the role of the solvent and the ‘Onleave from the Institute of Physics PAN, Al. Lotnikow 32/46,02-668 Warszawa, Poland. ‘Holder of the Goodyear Chair in Chemistry at The University of Akron.
dielectric relaxation on the C T ~ t a t e . ~ Singlet * ~ - ~ excimers comprise the third category in which the near-resonance exciton interactions (or exciton resonance in the case of dimers of high symmetry) are largely responsible for their stabilization.6 For the bridged diaryl compounds of the structure M-X-M, which cannot attain a parallel sandwich conformation favored by singlet excimers of aromatic hydrocarbons, a partial overlap of the two aromatic moieties may still lead to formation of weakly bound intramolecular singlet excimers, as in 1,2-dianthry1ethanes7 In the first paper of this series’ we demonstrated formation of intramolecular triplet excimers in 2,2’-dinaphthyl ether (2,2’-DNE) and 2,2’-dinaphthylmethane (2,2’-DNM). It was also shown that (1) Modiano, S. H.; Dresner, J.; Lim, E. C. J. Phys. Chem. 1991.95,9144. (2) Lim, E. C. Acc. Chem. Res. 1987, 20, 8. (3) Rettig, W.; Zander, M. Ber. Bumen-Ges. Phys. Chem. 1983,87, 1143. (4) (a) Simon, J. D.; Su,S. J. Chem. Phys. 1987, 87, 7016. (b) Rettig, W.; Chandross, E. A. J. Am. Chem. SOC.1986, 107, 5617. ( 5 ) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; Baumann, W. N o w . J. Chim. 1979, 3, 443. (6) (a) Hirayama, F. J. Chem. Phys. 1965,42,3163. (b) Chandross, E. A.; Dempster, C. J . Am. Chem. SOC.1970, 92, 3586. (c) Zachariasse, K.; Kiihnle, W. IUPAC Congress on Photochemistry, Enschede, Abstracts, 1974; p 197. (d) Klopffer, W. Chem. Phys. Lett. 1969,4, 193. (e) Masuhara, H.; Tamai, N.; Mataga, N.; De Schryver, F. C.; Vandendriessche, J. J. Am. Chem. SOC.1983, 105, 7256. (7) (a) Hayashi, T.; Suzuki, T.; Mataga, N.; Sakata, Y.; Misumi, S. J. Phys. Chem. 1977, 81, 420. (b) Hayashi, T.; Mataga, N.; Umemoto, T.; Sakata, Y.; Misumi, S. J. Phys. Chem. 1977, 81, 424. (8) (a) Lueck, H.; Windsor, M. W. J. Phys. Chem. 1990,94,4550. (b) Mataga, N.; Yao, H.; Okada, T.; Rettig, W. J . Phys. Chem. 1989, 93, 3383. (9) (a) Nagarajan, V.; Brearly, A. M.; Kang, T.; Barbara, P. F. J. Chem. Phys. 1987,86, 3183. (b) Kang, T.; Kahlow, M. A,: Giser, D.; Swallcn, S.; Nagarajan, V.; Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 6800. (c) Kang, T.; Jarzeba, W.; Barbara, P. F.; Fonstca, T. Chem. Phys. 1990, 149, 81.
0022-3654/92/2096-43 1Q%Q3.QQ/Q0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4311
Bridged Diary1 Compounds an annihilation process involving triplet excimers leads to the formation of stable, nonpolar, intramolecular singlet excimem (SI’) with a characteristic delayed fluorescence spectrum situated between that of monomeric naphthalene and singlet excimers of naphthalene. In both 2,2’-DNM and 2,2’-DNE, the interaction in SIis too weak for the direct SI SI’process to occur within the short lifetime of SI. This paper deals with the interactions in the lowest excited singlet states of 1,l’- and 2,2’-dinaphthylamine. We demonstrate the formation of an intramolecular CT state for these molecules through steady-state and time-resolved fluorescence studies. Differences in the rate of formation and relaxation of the CT state for 1,l’- and 2,2’-DNA indicate the importance of the bridge position in this process. The comparison of the fluorescence of the protonated and unprotonated forms of 2,2’-DNA as well as 2,2’-DNM and 2,2’-DNE show the role of the lonepair electrons of the amine bridge in the intermoiety chargetransfer interactions. The formation of an intramolecular CT state was previously demonstrated by K m w e r and co-workenl0 for a series of asymmetric and symmetric arylamino sulfonates but to our best knowledge has not been reported to date for unsubstituted dinaphthylamines.
-
Experimental Section Fluorescence lifetimes and time-resolved fluorescence spectra were obtained by the technique of timecorrelatedphoton a n t i n g . The excitation source was the frequency-doubled output of a synchronously pumped dye laser (Coherent Antares 70/702-CD). The fluorescence signal was detected at 90° from the direction of excitation by a microchannel plate PMT (Hamamatsu 1645), through a subtractive double monochromator (Spectramate 1680). The single photoelectron pulses were amplified 10-fold with a Philip Scientific 774 2-GHz amplifier and discriminated with a Tennelec 454 fast constant fraction discriminator (CFD), modified for fast response detectors. Stop pulses of 500 mV delivered by an avalanche photodiode (RCA C30902E) were discriminated with an Ortec 583 CFD. Start and stop pulses were input to an Ortec 437A time-to-amplitude converter (TAC) and analyzed with a Nucleus PCA I1 pulse height analyzer/multiscaler. The time-resolved spectra were obtained by further discrimination of the TAC output pulses with an Ortec 553 timing single-channel analyzer providing a variable time window. When the output from the TAC was amplified with an EG&G AN109 biased amplifier, the highest temporal resolution of 11 ps/channel was obtained. The overall instrument response function measured by the detection of the scattered laser pulse of ca. 1-ps duration was 90 ps. The fluorescence decay times were obtained by a standard least-squares optimization using the Marquardt algorithm.” A specially written program was used, which allows for a threeexponential model function with additive background fitting. The success of the fit was determined by a weighted analysis of the residuals and their autocorrelation function distribution, the Durbin-Watson (DW) parameter, and reduced x2. It was critical for the high timeresolution experiments that the excitation profile be measured with approximately the same counting rate as the corresponding decay signal. Under this condition, satisfactory fits were usually obtained in the entire rise and decay portion of the curve to yield values of 1 < x2 < 1.2. The steady-state fluorescence spectra were recorded on an Aminco SPF-500 spectrofluorometer. 1,l’- and 2,2’-dinaphthylamines were synthesized according to the method of Buu-Hoi.l* I-Naphthylamine (Eastman-Kodak) ~~
~
(IO) (a) Kosower, E. M.; Dodiuk, H.; Tanizawa, K.; Ottolenghi, M.; Orbach, N. J. Am. Chem. Soc. 1975, 97, 2167. (b) Dodiuk, H.; Kosower, E. M.J . Phys. Chem. 1977, 81, 50. (c) Kosower, E. M.;Dodiuk, H. J . Am. Chem. Soc. 1978, 100, 4173. (d) Kosower, E. M.; Dodiuk, H.; Kanety, H. J. Am. Chem. Soc. 1978,100,4179. (e) Dodiuk, H.; Kosower, E. M. J . Phys. Chem. 1979,83, 2053. (11) Marquardt. D. W.J. SOC.Ind. Appl. Math. 1963, 11, 431. (12) Buu-Hoi, N. P.J . Chem. SOC.1952, 4346.
2,Z’- DNA
3 K
NA
._
-
(A) Hexane
C
c 0
I
1
Wavelength, nm
Figure 1. Comparison of the steady-state, room-temperature fluorescence spectra of 2,2’-DNA and 2-NA in hexane (a) and acetonitrile (b) with the corresponding spectra for 1,l’-DNA and 1-NA in hexane (c) and acetonitrile (d). Concentration = 1 X M for 1,l’-DNA and 2,2’DNA. Concentration = 2 X M for 1-NA and 2-NA.
and 2-naphthylamine (Aldrich) were purified by recrystallization and vacuum sublimation. Before the start of the time-resolved measurements, the samples were degassed by bubbling purified N2 gas through the solution. The low-temperature studies were performed by cooling the sample in a quartz dewar using cold N2 vapor.
Results and Discussion 1. Steady-StateFluorescence. Figure 1 shows the steady-state fluorescence of 1,l’-dinaphthylamine (l,l’-DNA) and 2,2’-dinaphthylamine (2,2’-DNA) in the nonpolar and polar solvents of hexane and acetonitrile. Corresponding spectra of 1-naphthylamine (1-NA) and 2-naphthylamine (2-NA) are also shown for comparison. The emission spectra are clearly dependent upon solvent polarity. In nonpolar solvents such as hexane, the fluorescence of 2,2’-DNA resembles that of its reference compound 2-NA in the same solvent, and we assign it to the fluorescence from the locally excited (Le., naphthylamine-centered) excited state. In polar solvents such as acetonitrile, on the other hand, the emission is broader and Stokes shifted to a much greater extent than the 2-NA. We assign the broad Stokes-shifted fluorescence to the intramolecular CT state in which charge has been transferred from one moiety to the other (vide infra). Comparison of 1,l’-DNA and 2,2’-DNA suggests that there is a much stronger interaction in 1,l’-DNA than in 2,2’-DNA. This is evident not only in acetonitrile, where the emission of 1,l’-DNA is broader and more red-shifted than 2,2’-DNA, but also in hexane where the emission of 1,l’-DNA is already red-shifted with respect to 1-NA. In contrast, the steady-state fluorescence of 2,2’-DNA and 2-NA in hexane are quite similar in both shape and spectral position. The solvent dependence of the steady-state fluorescence for 2,2’-DNA can be more readily seen in Figure 2, which shows the strong solvatochromic shift of the emission for a series of solvent mixtures of 1,Cdioxane and methanol. Following the treatment (eq 1) of Lipped3 and Mataga,I4 the difference between
the excited-state dipole moment pe and the ground-state dipole moment p i.e., Ap = pc - p, can be obtained. The terms are I , is the energy in wavenumbers (cm-I) of the defined as
?allows:
(13) (a) Lippert, E. Z . Naturforsch. 1955,IOa, 5412. (b) Lippert, E. Z. Elektrochem. 1957, 61, 962. (14) Mataga, N.; Keifu, Y . ;Koizumi, M. Bull. Chem. Soc. Jpn. 1956,29,
465.
4312 The Journal of Physical Chemistry, Vol. 96. No. 11, 1992
Dresner et al.
TABLE I: Emission Data for Naphthylamines in Various Solvents
1,l'-DNA
2,2'-DNA Stokes
solvent dioxanemethanol' F .( D. A, b 100.00 0.02 99.0 0.049 98.0 0.072 96.0 0.107 90.0 0.17 84.0 0.203 70.0 0.243 60.0 0.26 40.0 0.281 10.0 0.302 hexane 0 butyl acetate 0.171 ethyl acetate 0.199 valeronitrile 0.264 0.28 1 propionitrile 0.3 acetonitrile
abs, cm-' 29 590 29 590 29 590 29 590 29 590 29 590 29 590 29 590 29 590 29 590 29 540 29410 29 530 29 590 29 540 29 530
FL, cm-] 23 710 23 710 23 660 23610 23410 23 320 22 860 22 500 22 140 21 710 25 380 23 660 23 430 22 460 22 070 21 740
shift VA - v p ,
cm5880 5880 5930 5980 6180 6270 6730 7090 7450 7880 4160 5750 6100 7130 7470 7790
1-NA Stokes shift
abs, cm-' 28 740 28 740 28 740 28 740 28 740 28 740 28 740 28 740 28 740 28 740 29 060 28 570 28 420 28 420 28 530 28 400
FL, cm-' 25 040 24 980 24 900 24 850 24 620 24 530 24 300 24 210 24 110 23 980 26 430 24 980 24 690 24 270 24 120 23 780
VA
2-NA Stokes shift
- VFL,
abs, cm-' 30210 30210 30210 30210 30210 30210 30 390 30 490 30 490 30 580 30 430 30 030 29 990 30 180 30 180 30 120
cm-' 3700 3760 3840 3890 4120 4210 4440 4530 4630 4760 2630 3590 3730 4150 4410 4620
FL, cm-' 24 890 24 890 24710 24 650 24 600 24 350 24 180 24 030 23 880 23 740 26 820 24 930 24 690 24 430 24 380 24 100
VA
- YFL,
cm-' 5320 5440 5500 5560 5610 5860 6210 6460 6610 6840 3610 5100 5300 5750 5800 6020
Stokes shift abs, cm-'
28990 28990 28990 28990 28990 29150 29150 29 150 29150 29 150 29150 28820 28830 28830 28940 28940
FL, cm-' 25 550 25 510 25 460 25 390 25 320 25 250 25 150 25 070 25010 24 930 26 670 25710 25 510 25 320 25310 25 190
VA
- YFL,
cm-1 3440 3480 3530 3600 3670 3900 4000 4080 4140 4220 2480 3110 3320 3510 3630 3750
'Percentage of dioxane by volume mixed with methanol. bF(D,n) = (D - 1)/(2D + 1) - (n2 - 1)/(2n2 + l), where D is the static dielectric constant of the solvent, and n is the refractive index of the solvent. 4800
+
1
y4400 E
2, 2'-DNA
e
4000.
B
15::3600
~
3200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0. F(D, n)
5000 7
1 4.
14000
"
2000
4
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 F(D, n)
360 380 400 420 440 460 480 500 520 540
Wavelength,
nm
Figure 2. Steady-state,room-temperature fluorescence spectra in a series of dioxane-methanol mixtures for 2-NA (a) and 2,2'-DNA (b). The solvent compositions, Le., the percentage of dioxane by volume mixed with methanol, are as follows: (1) 100% (2) 99% (3) 98% (4) 96%; (5) 90%; (6) 84%; (7) 70%; (8) 60%; (9) 40%; (10) 10%. Concentration = 1X M for 2,2'-DNA. Concentration = 2 X M for 2-NA.
lowest energy singlet-singlet absorption, i+is the energy in wave numbers (cm-I) of the fluorescence maximum, D and n are the static dielectric constant and refractive index for the solvent, and a is the cavity radius in Onsager's theory of the reaction field for the solute involved. Table I gives the observed values for the dioxane-methanol solvent mixtures as well as for other solvents for 2,2'-DNA and 2-NA. Figure 3 shows the plots of the Stokes shift (Fa - i+) versus the solvent function F(D,n) = (D - 1)/(2D
Figure 3. Plot of the Stokes shift of 2,2'-DNA and 2-NA vs the solvent function F(D,n) for a series of dioxane-methanol mixtures (a) and for a series of aprotic solvents (b). See Table I for the listing of the solvents.
+ 1) - (n2- 1)/(2nZ+ 1 ) .
The change in the dipole moments,
A p (=pe - p,), between the ground and excited states were com-
puted from the slopes of these graphs. The slopes along with the values of Ap are presented in Table 11. The values were calculated assuming a to be 6 and 3 A for 2,2'-DNA and 2-NA, respectively. If only the relative difference between 2,2'-DNA (Ap = 10.8 D) and 2-NA (Ah = -3 D) are considered, it can be seen that A p for the bichromophoric molecule is about 3 times that of the monomer. Although the ground-state dipole moment of 2,2'-DNA is not known, the large increase in the dipole moment in the excited state along with the strong solvatochromatic shift allows us to assign to emitting state to an intramolecular CT state. The large value of A p for 2,2'-DNA, relative to that of 2-NA, can be at-
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4313
Bridged Diary1 Compounds TABLE II: Calculrted Dipole Moment Cbange, Ab in Dioxane-Methanol Mixtures and Aprotic Solvents molecule solventC slope” dioxane-methanol 5447 2,2’-DNA dioxantmethanol 2819 2,2’-DNA aprotic solvent series 6402 2,2’-DNA dioxantmethanol 3265 2-NA dioxantmethanol 1576 2-NA aprotic solvent series 4128 2-NA dioxane-methanol 16342 1,l’-DNA dioxantmethanol 2287 1,l’-DNA aprotic solvent series 12028 1,1’-DNA dioxane-methanol 9869 1-NA dioxane-methanol 1811 1-NA aprotic solvent series 7898 1-NA
8000 _.
-
I
Ap,bD 10.8
7.8 1 1.7 3.0
2.1 3.3 18.7 7.0
+
1, 1 l - W
v
c w
6000 -
$5000
/
16.1 5.2
/
4000 ’ 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 F(D, n) 1
2.2 4.6
aSlopes measured from Figures 3 and 5 . * A p was computed using the assumed Onsager radii of 3 A for 1-NA and 2-NA and 6 A for 1,l’-DNA and 2,2’-DNA.
