6340 E. F. Ullman, Tetrahedron Lett., 961 (1973). (25)P. J. Wagner and R. G. Zepp, J. Am. Chem. SOC.,84,288(1972). (26)H. E. O'Neai, R. G. Miller, and E. Gunderson, J. Am. Chem. &c., OS, 3351 (1974). (27)M. Hamlty and J. C. Scalano, J. Photochem., 4, 229 (1975). (28)J. C. Scaiano, J. Am. Chem. SOC., 89, 1494 (1977). (29)P. J. Wagner and K . 4 . Llu, J. Am. Chem. SOC.,96, 5953 (1974). (30)S. Mizashlma, "Structure of Molecules and Internal Rotation", Academic Press, New York, N.Y., 1954. (31)F. D. Lewis, R . H. Hirsch, P. M. Roach, and D. E. Johnson, J. Am. Chem. SOC.,98, 8438 (1976). (32)(a) N. J. Turro, J. C. Dalton, K. Dawes, 0. Farrlngton, R . Hautala, D. Morton, M. Nlemczyk, and N. Schore, Acc. Chem. Res., 5,92(1972);(b) M. W. Wolf, K. D. Legg, R. E. Brown, L. A. Singer, and J. H. Parks, J. Am. Chem. SOC., 87, 4490 (1975). (33) R. L. Coffin,R. S. Givens, and R. G. Carlson, J. Am. Chem. SOC.,OS, 7556 (1974). (34)R. M. Hochstrasser and C. A. Marzzacco in "Molecular Luminescence",
E. C. Lim, Ed., W. A. Benjamin, New York, N.Y., 1989,p 631. (35)R. A. Caidweil, G. W. Sovocool, and R. P. GaJewski,J. Am. Chem. SOC., 95, 2549 (1973). (36)P. J. Wagner and D. J. Bucheck, J. Am. Chem. SOC., 91, 5090 (l969), (37)Although most evidence for the formation of trlpiet excipiexes and excimers is of an Indirect nature, Takemura and co-workers have now reported emission from triplet exclmers of naphthalene and 1-chloronaphthalene; T. Takemura, M. Alkawa, H. Baba, and Y. Shindo, J. Am. Chem. SOC.,Q8,
2205 (1976). (38) Gupta and Hammond have recently reported quantum yield evidence which suggests that certain alkenes and acetophenone triplets form exciplexes which can be quenched by cis-piperylene: A. Gupta and 0. S. Hammond, J. Am. Chem. Soc., 90, 1218 (1976). (39)F. G. Moses, R . S. H. Liu, and B. M. Monroe, Mol. Photochem., 1, 245
(1969). (40)R. E. Hunt and W. Davis, Jr., J. Am. Chem. SOC.,69, 1415 (1947). (41)C. G.HatchardandC. A. Parker, Roc. R. SOC.London, Ser. A, 235,518 (1956).
Role of the Charge Transfer Interactions in Photoreactions. 1. Exciplexes between Styrylnaphthalenes and Aminesla G. G. Aloisi,* U. Mazzucato, J. B. Birks,lb and L. Minuti Contribution from the Istituto di Chimica Fisica. Unioersit; di Perugia, 1-06]00 Perugia, Italy. Received July 9, 1976
Abstract: Measurements have been made of the fluorescence quantum yields ( ~ F M ) lifetimes, , spectra, and of the photoisomerization quantum yields ( 4 ~of) trans-styrylnaphthalenes(StN's) in deaerated and aerated solutions of n-hexane and acetonitrile, and the effect of the addition of amines on these properties has been studied. The fluorescence quenching is associated with exciplex emission, and the Stern-Volmer quenching coefficient increases with a decrease in the amine ionization potential. @C is reduced proportionately less than ~ F Mand , in some cases it is increased, by the addition of the amine. Analysis of the data for P-StN diethylaniline (DEA) in n-hexane provides extensive kinetic parameters which indicate that the photoisomerization occurs via the triplet state with a quantum efficiency q c T = 0.41, increased to qCT = 0.47 (fO.01) in the presence of DEA or oxygen. Similar results have been obtained for a-StN and 4-Br-a-StN. A model of the S I potential of 0-StN is proposed.
+
Interest in the photophysics and photochemistry of electron donor-acceptor (DA) complexes has been increasing in recent years.* The long term aim of our studies on charge transfer (CT) interactions in the ground3 and excited4 states is to investigate the role of DA complexes in the photochemical and photophysical behavior of the partner^.^,^ Substrates of particular interest are the styrylnaphthalenes (StN's) both because their study can give useful information concerning the photoreaction mechanism of stilbene-like molecules and because their relatively long fluorescence lifetime makes them particularly suitable for a quenching study. The fluorescence and photoGhemistry of StN's in the absence of quenchers has recently received much attentiox7-'] The fluorescence has a high quantum efficiency q F M which decreases in the presence of oxygen. The photoisomerization quantum yield 4~ increases with increase in concentration of StN and of dissolved oxygen. On the basis of the experimental results, all observers consider that the photoisomerization probably occurs by a triplet mechanism. The influence of perturbers other than oxygen, acting through C T interaction, on StN photochemistry, has not been investigated previously. This paper reports the results of a study of exciplexes formed by the two isomers a-StN and 0-StN in the first excited singlet state SI with some amine electron donors. a-StN, para-substituted with bromine in the phenyl group (4-Br-a-StN), was also studied. The quenchings of fluorescence and trans cis photoisomerization of StN's by amines were compared to obtain information on the photoreaction mechanism.
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Journal of the American Chemical Society
99:19
Experimental Section The StN's were synthesized for previous work and their preparation is described elsewhere.' The amine quenchers were commercial products (Fluka AG puriss. grade) distilled over N a O H under reduced pressure before use. The solvents were from Carlo Erba R S or R P grade, purified following conventional methods when necessary. The fluorescence spectra and quantum yields were measured with a Perkin-Elmer MPF-3 spectrofluorimeter with an accessory for spectrum correction using rhodamine B as a quantum counter. The measurements of the emission yields were carried out in dilute sohtions (absorbance 0.05 at 350 nm), using quinine bisulfate in 1 N H2SO4 as standard. A CGA DC-3000/1 spectrofluorimeter was also used. The solutions were deaerated by bubbling nitrogen. The Stern-Volmer (SV) fluorescence quenching coefficient K = ~ Q M T M (M-I) was obtained from observations of the fluorescence quantum . ratio of yield @FM as a function of the amine concentration [ Q ] The the fluorescence quantum yields was equated to the intensity ratio at the analytical wavelength. The fluorescence quenching rate parameter k Q M (M-I S - I ) can be obtained, if the S I lifetime T M . at infinite dilution, is known. The fluorescence lifetimes were measured in n-hexane by a sampling fluorometer with a resolution time of 1 ns.12 The decay curves, corrected for the instrumental response time, fit an exponential decay law for the (0-StN DEA) exciplex (studied in more detail because of its higher emission intensity) and for a-StN, but the data for P-StN cannot be expressed by a single exponent (the value of 15 ns reported below corresponds to the best fit observed). The photoisomeriza tion quenching was studied spectrophotometrically, using an Unicam SP500/2 spectrophotometer, under the same experimental conditions as the fluorescence studies. The trans cis
/ September 14, 1977
-
+
-
6341 quantum yield (@c) was determined at various amine concentrations [Q] and at a fixed concentration of StN (-1.5 X M), sufficiently high to assure a total absorption of the 350-nm incident light (the quenchers do not absorb at this irradiation wavelength).
is given by
Reaction Kinetics and Rate Parameter Notation. Exciplex Formation and Dissociation. The following kinetic scheme and notation are used to describe the behavior of the singlet excited molecule 'MI* and the singlet exciplex IE* (= I(M,.Q)*) formed by its interaction with the amine Q. The derivation of the equations is given e1~ewhere.l~
with X = F, G , C, or T. Observations of '#JxSas a function of [Q], and ofqxM and K , enable q X E to be determined. For X = For T, the total quantum yield @X = 4 ~ s . For X = C or G, there is also a contribution to @X from 3M1*, namely,
Subscripts t and c refer to trans and cis isomers, respectively. '#JXT
= @TSqXT
(12)
+ kGT.
where 9x-r = k X T / k T and k r = kCT The total quantum yield
4Jx = @ x S + @XT
- ( q X M + 9TMqXT)
9TE9XT)K[Q1
(qXE
1 + K[Q1
= ~ ' x M '+ ~
(1)
X E '
( 1 3)
for X = C or G . The case X = C (isomerization) is the more interesting. The total photoreaction quantum yield @ c at any [Q] is the sum of the two contributions, by the excited molecules which are not quenched and react in the monomeric form (@cM') and those which react through the complex (@cE'). At [ Q ] = 0, the quantum efficiency of each 'MI* process is ~ X = M
kxM/kM
(2)
where
where
@C = @Co = 9CM
+ 9TMqCT
(14a)
4~~ =
+ qTE9CT
( I4b)
at [Q] = 0, and kM =
X
kXM = l/TM
(3)
is the ' M I *decay parameter, T M is the 'MI* lifetime, and X = F, G , C, or T. When [Q] is large, the quantum efficiency of each 'E*process is 9XE = kXE/kE
(4)
where
kE = 2 k X E = X
is the ] E * decay parameter,
TE
l/TE
(5)
is the limiting value at high [Q]. Observations of K and of @C as a function of [Q] enable @cLim to be determined. Oxygen Perturbation. Superscript a indicates that a solution is aerated, ;.e., it is in equilibrium with the atmosphere at room temperature. An aerated solution contains a molar concentration [ 0 2 ] of oxygen, which introduces IMt* and 'E*quenching processes of rate parameters koM and koE, respectively, additional to those shown in eq I . When [ Q ] = 0
is the 'E* lifetime, and X = F, G , C,
9FM/9FMa
or T.
--
d'XM
1 =K[QI
(6)
where K is the SV coefficient describing the quenching of 'MI*by Q. K is usually determined from measurements of @FM as a function of [Q]. In terms of the parameters of eq 1, 3, and 5,13
K=
kEkEM
kM(kE I t may also be expressed as13
+ kME)
9FE/9FEa
KOM ( 1 5 )
E
TE/TEa
= 1+~
+ KOE
O E [ ~ Z ] =/ ~1 E
9xu/9xua = 1 + KOY
(16)
(17)
where Y = M or E, and X = F, G, or C, but nor T. Oxygen quenching of an excited singlet state S I is equivalent to catalyzed SI --*TI intersystem crossing (ISC),I3so that
(7)
(9TY
+ KoY)/9TYa
E
1
+ KOY
(18)
where Y = M or E. At intermediate [Q], K (eq 7) becomes Ka
kE P=(9) kE ikME is the quenching reaction probability per collision between 'MI* and
Q.
At intermediate values of [Q], the quantum yield @XE of each 'E* process is
where X = F, G , or T. Observations of &=E as a function of [Q] thus provide an independent means of determining K. The total singlet ('MI* ]E*) quantum yield @xS of each process
+
O M [ ~ Z ] / =~ IM-k
In general
=
kEM(kE
where k E M is the diffusion-controlled colhsional rate parameter, kQM is the ' M I * quenching rate parameter, and
c,
= TM/TMa = 1 + ~
When [Q] is large
At intermediate values of [ Q ] , the quantum yield @XM of each 'MI* process is given by 9XM
= 9CE
( k M -t ~
+ ko~[oz]) + ~OE[OZI) - K(I + KOE) ( 1 + K o M ) ( ~+ KoE')
O M [ ~ Z ] ) -k ( ~k EM E
(19)
where KOE' = ~
+
O E [ ~ Z ] / ( ~ kEM E )
= PKOE
(20)
Results Fluorescence and Photoisomerization of StN's. The fluorescence and photoisomerization quantum yields of the trans-StN's were observed in deaerated and aerated n-hexane and acetonitrile solutions at an excitation wavelength of A,, = 350 nm in the absence of the amine. The values of qFM and #c0 for the deaerated solutions and of qFM" and for the
AIoisi et al.
/ Exciplexes between Styrylnaphthalenes and Amines
6342 Table 1.
Fluorescence and Photoisomerization Quantum Yields of Dilute Solutions of trans-StN's (Aex = 350 nm) Deaerated
Aerated qCT
Solute
P-StN
0.36 0.32
0.3 1 0.19
0.48 0.28
0.14 0.28
0.39 0.37
0.5 1 0.23
0.20 0.27
0.4 I 0.35
0.23 0.28
0.43 0.40
0.40 0.27
0.23 0.27
0.38 0.37
(eq 25)
n-Hexane
0.7 1 0.5 1
0.12 0.18
0.64 0.25 0.47 0.30
Acetonitrile 4-Br-a-StN
0.41 0.37
4Ca
n-Hexane
a-StN
koa (eq 25a)
qFM
Acetonitrile
n-Hexane Acetonitrile
qCT"
~ F M "
Solvent
in n-Hexane and Acetonitrile Solution. Fluorescence Lifetime ( T M . in ns) and DEA Fluorescence Quenching Coefficients ( K , K", in M-I) and Rate Parameters
Table 11. StN's
(kQM,
in M-'
ComDound P-StN
tu-StN 4-Br-a-StN
./
S-I) 4
TM
15 1.9 I.1
n-Hexane K knu 135 35 28
9 . 0 X IO9 1.8 X 10'O 2.5 X 1Olo
Acetonitrile Ka
K
K"
76 35 28
139 28 32
93 28 32
qF. OF,
3
2
i n
1
Figure 2. Stern-Volmer plots for fluorescence quenching of 0-StN in acetonitrile by amines of different ionization potential (IP) (diethylaniline. DEA. IP = 6.98 eV; triethylamine, TEA, I P = 7.50 eV; diethylamine, D E A M . I P = 8.01 eV; monobutylamine, MBA, IP = 8.71 eV). Amine concentration is designated by [Q].
Figure 1. Emission spectra of P-StN in n-hexane, alone ( I ) and with increasing amounts of diethylaniline: [DEA] = 0.34 X M (2), 1.5 X IO-2 M (3), 5.0 X IO-* M ( 4 ) .
aerated solutions are listed in Table I. The values of T M for the deaerated n-hexane solutions are listed in Table 11. Fluorescence Quenching. The absorption and fluorescence spectra of StN's in the presence of amines are consistent with the formation of excited molecular complexes (exciplexes) by CT interaction. N o changes were observed in the absorption spectra with increasing amine concentration, showing the complexes to be dissociated in the ground state. The quenching of the S t N fluorescence is associated with the appearance of a structureless exciplex fluorescence band a t longer wavelengths. Figure 1 shows typical emission spectra for the 0-StN diethylaniline (DEA) system. The S V quenching coefficient K decreases with increase in the ionization potential of the amine quencher, as shown in Figure 2 for P-StN. This behavior shows that the amine acts as a n electron donor, and the S t N as an electron acceptor, in forming an A D e~cip1ex.l~ Similar behavior was observed for a-StN and its 4-Br derivative. Table I1 lists the S V quenching parameters K and Kd for 0-StN, a-StN and its 4-Br derivative in deaerated and aerated
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Journal of the American Chemical Society
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99:19
n-hexane solutions, respectively. The values of ~ Q M obtained , from K , T M and eq 8, are also listed in Table 11. Table I11 lists the values of ~ F and M q F M / 4 F M for deaerated solutions of the StN's, in different solvents, as a function of the DEA concentration [ Q J . PhotoisomerizationYields. The presence of the amine also affects the S t N photoisomerization quantum yield 4 ~Table . I I1 lists the values of $c a t different DEA concentrations [Q] for deaerated solutions of the StN's in different solvents. SV-type plots of $c"/$c against [Q] show more or less strong deviations from linearity, compared with the linear S V plots of (IFM/$FM against [Q], and their gradients are always less than K , showing that amines do not reduce ~ F and M 4~ proportionately. Figure 3 compares the plots of $c0/4c and ~ F M / $ F M against DEA concentration [Q] for deaerated nhexane solutions of P-StN and a-StN. Table IV compares the effective SV coefficient of photoisomerization quenching KQC with the SV fluorescence quenching coefficient K for P-StN in three solvents (deaerated and aerated) quenched by DEA and triethylamine (TEA). Figure 4 presents the corresponding SV-type plots for 0-StN + DEA in benzene and acetonitrile, showing the strong influence of the solvent on the behavior. Exciplex Fluorescence. The wavenumber V F E of~ the ~ ex~ ciplex fluorescence maximum is markedly solvent dependent, and it shifts to lower energies with increase in solvent polarity. Table V lists observations of YFE"'~' for the exciplexes of p-StN, a-StN, and 4-Br-a-StN with DEA in ten solvents a t room temperature. The exciplex dipole moment was estimated by comparison of the effect of solvent polarizability on V F E ~ ~for ' the exciplexes of the StN's and anthracene with the same donor DEA.I4 Assuming the exciplexes to have the same volume, dipole moments of about 10 D are obtained for both (a-StN DEA) and (6-StN DEA). Similar values have been re-
+
/ September 14, I977
+
6343 Yields of Fluorescence Concentration ([Q], in IO-* M)
Table 111. Quantum
Solute
(&M)
Solvent
0-StN
a-StN
[QI
~ F M
0.71 0.33 0.21 0.13 0.069
0.12 0.12 0.12 0.13 0.13
0.12 0.055 0.036 0.022 0.01 1
0 0.065 0.083 0.099 0.1 10
1.oo 2.55 4.10 5.48 7.18
0.125 0.163 0.19 0.235 0.235
0.125 0.049 0.03 1 0.023 0.017
0 0.14 0.185 0.200 0.210
1.oo
Acetonitrile
0 0.65 1.99 2.99 3.99
0.51 0.27 0.135 0.10 0.078
1.90 3.77 5.15 6.55
0.18 0.1 16 0.080 0.060 0.050
0.18 0.095 0.048 0.035 0.027
0 0.015 0.024 0.027 0.028
0 1.95 3.90 7.80 0 1.53 4.54 7.65
0.64 0.38 0.27 0.17 0.25 0.175 0.1 1 0.08
1 .oo 1.68 2.36 3.73 1 .oo 1.43 2.30 3.12
0.14 0.20 0.19 0.19 0.28 0.21 5 0.17 0.125
0.14 0.083 0.060 0.037 0.28 0.195 0.122 0.090
0 0.094 0.138 0. I70 0 0.021 0.037 0.044
n-Hexane
0 2.00 3.82 5.00 7.64
0.47 0.30 0.23 0.196 0.15
1 .oo 1.56 2.07 2.40 3.14
0.23 0.24 0.245 0.24 0.25
0.23 0.148 0.1 1 1 0.096 0.073
0 0.088 0.13 0.148 0.173
Acetonitrile
0 3.83 5.75
0.30 0.135 0.105 0.074
1 .oo
2.22 2.84 4.06
0.28 0.205 0.170 0.143
0.28 0.126 0.099 0.069
0 0.058 0.072 0.080
Quenching of @StN by Amines. Photoisomerization Quenching Coefficient KQC (Me'), from Gradient of SV Plot of @ c " / ~against c Amine Concentration ([Q]), Compared with SV Fluorescence Ouenchine Coefficient K (M-l)
DEA KQC K
65
4CE'
0 1.78 3.56 5.16 7.12
I
Table IV.
