276
J. Phys. Chem. 1900, 84, 276-281
Fluorescence Data of 0,y-Unsaturated Ketones Jens Erlksen Depaflment of Cbmistw, New York University, Washington Square, New York, New York 10003 (Received June 28, 1979)
Fluorescence has been observed from 20 P,y-unsaturated ketones of widely different structures in cyclohexane solution at room temperature. The emission data obtained include maxima of corrected emission and excitation spectra, lifetimes of singlet excited states, fluorescence quantum yields and rate constants, experimental and calculated natural fluorescence lifetimes,average fluorescence wavelengths, fluorescence centers of gravity,Stokes shifts, fluorescence bandwidths, and fluorescence quenching efficiency by oxygen. In addition, oscillator strengths of the absorption bands are determined.
Introduction Although light emission from organic molecules has been known for centuries, most fluorescence studies have concerned aromatic molecules, perhaps most systematically through the work of Ber1man.l Far less is known about fluorescence from other classes of organic molecules. As part of an investigation of the photophysical and photochemical properties of P,y-unsaturated ketones,2 we discovered that, as a general rule, these ketones fluoresce in dilute solution at room temperature, although the emimion efficiencies were lower than those typically found for aromatic molecules. Prior to our investigations only scattered reports on fluorescence of P,y-unsaturated ketones had appeared in the literatures3 There are probably several reasons for the scarcity of such reports: (a) while many aromatic molecules are commercially available, few @,y-unsaturatedketones are available from chemical supply houses and the routes to synthesize these ketones often require lengthy synthetic schemes; (b) P,y-unsaturated ketones are generally less stable and harder to purify than aromatic molecules; (c) since most P,y-unsaturated ketones are not Commercially available, these compounds have most often been in the hands of organic chemists who have not been primarily interested in photophysical properties of the compounds; and (d) because of the low fluorescence efficiency of P,y-unsaturated ketones the emission of these ketones is not as conspicuous as the visible emission seen from many aromatic molecules. The photochemical behavior of P,y-unsaturated ketones has been of interest to numerous researchers in the last two decade^.^ Our own studies in this area have involved 20 P,y-unsaturated ketones of widely different structures, suggesting that conclusions and generalizations obtained for these compounds could be extrapolated to other P,yunsaturated ketones. The observation of fluorescence from @,y-unsaturated ketones should prove useful in photochemical as well as photophysical studies of these compounds. The quantum yield (ai)in the absence of quenchers for any singlet derived process is given by
where ki is the rate constant for this process, the denominator contains the sum of the rate constants for fluorescence (kf), radiationless decay (kd), intersystem crossing (ksT),and reaction (k,) from this state, respectively, and 7s is the singlet excited state lifetime. For fluorescing compounds, 7s can usually be measured (see below) and if can be determined in some way, ki can be obtained *Address correspondence to the author at the Department of Chemistry, University of Aarhus, DK-8000Aarhus C, Denmark. 0022-3654/80/2084-0276$01.00/0
Chart I
2
1
4
3
5
7
6
a opp
0
0
8
11
15
19
22
10
9
12
13
14
16
17
18
21
20
23
24
from eq 1. It is clear that such rate constants are necessary for studying structure-reactivity relationships in photochemical reactions, just as absolute rate constants have been used in studies of ground state reactions. The structures of the @,y-unsaturatedketones used in this study as well as those of a few other compounds for 0 1980 Amerlcan Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 277
Fluorescence of fl,y-Unsalurated Ketones
TABLE I: Fluorescence Data of p,r-Unsaturated Ketonesa compd
Exmax,b nm
1 2 3 4
5 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
305 305 305 305 305 310 310 300 300 300
310 310 310
310 i 305 310 310 320 320
i i i 2
Em,,,C nm 406 398 398 39 6 398 398 414 400 400 400 414 420 416 408 406 400 388 39 0 370 386 396 406 340 387
A , ~ , nm ~
A F , ~cm-'
W? cm-'
410 402 402 402 402 399 416
41 3 404 404 403 404 400 41 7
4890 4604 4548 4535 4510 4824 4830
2379 2502 2462 2555 2501 2614 2427
I
i i i
1
i
i i i
415 422 416 412 409 405 394
417 423 41 7 41 3 411 408 39 6
4487 4650 4420 4438 5200 4690
2460 2393 2497 2506 2422 2260 2546
I
1
nm
1
37 3 392 399 409 343k 390'
374 393 391 41 0 345 39 2
II j j
4820 5300 3790' j
103p 2.6 2.6 2.3 2.7 2.6 1.6 1.5
i
i
i
2
2
i 2725 2904 2514 2525 1570m 1483p
3..3 4.5 3.7 2.5 2.2 2.5
r
2
j j
0.43 0.54 884n j
Maximum of corrected fluorescence excitation spectrum. Maximum of corIn cyclohexane a t room temperature. rected fluorescence emission spectrum. d Average wavelength of fluorescence curve, see eq 2. e Center of gravity of the Oscillator strength of the fluorescence curve, see eq 3. f Stokes shift, see eq 4. g, Fluorescence band width, see eq 5. N o t determined. Not determined since the So -+ SI and So -+ S, absorption bands So -+ S I absorption band, see eq 7. Reported value 341.8 nm, ref 14. Reported value 2960 cm-', ref 14. Reported value 1611 were not well separated. Reported value 1411 cm-', ref 13. cm-', ref 14. ' Reported value 0.879, ref 21. O Reported value 392.9 nm, ref 13. a
'
J
comparison and checking purposes are given in Chart I. The names of these compounds are given in the Experimental Section.
Results and I[)iscussion Spectra. Fluorescence emission and excitation spectra were obtained at room temperature in cyclohexane solution. All spectra were corrected for wavelength-dependent instrumental factors as described in the Experimental Section, Maxima for excitation and emission spectra are given in Table I and typical examples of fluorescence spectra of P,y-unsaturated ketones are shown in Figure 1 for compounds 7 and 16. The excitation spectra centered a t -300 nm (Table I) correspond to the first absorption band of ,O,y-unsaturated ketone^.^ The emission spectra of all the ketones studied have maxima close to 400 nm and in most cases a weak blue fluorescence is visible. It should be noted that the shape and the position of the fluorescence curves for P,y-unsaturated ketones (1-20) and saturateld ketones (21 and 22) are quite similar (Table I and Figure 1). Average Fluorescence Wavelength (Aav). The quantity A,, was (obtained as the recipical of P, computed from
where f(p) is a plot of the fluorescence intensity vs. wave are the limits of the number (P) and bmin and,,P fluorescence Values of A, are given in Table I. The fluorescence center of gravity (A,) was computed from
WAVELENGTH, nm
WAVE NUMBER. kcm-'
WAVELENGTH, nm
WAVE NUMBER, kcri'
(3)
Figure 1. Absorption and corrected fluorescence spectra of 7 and 16: excltatlon wavelength, 300 nm; concentration, 0.01 M In cyclohexane.
where f ( i i ) is defined as above.7 Values of ,A, are given in Table I. The Stokes shift (or Stokes loss, AP) is given by AP = Po - P, (4)
where ir0 is the wave number of the line of symmetry lbetween the absorption and the fluorescence band and scg is defined in eq 3.8 The Stokes shift represents a loss in excess vibrational energy over the equilibrium configuration of the molecule in SI during absorption and in So
XCg-l = ,b
=
D~(P) dv/ l f ( s ) dP
278
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980
Eriksen
TABLE 11: Singlet Lifetimes, Fluorescence Quantum Yields, Fluorescence Rate Constants, and Experimental and Calculated Natural Lifetimes of 0.r-Unsaturated Ketonesa exptl values
calcd values
compd
T~ ,b ns lO4Qfc 1 0 6 k , ds-' T ~ ns, ~ T , ( B W ) ,ns ~ T , ( F ) ,ns ~ T , ( S B ) , ns ~ 1.56 i- 0.05 21 1.3 744 247 7 28 730 0.41 i- 0.05 9.6 2.4 427 246 669 682 0 . 3 3 k 0.05 4.6 1.3 717 27 9 774 7 67 0.61 i 0.05 9.2 1.5 663 237 670 654 0.47 f 0.05 6.9 1.5 681 248 679 67 8 0.66 k 0.05 5.4 0.82 1222 382 984 1088 2.25 f 0.05 20 0.89 1125 436 1347 1317 1.7 i- 0.1 i i i i i i 2.4 f 0.1 1 1 i i 1 1 3.4 f 0 . 1 i 1 i 1 i 1 11 5.0 ?: 0.1 35 0.70 1429 206 528 571 12 4.91 k 0.05 72 1.5 682 149 433 438 13 4.47 i 0.05 53 1.2 843 182 475 505 14 4.16 i 0.05 46 1.1 904 294 785 737 16 0.96 i- 0.05 32 3.3 303 262 662 718 17 0.11 i- 0.05 3.2 2.9 344 j j j 18 1.06 f 0.05 12.5 1.2 804 j j j 19 0.82 i- 0.05 6.7 0.82 1224 j j j 20 1.29 i 0.05 6.7 0.52 1912 j j j 21 2.45 i 0.05 16 0.65 1531 1429 3822 4057 23 0.96 k 0.05k 93001 970 1.03 0.61 1.18 1.22m a In cyclohexane at room temperature. Singlet lifetime measured by single photon counting. Fluorescence quantum iield. Fluorescence rat,e constant, see eq 9: e Natural lifetime, see eq 10. f Bowen-Wokes, eq 11. Forster, eq 12. Strickler-Berg, eq 1 3 . Not determined. Not determined since So -+ S, and So + S, absorption band were not well separated. Reported value 0.95 ns, ref 14. Reference 14. Reported value 1.21 ns, ref 14.
1 2 3 4 5 6 7 8 9 10
J
during emission. The more the equilibrium configurations of the molecule differ in the two electronic states, the wider the separation between the absorption and emission spectra, and the greater the Stokes shift. In the present study D~ in eq 4 was approximated by the average of the position in wave numbers of the absorption and fluorescence maxima. Since AD is a small difference between two ) accuracy of this calculation is large values (p0 and D ~ the rather poor. Stokes shifts given in Table I show larger energy losses for @,y-unsaturatedketones (1-20) and saturated ketones (21 and 22) than for aromatic compounds (23),as expected for n,r* vs. T,T* transitions. Stokes shifts were not calculated for compounds where the absorption maximum of the S1 band was not clearly separated from the So S2transition. Fluorescence band widths ( W 6were calculated from = (p(2) - -*cg 2 ) 1/2 (5)
-
w
where
vcg is given in eq 3 and d2)is computed from P@)
= S p 2 f ( v ) dD/Sf(v) dr,
(6)
The bandwidths given in Table I for @,y-unsaturatedketones (1-20) and saturated ketones (21 and 22) are much greater than those of the aromatic compounds 23 and 24. Oscillator strengths (f) of the So S1transitions were calculated from the following equation in the cases where So Si was clearly separated from higher transition^:^ f = (4.32 X 1 0 - 9 ) j c ( ~dD ) (7)
-
-
where E(D) is a plot of the molar extinction coefficient vs. wave number. As expected (Table I), values off obtained for @,y-unsaturatedketones (1-20) are about an order of magnitude higher than values for saturated ketones (21 and 22). This enhancement of the n,r* transitions in @,y-unsaturated ketones has received much attention among theoretician^.^ Singlet Lifetimes. The lifetimes of the singlet excited state of P,y-unsaturated ketones were determined in cyclohexane at room temperature by using single photon counting.l0 Because of the short lifetimes (15.0 ns) of these
Flgure 2. Lifetime measurements by single photon counting. Lamp profiles, observed fluorescence decays (-e), and convoluted decays of 7 and 18. Concentration, 0.01 M in cyclohexane.
ketones, an iterative convolution technique was used in the data treatment to extract the true lifetimes from the observed fluorescence decays.l1 The procedure for determining lifetimes in this way has been described elsewhere." Examples of the results of this technique are shown in Figure 2 for compounds 7 and 16. All lifetime measurements were performed at least twice and the obtained values were reproducible to within f0.05 ns. Thus, the results of three lifetime measurements on 16 were 0.96, 0.94, and 0.99 ns. Lifetimes down to 0.1 ns could be ob~ 0.11 f 0.05 ns). To check the technique tained ( 1 7 , = a few model compounds were tested. With reported values from the literature given in parentheses, lifetimes determined in cyclohexane solutions were as follows: anthracene, 4.9 f 0.1 ns (4.9 nsl'); 24, 4.28 f 0.05 ns (4.3 ns13); 23, 0.96 f 0.05 ns (0.95 ns14). Values obtained for compounds 8-10 are also in good agreement with those reported independently by Dalton et al.% Table I1 lists the singlet lifetimes obtained for &y-unsaturated ketones in this study. Because of fast photochemical reactions4 the excited states of these ketones are quite short-lived. Fluorescence quantum yields (@f) were measured by comparing the corrected fluorescence intensity of a P,y-
Fluorescence of F,y-Unsaturated Ketones
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 270
unsaturated ketone of unknown af to the intensity of a reference compound with known ‘Pf. If the solvent, excitation wavelength, and slit widths are kept constant, the ratio of the two fluorescence intensities is given by I’”, a,, light absorbed by 2 -- = (area), -(8) F1 (area)l @f,l light absorbed by 1 where F2/Flis the ratio of the areas under the two corrected ernissioin curves plotted vs. wave number. In the present study, 24 was chosen as the reference compound for the following reasons: (a) the afof 24 has been reported in cyclohexane (af= 0.1813); (b) the fluorescence of 24 is centered at 3910 nm,13 close to the emission maxima of 0,y-unsaturated ketones, thus minimizing experimental errors due to correction of the observed spectra; ( c ) the value of affor 24 was determined at an excitation wavelength of 313 nm,13which could also be used for excitation of /3,y-urisaturated ketones; and (d) 7s of 24 is 4.28 ns, comparable to that of most &y-unsaturated ketones in this study. H[ence, in measuring the relative fluorescence intensities of 24 and 1-21, errors introduced by oxygen quenching of the fluorescence would be negligible after the solutions were purged with nitrogen. Using nitrogensaturated cyclohexane solutions of 24 and 12 of identical optical density (1.00) at the excitation wavelength (313 nm), we obtained a value of 24.9 for the ratio of the areas of the clorrected fluorescence bands plotted vs. wave number. From eq 8 and @f,2 = 0.18, the @f of 12 was determined as 7.23 X W. This latter compound was then used as a secondary standard for determining of all other &y-unsaturated ketones in Table 11. Each afvalue was determined twice and the estimated error is &lo%. Fluorescence rate constants ( k f )were calculated from eq 9, which is a variant of the more general eq 1. The kf kf = @f/7S (9) values listed in Table I1 for many &y-unsaturated ketones show a remarkable insensitivity to molecular structure. Indeed, almost, all values of kf in Table I1 are within a factor of 2 of lo6 s-l. Natural Fluorescence Lifetimes. The natural (or mean radiative) lifetime, 70, is defined as the singlet excited state lifetime a compound would have if its fluorescence quantum yield was unity. Hence, from eq 9 To = kfl = 7S/@f (10) Natural Lifetimes calculated from experimental data according to eq I O are listed in Table 11. A variety of formulae have been derived for calculation of 70 from the So S1 absorption and the fluorescence spectra. Four approaches are described below. (a) The simpliest equation is that of Bowen and Wokes.15 For wave numbers (in cm-l) and lifetimes (in ns), this equation takes the form of
-
kf = T
~
=- 2.9nZn2$~ ~ do
(11)
where n is the refractive index of the solvent, no is the wave number of the maximum of the absorption band, and .fc dii is the area under a plot of the molar extinction coefficient vs. wave number. (b) A more complex equation has been proposed by Forster l6
where -J is the wave number of the mirror symmetry point between ,the absorption and the fluorescence bands. Although Forster’s formula corrects for asymmetry in the
absorption band, it assumes mirror symmetry between the absorption and the emission band. ( c ) A more general equation has been derived Iby Strickler and Berg17 2.9n2 f ( c / n ) dn where ( P ~ ) , , =: ( . f ~ - ~ f (dp)/(.ff(v) n) dn) and .ff(n)dn is the area under the fluorescence curve plotted vs. wave number. Since f(v) appears in both the numerator and the denoininator of ( F - ~ ) , ~ the , formula does not require absolute values of the fluorescence intensities. (d) An almost identical equation is given by Birks a i d Dyson.le The only difference from eq 13 is that n2 is replaced by nf3/na, where nf and n, are the refractive indices of the solvent at the maximum of the fluorescence and the absorption bands, respectively. Values of 7o were calculated from eq 11,12, and 13 for a large number of P,y-unsaturated ketones (Table 11) as well as compound 23 which served as a check of the calculation method. Thus, T~ for 23 obtained from eq 13 was 1.22 ns in good agreement with a value of 1.21 ns calculated by Berlman14by use of the same equation. Calculation of T~ was only performed on compounds with clearly separated So S1 and So S2absorption bands. Refractive indices (n) of the solvent (cyclohexane) were calculated fromlg n2 = 1 + (a - bx)-l (14) where a = 0.9965, b = 0.01025, and x = v,: X where nav is the average wave number (in cm-’) of the fluorescence spectrum as defined in eq 2. For 8, = 25000 cm-l (Aav = 400 nm), n = 1.4396. Good agreement (generally within a factor of 2) is found between experimental T~ values of 0,y-unsaturated ketones and values calculated from the Forster (eq 12) or the Strickler-Berg (eq 13) formulae (see Table 11). This implies that, if 7s cannot be measured directly, a satisfactory value can be estimated from eq 10 if afcan be measured and if T~ can be calculated from eq 12 or 13. For the saturated ketone 21 the best agreement is found between the experimental T~ value and the value calculated from the Bowen-Wokes equation (eq 11,Table 11). The generality of this finding was not investigated. Fluorescence Quenching by Oxygen. The relative sensitivity of a compound to oxygen quenching is given by Lo/L = 1 + Pk,7,[0,] (15) where Lo and L are the relative fluorescence intensities in the absence and presence of oxygen, P is the probability of quenching per encounter, k, is the encounter rate constant, and 7s is the singlet lifetime of the substrate. From a study of the effect of air on the fluorescence of a large number of aromatic molecules in cyclohexane, Berlmanm presents a graph of Lo/L vs. TS from which eq 16 can be obtained, where 7s is in nsec. Lo/L = 1 + 0.060(7~) (16)
-
-
Ratios of LOIL were measured for a number of &-punsaturated ketones (Table 111). In all cases cyclohexane solutions of the ketones were purged with nitrogen, air, or oxygen for 3 min prior to the fluorescence measurements. In such a short time, no appreciable change in concentration of the solutes due to solvent evaporation occurred and purging for longer times did not change the fluorescence intensities. From Table I11 it can be concluded that oxygen has less effect on the fluorescence yield of B,yunsaturated ketones and alkanones than of aromatic
280
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980
Eriksen
TABLE 111: Effect of Oxygen Quenching on Fluorescence Yielda
New York) were recrystallized from ethanol. Compounds 21 and 22 (Aldrich) were purified by spinning band distillation. Compound 23 (Aldrich) was recrystallized from T S , ns ~ L,/L (air)c L,>L (O,)d compd cyclohexane, and 24 (Aldrich)was purified by sublimation. 11 5.0 1.06 1.18 Cyclohexane (spectrophotometric grade, Aldrich, Gold 12 4.91 1.06 13 4.47 1.22 Label) showed only end absorption in UV and no de14 4.16 1.26 tectable fluorescing impurities and was used as received. 16 0.97 1.12 A Raman scattering band due to the C-H vibrations is 17 0.11 1.03 observed at a frequency shift of -2880 cm-l relative to the 21 2.45 1.12 frequency of e x c i t a t i ~ n Thus, . ~ ~ excitation at 300 nm leads 24 4.28 1.32e 2.98 to a Raman band at 328 nm. At high spectrofluorimeter a In cyclohexane a t room temperature. b Singlet lifesensitivities this band was clearly present and was subtime determined by single photon counting. ' Nitrogen tracted before data treatment. vs. air-purged fluorescence intensities. Nitrogen vs. oxygen-purged fluorescence intensities. Reported value Purity Considerations. Each solute was carefully pu1.35, ref. 13. rified as described above until a single peak was obtained on two different GLPC columns corresponding to a purity molecules (cf. eq 16). Thus, it appears that, for a given of >99.9%. UV absorption spectra of the purified samples lifetime, n,r*co states are less sensitive to oxygen were in good agreement with published data where quenching than are . R , . R * ~states, ~ possibly due to a lower available. To test for the presence of fluorescing impur~~ value of P in eq 15 for n,n*co states than for T , . R * states. ities, several samples (1-5,16,17) were repurified, resulting The present study is not comprehensive enough, however, in quantitatively reproducible spectra. It should be noted to allow for the presentation of an equation of the form that fluorescencefrom P,y-unsaturated ketones (including of eq 16 for fluorescence quenching of P,y-unsaturated compounds in this study) has been reported from other ketones by oxygen. lab~ratories.~ Emission spectra were recorded at room temperature Experimental Section in cyclohexane on a Hitachi/Perkin-Elmer Model MPF-2A Materials. The names of the compounds used in this spectrofluorimeter operating at a geometry of 90". This study are as follows: 3-(l-cyclobutenyl)-3-methyl-2-buta- instrument allows for scanning of grating emission and none (I), 3-(l-cyclopentenyl)-3-methyl-2-butanone (2), excitation monochromators for recording fluorescence 3-(l-cyclohexenyl)-3-methyl-2-butanone (3), 3-(l-cycloemission and excitation spectra, respectively. The sample heptenyl)-3-methyl-2-butanone(4), 3-(l-cyclooctenyl)-3holder was a 1-cm quartz cuvet with four polished sides. methyl-2-butanone ( 5 ) , l-(l-cyclopentenyl)-2-propanone An 8-cm stem allowed for easy handling of the cuvet and (6), 3-methylene-2,2,5,5-tetramethylcyclohexanone (7), for purging samples with nitrogen, air, or oxygen. A9~10-2-octalone (8), A9~10-l-methyl-2-octalone (9), A9710Optical densities (OD) of -1.0, corresponding to a l,l-dimethyl-2-octalone ( l o ) , 1,4,4-trimethylbicycloconcentration of M of the substrates, were used [3.2.0]hept-6-en-2-one ( l l ) , 6,7-dimethylbicyclo[3.2.0]throughout this study. Such high ODs could be allowed hept-6-en-2-one (12), 7-tert-butylbicyclo[3.2.0]hept-6-enfor two reasons: (a) reabsorption of emitted light was 2-one (13), 6-tert-butylbicyclo[3.2.0]hept-6-en-2-one (14), unimportant because of the large Stokes shifts (see Figure 3,3-dimethyl-4-penten-2-one (E),3,3,5,5-tetramethyl-1,6heptadien-4-one (161, 2,2,7,7-tetramethyl-3,5-cyclo- 1 and Table I) and (b) because of the low @f values ( 104-10-2, Table 11) of P,y-unsaturated ketones, high ODs heptadienone (171, 3,5-~ycloheptadienone(18), tricyclowere necessary in order to observe the fluorescence. [4.4.1.12~5]dodeca-3,7,9-trien-ll-one (19), tricycloCorrection of Fluorescence Emission Spectra. A relative [4.4.1.12b]dodeca-7,9-dien-ll-one (20), cycloheptanone (21), emission sensitivity curve of the wavelength-dependent 3,3-dimethyl-2-butanone (22), p-terphenyl (23), and N,instrumental factors (sensitivity of the photomultiplier, N,N',N'-tetramethyl-p-phenylenediamine (24). the slit width, and the transmission efficiency of the Compounds 1-10,15, and 16 were obtained from Promonochromator) were obtained by a phototube calibration fessor P. s. Engel, Rice University, Houston, Texas. technique.25 This curve was permanently stored in the Compounds 1-5 were purified by column chromatography computer and all fluorescence emission spectra were cor(Silica Gel, eluent hexane/ether 19:l). Compound 6 was rected accordingly. purified by preparative scale VPC on a 0.25 in. X 300 cm Correction of Fluorescence Excitation Spectra. A column packed with 20% PDEAS on W100 Chromosorb calibration curve for wavelength-dependent excitation P (150 "C). Compound 7 was very pure (199.99%) and factors was obtained as follows. It has been reportedz6that was used as received. This compound had been repeatedly a uniform layer of sodium salicylate on a microscope slide purified until no phosphorescence could be detected at 77 has a constant fluorescence quantum yield (@f = 0.99 f K.z2 However, the fluorescence of the same sample was 0.03) with an emission centered at 443 nm when excited easily detectable (Figure 1and Table 11). Compounds 8-10 in the range 60-360 nm. Since light is absorbed totally were purified by preparative scale VPC on a 0.25 X 200 throughout the excitation range, the observed excitation cm column packed with 20% Apeizon L on 60-80 Chrospectrum monitoring emission at 443 nm directly gives the mosorb W (150 "C). Compounds 11-14 were obtained in desired calibration curve. pure form from Mr. Saadat Hussain, New York University, Uniform coatings on two glass slides were obtained from New York, New York. Compounds 15 and 16 were purified drops of a saturated methanol solution of sodium salicyby column chromatography (Silica Gel, eluent hexane/ late.27The two slides gave identical excitation curves when ether 99:l). Compound 17, synthesized as described earplaced at 45' angle to the excitation beam in the speclier,2ewas purified by spinning band distillation (Perkintrofluorimeter, indicating that the technique is reproduElmer Model 151 annular still), bp 75 "C (7 mm) [lit. bp cible. 48-51 "C (1.5 mm)23]. Compound 18, synthesized as deThe fluorescence excitation spectra were corrected acscribed earlier,2ewas purified by column chromatography cording to this calibration curve in the range 240-360 nm. (Silica Gel, eluent hexane/ether 99:l). Compounds 19 and Since high ODs ( 1.0) and large slit widths were necessary 20 (from Dr. C. W. Kim, New York University, New York,
-
Fluorescence of 0,yUnsaturated Ketones
to obtain the excitation spectra of P,y-unsaturated ketones, the maxima reported in Table I are only approximate. Lifetime Measurements. Singlet excited state lifetimes were me,asured by single photon countinglo with iterative convolution in the computer-assisted data treatment.ll The procedure used has been described elsewhere.2eThe filter used to isolate the excitation band consisted of Corning glass filter 7-54 (2 mm) + 2 cm of a solution of NiS04.6Hz0 (50 g) and KCr(S04)2-12Hz0(15 g) in 100 mL of HzO. This filter transmits 140% of the light at 300-320 nm and has no transmittance above 350 nm. The emission filter, which consisted of Corning glass filter 4-70 (2 mm) + 1 cm solution of NaNOz (75 g) in 100 mL of H20,had no transmittance below 400 nm and transmitted 150% of the light at 420-530 nm. This filter combination essentially eliminated all scattered light. Sample ODs of 1-2 at 300 nm were used corresponding to concentration of the solutes of M. All measurements were done in cyclohexane which was saturated with nitrogen by bubbling nitrogen through the solution for 5 min. Computer Program. A computer program was designed for performing the following computations: (a) correction of fluorescence emission spectra; (b) determination of fluorescence maxima of corrected spectra; (c) calculation of average fluorescence wavelength from eq 2; (d) calculation of fluorescence center of gravity from eq 3; (e) calculation of Stokes shifts from eq 4; (f) calculation of fluorescence band widths from eq 5; (g) calculations of oscillator strengths of absorption bands from eq 7; (h) calculatilon of natural lifetimes by using the Bowen-Wokes (eq ll),the Forster (eq 12), and the Strickler-Berg (eq 13) formulae; and (i) plotting of absorption and corrected fluorescence spectra on an x,y recorder (Figure 1). The fluorescence curves represent relative emission intensity vs. wave number and the absorption spectra are plots of molar extinction coefficients (in M-l cm-l) vs. wave number. The observed fluorescence and absorption spectra were read into permanent files point by point. On the average, 80 points were used for an absorption spectrum and 60 ]points for a fluorescence spectrum. Whenever possible, the results of the calculations were checked against literature values (see Tables I and 11). Acknowledgment. Financial support of this work by a Cottrell Grant from Research Corporation and Grant CHE-76-09566 from the National Science Foundation is gratefully acknowledged. I thank Professor Paul S. Engel, Mr. Saadat Hussain, and Dr. C. W. Kim for supplying compounds foir this study. I also thank Professor David I. Schuster foir helpful and critical comments and suggestions and for support throughout this work. I finally N
The Journal of Physical Chemistty, Vol. 84, No.
3, 1980 :281
thank Professor N. Geacintov for helpful discussions on fluorescence and single photon counting and Professor Christopher S. Foote for critical comments on the final manuscript. References and Notes I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Molecules”, 2nd ed, Academic Press, New York, 1971. (a) J. Eriksen, Ph.D. Thesis, New York University, Sept, 1976; (b) J. Eriksen, K. KroghJespersen. M. A. Ratner, and D. I. Schuster, J. Am. Chem. Soc.,97, 5596 (1975); (c) D. I. Schuster, J. Erksen, P. S. Engel, and M. A. Schexnayder, lbM., 98, 5025 (1976); (dl) J. Eriksen and D. I. Schuster, Mol. Photochem., Q, 83 (1978); (e] D. I.Schuster and J. Erlksen, J . Org. Chem., 44, 4254 (1979). (a) K. 0. Hancock and R. 0. Griier, Tetrahedron Left., 4281 (1971); (b) S. R. Kurowsky and H. Morrison, J . Am. Chem. Soc., 94, 607 (1972); (c) D. I. Schuster and C. W. Kim, ibld., 96, 7437 (1974); (d) T. R. Darling, J. Poullquen, and N. J. Two, lbkl., 96, 1247 (1974); (e) J. C, Dalton, M. Shen, and J. J. Snyder, IbM., 98, 5023 (1976). For reviews, see (a) K. N. Houk, Chem. Rev., 76, 1 (1976); (b) S. S. Hixson, P. S. Mariano, and H. E. Zimrnerman, Ibkl., 73, 531 (1973); (c) W. G. Dauben, G. Lodder, and J. Ipaktschi, Top. Curr. Chern., 54 (1975); (d) K. Schaffner, Tetrahedron, 32, 641 (1976). For a discusskn on the absorptbn spectra of p,yunsatwated ketones, see ref 4a, pp 45-48. Reference 1, p 28. V. L. Ermolaev, Opt. Spectrosc. (USSR), 16, 383 (1964). Reference 1, p 27. Reference 1, p 20. (a) W. R. Ware in “Creation and Detection of the Excited State”, A. A. Lamola, Ed., Vol. 1, Part A, Marcel Dekker, New York, 1971, Chapter 5; (b) L. J. Cline Love and L. A. Shaver, Anal. Chem., 48, 365 A (1976). H. E. Zimmennan, D. P. Werthemann, and K. S. Kamn, J. Am. Wpm. SOC.,96, 439 (1974); H. E. Zimmerman and T. P. Cutler, J. Cht?rn. SOC., Chem. Commun., 598 (1975). Reference 1, p 356. Reference 1, p 168. Reference 1, p 220. E. J. Bowen and F. Wokes, “Fluorescence of Solutions”, Longmans Green, London, 1953. Th. Forster, Discuss. Faraday Soc., 27, 7 (1959). S. J. Strlckler and R. A. Berg, J . Chem. Phys., 37, 814 (1962). J. B. Birks and D. J. Dyson, Proc. R . SOC.London, Ser. A , 275, 135 (1963). M. L. Snaverly, 0. G. Peterson, and R. F. Reithel, Appl. Phys. Lett., 11, 275 (1967). Reference 1, p 59. Reference 1, p 423. M. A. Schexnayder and P. S. Engel, Tetrahedron Left., 1153 (1975). L. A. Paquofie, R. F. Eizember, and 0. Cox, J . Am. Chem. Soc., 90, 5153 (1968). C. A. Parker, “Photoluminescence of Solutions”, Elsevier, New Ytxk, 1968, p 66. Professor N. Geacintov, New York University, private communlcatbn. Using an RCA 1P28 multiplier phototube, we obtained a response curve for the emission monochromator from 350 to 620 nm with 5-nm intervals. WHhln each interval the response curve was assumed to be linear, See also C. E. White, M. Ho, and E. 0.Welmer, Anal. Chem., 32, 438 (1960). R. Allison, J. Burns, and A. J. Tuzzolino, J . Opt. SOC. Am., 54, 747 (1964). F. S. Johnson, K. Watanabe, and R. Tousey, J. Opt. Soc. Am., 41, 702 (1951)
