Department o/Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180. (Received 25 ... electric field E. In organic materials, (3 may be significantly ... Downloaded to IP: ...... 33 A. Samata, C. Devadoss, and R. Fessenden, J. Phys.
The low-energy, charge-transfer excited states of 4-amino-4' -nitrodiphenyl sulfide Donald B. O'Connor, Gary W. Scott, and Kim Tran Department 0/ Chemistry, University 0/ California, Riverside, Riverside, California 92521
Daniel R. Coulter, Vincent M. Miskowski, and Albert E. Stieg man Space Materials Science and Technology Section, Jet Propulsion Laboratory, California Institute 0/ Technology, Pasadena, California 91109
Gary E. Wnek Department o/Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180
(Received 25 March 1992; accepted 1 June 1992) Absorption and emission spectra of 4-amino-4' -nitrodiphenyl sulfide in polar and nonpolar solvents were used to characterize and assign the low-energy excited states of the molecule. Fluorescence-excitation anisotropy spectra, fluorescence and phosphorescence lifetimes, and fluorescence quantum yields were also used to characterize the photophysics of these states. The lowest-energy, fluorescent singlet state was determined to be an intramolecular charge transfer (lCT) state involving transfer of a full electron charge from the amino to the nitro group yielding a dipole moment of ~50 D. A low-energy, intense absorption band is assigned as a transition to a different ICT state involving a partial electron charge transfer from sulfur to the nitro group.
I. INTRODUCTION
The search for new organic materials possessing nonlinear optical properties has grown tremendously in recent years. l -4 There have been various attempts to gain fundamental understanding in designing new materials with larger nonlinear properties. 5- 7 On the molecular level, the nonlinear properties of these materials are characterized by the values of the nonlinear terms {3, y, ... in the following expression 2: (1)
in which P is the induced molecular polarization by an electric field E. In organic materials, (3 may be significantly enhanced by excited-state charge-transfer (CT) interactions. Such interactions occur when an electron-donating group and an electron-accepting group are connected through a 1T-electron linkage. f3 is then the sum of f3add and (3er, in which f3add is due to the substituent induced asymmetry on the 1T-cloud charge distribution and the C1 skeleton while f3eT is a charge-transfer contribution due to mixing of a polar excited state with the ground state in an external field. 2 f3er can be written as 8
f3 cT =
(3e2h2/8m) r(UJ )filJLg,e,
standing the nonlinear properties of such materials is a study of their excited charge-transfer states. In this paper, we report the excited-state photophysics of 4-amino-4'-nitrodiphenyl sulfide (ANDS) in both polar and nonpolar solvents. This molecule has a relatively high nonlinear optical response. 4,9 Excited-state photophysics are also reported for related model compounds of ANDS, namely methyl-4-nitrophenylsulfide (MNPS) and 4amino-4'-nitro-diphenyl (ANDP). The structures of these molecules are shown in Fig. 1. The present spectroscopic investigation results in a characterization of the lowenergy, charge-transfer excited states of the ANDS molecule.
H2N
(
)
with
in which W is the optical transition energy, f is the oscillator strength, UJ is the optical frequency, and ilJLg,e is the difference between ground and excited state dipole moments. Equation (2) illustrates the direct relationship between the excited-state CT character of donor-acceptor molecules and their nonlinear optical response. A key to under-
(
)
N0 2
ANDP
/s-o-'
H3 C
N0 2
-
MNPS FIG. 1. Molecular structures of 4-amino-4'-nitrodiphenyl sulfide (ANDS), methyl-nitrophenyl sulfide (MNPS), and 4-amino-4'-nitrodiphenyl (ANDP).
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O'Connor et al.: Charge-transfer excited states
II. EXPERIMENT A. Materials
The solvents-isooctane (Aldrich, 99+%, A.C.S. spectrophotometric grade), toluene (Aldrich, 99+%, A.e.S. spectrophotometric grade), methyl cyclohexane (Aldrich, 99%, spectrophotometric grade), and ethanol (Quantum, 200 proof dehydrated)-were used without further purification. ANOS was synthesized according to the procedure given by Hyne and Greidanns. 1O The preparation of ANOP was previously reported. II MNPS, obtained from Aldrich, was used without further purification.
4019
Fluorescence-excitation spectra were obtained using the spectrofluorimeter described above. These excitation spectra were collected by monitoring the emission intensity at the peak of the emission bands while the excitation monochromator was scanned over the absorption bands. 2. Excitation spectra anisotropy
The steady-state excitation polarization spectra were obtained with the spectrofluorimeter by adding a calcite Glan-Taylor prism polarizer and a polarizer (Optics for Research; PUM-51) as excitation and emission polarizers, respectively. The L-format method l6 was utilized and the excitation anisotropy at each wavelength is given by
(4)
r= (l vv- G/ VH)/(l vv+2G/ VH),
B. Methods
1. Absorption, emission, and fluorescence-excitation spectroscopy
Absorption spectra were obtained using a Varian OMS 100S UV visible spectrometer in a double-beam configuration. Steady-state emission spectra were recorded with a spectrofluorimeter (Spex Fluorolog 2, model F212) equipped with double grating emission and excitation monochromators, a high-pressure xenon lamp (450 W) for excitation, a quantum counter (Rhodamine B as scintillator) to monitor the excitation beam intensity, and a cooled photomultiplier tube (Hamamatsu R928-P) used in photon-counting mode to monitor the emission intensity. The excitation monochromator wavelength was set at either 337 or 355 nm with a 1.8 nm bandwidth, and emission was collected at a 90· angle with respect to the excitation. Schott KV nonfluorescing filters were used to eliminate the interference of scattered excitation light. Emission intensities were quantum counter ratioed and corrected for wavelength variation of detector efficiency. Each sample emission spectrum was baseline corrected with an emission blank (solvent only) spectrum. Emission quantum yields were referenced against yields from rhodamine 6G (in ethanol), sulfurhodamine 101 (in ethanol) and 4-(dicyanomethylene)-2-methyl-6(p-dimethyl aminostyryl)-4H-pyran (OCM in dimethyl sulfoxide) used as standards. Concentrations of the standard solutions were adjusted to have an absorbance of ..
oj.)
.....> oj.)
0... L
0
en
.0
('
. ..-t
+J
~ ...... (j) c
o
(j)
..0 -..
0... L
OJ
~
2
C
I
/
.......
C
L
o o
OJ
> ~
0
L
....-1
OJ 0:::
450
500
WovelengthCnm)
WavelengthCnm) FIG. 3. Absorption spectra of (a) ANDS; (b) ANDP; and (c) MNPS in ethanol (-) and in protonated ethanol (- - -). All were obtained with a I nm bandpass.
FIG. 4. Room temperature steady-state emission spectra (Aex=355 nm) of ANDS in isooctane/toluene solvent mixtures with the following toluene mole fractions: Xto\=O.OO (-); 0.15 (_._); 0.61 (---); and J.()() (_. '-).
J. Chern. Phys., Vol. is97, No.6,to15 1992 This article is copyrighted as indicated in the article. Reuse of AIP content subject theSeptember terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.22.1.19 On: Fri, 10 Jul 2015 13:07:59
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O'Connor et al.: Charge-transfer excited states
corresponding molar absorptivities of ANDS in each mixture are given in Table I. The wavelength of the lowest energy absorption band at its maximum and the molar extinction coefficient are solvent sensitive. As the mole fraction of toluene in the solvent mixture increases, the absorption band undergoes a minor red shift and a slight reduction in absorptivity. Figure 3 shows absorption spectra of ANDS and the model compounds, ANDP and MNPS, in ethanol and "protonated" ethanol. ("Protonated" ethanol is a solution of 0.1 M Hel in ethanol.) The emission spectra of ANDS in the same solvent mixtures of toluene and isooctane are shown in Fig. 4. The wavelengths corresponding to the emission maxima in these solutions are given in Table I. Figure 5 shows the emission spectra of ANDS, ANDP, and MNPS at 77 K in both ethanol and "protonated" ethanol. For ANDS in ethanol, the emission was only detectable at low temperature. At 77 K, the emission of ANDS in ethanol [Fig. 5(a)] is
broad and featureless, with a maximum intensity at 608 nm. The fluorescence-excitation spectrum, shown in Fig. 6(a), closely matches the absorption spectrum and is independent of the emission wavelength monitored. The fluorescence-excitation polarization spectrum of ANDS in methyl cyclohexane at 77 K is shown in Fig. 6(b). As shown, across the lowest-energy absorption band of ANDS (320-410 nm), the anisotropy remains essentially constant, with an average anisotropy, r, of 0.24±0.02. Under identical conditions, the excitation polarization spectra of ANDP in methyl cyclohexane at 77 K were obtained, as shown in Fig. 6(c). In the spectral region of 320-430 nm, across the lowest energy absorption band of ANDP, the average anisotropy was r=0.40±0.02. ANDS emits with low quantum efficiency in nonpolar solvents at room temperature. In solvent mixtures of isooctane and toluene, the Stokes shifts and the emission quantum yield, 4>, both vary with the mole fraction of toluene.
>-
I\
+J ..... (J) c
a
I \,
OJ
\
{./
I I I I
a
+J C
.......
\
OJ
.....>
"
+J
a ...... OJ
a:::
>-
b
.4
0...
0
L +J
.2
0
.....(J)
OJ
c
-
-.2
I \
OJ 0:::
/"
.4
>-
0... 0 L +J 0
\
I I 0 450
c ,~
\
"-
,, ......
500
550
600
-
650
.....(J) c
~~~
-
+'
a
...... Q)
0:::
As shown in Tables I and II and in Fig. 7, the Stokes shifts (AAI - AEI in cm- I ) increase monotonically with toluene mole fraction but the dependence of cfJ upon the toluene concentration is not monotonic. As shown in Fig. 7(b), cfJ increases as the mole fraction of toluene in the mixtures increases, up to -0.22. However, when the mole fraction of toluene exceeds -0.28 in the mixtures, the emission quantum yield begins to decrease. Furthermore, when a higher polarity solvent is used at room temperature (e.g., ethanol), the emission efficiency of ANDS becomes so low that the emission is undetectable (e.g., ANDS in ethanol). A time-resolved emission profile of ANDS in a nonpolar solvent mixture at room temperature is shown in Fig. 8. A least-square fit with a single-exponential decay function and deconvolution l9 ,20 gives the fluorescence lifetimes in isooctane/toluene solvent mixtures as presented in Table II. The time-resolved emission profiles of ANDS in ethanol at 77 K were also obtained, an average decay profile being shown in Fig. 9(a). Deconvolution of this emission
-2
-1
o
2
5
4
3
Time (ns) FIG. 8. Room temperature time-resolved fluorescence profile (A. ex =355 nm) of ANDS in solution of 90% isooctane+ 10% toluene by volume (Xtol=0.15).
kinetics profile from the detector response to the scattered laser pulse with simultaneous fitting to a single-exponential decay20 gives a fluorescence lifetime of 500 ± 170 ps. For comparison, an average emission decay curve of ANDP in
OJ
....>
16 E -.
....""
30
(J)
c
OJ
"" C
20
"
""0
10
OJ
0:::
0 (bl
0
0.0
0.2
0.4
0.6
0.8
1.0
Mole Fraction of Toluene
FIG. 7. (a) Stokes shifts and (b) fluorescence quantum yield of ANDS in various mixtures of isooctane:toluene at different mole fractions of toluene.
0
100
200
300
400
500
TimQ ems) FIG. 9. Fluorescence decay profile (A. ex =355 nm) of (a) ANDS in ethanol and (b) phosphorescence decay profile (Aex=337 nm) of ANDS in protonated ethanol at 77 K.
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O'Connor et a/.: Charge-transfer excited states
a
>..
4023
TABLE III. Fluorescence and phosphorescence lifetimes of ANDS, ANDP, and MNPS at 77 K.
.jJ
Compound
(f)
C 01
Tf
ANDS ANDP MNPS
.jJ
C 01
(ps)"
500± 170 3360±660
62.0± 12.1 95.4± 17.6
> "Measured in ethanol. bMeasured in "protonated" ethanol.
.jJ
.....a 01
~
0~
-2
__
~~~
o
-1
__L -_ _ 2
~~
_ _~_ _-L~
3
4
5
Time (ns)
b
>.. .jJ
..... (f)
C 01
.j.J
C ...... 01
.....> .j.J
.....0 01
0::
0
0
100
200
300
400
500
Time (ms)
FIG. 10. Fluorescence decay profile (A e,=355 nm) of (a) ANDP, and (b) phosphorescence decay profile (A. x =337 nm) ofMNPS in ethanol at 77 K.
ethanol at 77 K was also obtained as shown in Fig. lO(a). Similar data analysis performed on this average profile yielded a fluorescence lifetime of 3350 ± 660 ps. Upon protonation, the emission spectrum and lifetime of ANDS in ethanol changed significantly. An average decay profile of ANDS emission in "protonated" ethanol at 77 K is shown in Fig. 9(b). Weighted, single-exponential, least-square fits applied to this significantly longer decay profile gives a phosphorescence lifetime of 62.0± 12.1 ms. Under identical conditions, the phosphorescence decay lifetime of MNPS in ethanol was found to be 95.4± 17.6 ms. For MNPS in ethanol, the decay profile [Fig. lO(b)] and lifetime remain essentially unchanged after the solution is protonated. These results are summarized in Table III.
for the identical substitution in diphenyl ether. Szmant and McIntosh 22 speculated that this red shift might be caused by the reduction of the electronic excitation energy in ANDS due to the presence of the two resonance structures shown in Fig. 11. The coplanarity of the two phenyl rings shown in these structures suggest that an intramolecular charge-transfer (lCT) between the amino and the nitro groups in ANDS is feasible, and therefore the lowestenergy absorption band may be due to an amino-to-nitro CT band. However, a recent study of the molecular geometry of ANDS revealed that the phenyl rings are exactly perpendicular to each other in the ANDS crystal. 9 An approximately tetrahedral structure with a 104° C-S-C bond angle was obtained at the sulfur center. Those authors9 concluded, therefore, that conjugation between the aromatic rings is absent in the ground state. Consequently, they inferred that the lowest energy absorption band is not due to amino to nitro CT in ANDS; instead, this band was attributed to a CT transition from the sulfur to the nitro group. This attribution was supported by the results of a complete neglect of differential overlap (CNDO) approximate molecular orbital calculation. 9 Comparing Figs. 2 and 3(a), it is apparent that the position of the lowest energy absorption band of ANDS undergoes a red shift as the solvent polarity increases. However, from a comparison of Figs. 3(a) and 3(b), this band for ANDS is at higher energy than that of ANDP in the same solvent system. Furthermore, a comparison of Figs. 3(a) and 3(c) reveals that the lowest-energy band for both ANDS and MNPS are quite similar in pure ethanol. Since it is known that the extended conjugation of the 7T-electron network between the two phenyl rings in ANDP is responsible for the shift of its So -+ S I absorption
H2N
N0 2
t
IV. DISCUSSION
The electronic absorption spectra of ANDS and the model compounds have been previously reportedY-24 A red shift of the longest-wavelength band of the absorption spectra of diphenyl sulfide was previously observed 21 when substituted by amino and nitro groups at the p and pi locations, respectively. However, no such a shift was seen
~.-
N02 one-electron transfer. As shown in Fig. 7(b), the emission quantum yield, 0/, of ANDS in isooctane/toluene solvent mixtures changes with solvent composition of the mixtures. At first, 0/ increases with increasing toluene mole fraction, up to about Xtol=O.22. From the observed fluorescence quantum
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O'Connor et al.: Charge-transfer excited states
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yields, ifJ, and the fluorescence lifetimes, T, the experimental radiative rate constants, k" were determined using the relation kr = T-lifJ. These values of kr are presented in Table II. The kr values are essentially constant, within experimental error, for all solvent mixtures. Thus it is clear that the nonradiative decay rate, k nn of ANDS first decreases, then increases as the mole fraction of toluene increases in the solvent mixture. Since the energy of the fluorescent state continuously decreases as the toluene concentration increases, this suggests that at low toluene mole fraction (X tol = 0.0-0.22) a nonradiative decay pathway [perhaps intersystem crossing to a 31T1T* state (vida infra)] may be shut off as the toluene mole fraction increases. As the excited-state energy decreases further, the reduced energy gap between this state and the ground electronic state causes increased coupling between these two states. Thus, the rate of another nonradiative decay process, internal conversion, eventually increases. Therefore, a significant reduction of the emission quantum yield is observed when the mole fraction of toluene in the solvent mixture exceeds 0.22. The polarized fluorescence-excitation spectrum of ANDS in methylcyclohexane at 77 K gave an average anisotropy across the lowest energy absorption band of r =0.24±0.02. For fluorophores with parallel absorption and emission dipoles in a dilute, vitrified solution that prevents energy transfer and reabsorption, the anisotropy should be 0.40. 16 The difference in these values is neither due to energy transfer nor reabsorption since a dilute solution was used (M < 10- 5 mol/I) and the Stokes shift is quite large. At 77 K, the solution is rigid so as to preclude rotational diffusion during the excited-state lifetime from reducing r. Thus, the experimental anisotropy value indicates that the absorption and emission transition dipoles are not collinear. Using the relationship l6 r=0.2(3 cos2 a-l),
(7)
in which r is the anisotropy and a is the angular rotation of the emission dipole with respect to the absorption dipole, a is calculated to be 31 ± 2° for ANDS in methyl cyclohexane at 77 K. If it is assumed that the transition observed in absorption corresponds to a sulfur-to-nitro CT transition with the transition dipole along the S -+ N0 2 direction, and the transition observed in emission to an amino-to-nitro CT transition with the transition dipole along the NH2 -+ N0 2 direction, the ground state molecular geometry9 predicts 38° for the angle between these two transition dipoles. (The same value for this angle would be predicted if the molecule became linear in the excited state along the C-S-C bond in a symmetrical fashion.) Under identical experimental conditions, the average fluorescence-excitation anisotropy over the lowest energy absorption band for ANDP in methylcyclohexane at 77 K was found to be 0.40. This result is consistent with the value expected for ANDP since a direct charge-transfer process occurs in the excited state between the NH2 and N0 2 moieties through the two phenyl rings in ANDP.
Thus, the lowest energy Franck-Condon excited singlet state is the same as the fluorescing state for ANDP, and the angle between the absorption and emission transition dipoles is expected to be zero as observed. ANDS displays a weak, visible emission only in nonpolar solvents and no emission in polar solvents at room temperature. As shown in Figs. 6(a) and 12, the red shift of the fluorescence band of ANDS is directly proportional to the solvent polarity function, due to the lowering of the excited-state energy during the solvation process. In terms of the excited-state dipole moment, ILl!' solvent dielectric constant, D, and the Onsager radius, a, the solvation en. gtven . by30 ergy, Esob IS
(8) Thus, the solvation energy is directly proportional to the polarity of the solvent as well as the excited-state dipole moment of the solute molecule. Since the results indicate that the fluorescent state of ANDS is fully chargeseparated between the amino and nitro group, it is better stabilized in high polarity solvents. Consequently, a smaller energy gap occurs between the lowest excited singlet state and the ground electronic state for ANDS in ethanol than in nonpolar solvents. Thus, the nonradiative decay rate due to internal conversion becomes quite rapid, resulting in the absence of detectable emission from ANDS in polar solvents at room temperature. At 77 K, the emission band detected for ANDS in ethanol is broad, featureless and similar to the emission of ANDP in ethanol at this temperature (see Fig. 5). For ANDP in ethanol, emission disappears after the solution is protonated, probably due to the blocking of the NHr .. N02 CT state upon protonation of the amino group. For ANDS, however, protonation causes the emission band to undergo a blue shift, and it becomes structured. The shape and position of this emission band of ANDS in protonated alcohol closely resembles those of the emission band of MNPS in ethanol at 77 K. The characteristics of the MNPS emission remain essentially unchanged upon protonation. The emission lifetimes of ANDS in protonated ethanol and MNPS in ethanol are relatively long, identifying this emission as phosphorescence (see Table III). Examination of the structured phosphorescence bands of both ANDS and MNPS indicates that these bands are similar to those reported for other nitro-substituted aromatic molecules. As shown by Khalik et al.,31 the observed phosphorescence occurs following rapid intersystem crossing, 1(n, 1T*) -+ 3 ( 1T,1T*), in these systems. As shown in Fig. 5, for ANDS in unprotonated ethanol at 77 K, however, only fluorescence is seen since the NH2 -+ N0 2 CT singlet state is apparently located at lower energy than the 3 (1T,1T*) state. Thus, the nonradiative transition of going from the S -+ N0 2 CT state to the NH2 -+ N0 2 CT singlet state must occur with higher probability than intersystem crossing. On the contrary, for ANDS in protonated ethanol and for MNPS in ethanol, the NH 2-+N0 2 CT transition may not occur, causing the 3 ( 1T,1T*) state to be the lowest-energy excited state. The rate for intersystem crossing from the S -+ N0 2 CT state to
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O'Connor et al.: Charge-transfer excited states
the 3( 17",17"*) state is likely enhanced by an intramolecular heavy-atom effect due to sulfur in both ANDS and MNPS. Thus, intense phosphorescence is observed for these two species in protonated ethanol. For ANDP, on the other hand, the lack of a heavy atom in its structure accounts for a decreased transition rate for the intersystem crossing process. The phosphorescence of ANDP in protonated ethanol was not detected. Least-squares fitting analysis, performed on the timeresolved emission profiles obtained for ANDS in different solvent systems and at different temperatures, indicates that both fluorescence and phosphorescence decay profiles are well-fit by single-exponential decay functions. Thus, in each case, it may be assumed that only one emitting state is responsible for each of the observed emissions. For ANDS in nonpolar hydrocarbon solvents or in an unprotonated polar solvent, the emission likely originates from the NH 2-+N02 ICT state. For ANDS in protonated alcohol, however, the emissive state is undoubtedly the 3(17",17"*) state. Theoretical radiative decay rates, k~alc, were calculated for the lowest-energy absorption bands of ANDS in the toluene/isooctane mixed solvent solutions. The k~alc were obtained, after deconvolving the lowest-energy band from higher energy absorption, using the Strickler-Berg formula 32
k~alc=2.880X 1O-9n2 in which kr is the radiative rate constant, k ISC is the rate constant for the intersystem crossing, and k IC is the internal conversion rate. Since ¢< 1, k ISC + k IC = k nr ::::: 1/T. The knrare shown in Table II for ANDS in isooctaneltoluene solvent mixtures at room temperature. An explanation for the variation in the fluorescence quantum yield with the composition of these solvent mixtures [Fig. 7(b)] is as follows: Evidence presented above suggests that the intramolecular NH2 -+ N0 2 CT state lies below the S -+ N0 2 CT state. Furthermore, a 3 ( 17"17"*) state lies only slightly above the NH2 -+ N02 CT state. Intersystem crossing to this triplet state, perhaps thermally assisted at room temperature, may contribute significantly to the
4027
non radiative decay of the ANDS fluorescent singlet in isooctane. As the toluene mole fraction is increased in the solvent mixture, the energy of the NH2 -+ N0 2 CT state drops relative to the 3 (-1T17"*) state, and nonradiative decay via intersystem crossing is blocked. Thus, the fluorescence quantum yield increases with toluene concentration at low values of toluene mole fraction. As the energy of the CT singlet drops still further, relative to the ground state, this reduced energy gap enhances the rate of internal conversion, and the fluorescence quantum yield again drops as knr again increases. Within experimental error, kr stays essentially constant. From the steady-state emission spectra, the energy gap between the lowest-energy fluorescing CT excited-state and the ground state is EIhe::::: 17 000 cm - I. This small energy gap undoubtedly causes the internal conversion process to compete favorably with that of fluorescence. The high internal conversion rate and large difference between kr and k~alc may be due to a change in the excited state character andlor molecular geometry. Hence the radiative rate for a S -+ N0 2 CT state is larger than that for a NH2 -+ N02. Another reasonable possibility is that the molecule becomes linear and perhaps planar in the lowest energy CT state. However, this hypothesis can not be proven by the present results. A similar account has been used to explain the rapid internal conversion process as seen in acenaphthylene 33 and biphenylene. 34 For nonlinear optical materials which are capable of forming an intramolecular CT state, at the molecular level, the second-order hyperpolarizability, {3, is considered to be (Ref. 2) {3={3add +{3er. From Eq. (1), the {3er term is directly proportional to A/-Lg,e> the difference of the excitedstate and ground-state dipole moments. For ANDS, from Table IV, A/-Lg,e ::::: 50 D. Thus, the highly polar character of the lowest CT excited-state of ANDS makes a significant contribution to its high value of the second-order susceptibility as previously measured. 9 This conclusion, based primarily upon information obtained from the spectroscopic behavior of ANDS in its excited-state, is contrary to that made by Robinson et al.,4 who suggested that the high efficiency of second harmonic generation of ANDS is attributed solely to its molecular packing in the crystal. V. SUMMARY AND CONCLUSION
The results obtained from our investigation of the spectroscopy and emission kinetics of ANDS and its model compounds in various solvent media at different temperatures support the following conclusions: ( 1) The absorption spectra of ANDS are similar to those of ANDP and MNPS model compounds, suggesting that the lowest energy absorption band is due to a S-+N02 CT transition. However, the red edge of this band, which disappears upon protonation, is likely due to the NH2 -+ N02 CT transition. (2) Upon excitation, ANDS emits with a low fluorescence quantum yield and its emission characteristics (i.e., band energy, quantum yield, and lifetime) are highly solvent sensitive. This evidence suggests that the emitting
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4028
O'Connor et al.: Charge-transfer excited states (+)
(-)
sl H2N-cp-S-cp-N02 )
(+)
(-)
,,
,, ,, ,, ,, ,
,,
,, ,, ,
S 1 (H2N-CP-S-cp-N02 )
FIG. 13. Schematic energy level diagram for ANDS.
state is a cr state which is different from the CT absorbing state. (3) The results of the solvatochromism study suggests that a full electron charge is transferred from the NH2 group to the N0 2 group in ANDS prior to deexcitation of the fluorescent state, giving an excited-state dipole moment that is significantly larger than the ground state dipole moment. ( 4) The anisotropy obtained from the polarized fluorescence-excitation spectra of ANDS indicates the angle between the emission and absorption dipoles is _ 31°. This angle is consistent with conclusions (1) and (2). (5) Large differences between the radiative decay rates, kr and k~alc, also indicate that the emitting state of ANDS is different from the lowest absorbing state. In addition, a significant geometry change may occur during the excited-state lifetime. Evidence gathered above supports the conclusion that for ANDS there exists an intramolecular CT state (NHr +N0 2 ) located below the S-+N02 cr state, as shown in Fig. 13. Due to the molecular geometry of ANDS in its ground state, electronic absorption for the direct transition from the ground state to this amino-to-nitro CT state is forbidden. However, upon excitation to the sulfurto-nitro CT state, the excited ANDS molecules undergo rapid relaxation to the amino-to-nitro cr state prior to emission. ACKNOWLEDGMENTS
Work at the University of California, Riverside was sponsored by the Committee on Research, University of
California, Riverside. This work was performed in part by the Jet Propulsion Laboratory, California Institute of Technology, as part of its Center for Space Microelectronics Technology, which is supported by the Strategic Defense Initiative Organization, Innovative Science and Technology Office through an agreement with the National Aeronautics and Space Administration. G. E. W. wishes to thank the Air Force Office of Scientific Research for partial support. We thank Dr. I. Gorodisher and Dr. P. Kitipichai for the preparation and purification of the ANDS. 1M. Sigelle, J. Zyss, and R. Hierle, J. Non-Cryst. Solids 47,287 (1982). 2D. J. Williams, Angew. Chern. Int. Ed. Eng. 23, 690 (1984). 3y. Wang, W. Tam, S. H. Stevenson, R. A. Clement, and J. Calabrese, Chern. Phys. Lett. 148, 136 (1988). 4D. W. Robinson, H. Abdel-Halirn, S. Inoue, M. Kimura, and D. O. Cowan, J. Chern. Phys. 90, 3427 (1989). sB. F. Levine and C. G. Bethea, J. Chern. Phys. 66, 1070 (1977). 6J. Zyss and J. L. Oudar, Phys. Rev. A 26, 2028 (1982). 1S. K. Kurtz and T. T. Perry, J. Appl. Phys. 39, 3798 (1968). 8J. L. Oudar and D. S. Chernla, J. Chern. Phys. 66, 2664 (1977). 9H. Abdel-Halirn, D. O. Cowan, D. W. Robinson, F. M. Wiygul and M. Kimura, J. Phys. Chern. 90,5654 (1986). IOJ. B. Hyne and J. W. Greidanus, Can. J. Chern. 47, 803 (1969). 11 E. M. Graham, V. M. Miskowski, J. W. Perry, D. R. Coulter, A. E. Stiegman, W. P. Schaefer, and R. E. Marsh, J. Am. Chern. Soc. 111, 8771 (1989). 12J. N. Demas and G. A. Crosby, J. Phys. Chern. 75, 991 (1971). 13R. F. Kubin and A. N. Fletcher, J. Lurnin. 27, 455 (1982). 14Eastman Kodak Company, Kodak Laser Dyes, Kodak Publication JJ169, 1987, p. 38. 15p. R. Hammond, Opt. Cornmun. 29, 331 (1979). 16J. R. Lackowitz, in Principles of Fluorescence Spectroscopy (Plenum, New York, 1983). 11M. Shinitzky and Y. Barenholz, J. Bio. Chern. 249, 2652 (1974). 18D. E. Damschen, C. D. Merritt, D. L. Perry, G. W. Scott, and L. D. Talley, J. Phys. Chern. 82, 2268 (1978). 19D. B. O'Connor, G. W. Scott, D. R. Coulter, V. M. Miskowski, and A. Yavrouian, J. Phys. Chern. 94, 6495 (1990). 20D. B. O'Connor, G. W. Scott, D. R. Coulter, and A. Yavrouian, Macromolecules 24, 2355 (1991). 21 A. Mangini and R. Passerini, J. Chern. Soc. 1952, 1168. 22H. H. Szmant and J. J. McIntosh, J. Am. Chern. Soc. 73, 4356 (1951). 23D. W. Sherwood and M. Calvin, J. Am. Chern. Soc. 64, 1350 (1942). 24E. A. Fehnel and M. Carmack, J. Am. Chern. Soc. 71, 2889 (1949). 25T. C. Werner, in Modern Fluorescence Spectroscopy (Plenum, New York, 1976), Vol. 2, Chap. 7. 26H. Mataga, Y. Keifu, and M. Koizumi, Bull. Chern. Soc. Jpn. 29, 465 (1956). 21E. McRae, J. Phys. Chern. 61, 252 (1957). 28W. Rettig, R. Haag, and J. Wirz, Chern. Phys. Lett. 180,216 (1991). 29 Handbook of Chemistry and PhYSics, 56th ed. (Chemical Rubber, Cleveland, 1975). 3OE. Lippert, Z. Naturforsch. Teil A 10, 287 (1955). 31 0. S. Khalik, H. G. Bach, and S. P. McGlynn, J. Mol. Spectrosc. 35, 455 (1970). 32S. J. Strickler and R. A. Berg, J. Chern. Phys. 37, 814 (1962). 33 A. Samata, C. Devadoss, and R. Fessenden, J. Phys. Chern. 94, 7106 (1990). 34N. Ohta, M. Fujita, H. Baba, and H. Shizuka, Chern. Phys. 47, 389 (1980).
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