J. Phys. Chem. 1993,97,8152-8157
8152
Fluorescence Emission and Conformational Changes of 1,3,5,7-Tetra-t-butyl-s-indacene(TTBI) C.Gellini and P. R. Salvi' Laboratorio di Spettroscopia Molecolare, Dipartimento di Chimica, Universith di Firenze, Via Gino Capponi 9, 50121 Florence, Italy K. Hafner Znstitut fur Organische Chemie der Technischen Hochschule, Petersenstrasse 22, D-6100 Darmstadt, Germany
Received: February I , 1993; In Final Form: April 26, I993
The electronic properties of 1,3,5,7-tetra-t-butyl-s-indacene (TTBI) in the second excited singlet state have been studied by absorption and fluorescence spectroscopy. Fluorescence from S2 may be related, as in the azulene case, to the large energy gap between S2 and SIand to the high SO S2 oscillator strength. Absorption, fluorescence excitation, and fluorescence spectra have been measured on rigid TTBI solutions at 77 K. Several emissions have been observed which have been assigned to different conformers of TTBI. Photoselection data indicate that the symmetry of TTBI differs in the SZstate with respect to the ground state with minor structural changes.
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Introduction The fluorescence emission of large organic molecules usually occurs from the lowest excited singlet state as SI-SO transition (Kasha rule).' Nonradiative transitions are responsible for the low quantum yield of fluorescence of higher excited electronic states." In the Bixon-Jortner models the efficiency of these processes depends on the density of thevibronic manifold in quasiresonance with the initially excited level. In addition, the interaction strength is larger as the energy difference between electronic states becomes smaller.2 As a consequence, for S, (n > 1) excitation, the system relaxes within picoseconds (or less) to si. Exceptions of this rule are of great interest as they represent a direct probe of relaxation properties of upper electronic states. It has been already pointed out that most of the S, states are potentially l~minescent.6~~ In fact, a weak fluorescence from highly absorbing S, states may be observed before deactivation using ultrashort excitation pulses. Experimental evidence has been reported in a number of molecules."g A second possibility arises when the energy gap AE(S&) is %hE(s~-So).In this case the Franck-Condon factors of the lowest vibrational level of S2 with the vibrational states of S1 are small and the S2 SO radiative decay channel may compete efficiently with the nonradiativedeactivationfrom S2. Under low-intensity excitation conditions,fluorescencefrom SZhas been reported for azulene1@'3 and several thiones.I4J5 Quite recently, the emission from the second excited level of nonalternant 1,3,5,7-tetra-t-butyl-s-indacene (TTBI) has been reported.16 Emphasis was given to the photophysical behavior of TTBI with a discussionof major deactivation routes of Sz from picosecond transient absorption data. Here we wish to report more specifically on the spectroscopic properties of S2, following a previous investigation on the ground state.17 Absorption, fluorescenceexcitation, and fluorescencespectra of TTBI in fluid solution at 77 K have been measured. It comes out from these experimentsthat TTBI has different S2 SOemissions, depending upon the excitation wavelength. These are interpreted as due to different conformers of TTBI arising from the hindered rotation of butyl groups with respect to the ring moiety. The S2 state has been probed also by fluorescence depolarization.'* These data and theabsorption spectrum indicate that the molecular geometry of TTBI in the S2 state, although of higher symmetry, does not differ appreciably from that in the ground state.
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0022-3654/93/2097-8 152$04.00/0
Experimental Section TTBI was prepared according to the reaction scheme described by Hafner et al.I9 It has been already reported on the thermal instability and sensitivity of TTBI toward air and light.20 During our experiments we have found that aerated solution samples degrade easily, as shown by the bleaching of their red color. Stability of TTBI over a sufficiently long period of time was obtained by repeated thaw-and-freeze processes on solutions kept under vacuum. The room temperature absorption and fluorescence spectra of these samples are coincident with those of past reports.16 The choice of solvent and concentration is important for lowtemperature fluorescence measurements. Dissolving TTBI in cyclohexane or n-alkanes, a polycrystalline matrix is obtained, which strongly scatters the incident light. On the contrary, on cooling rapidly a diethyl ether-isopentane solution (3:7 ratio, respectively) of TTBI to liquid nitrogen temperature, a perfectly clear glassy sample is formed. The optical quality may be optimized for concentrations 110-4 M. In these conditions we have been able to measure fluorescence spectra with excitation wavelengths very close to the S2 origin, without interference of stray light. Due to the glassy nature of our matrix the molecules may undergo rotational reorientation.*8*21,22 The matrix was allowed to relax at 77 K = 1-2 h before the start of each experiment. A tunable pulsed dye laser operating at =7 Hz was used as excitation source. With commercially available dyes it was possible to study the fluorescence spectrum of TTBI as a function of wavelength from the S2 origin to 337 nm, corresponding to the N2 laser emission. Generally, each dye was tuned to its intensity maximum to enhancethe fluorescence signalexcept when scanning through the vibronic structure of S2. In this case the excitation wavelength was set in correspondence of the absorption band maxima. The laser beam was first collimated with a system of two lenses and then focused on the sample with a 170-mm converging lens. A small portion of the incident beam was diverted, before striking on the sample, by a beam splitter, sent to a photodiode, and taken as normalization signal for the fluorescencespectra. The quartz cell, 1 mm thick and containing the solution sample, is immersed in liquid nitrogen in a homemade cryostat and rotated -45O around the vertical axis with respect to the incidence direction. Bubbling of the cryogenic liquid was almost completely eliminated by slow evacuation of the sample 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8153
,
(0-0) 183.50 s m
I
-1
7 17.5.50 c m -1
ri
1
.05
0 ' 15000
-
10000
-22000
20000
18000
w a v e n u m b e r (cm
w a v e n umber
( cm - ')
Figure 1. SO S1 absorption spectrum of TTBI in isopentane-ether solution at room temperature. The vibronic intervals (cm-l) from the (0-0) band at 9270 cm-1 arc also shown in the spectrum.
compartment with a rotary pump. With this arrangement, the signal is collected 90° from the incident beam quite easily and focused onto the entrance slits of the monochromator, kept fixed, together with the exit slits, at 100 pm. Both signals, from the PMTconnected to the monochromator and from the photodiode, were sent to a box-car averager and fed to a PC for data processing. The fluorescence excitation spectra in the range 5250-5600 A were obtained with the same experimental apparatus, using Cumarine 540A as tunable exciting dye and with the monochromator wavelength set at 5575, 5600, or 5700 A, according to the experimental requirements. By insertion of the polarizing optics along the incidence and emission directions, the fluorescence polarization ratio of the S1 SOtransition was determined. We performed two types of experiment, depending upon the relative orientation of the polarizer and the analyzer. The fluorescence intensities I,, with both incidence and emission radiation vertically polarized, and Ivh, with the analyzer horizontal with respect to the polarizer, were measured. A scrambler was placed in front of the monochromator to eliminate fluorescence depolarization due to holographic gratings. As already noted,ll other sources of depolarization are present in this experiment. Their effect may be taken into account through a correction factor calculated repeating previous experiments with the incident beam horizontally polarized. In these conditions Zu and 1,hare measured and from the theory's it results Zu = Zhv. The deviation from the ideal rhh/Zhv = 1 ratio expresses depolarizing effects correlated to the instrumentation or to the sample and must be considered for the determination of the polarization ratio &,,/Zvh. The TTBI absorption spectrum has been measured at room temperature in various solvents and at 77 K in isopentane-ether with a Cary 5 spectrophotometer under ~20-cm-1experimental resolution. Due to the strong variation of the molar extinction coefficient through the spectrum, different TTBI concentrations were chosen according to the oscillator strength of the electronic transition.
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Electronic Transitions of lTBI In this sectionan overview of the electronic absorption spectrum of TTBI and qualitativeconsiderationson the S2- SOfluorescence
will be presented. The TTBI spectrum in cyclohexane solution at room temperature has been reported to discuss deactivation processes from S2.16 Our results complement these data and provide a more detailed spectroscopic analysis of excited state properties. The absorption spectrum of TTBI in isopentane-ether solution at room temperature is shown in Figures 1-3. Three main absorption regions are found in the range 200-1200 nm and have been assigned as SO S1,SO SZ,and SO Sg transitions,
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-
16000
-')
Figure 2. Absorption (A, left) and fluorescence (F, right) spectra of TTBI in isopentane-ether solution at room temperature in the region of the second electronic transition. (0-0) 32605 cm-l'
I
A.5
J 21 I 5
2930
I I
04
3640Y I L. ,
38000
36000
32000
34000
w o v e n u m b e r ( c m - ')
F i p e 3. Absorption spectrum of TTBI in isopentanbether solution at room temperature in the region 38 m 3 2 OOO cm-I. according to semiempirical MO calculations.16 The first, SO SI, has origin at -9270 cm-l and extends in the near-infrared up to 16 500 cm-l ( ~ 6 0 nm) 0 (see Figure 1). The SO S1 oscillator strength has been approximately determined from the observed spectrum to be 0.004, suggesting that the transition is very weakly allowed or forbidden (or, alternatively,a combination of both). A Franck-Condon band structure is observed with maxima at 9520 and 9830 cm-l. The corresponding vibronic intervals, 250 and 560 cm-I, fit the totally symmetric vibrations 245 and 572 cm-I, active in the Raman spectrum.17 At higher energies a strong band is observed at 10 655 cm-I followed by a vibronic profile quite similar to that accompanying the electronic origin. On an intensity basis, this band should be assigned as false origin shifted -1385 cm-l from (0-0). Since S2 has a relatively high oscillator strength cf = 0.22, from our own measurements), it is plausible a borrowing mechanism from S2 to SI induced by the 1385-cm-1 mode. A third band is found at 12 900 cm-l with a long tail up to -16 500 cm-l. The energy difference (3630 cm-l from (0-0); 2245 cm-l from the falseorigin) cannot be related easily to vibrational frequencies of TTBI. Figure 2 shows the absorption spectrum of TTBI in the region of the second electronic transition and the corresponding fluorescence emission, at room temperature. The two spectra are similar each to the other, a mirror symmetry being evident between them. The fluorescence spectrum has vanishing intensity below 600 nm and no evidence of a band counterpart of that observed in absorption at 19 810 cm-1. This suggests that the latter is the origin of a new electronic state, in agreement with MO predictions about the occurrence of the S3 state -2000 cm-1 above S2 with calculated small oscillator strength.'6 The S2 SOfluorescence emission of TTBI represents a violation of Kasha rule. It was suggested16 that this effect is related to the large energy gap between S1 and S2, -9100 cm-1, which decreases the rate of internal S2 S1 conversion, and to the relatively large SO S2
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8154 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
A
3
/
C
I?
217
ES”’
-1
-0) 1 8 1 6 2 c m
2
ES” 1
560 nm E
I 0 2 1000
20000
-
19000
18000
c-c
ES’
wavenumber (cm-’)
557 nm
Figun 4. &J S2 absorption spectrum of TTBI in isopentanbcther M. solution at 77 K. Lower trace, 1W M;upper trace,
oscillator strength. This is peculiar also of the S2- SOfluorescence of azulene.lG13 In both cases comparable energy gaps AE(Sr SI) and AE(S1So) are found. The third absorption region extends in the range 3 10-270 nm with oscillator strength f Y 0.06 and has been assigned as SO S9 transition.16 The origin occurs at 32 605 cm-l. As shown in Figure 3, a vibronicprogression on the 7 10-cm-1 mode is distinctly seen up to u = 5, although for the last two members it is difficult to establish the band maximum. The activity of this mode in SS indicates a large distortion of the equilibrium structure in the excited state with respect to the ground configuration along the corresponding normal coordinate. It is worth noting, on the contrary, that most of the SO S2 intensity is carried by the origin band.
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1 a500 wavonumbmr
18000
(em-’)
S2 fluorescenceexcitation spectra of TTBI at 77 K.ES’, ES”,and ES”’have been measured with the fluorcscenoc window set at 557, 560, and 570 nm, respectively. The letters A to G correspond to ( 0 4 ) transitions of TTBI conformers,as discussed in the text. Vibronic from the strongest (0-0) bands are also shown for each intervals (c”) spectrum. Figure 5. &J
m
TBI
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The Second Excited Singlet State of TTBI
A. Absorption and Fluorescence Excitation Spectra. The absorption spectrum of TTBI in the S2 region in isopentane ether solution at 77 Kis shown in Figure4. At low concentrations ( Y 10-4M) only one sharp single band is observed at 18 162cm-l (Figure 4, lower trace), which is thereforeassigned as theelectronic So S2origin. The band is red-shifted with respect to the absorptionmaximum at room temperature by e 2 2 0 cm-l. Note, however, that it roughly corresponds to the crossingpoint between the absorption and fluorescence spectra of Figure 2. When the concentration is increased to 10-3 M, a vibronic structure is seen with peaks 247,389,547,742, and 1028cm-l from the electronic origin, which are easily correlated to ground state totallysymmetric modes.17 The 547-cm-’ frequency is intermediate between the two Raman modes 572 and 522 cm-l.17 We have measured three distinct SO S2 fluorescence excitation spectra varying the detection window through the fluorescence emission. They are shown in Figure 5 as ES’,ES”, and ES‘” with detection at 557, 560, and 570 nm, respectively. The ES’ spectrum has origin at 18 170 cm-l in close agreement with the absorption result. Its vibronic structure is coincident with that of the absorption spectrum although with different intensity. The ES’’spectrum is red-shifted with respect to ES’ as much as the fluorescence window, e 8 0 cm-’, and has a similar band profile. However, a band, 125 cm-1 from the origin at 18 090 cm-1, has no counterpart in the ES’ spectrum. On shifting the fluorescence window toward 570 nm, the strongest remains unalteredat 18 090cm-’withashoulderat 18 040cm-l. Weaker transitions occur at 17 900 and 18 260 cm-1. The analysis of the absorption and excitation spectra is not simple. It may be tentatively assumed that the ES’’spectrum is due to hot transitions, Le., from a low-frequency torsional mode of SOto S2,SO,” S2. Being the energy difference between the ES’and ES”origins e 8 0 cm-1, the Sop S2 transition at 18 090 cm-l should have at 77 K an intensity -0.22 that of the
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I \ (bbbb)
(1)
Figure 6. Conformational isomers of 1-t-butylindacene(TBI,left) and 1,3,5,7-tetra-r-butyle (TTBI,right). The number of isocnergetic conformers is also shown in parentheses assuming & ring symmetry.
18 170-cm-l transition and then be visible in a absorption experiment. However, the spectrum of Figure 4 does not show any peak (or, at least, shoulder) on the red side of the 18 162-cm-1 band. In addition, in this hypothesis, on exciting the sample with photons of energy 18 090 cm-1, the fluorescence spectrum should be coincident with that obtained exciting directly at 18 170cm-l. On the contrary, as weshall seebetter in the following, fluorescence emission may be observed from the 18 090-cm-l level in various excitation conditions without observing that from 18 170 cm-1. This suggests that ES’ and ES“ refer to two different emitting species. In a previous paper17 we have concluded from MO ab initio calculations that the torsional motion of the butyl group in l-tbutyl-indacene (TBI) with respect to the indacene ring gives rise totwostableconformen (seeFigure6),one(a) with theequatorial Cbut-Cm bond toward the ring H in position 2 and the second (b) with the same bond looking at H in position 8. The energy difference A E between the two ground-state conformations as a function of the torsional angle is large, ~ 3 4 cm-’, 0 according to 3-21G calculation^,^^ (a) being more stable than (b). When the calculation is repeated with the 4-31G basis set, a similar energy difference, ~ 3 6 cm-1, 0 is obtained. In both cases, however, AE decreases considerably, up to N 100 cm-l, when the structure of
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8155
1,3,5,7-Tetra-t-butyl-s-indacene (0-0) 18170 cm-’
rC
(0-0) 18090 cm-1 1
1 El 1
h
E
- 253
‘ I
E-387
(0-0) t 252
n
- 240
/!(
1
I-
F
- 738
(0-0) t 420
(0-0)t 513
18500
18000
17500
wavenumber (cm -’)
& fluorescence spectra of TTBI in isopentanbether solution at 77 K as a function of the exceas energy with respect to Sz.For each fluorescence spectrum the vertical bar locates the excitation wavenumber, corresponding to the band maxima of the ES‘ spectrum (see Figure 5). The letters, E, B, and C have the same meaning as in Figure 7. S2
+
&
i! A
I
I
18500
18000
17500
-
wavenumber .~ (cm -’) F’igure 8. S2 & fluorescence spectra of TTBI in isopentanbether solution at 77 K as a function of the excess energy with respect to S2.For
Fwre 5.
each fluorescence spectrum the vertical bar locates the excitation wavenumber, corresponding to the band maxima of the ES” spectrum. The letters C, D, and F have the same meaning as in Figure 5.
the two conformations is optimized. It follows, on the basis of the Boltzmann distribution,that TTBI should exist at 77 K almost completely in the (aaaa) conformation (Le., with all equatorial bonds directed outwards) in the ground state. Therefore, the absorption spectrum of Figure 4 must be assigned to the most stable conformer of TTBI, (aaaa). On the other hand, since the excitation spectrum depends not only on the number of absorbing species but also on their fluorescencequantum yield, if the latter quantity is sufficiently high, transitions due to less populated conformers may be observed. It may be noted that, assuming an energy difference between the (a) and (b) conformations in the glassy matrix of e 2 0 0 cm-1, the fluorescence quantum yield of a %on-aaaa” conformer should increase with respect to that of (aaaa) by a factor of =SO, in order to observe the excitation spectrum with intensity comparable to (aaaa), as it results for ES’and ES” of Figure 5 . The relaxation kinetics of TTBI from the S2 state has been discussed in terms of fast S2 SOinternal deactivation.16 The conformationaleffect on quantum yields may be related to the activation of the S2 SI conversion, larger in the (aaaa) case than for the other conformers. B. Emission Spectra. If the excitation spectra are due to different conformers of TTBI, as is proposed in the last section, evidence of this should be found also in the S2 SOfluorescence spectra. These latter have been measured at 77 K as a function of the excess energy with respect to Sz,up to 337 nm. In Figures 7 and 8 we report the fluorescence spectra exciting in correspondence of the ES’and ES” band maxima of Figure 5, respectively. In the first case, most of the spectra may be assigned assuming that the fluorescent level is 18 170 cm-l, whatever the excitation energy. However, on exciting at (0-0) + 390 cm-1, one band is observed at 18 050 cm-1, not related to the vibronic structure from the 18 170-cm-l level, in good correspondence with the B shoulder of Figure 5 . With (0-0) +
505-cm-l excitation, a second band is found at 18 090 cm-1, denoted by C (Figure 7, fourth spectrum from the top), in addition to the usual fluorescence emission from 18 170 cm-l. With (00) 747 cm-l excitation the two emissionsbecome less resolvable in the spectrum. More complex fluorescence profiles are observed exciting into the ES” structure. The fluorescence from the 18 090-cm-1 level is seen with (0-0)excitation (Figure 8, uppermost spectrum)and with (0-0) 252 cm-I (Figure 8, third spectrum from the top). In addition, new emissions may be isolated. Consider,for instance, the (0-0) + 513-cm-1 excitation. Here also the fluorescence from 18 220 cm-l (denoted by F in Figures 5 and 8) is clearly seen. A third independent fluorescence is associated with the (0-0)+ 420-cm-l excitation,starting with D (18 13Ocm-1; Figure 8, third spectrum from the top). Note that in this case also the F fluorescence may be barely seen. Therefore, the independent emissionsare at least five, from the B, C, D, E,F levels of Figures 7 and 8. Fluorescence properties may be discussed on the basis of the molecular geometry of TTBI. According to calculation^,^^^ while s-indacene has in the ground-state CU symmetry with alternating doubleand singleC-C bonds, the TTBI ring structure has been approximated by DU with more balanced bond lengths due to the inductive effect of the butyl substitution. Experimentally,19 the TTBI ring structure is in good agreement with these predictions. Since two conformational minima are found for each butyl rotation in 1-r-butyl-indacene,2‘ = 16 conformers must be associated to TTBI. The most stable conformer, (aaaa) with all butyl group a8 in (a), has energy E. Next, flipping one group from (a) to (b), four conformers (aaab) are obtained of energy E‘ higher than E. Repeating the procedure with two (aabb), three (abbb) and four (bbbb) butyls, the number of isoenergetic conformers, i.e., their degeneracy, is 6, 4, 1, respectively,with energy 2(E”-E),3(E’-E),4(E’-h‘)above (aaaa)
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8156 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
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TABLE I: Observed SO S, Energies ( E L in cm-9 of the lTBI Conformers and Shifts A = E L - 18 170 cm-1’ A EA ~XIJ Calc assignment a1a3asa7 18 170 0 0 0 E bla3a~a.l 18 220 +50 A1 +45 F ambsa7 A2 -135 B a1bsasa.l 18050 -120 alalasb 18090 -80 AI + A2 -90 C blb3asa7
B
I I
bla3bp.l albsasb, bla3ash
18260
+90
17 900 18 130
-270 -40
281 2Az AI + A2
+90
G
-270
A
-90
D
IWI
1 ’ 1
c
~
a1asbsb7
A
504 nm
465 nm
81bsbw
2A1 + A 2 -45 blbsap7 a1ahb7 2Az+A1 -225 albsbsb7 blb3asb7 2(A1+ Az) -180 bibsbsb7 0 The value 18 170 cm-1 is the transition energy of the most stable TTBI conformer. The calculated shifts result assuming a Cs ring symmetryfor the %state. A1 is the energy shift of the bla3ap7 transition and A2 of the albsasa7 transition with respect to ala3asa7, with best fit values A1 = 45 cm-1 and A2 = -135 cm-l (see text for details). Capital letters in the last column identify the ( 0 4 ) transitions of Figure 5.
(see Figure 6). It is plausible that the torsional potential of the butyl group in SZis different from that in SO.Therefore, five SZ ) are expected. The most stable conformer -SO (04transitions (aaaa) has (04) at 18 170cm-l, as already pointed out. Assume now that (aaab) has an energy minimum closer to that of (aaaa) in SZthan in SO,Le., their energy difference is lower than (E’-E) in SZby A. Then the (aaab) (0-0)transition would shift to lower energy by A with respect to 18 170 cm-I, and, as a consequence, the (aabb), (abbb), (bbbb) transitions by 2A, 36, and 4 4 respectively. If, on theother hand, the energy difference between the (aaaa) and (aaab) minima in Sz were larger than in SOby A, the shift would be on the blue by the same amount, Le., A,2 4 3 4 4 4 according to the number of flipped butyls. Summarizing, the four origins, in addition to 18 170 cm-’, would be all, in this hypothesis, either on the blue side or on the red side of the (aaaa) origin. The fact that we observe fluorescence emissions both on the blue (i.e., the F peak) and on the red side (B, C and D) of the band at 18 170 cm-l indicates that the ring geometry of TTBI is slightly less symmetrical in SOthan in SZ. A better agreement with experimental results is obtained assuming that the butyl substituent has different torsional potentials in SO, according to the site of substitution. If the (0-0) transitionis shiftedto higher energy by A1 in the (blaaa) conformer and to lower energy by A2 in (abaaa), then the transition energies of allother conformersmay bepredicted. The results arereported in Table I. To verify the hypothesis, we have performed conformational calculations on 1-t-butyl- and 3-t-butylindacene with the optimized STO-3G Ca ring structure.17 It turns out that the energy difference between the (a) and (b) conformers increases in 1-t-butyl- and decreases in 3-t-butyl-indacene, using this structure, with respect to that calculated for the ground state Dzr symmetry. It is expected that, with a CU ring geometry more closelyapproaching that of SO,the energy differences become smaller than in the present calculation but still of opposite trend, in agreement with previous speculations. In this hypothesis the transition energies of the conformers may occur on both sides of the (aaaa) origin, 18 170 cm-l. In addition, the largest shifts, 2A1 and -2A2, occur for the conformers with only two butyls rotated from (a) to (b), blasbsa7 and albsasb, respectively (butyl rotation in the site indicated by the subscript). Therefore, in principle we need to identify only the transition energies corresponding to these conformers for a complete assignment. Most probably, the largest downshift does not correspond to any
I
I
I
I
18500
18000
17500
17000
-
I
I
I
16500
16000
wavenumber (cm -’)
Figure 9. Sz fluorescence spectra of TTBI in isopentanbether solution at 77 K with excitationwavelength 504,465, and 337 nm (from top to bottom). The letters A, B, and G have the same meaning as in Figure 5.
of the previous fluorescence origins, from B to D. Increasing the excitation energy, the whole fluorescencespectrum becomes more unstructured, as expected, and has a marked red shift. The fluorescence emissions with excitation at 504,465, and 337 nm are shown in Figure 9. Most of the fluorescence due to previous conformers disappears in these spectra. Only the B origin at 18 050 cm-1 is observed. To the red of this a new emission is found at h17 900 cm-1, which is predominant with excitation at 337 nm. The latter spectrum is quite similar to that measured at room temperature (see Figure 2) and must be interpreted, on the basis of past considerations, as due to a1bsa5b. Note that the fluorescence origin has a weak counterpart in the A feature of the ES’” excitation spectrum of Figure 5 . As a consequence we have 2Az = -270 cm-I. It is more difficult to locate the most blue-shifted fluorescence origin. In the spectra of Figure 9 a band at 18 260 cm-l is observed, independent of the excitation wavelength, in good coincidence with band G (Figure 5; ESNf spectrum). This may be taken as the origin of b1a3b5a7, corresponding to the 2A1 shift. In this case 2A1 = 90 cm-1. A1 and AZ being known, the fluorescence shifts may be calculated for all conformers. They are reported in Table I and compare satisfactorily with experimental results. According to this assignment, all theobserved originsare associated withconfor“ having at most two rotated butyls. C. Wotosaection Mel8mewpts. It has been known for a long timel*al that the fluorescence emission of isotropic rigid solutions is polarized and that the polarization ratio
with Z, and IVhas defined in the Experimental Section gives information on the symmetry of the emitting state. This is particularly useful in the TTBI case in order to assess how large is the difference between the S, and SZequilibrium geometries. If both states have D2r symmetry, the transition moments for absorption and emission may be along the short (S),long (L)
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8157
1,3,5,7-Tetra-t-butyl-s-indacene
Conclusions In this paper we have considered the fluorescence emission of 1,3,5,7-tetra-t-butyl-s-indacene (TTBI) from its second excited singlet state. The observed emission arises from Sz,in contrast with the Kasha rule, and may be related to (i) the large energy gap between the SIand S2 states and (ii) the high oscillator strength of the SO Sz transition. The present studycontains two major results. From comparison of So S2 absorption and fluorescence excitation spectra and analysis of the fluorescence spectra, evidence of different conformations of TTBI, due to the t-butyl torsion with respect to the molecular plane, has been reached, in agreement with MO calculations. Second, TTBI has in SOa structure similar to that of S2, though less symmetrical as its results from fluorescence spectra. In conclusion, electronic properties of antiaromatic nonalternant molecules are a promising field of spectroscopic research. It may be hoped that further investigation on similar systems will give a more complete understanding of their electronic structure.
-
-I
0.1
04
17250
I
r
17460
,
1
I
I
1
I
17660 17W 18060 wavenumber (cm-1)
,
1&50
' 18/60
Figure 10. The polarization ratio P of the TTBI fluorescence emission as a function of wavenumber for a rigid 10-4 M isopcntane-ether solution a t 17 K.
in-plane,or normal (N) to-planeaxes and from theory's it follows that P assumes only two values, and -1/3, depending upon the relative orientations (parallel or perpendicular, respectively) of the absorption and emission directions. On the other hand, for c 2 h symmetry there is a unique symmetry axis and the transition moments may be parallel (A,) or perpendicular (B,) to this axis. Any direction in the molecular plane normal to the unique axis is allowed for one-photon fluorescenceand absorption processes, with no relationship between them. The polarization ratio depends therefore in this case on two parameters. Considering the mirror symmetry between absorption and emission spectra and their Franck-Condon origin, we may limit our discussion to the case of parallel absorption and emission directions. On the basis of theoretical considerations,'* it may be shown that for in-plane parallel transition moments
+
4a(a - 1) 3 = 2 4 1- a) 1 where a is the probability of emission (or absorption) along the in-planelong axis. Ifa = l;O (i.e.,absorptionandemissionmoment both along L or both along S) (rw/Ivh) = 3 and therefore P = Equation 2 is symmetric around a = '/z(i.e., both absorption and emission transition moments 4 5 O with respect to L) where (Zw/Ivb) = 4/3 and P = l/7. Summarizing, while in the DU symmetry the expected ratio is l/2, in C Z h the ratio depends on the transition direction and may vary from I / z to l/7. The polarization ratio of the TTBI fluorescence spectrum in a rigid 10-4 M isopentane/ether solution at 77 K is shown in Figure 10. The polarization ratio has an almost constant behavior around 0.4 as a function of wavegumber with maxima roughly corresponding to the fluorescence bands. This result indicates that no large departure occurs in SOfrom the D 2 h ring symmetry. IW/IVh
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Acknowledgment. This work was supported by the Italian Minister0 Universiti e Ricerca Scientifica e Tecnologica (MURST) and ConsiglioNazionale delle Ricerche (CNR) under the P.F.-M.S.T.A. program. References and Notes (1) Berlman, I. B. Handbook of Fluorescence Spectra of Aromaric Molecules; Academic Prws, New York, 1971. (2) Henry, B. R.; Siebrand, W. In Organic Molecular Phorophysics, J. Wiley: New York, 1973; Vol. 1, 153 p. (3) Freed, K. F. In RadiarionlessProcesses in Moleculesand Condensed Phases; Topics in Applied Physics, Vol. 15; Springer-Verlag: Berlin, 1973. (4) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Reo. 1977,77, 793. (5) Bixon, M.; Jortner, J. J . Chem. Phys. 1968, 48, 715. (6) Rentzepis, P. M.; Jortner, J.; Jones, R. P. Chem. Phys. Lerr. 1970, 4, 599. (7) Omer, G. C.; Topp, M. R. Chem. Phys. Lett. 1975,36,295. (8) Topp, M. R.; Lm,H. B. Chem. Phys. Lerr. 1977,50,412. (9) Ho, C.-J.; Babbitt, R. J.;Topp, M. R.J. Phys. Chem. 1987,91,5599. (10) Beer, M.; Longuet-Higgins, H. C. J. Chem. Phys. 1955,23, 1390. (11) Murata, S.;Iwanaga, C.; Toda, I.; Kokubun, H. Chem. Phys. Lett. 1972, IS, 152. (12) Rentzepis, P. M. Chem. Phys. Lerr. 1969, 3, 717. (13) Gillespie, G. D.; Lim, E. C. J. Chem. Phys. 1978,68,4578. (14) Huber, J. R.; Mahaney, M. Chem. Phys. Lerr. 1975, 30,410. (15) Hui, M. H.; De Mayo, P.; Suau, R.; Ware, W. R. Chem. Phys. Leu. 1975, 31, 257. (16) Klann, R.; Bauerlc, R. J.; Laermer, F.; Elsaesser, T.; Niemeyer, M.; Luttke, W. Chem. Phys. Lerr. 1990,169, 172. (17) Gellini, C.; Cardini, G.;Salvi, P. R.; Marconi, G.; Hafner, K. J. Phys. Chem. 1993, 97, 1286. (18) Albrecht, A. C. J. Mol. Specrrosc. 1961, 6, 84. (19) Hafner, K.;Stowasser, B.; Krimmer, H. P.; Fisher, S.;Bohm, M. C.; Lindner, H. J. Angew. Chem., Inr. Ed. Engl. 1986, 25, 630. (20) Hafner, K. Pure Appl. Chem. 1982, 54, 939. (21) Russel, P. G.; Albrecht, A. C. J . Chem. Phys. 1964, 42, 2536. (22) Scott, T. W.; Haber, K. S.;Albrecht, A. C. J . Chem. Phys. 1983,78, 150. (23) Heilbronner, E.; Yang, Z . Z . Angew. Chem.,fnr.Ed. Engl. 1987,26, 360.
