35
J. Phys. Chem. 1994,98, 3 5 4 6
Fluorescence, Fluorescence-Excitation, and Ultraviolet Absorption Spectra of brras-l-(2-Naphthyl)-ZphenyletheneConformed J. saltiel,' D. F. Sears, Jr., J.-0. Choi, Y.-P. Sun, and D. W.Eaker Department of Chemistry, The Florida State University, Tallahassee, Florida 32306-3006 Received: September 16, 1993' Fluorescence spectra of trans-l-(2-naphthyl)-2-phenylethene (NPE) obtained under varying conditions of excitation wavelength and oxygen concentration in methylcyclohexane are resolved into two distinct components by application of principal component analysis with self-modeling. The key to obtaining unique spectral solutions is the constraint that Stern-Volmer quenching plots for the individual conformers be independent of excitation wavelength. Resolved conformer fluorescenwxcitation spectra are obtained by application of principal component analysis on a matrix of fluorescence-excitation spectra. Consistency between pure component fluorescenceand fluorescen-xcitation spectrais established by use of a two-dimensionalfluorescenceexcitationemission matrix. Fluorescence quantum yields and the pure component fluorescencbexcitation spectra are employed to resolve the UV absorption spectrum of N P E into pure component conformer absorption spectra. The results are compared with those from earlier studies based on the analysis of fluorescence decay data.
Flexible molecules containing conjugated double bonds undergo facile rotation about essential single bonds and exist, in the ground state, as an equilibrium mixture of different conformations. The
SM and the KFA approaches are critically compared. Finally, we conclude with a short discussion of the mechanism for trans cis photoisomerization.
-
ReSUltS
s-cis and s-trans conformersor rotamers of 1,3-butadieneprovide the prototypical example.* Electronic excitation, in many such systems, leads to a reversal of single/double bond order, and as a consequence freely equilibrating ground-state conformers give noninterconvertingexcited molecules with different structures and properties. Since ground-state conformers have different absorption spectra, different excitation wavelengths, Lxc, usually lead to different populations of excited conformers and consequently to different photochemical and photophysical responses. Control of photochemical response through selective excitation of ground-state conformerswas first postulated for trienes related to vitamin D by Havinga, who generalized this concept in his principle of nonequilibrating excited rotamers (NEER).3 In achieving a quantitative elucidation of the photochemical behavior of such flexible molecules, the photochemist is faced with the challengeof determiningthe absorptionand fluorescence spectra of each significantconformer. In our laboratory we have shown that matrices of spectra reflecting different mixtures of a set of conformers can be decomposed into pure component spectra by use of principalcomponent analysiswith self-modeling ( E A - S M ) , a method first p r o p a d by Lawton and Sylvestre.4 The first application of PCA-SM to a conformational problem involved a matrixoffluorescencespectraoftranr-l-(2-naphthyl)2-phenylethene (NPE)obtained for different Lxc and oxygen concentrati~ns.~The pure component spectra obtainedwere later shown to be at variance with spectra obtained by Bartocci, Mazzucato, and-workers6 through the treatment of fluorescence decay curves introduced by Birks et al., the KFA method.' In thispaper we report a refinement of the PCA-SM approach to the NPE system and describe pitfalls of the method and how they can be avoided. We extend the PCA-SM method to fluorescence-cxcitationtwo-dimensional matrices and show that these lead to pure component fluorescencbexcitation spectra. Fluorescence quantum yields of the individual conformers are determined and together with the fluorescencbexcitation spectra are employed to resolve N P E s UV absorption spectrum into conformer-specific UV absorption spectra. Results from the PCA~
Abstract published in Advance ACS Abstracts. December 15, 1993.
Mathematical Procedures. PCA is an algebraic method for extracting the principal componentsfrom mixturesthat are linear combinations of the components. It represents the mixtures in eigenvector space with the number of coordinatescorresponding to the number of principal comp0nents.8.~It begins with a m X n input matrix Y constructed from experimental spectra such that each row vector, S,corresponds to the n o r m a l i i intensities (sum of intensities equal unity) of a spectrum at n wavelengths. Each spectrum is a linear superposition of k pure component spectra, and them row vectors, m 2 k,correspond to m different experimental spectra having different pure component spectral contributions. Diagonalizationof the second moment matrix M
M = YTY
(2)
where F is the transpose of Y,gives k nonzero eigenvalues and their correspondingeigenvectors. The initial mdimensional space is reduced to a k-dimensional space in which each of the m row vectors is represented as a linear combination of the k eigenvectors. A theoretical matrix P, which is the best approximation of the input matrix in the k-dimensional space but contains less random experimental noise,1° can be constructed by use of the k X n eigenvector matrix V and the m X k combination coefficient matrix A:
P=AV
(3) The correct number of principal components, k, can often be suspected based on chemical knowledge. In practice, it is found by applying one or more criteria such as (a) the relative magnitude of the eigenvalues, (b) the residuals between the input matrix, Y,and the recovered matrix, P,' and (c) the significance of the (k 1)th eigenvector which is supposed to contain only random noise. For a two-component system each spectrum, St, can be representedby a point (a,, 01)in 2-dimensional eigenvectorspace. Since the normalization of spectral areas in Y
+
(4)
requires that the sum of the elements in S, be unity, the combination coefficients (at,@,)are constrained to fall on the normalization line defined by the eigenvectors V, and V, where
0022-3654/94/209~-0035~04.50/0 (B 1994 American Chemical Society
Saltiel et al.
36 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 Standard Deviation x
1
2.00
/
I I
ku
-1.00
-2.00 0.00
0.20
0.40
0.60
0.80
1.00
cx x 101 Figure 1. Combinationcoefficientsfor NPE fluorescencespectra in MCH at 30 ‘C for different bxc and [Oz]. The normalization line is defined by the eigenvectorsin Figure 2. The curves give the standard deviations of the global Stern-Volmer plots as a function of 8. LawtonSylvestre limits are designated by A I ,Az, El, and Bz; see text. the V, and Vbj are the j t h elements in each vector. The pure spectra SAand SBof components A and B, respectively, are also linear combinations of the eigenvectors, and their combination coefficients (a~,j3~) and (a~,j3~) fall on the normalization line, eq 5 .
The task of the self-modeling curve resolution procedure is the determination of the pure component combination coefficients. The initial method, proposed by Lawton and Sylvestre for a twocomponent system: and its somewhat modified versions1I-l4 are based on two constraints corresponding to two spectral properties.
The first requires that the fractional contribution X A and ~ X B be ~ 10, and the second requires that SA and SB have no negative elements. These two constraints define two regions in Figure 1, for acceptable pure component coefficients at the ends of the normalization line. Unique determination of pure component coefficients requires additional constraints. Pure component coefficients are identical to the outer limit coefficients determined bytheLawtonandSylvestreapproachat pointsAl andB1 (Figure 1) only if there exists a t least one wavelength where spectrum SA has zero intensity and spectrum SB is positive and at least one wavelength where the reverse applies. In many cases, the two pure component spectra are actually shifted with respect to one another so only one component contributes at the onset region WI and only the second component contributes at the tail region W2. By simultaneously seeking several zero or small negative intensities separately in the W1 and W2 regions, erroneous limits due to the presence of random noise in the spectra can be minimized.15--” When WI and WZregions are not well-defined, the Lawton and Sylvestre approach may result in fairly wide regions of acceptable pure component combination coefficients. Unambiguous determination of pure component spectra may then be achieved by finding and applying as many additional chemical constraints as possible for each specific p r ~ b l e m . ~ J S l ~ The Stern-Volmer Constant Constraint. When one of the experimental variables employed in generating the spectra in Y is the concentration of a quencher, it can be assumed that for an equilibrium mixtureof conformersa t constant Tthe Stern-Volmer constant Ksv of each individual conformer will be independent of A+,, . This condition provides independent constraints that can
aid in the determination of pure component coefficients.16 We now show that, for a system of two conformers, each conformer’s Stern-Volmer plot determines the other conformer’s pure component coefficients. The Stern-Volmer equation for component i is
where F and F a r e areas of the fluorescence (or fluorescenceexcitation) spectra in the absence and presence of quencher, Q, prior to normalization, respectively, x,‘J and xi are fractional contributions of the ith component without and with added quencher, and (Ksv)~= k.& is the Stern-Volmer constant of the ith component where k,,i and T,‘J are the quenching rate constant and the singlet excited-state lifetime of the ith component, respectively.16 For a two-component system the xA and hence the X B = 1 - X A can be calculated according to the lever rule
where 80 and j3 are coefficients for experimental spectra without and with added quencher, respectively, for a specific )hxcand j 3 ~ and j 3 are ~ the coefficients selected for components A and B, respectively. Substitution of eq 8 into eq 7 gives
(9) Thus, the Stern-Volmer equation for conformer A depends only on the combination Coefficients of conformer B, and vice versa. Minimizing the standard deviation of all points from common Stern-Volmer lines for each conformer yields j 3 and ~ 88. The corresponding a’s are obtained using the normalization condition, eq 5 . Fluorescence-Excitation Spectra via the Two-Mmensional Matrix Method. Two-dimensional matrices have found many applications, especially in rank annihilation factor Fluorescence/fluorescence-excitation two-dimensional matrices are referred to as EEM by Warner, who has exploited them in the analysis of mixtures of fluorescent compounds.22.23 In the present application weshow that an EEM can beused todetermine pure component fluorescence-excitation from predetermined pure component fluorescence spectra. The input matrix M is obtained by measuring fluorescence spectra a t systematically increasing excitation wavelengths, so that each row vector of M is a fluorescence spectrum (not normalized) at a single &,, and each column of M is an excitation spectrum at a single emission wavelength A+,,. The matrix M for a k-component system can be expressed as k
M = caizpi I= 1
where Zf and Sf are column and row vectors corresponding to normalized fluorescenceexcitation and fluorescence spectra of the ith component, respectively, and ai is a scaling factor related to the relative concentration of the components. If, due to nonlinearity in instrumental response, the raw spectra are distorted, the pure component spectra 2,and Si in eq 10 are correspondingly distorted. To avoid magnification of spectral random noise, we have found it advantageousto relegate correction of the resolved spectra to the last step in the analysis. When the pure component fluorescencespectra, Si, are known, their excitation spectra can be obtained easily using either of the following procedures. In the first, least-squares fitting the pure spectra to the row spectra in M gives fractional contributions of each component for each element in M and thus resolves each column in M into pure component fractional contributions, each
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 37
Spectra of NPE Conformers
1.50 L
1
h
2v)
1.20
23 0.90 CI
0.60 N .3 3
m
E
0.30
L 0
0.00
320
360
A (nm) Figure 2. Twosignificant eigenvectors from the NPE fluorescencespectral marrix based on different Lcand [Oz]for MCH at 30 OC. The largest and 0.92 X l k 5 . eigenvalues are 0.649, 1.06 X l k 3 , 2.34 X
proportional to a resolved 2,.In the second procedure normalization of each row vector (equivalent to division by its area F) converts M to Y. Performing PCA on Y leads to the theoretical matrix q, which can be expressed as
440
400
480
(nm) Figure 3, Pure component NPEA (broad) and NPEB (sharp) spectra for MCH at 30 OC based on the Stern-Volmer limits in Figure 1. The spectra are corrected for nonlinearity in instrumental response. Resolved spectra6 from the KFA treatment of decay data for NPE in 3-methylpentane at 20 OC are shown in the insert.
TABLE 1: Representative Fractional CoatributioM for Fluorescence Spectra and 0 2 Stem-Volmer ConstenQ for NPE Conformers in MCH. 30.0
k
P=CXSi i= I
where X,is a column vector composed of the fractional contributions, xi], of the ith component at thejth excitation wavelength. Multiplication of each xIj by the corresponding normalization factors Fj reverses the normalization procedure and converts P to M and the X,to excitation spectra 2,. NPE Fluorescence Spectra at Several bx,,[O,]. A set of fluorescence spectra (320-560 nm in 0.25-nm increments) of NPE (6.1 X 10-6 or 7.3 X 10-6 M) in methylcyclohexane(MCH) at 30.0 OC was obtained by using Ar-, air-, and Orsaturated solutions and exciting at 240-320 nm in 2.0-nm increments. Spectra for 240 Ib,,, < 260 nm were not utilized in PCA-SM analysis because they were relatively more noisy. The final input matrix (105 X 361) consisted of 105 spectra with intensities recorded at each 0.5-nm interval in the 320-500-nm range with 260 I bxc I 318 nm. Resulting significant eigenvectors and corresponding eigenvalues for the two-component solution are shown in Figure 2. Combination coefficients for experimental spectra adhere closely to the normalization line (Figure 1). Also shown on the normalization line are Lawton and Sylvestre limiting coefficients as well as coefficients based on the minima of plots of standard deviation from the two common Stern-Volmer lines as a function of the combination coefficient 0 (Figure 1). Pure componentspectra based on the best Stern-Volmer linesareshown in Figure 3 and, though strongly overlapping,exhibit a W1 region at theonset as required for a unique Lawton and Sylvestre solution. Table 1 lists fractional contributions for component A at different [02] and K ~ values v at selected bXc; XB = 1 - xA. Stern-Volmer plots for all the fluorescence data combined are shown in Figure 4. Two-DimensionalFluorescence/Fluorescence-ExcitationNPE Matrix. The input matrix (51 X 140) consisted of fluorescence spectra obtained from NPE in degassed MCH solution at 25.0 OC by recording fluorescence intensity at each 1-nm interval from 340 to 479 nm for 51 &,, at each 2-nm interval in the 260360-nm range. Fluorescence spectra for 340 1 bxc I 360 nm were corrected for prominent scattered light peaks by least-squares fitting pure component spectra and using the resulting fractional contributions and pure component spectra to reconstruct the shorter wavelength portions of the experimental spectra. Dot products between the eigenvectors obtained from PCA of this EEM and the pure component spectra in Figure 3 were used to define the pure component coefficientson the normalization line. The predicted pure component fluorescence spectra based on the
bxc, nm
Ar
air
0 2
260.0 266.0 272.0 278.0 284.0 290.0 296.0 302.0 308.0 314.0
0.423 0.422 0.420 0.396 0.336 0.204 0.192 0.179 0.203 0.198
0.584 0.592 0.596 0.569 0.509 0.342 0.319 0.310 0.341 0.336
0.733 0.729 0.719 0.707 0.634 0.469 0.461 0.440 0.484 0.472
B
A
145 131 136 136 145 151 135 140 140 137 140
806 73 1 716 751 716 748 726 739 766 735 750
5'
* 25'
Results are corrected for nonlinear instrumental response. The 0 2 concentration increases in the order 1.43 X 10-4, 2.31 X 10-3, and 1.16 X 1k2M. e Data for all bxc combined.
4'01 2.0
0.0 0.00
0.30
0.60
0.90
1.20
[02] x 102 Figure 4. Global Stern-Volmer plots for 02quenching of resolved NPEA and NPEB fluorescence in MCH at 30 OC.
eigenvectors from the normalized EEM were indistinguishable from the spectra in Figure 3. Since inconsistent responses may occur due to instrumental response drift over the long time period required to measure the 51 fluorescence spectra, a set of drift correction factors was generated by comparison of the NPE fluorescence-excitation spectrum for 260 1 bxo 5 360 nm and b,,, = 368 nm with the corresponding relative responses in the 368-nm column of the EEM. The drift correction was generally 1 2 % of the recorded intensity. Treatment of the EEM by the two procedures described above yielded identical fractional contributions for the compositions of the fluorescencespectra as
38 The Journal of Physical Chemistry, Vol. 98, No. I , 1994
Saltiel et al.
Standard Deviation x
,x
1.00
3.00
0.60
2.00
.3
v)
$
c,
c a
c-(
0.60 0.40
.M I
$
260
290
320
350
0.00
X
Q-1.00
0.00
230
1.00
41
0.20
L
g
cu
-2.00
-3.00
380
0.00
A (nm)
Figure 5. Pure component NPEA and NPEB fluorescencbexcitation spectra, uncorrected for nonlinearity of instrumental response. Points are from the EEM matrix for MCH at 25 OC, and lines are from PCASM of the fluorcscencbexcitation spectral matrix for MCH at 30 OC. 0.18
A
x
0.20
0.40
a
0.60
0.80
x 101
Figure 7. Normalization line for the eigenvectors in Figure 6 and combination coefficients for the experimental fluorescencbexcitation spectra and pure component combination coefficients. The curves give standard deviations of the global plots as a function of 6 (see text). The LawtonSylvtstre limits for NPEB are designated by B1 (first zero intensity) and Bz (first slightly positive intensity).
TABLE 2 Representative Fractio~lContributions for Fluorescence-Excitation Spectra and 0, Stern-Volmer C ~ ~ t a nfor t s NPE Conformers h MCH, 30.0 OC* XAb Ksv, M-' Xac,nm Ar air 0 2 A B
-0.12 t;,,l,,,l,,,l..,l.,.llllllllllllllllllll 230 260 290 320 350 380
A (nm) Figure 6. Two significant eigenvectors from the NPE fluorescence excitation spectral matrix based on different & and [Oz] for MCH at 30 OC. The four largest eigenvaluesare 0.452,2.03X lk3, 4.27 X lk5, and 1.19 X lks.
a function of &. The fractional contribution vectors were converted to resolved fluorescence-excitation spectra Z1 and Z2 (Figure 5) by multiplication with the normalization factors F. NPE Fluorescence-Excitation Spectra at Several X, [Oz]. In order to test the above method for obtaining pure component fluorescencGexcitation spectra and to improve the resolution of these spectra, a set of fluorescenceexcitation spectra of NPE (6.1 X 10-6 or 7.3 X 10-6 M) in MCH at 30.0 OC was obtained by using Ar-, air-, and 02-saturated solutions and recording emission at 4-50 nm in 2-nm increments. The input matrix (78 X 341) consisted of 78 base line corrected fluorescenceexcitation spectra with intensitiesrecorded at each 0.5-nm interval in the 220 I hxc I 390 nm range. Resulting significant eigenvectors and corresponding eigenvectors for the two-component solution are shown in Figure 6. The normalization line and the combination coefficients for the experimental spectra are shown in Figure 7. Also shown in Figure 7 are Lawton and Sylvestrelimitingcoefficientsas well as plots of standard deviation from the two conformer Stern-Volmer lines as a function of 8. Pure component fluorescence-xcitation spectra based on the best Stern-Volmer lines are shown in Figure 5. The spectra exhibit a W Iregionat theonset whereonlyoneof thecomponents contributes, as required for a unique Lawton and Sylvestre solution. The approach of the intensities to zero at the short wavelength portion of the spectra is artificial, reflecting loss of instrumental sensitivity in that region. Table 2 lists fractional contributions for component A at different [02] and Ksv values at selected L.Stern-Volmer plots for all the fluorescenceexcitation data combined are shown in Figure 8.
400.0 406.0 412.0 418.0 424.0 430.0 436.0 442.0 448.0
0.214 0.279 0.283 0.235 0.219 0.256 0.281 0.258 0.233
0.317 0.393 0.413 0.356 0.329 0.366 0.404 0.389 0.356
0.502 0.579 0.570 0.515 0.505 0.550 0.569 0.551 0.522
130 136 138 136 129 135 148 142 134 138 6'
*
748 734 687 708 724 735 727 742 735 728 38'
*
* Results are uncorrected for nonlinear instrumental response. b See footnote b, Table 1. See footnote c, Table 1.
10.0
8.0
ao
-
a
6.0 4.0
2.0
0.0P 0.00 0.30 0.60 0.90 1.20
[ O Z ] x 102 F@re 8. Global Stem-Volmer plots for 02fluorescencequenching based on resolved fluoracencbexcitationareas of NPEAand NPEs in MCH at 30 O C .
NPE Fluorescence Quantum Yields. Two procedures were employed to determine the Lcdependence of &of NPE. In the first, air-saturated MCH solutions of 9,lO-diphenylanthracene (DPA) in a polished 1-cm square quartz cell matched to the absorbance of NPE solutions were employed as fluorescence standards. The reference quantum yield for DPA, t$fi = 0.637, was based on comparison with a degassed DPA solution in cyclohexane for which the average reported t$f = 0.925 was as~umed.2*.~~ Fresh NPE degassed (nine freeze-pumpthaw cycles to 6.2 X 10-6 Torr) solutions in MCH (1.01 X 10-5 M for
Spectra of NPE Conformers
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 39
TABLE 3: The X, Dependence of NPE’s Fluorescence Quantum Yields in Degassed MCH,30.0 OC L n m XA 4, &,nm x.4 k
TABLE 4 T,K
270.00 276.00 280.00 284.p 286.06 288.06 290.00 290.26 295.86 300.06
0.421 0.395 0.391 0.336 0.266 0.218 0.204 0.203 0.190 0.178
0.679 0.705 0.699 0.685 0.687 0.693 0.724 0.751 0.757 0.710
310.W 318.06 320.00 325.g6
331.p 343.6O 354.00 358.06 360.06
0.203 0.214 0.224 0.232 0.204 0.616 0.685 0.678 0.753
0.749 0.749 0.702 0.722 0.732 0.663 0.642 0.630 0.645
a Quinine bisulfate actinometer. 9,lO-Diphenylanthraceneactinometer.
263 273 283 293 303 313 323 333
0.86 0.12 0.87 0.82 0.14 0.81 0.77 0.16 0.76 0.73 0.18 0.73‘ 0.65‘ 0.20 0.68 0.61 0.22 0.56 0.54 0.25 0.51 0.45 0.27 a Estimated uncertain_tyin 3js is i1096.6 From Figure 2 H in ref 59. e Assumed values from 4Ahc) = f ~ ( & ) 4fA+ f&&) 41, from data in Table 3.
SCHEME 1
bxc> 350.0nm; 1.01 X lWMfor&,,,< 350.0nm)inhomemade quartz (350.0 nm) 1-cm square cells were employed. Fluorescencespectra (triplicates) were measured using 0.5-nm bm increments. The position of each Lcwas checked followingeach set of three spectra by repeating the scan of the Rayleigh line using 0.1-nm b,,, increments. Though these slow scans always gave Rayleigh maxima that were within f0.3 nm of the programmed excitation wavelength, the position of peaks in sequentiallymeasured fast- and slow-scan spectra differed by as much as 1.2 nm. Accordingly, all previous NPE spectra were shifted to correct them for distortions in the wavelength scale due todifferences in instrumental responsetimes. Fractional contributions of NPE components in the quantum yield spectra were determined by including them in the matrix corresponding to Figures 1-3. When scattered light from the excitation source strongly overlapped NPE emission, fractional contributions were first based on a truncated matrix for the 370-500-nm spectral range and then adjusted to the full spectra. Fluorescencequantum yields and fractional contributions were based on spectra that were corrected for nonlinearity of instrumental response (Table 3). In the second procedure, air-saturated solutions of quinine bisulfate (QBS) in 1 N H2SO4 (deionized water was refluxed over KMn04 overnight, 10 g L-I, and then triply distilled) in a polished 1-cm square quartz cell matched to the absorbance of NPE solutions were employed as fluorescence standards, 4f= 0.546.z4 Fresh NPEsolutions in MCH (1.423 X 1W M) degassed in homemade clear fused quartz 1-cm square cells were employed. NPE photoisomerization was minimized by monitoring light intensity at a single bm.The maximum signal, attained in 2-3 s as the internal shutter opened completely, was used to determine the relative fluorescence intensity of NPE. Each measurement was complete within -6 s, and no drop in intensity was noted during that time interval. The statistical average of 25 A/D conversions collected after the complete opening of the shutter was used for QBS. These intensities were converted to full areas of fluorescencespectra, corrected for nonlinearity in instrumental response, from which 4s; for NPE were calculated as previously described.26 Fluorescence quantum yields as a function of bxc and corresponding fractional contributions based on spectra that were corrected for nonlinearity of instrumental response are given in Table 3. The temperature dependence of 3~was approximately determined using MCH solutions degassed in ampules equipped with 1-cm square Pyrex side arms. NPE concentrations of the two solutions employed were adjusted so that absorbance was 0.041 at each of the two excitation wavelengths, 316 and 355 nm. The relative intensity at ,A, = 378 nm of the fluorescence spectra was monitored and assumed proportional to the quantum yield. Except at 333 K where a small loss of signal intensity with time was noted, intensity signals remained constant during the time of the measurements. The results are given in Table 4.
Discussion A comprehensive review of the photophysical manifestation of rotational isomerism in trans-1 ,Zdiarylethenes appeared recent-
Temperature Dependence of & A L d in MCHI &3 16 nm) &355 nm) & (366 nm)b
*
&?; ,&
0
0
SI
NPK,
NPK,
and earlier reviews are also available.28-30 Distinct photophysical behavior of s-cis and s-trans isomers arising by rotation about the essential single bond in vinyl-substituted arenes was first proposed by Cherkasov to account for the spectroscopic behavior of 1-, 2-, and 9-vinylanthracenes.3’ In the absence of steric constraints the phenomenon of rotamerism is general for 1,Zdiarylethenes. It has been studied most thoroughly in NPE. In the ground state NPE exists as an equilibrium mixture of two nearly isoenergetic conformers, NPEA and NPEB (Scheme l), that are analogous to s-cis- and s-truns-polyene structures, respectively, since the ClCz bond in naphthalenes is somewhat shorter than the c2c3 bond.29b3r33 Though the possible involvement of NPEA and NPE, excited states was considered by Hammond et al. in the first study of NPE photoisomerization,” and &dependent photoisomerizationquantum yields were also reported by Fischer and co-workers?S it was Kovalenko et al. who first concluded that they reflected different photoisomerization efficiencies of the two conformers.36 The difference in fluorescence behavior of the two conformers was first described by Scheck and Alfimov et a1.28337 and more quantitatively, employing biexponential fluorescence decay analysis, by Haas and the Fi~chers.3~ The applicationof kinetic fluorescenceanalysis (KFA) to the determination of pure component fluorescenceand absorption spectra of NPEA and NPEB in n-hexane solution at 20 OC was introduced by Birks et aL7 and extended to other solvents and temperatures by Mazzucato and co-workers.6~3~ Steady-state fluorescence spectra of NPE obtained in MCH for different L,,, and oxygen concentrations also served as the first spectral input matrix for the application of the PCA-SM method to the resolution of pure conformer fluorescence ~ p e c t r a .A ~ comparative general description and evaluation of the KFA and PCA-SM methods has been given in a recent review.27 The present work allows a quantitative comparison of these complementary approaches to the resolution of the fluorescence, fluorescencb excitation, and absorption spectra of NPEA and NPEB.
Saltiel et al.
40 The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994
Relationship of KFA and PCA-SMParameters. The comparison of spectra obtained by the KFA and PCA-SM approaches must be based on the quantitative relationship of the experimental parameters derived and the assumptions made in each method. Since it is well established that NPEobeys the NEER principle38 (Scheme l), the fluorescence decay functions used in KFA are given by
-
~ ( t ) ~ ~ e x c ,= ~ eA,m )exp(-kAt)
+ ABexp(-k,t)
(12)
where j(t)(bxc,km) is the fluorescence intensity a t a specific bm from the conformer mixture (the bar over the symbol is used to identify quantities referring to the response of the conformer mixture) prepared by excitation at bXc as a function of time following a 6 excitation pulse. The biexponential decay parameters give conformer lifetimes 7~ = k ~ - land 7 g = k ~ ? ,and the preexponential factors give ratios of intensities of the fast and slow components.7J7
In eq 13 thefi(bxc) represent fractions of conformers i = A or B molecules excited at each specific A,,
where ;(bxc) are effective molar absorptivity coefficients of the conformer mixture, and the F,(bm) represent fluorescence intensities of each conformer at b,,,. As presently applied, the KFA method requires that the pure component spectra and the and 4fB of the individual fluorescence quantum yields conformers be sufficiently similar that one or more isoemissive wavelengths, bm’, exist where F(bm’)= FA(^^') = FB(~,,,’). The decay parameters in eq 12 monitored a t bm’ then give
which combined with eqs 13 and 14 definesf A andfBas a function of b,,, and, provided that the conformer fluorescence quantum yields are independent of bx,,thus gives the relative absorption spectra of the two conformer^.^^ Substitution of the decay rate constants together with FA(&xc)/fB(lexc) into eq 13 allows the determination of FA/FB as a function of bmand leads to the decomposition of the mixture fluorescence spectra into pure conformer fluorescence spectra.27 The procedure is somewhat tedious as it is done on a point by point basis. It can readily be shown27 that the relationship between fluorescence area fractional contributions, X I , from PCA-SM to excitation fractional contributions, fi, from KFA is
Assuming, as before, that 4fAand 4~.are independent of bx,, these pure component fluorescence quantum yields can be obtained from experimental quantum yields, &(kxc), and the corresponding XI(^^^) or J(bxc). For the KFA approach
can be rearranged to give
and the plot of &(Aexc) vsf A ( b x c ) should be linear with intercept
4fBand slope (4fA- 4f-h For the PCA-SM method is analogous to eq 17, and as it also allows the separation of the mixture quantum yields into individual conformer contributions, it follows that@
Use of eq 20 to eliminate theJ(b,,) and rearrangement gives
in fA(bxc) +j6(bXc) =1
and, accordingly, the slope and intercept of a plot of l/&,&,) vs xA(AcXc)should also give the pure component fluorescence quantum yields. Pure Component Fluorescence Spectra. The combination coefficients based on the Stern-Volmer constraints (Figure 1) give the corrected normalized pure component NPEAand NPEB fluorescence spectra shown in Figure 3. The small deviation of these spectra from a previously published set based on a somewhat more noisy spectral matrix is not real.41 It is due to an error in the application of correction factors for nonlinearity in instrumental response in our earlier worke42* Examination of Figure 3 shows that the fluorescence spectra of NPEA and NPEB span nearly identical wavelength ranges. It is precisely in such cases that application of the Lawton and Sylvestre approach fails to define uniquely the pure component combination coefficients. This is illustrated in Figure 1 where the first zero intensities at the tail and onset spectral regions are reached a t points A I and B I , respectively (designated by arrows). Thus, even the slight blue shift of SB relative to SAat the very onset of the spectra (Figure 3) does not sufficein giving the correct limitingcoefficients for SAwhen applying the Lawton and Sylvestre constraints. The reason is that both the onset and tail regions are highly sensitive to base line errors. For instance, if one seeks the first slightly negative intensity (99.9% purity by GLC). Methylcyclohexane from Baker or Aldrich, reagent grade, was washed with concentrated sulfuric acid, stirred over several portions of fuming sulfuric acid, washed with deionized water followed by aqueous sodium bicarbonate, dried over sodium sulfate, and distilled. In all instances, the UV transparency of purified MCH matched or exceeded that of Baker PHOTREX MCH. 9,lO-Diphenylanthracene (Aldrich) was purified by alumina chromatography yielding a white solid, mp 245.0-246.5 OC. Quinine sulfate (Matheson, Coleman and Bell, reagent) was recrystallized three times from water. Absorption Spectra. Absorption spectra were measured using a Perkin-Elmer Lambda-5 spectrophotometer. The NPE spectrum was recorded at 30.0 f 0.1 OC using matched jacketed 1.OO-cm Supracil quartz cells (Scientific Cell Co.). The temperature was regulated with a Haake FN constant-temperature circulator. Absorbances at 1.O-nm increments were transferred directly to a Dell Corp. 12-MHz 80286/87 microcomputer. Fluorescence Spectra. Fluorescence spectra were measured as previously described26with a Hitachi/Perkin-Elmer MPF-2A spectrophotometer extensively modified to interface with a Dell Corp. 12-MHz80286/87 microcomputer. Modifications include useof Bertan Associates PMT- 10A-N programmable (0 to-1000 V, remotely controlled via microcomputer digital analog operation) PMT power supplies, Slo-Syn M062-LE09 1.8O stepping motors equipped with 230-TH translator drives, a Data Translation DT2801-A A/D board, and home-built PMT amplifiers. The ratio mode was used in recording spectra. For spectra used in fluorescenceand fluorescen-xcitation matrices, 225-mL NPE solutions were circulated through a quartz cell (prepared from Kontes thin-walled square quartz tubing) using a Cole-Parmer N-07 149-10 all-Teflon piston pump with a homebuilt thermostated flow apparatus. (All parts in contact with the solution are either glass or Teflon, with the exception of the stainless steel temperature probe.) Temperature was maintained using a Neslab RTE-4DD refrigerated circulating bath. The temperature of the circulating solution was monitored about 1.5 in. above the path of the exciting light beam on the output side of the flow cell by means of an Omega Engineering Inc. Model 199P2 RTD digital thermometer equipped with a HYP-4 RTD hypodermic probe fitted with a Swagelok l/4-in. Teflon T-fitting. Argon or oxygen outgassing was achieved by bubbling the appropriate gas via a glass frit bubbler positioned near the bottom of the jacketed circulating cell sample outlet. Gas was bubbled for 2 h prior to recording spectra and continuously during the measurements using only brass fittings (regulator) and Teflon tubing and Swagelok fittings attached to the glass cell. The circulating cell reservoir was equipped with an Allihn condenser cooled with circulating ice-chilled water to eliminate evaporation loss. When outgassing, a bubbler filled with high-purity silicone oil was used to maintain a slight pressure a t the top of the condenser. UV spectra of the solutions before and after outgassing/fluorescence work showed no change in NPE absorbance. The bubbling rate was -1 bubble per second for the initial 2 h and was reduced to 1 bubble every 3 s during spectral scans. The residual oxygen concentration in Ar outgassed solutions was estimated by comparing the spectral intensity enhancement relative to an air-saturated solution for the Ar
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46 The Journal of Physical Chemistry, Vol. 98, No. 1. 1994
Saltiel et al.
bubbled and for a degassed NPE solution (six freeze-pump thaw cycles to 7.6 X 10-6 Torr). Degassed MCH solutions of NPE in quartz cells were employed to generate spectra used in the EEM matrix and for quantum yield determinations.
(36) Kovalenko, N. P.; Alfimov, M. V.; AWukaduov, A.; Schcck, Yu. B. Bull. Acad. Sci. USSR Ser. Chem. (Engl. Transl.) 1!V!), 11641167. (37) Alfimov, M. V.;Scheck, Yu. B.; Kovalenko, N.P. Chem. Phys. fett. 1976,43, 154-156. (38) Haas, E.; Fisher, G.; Fisher, E. J . Phys. Chem. 1978,82, 16381643. (39) Bartocci, G.; Masetti, F.; Mazzucato, U.; Marooni, G. J. J. Chem. Acknowledgment. This research was supported by the NSF, Soc., Faraday Trans. 2 1984,80, 1093-1 105. most recently by Grants CHE 93-12918 and CHE 90-14060. (40)Note that since normalization of the Spectralvectors in the E A - S M D.W.E. thanks the NSF for a predoctoral graduate research spectral matrix is accomplished by dividing the spectral area, N(&)S(&) = 1 in eqs 25 and 26 of ref 27. fellowship. (4 1) Saltiel, J.; Sun, Y.-P. Photochromism,MoleculesandSysteu; DOrr, H., Bouas-Laurent, H., Us.; Elsevier: Amsterdam, 1990, pp 64164. References and Notes (42) Corrected luminescence spectra published from our laboratory prior to 1991 were based on correction factors wed in the frequency domain to (1) Presented in part at the EUCHEM Conference on 'Photoisomerism obtain quantum yields. Application of these factors to the wavelength domain and Rotamerism in Organic Molecules", July 13, 1988, Assisi, Italy. (2) Sun, Y.-P.;Saltiel, J.;Scars, D. F., Jr. J. Am. Chem.Soc. 1988,110, distortsthe relative intensities by A2, thus overcompensatingfor loac of sensitivity 62776279 and references therein. at longer X; see e.g. refs 17, 41, and 43. (3) For a review see: Jacobs, H. J.; Havinga, E. Ado. Phorochem. 1979, (43) Saltiel, J.; Curtis, H. C.; Metta, L.; Miley, J. W.; Winterle, J.; I I, 305-373. Wrighton, M. J. Am. Chem. Soc. 1970,92,410411. (4) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 13,617633. (44)Parker, C. A. Photo1umfncsce~eofSolufions;Elsevia: Amsterdam, (5) Saltiel, J.; Eaker, D. W. J. Am. Chem. Soc. 1984,106,76247626. 1968; pp 252-258. (6) Bartocci, G.; Mazzucato, U.; Masetti, F.; Aloisi, G. G. Chem. Phys. (45) Saltiel, J.; Atwater, B. W. Ado. Photochem. 1988, 14, 1-90. 1986. 101. -.- -,. .., 461466. ... ... (46)Similar 02 quenching rate constants in MCH were obtained earlier (7) Birb, J. B.; Bartocci, G.; Aloisi, G. G.; Dellontc, S.;Barigelletti, F. for each of the three conformers of trans-l,2-di(2-naphthyl)ethene.*6 Chem. Phys. 1980, 51, 113-120. (47) Wismontski-Knittel, T.; Sofcr, I.; Das, P. K. J. Phys. Chem. 1983, (8) Malinowski, E. R.; Howery, D. G. Factor Analysis in Chemistry; 87, 1745-1753. Wileyi New York, 1980. (48) (a) Saltiel, J.; Scars,D. F., Jr.; Choi, J.-0.; Sun, Y.-P.; Mallory, F. (9) Sharaf, M. A.; Illman, D. L.; Kowalski, B. R. Chemometrics;Wiley: E.;Mallory, C. W. Unpublished results. (b) Eaker,D. W.W.D. Dissertation, New York, 1986. The Florida State University, Tallahassee, FL, 1984. (c) Choi, J.-0. M.S. (10) Beebe,K. R.;Kowalski,B. R. Anal. Chem. 1987,59,1007A-l017A. Thesis, The Florida State University, Tallahassee, FL,1989. (d) Choi, J.-0. (1 1) Sylvestre, E. A.; Lawton, W. H.; Maggio, M. S.Technometrics 1974, Ph.D. Dissertation, The Florida State University, Tallahassee, FL,1992. 16, 353-368. (49) Lamotte, M.; Morgan, F. J.; Muszkat, K. A.; Wismontski-Knittel, (12) Metzlcr.D.E.;Hams.C.M.;Reeves,R.L.;Lawton,W.H.;Maggio, T. J. Phys. Chem. 1990,94, 1302-1309. M.S.Anal. Chem. 1977,49,864A-872A. (50) Bartocci, G.; Masetti, F.; Mazzucato, U.; Spalletti, A.; Baraldi, I.; (13) Osten, D. W.; Kowalski, B. R. Anal. Chem. 1984, 56,991-995. Momicchioli, F. J. Phys. Chem. 1987, 91. 4733-4743. (14) Ramos, L. S.;Beebe, K.R.; Carey, W. P.; Sbnchez,E. M.; Erickson, (51) Wcare grateful toProfeaPor Mazzucato for bringing this adjustment B. C.; Wilson, B. E.; Wangen, L.E.; Kowalski, B. R. Anal. Chem. 1986,58, to our attention. 294R-3 15R. (52) A somewhat lower $1= 0.65 for a-NPD in cyclohexane at 293 K can (15) Sun,Y.-P.;Sears,D. F., Jr.;Saltiel, J. Anal. Chem. 1987,59,2515be based on Berlman53 by adjusting his value to +f = 0.93 for his 2519. 9, IMiphenylanthracene fluorescence standard.u* (16) Sun, Y.-P.; Sears,D. F., Jr.; Saltiel, J.; Mallory, F. B.; Mallory, C. (53) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic W.; Buser, C. A. J. Am. Chem. Soc. 1988,110.6974-6984. Molecuks; Academic: New York, 1971. (17) Sun,Y.-P.;Sears,D. F., Jr.;Saltiel, J. J. Am. Chem.Soc. 1989,111, (54) The&for & = 310 nmin ref 7 and for X, = 315 nmin ref 6 were 706-711. ._ omitted, fA 0.21 and 0.13 for X,= 290 and 310 nm were obtained by (18)-(a) Ho, C.-N.; Christian, G. D.; Davidson, E. R. Anal. Chem. 1978, interpolation from Figure 9b. 50, 1108-1 113. (b) 1980,52, 1071-1079. (55) Schulze, J.; Gerson, F.; Murrell, J. N.; Heilbronner, E. Helv. Chim. (19) McCue. M.;Malinowski, E. R. J. Chromatow. Acta 1961, 44, 428441. - Sci. 1983,21,229246, (56) Wettennark, G.; TegnCr, L.; Mirtensson, 0.Ark. Kem. 1968, 30, (20) (a) Lorber, A. Anal. Chim. Acta. 1984, 164, 293-297. (b) Anal. 185-212. Chem. 1985.57, 2395-2397. (57) Baraldi, I.; Momicchioli, F.; Ponterini, G. J. Mol. Struct. (21) Sbnchez, E.; Kowalski, B. R. Anal. Chem. 1986.58.496499. (THEOCHEM) 1984,110, 187-202. (22) Warner, I. M.; Christian, G. D.; Davidson, E. R.; Callis, J. B. Anal. (58) Scoponi, M.; Galinella, E.; Momicchioli, F. J. Chem. Soc.,Faraday Chem. 1977, 49, 564-573. Trans. 2 1988,84,95-103. (23) Neal, S.L.; Patonay,G.;Thomas, M. P.; Warner, I. M. Spectroscopy (59) Wismontski, T.; Fischer, G.; Fischer, E. J. Chem. Soc., Faraday 1986,1, 22-28. Trans. 2 1974, 70, 1930-1940. (24) M w h , S.R.; Phillips, D. J. Photochem. 1983, 23, 193-217. (60) (a) Saltiel, J.; D'Agostino, J. T.; Megarity, E. D.; Met& L.;Neuberga, (25) Maciejewski, A.; Steer, R. P. J. Photochem. 1986, 35, 59-69. K. R.; Wrighton, M.; Zafiriou, 0.C. Org. Photochem. 1973,3, 1-113. (b) (26) Saltiel, J.; Waller, A. S.;Sears,D. F., Jr.; Garrett, C. 2.J. Phys. Saltiel, J.; Charlton, J. L. Rearrangementsin GroundandExcited States, de Chem. 1993, 97,2516-2522. Mayo, P., Ed.; 1980, 3, 25-89. (c) Waldeck D. H. Chem. Rev. 1991, 91, (27) Mazzucato, U.; Momicchioli, F. Chem. Rev. 1991, 91, 1679-1719. 415436. (28) Scheck, Yu. B.; Kovalenko, N. P.; Alfimov, M. V. J. Lumin. 1977, (61) Sumitani, M.; Nagakura, S.; Yoshihara, K. Chem. Phys. Lett. 1974, 15, 157-168. 29.410413. (29) (a) Fischcr, E. J. Photochem. 1981,17,331-340. (b) Fisher, G.; (62) Aloisi, G. G.; Mazzucato, U.;Birb, J. B.; Minuti, L. J. Am. Chem. Fischer, E. J. Phys. Chem. 1981,85, 2611-2613. Soc. 1977.99. 6340-6347. (30) Mazzucato, U. Pure Appl. Chem. 1982,54, 1705-1721. (63) Saltiel, J.;-Eaker, D. W. Chem. Phys. Lett. 1980, 75, 209-213. (31) (a) Chcrhsov, A. S. Dokl. Acad. NAUK USSR (Engl. Transl.) (64) G&", H.; Eaker, D. W.; Saltiel, J. J. Am. Chem. Soc. 1981, 103, 1962,146,716-719. (b) Cherkasov, A. S.;Voldaykina, K. G. Bull. Acad. 7164-7169. Sci. USSR Phys. Ser. (Engl. Transl.) 1963, 27,630635. (65) Krysanov, S.A,; Alfimov, M. V. Chem. Phys. Left. 1983,98, 176(32) Shakked, 2. Unpublished results on the X-ray structure of trans178. 1,2-di(2-naphthyl)ethene quoted in ref 29a. (66) Wismontski-Knittel, T.; Das, P. K. J. Phys. Chem. 1984,88,2803(33) (a) Muszht, K. A.; Wismontski-Knitte1,T. Chem. Phys. Lett. 1981, 2808. 83.87-90. (b) Muszkat, K. A.; Wismontski-Knittcl. T.J. Phys. Chem. 1981, (67) Fischcr, E.;Castel, N. J. Mol. Srnrcr. 1986, 145, 367-376. 85, 3427-343 1 . (68) Saltiel, J.; Marinari, A.;Chang, D. W.-L.; Mitchmer, J. C.; Megarity, (34) Hammond, G. S.;Shim, S.C.; Van, S . P. Mol. Photochem. 1969.1, E. D. J. Am. Chem. Soc. 1979, 101,2982-2996. 89-106. (69) Elisei, F.; Mazzucato, U.;G(lmner,H . J. Chem.Soc.,Faraday Trans. (35) Kaganowich, M.;Fischer, G.; Fischer, E.; Goedicke, Ch.; Stegemeyer, I 1989,85, 1469-1483. H. Z . Phys. Chem. (Munich) 1971, 76, 79-84. (70) Charlton, J. L.; Saltiel, J. J. Phys. Chem. 1977, 81, 1940-1944.
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