J. Phys. Chem. 1983, 87, 1579-1582
group-transfer reactions, pericyclic reactions, cheletropic fragmentations, additions, eliminations, and conformational equilibria. Acknowledgment. The author thanks Prof. Saunders
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and Prof. Marcus for helpful discussions. This work was supported by grants from Research Corporation (through Pennwalt Corporation), the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation.
Electronic Excited States and Anomalous Fluorescence of Cyclopenta[ cdlpyrene Benjamin F. Plummer' and Zekl Y. Ai-Saigh Department of Chemistty, Trinity University, San Antonio, Texas 78284 (Received: July 22, 1982; In Final Form: November 1, 1982)
The electronic excited states of the yellow hydrocarbon cyclopenta[cd]pyrene(CPP) are characterizedby magnetic circular dichroism and UV spectroscopy. The signs of the B terms are calculated by application of PPP SCF CI calculations. Qualitative MO theory for a [15]annuleneperimeter is used as a basis to understand the origin of the B terms for the L and B states in CPP and to characterize the long-wavelength low-energy transition. An emission from CPP at 25 000 cm-' with a quantum yield of about and a lifetime of 3 ns is identified as an S2 So fluorescence. It is verified by an excitation spectrum.
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Introduction We are currently trying to identify polycyclic compounds with specific molecular architecture that leads to enhanced radiative emission from higher excited-state surfaces. Such emissions are often called anomalous fluorescence, although the phenomenon is increasingly reported. Azulene is the first example of this phenomenon and its verified fluorescence from its second excited state, S2 So, is well-known1* A similar radiative behavior of the second excited state in the naphthothiadiazines was recently verified by us.' Since theoretical considerations suggest that upper excited-state fluorescence is a function of Es, - E8,= AEgap,Bwe seek molecules that have a perturbation which enlarges AE,, from that of the parent compound. By perturbation of naphthalene with the fusion of an unsaturated bridge at the peri positions, acenaphthylene with AEBBp x 9000 cm-' is created and a very weak S2 So emission can be detected by laser ex~itation.~ Pyrene with AEgap= 1400 cm-' shows a Stokes fluorescence from S1 with a quantum yield 4F between 0.14 and 0.08 regardless of the excitation energy.'&13 The radiative emission yield from the second excited state of pyrene increases with excitation energy and with increasing temperature demonstrating a thermal repopulation mechanism for the very weak S2 So fluorescence. The annelated derivative 3,4-benzpyrene AEgap< 1000 cm-l also emits a very weak fluorescence from S2.14
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(1) M. Beer and H. C. Longuet-Higgins, J. Chem. Phys., 23, 1390 (1955). (2)G.Viswanath and M. Kasha, J. Chem. Phys., 24,574 (1956). (3)G.Binsch, E. Heilbronner, R. Jankow, and S. S. Schmidt, Chem. Phys. Lett., 1, 135 (1967). (4)R. C. Dhringra and J. A. Poole, J. Chem. Phys., 48,4829 (1968). (5)R. C. Dhringra and J. A. Poole, J. Phys. Chem., 72,4577 (1968). (6)J. Griesser and U. Wild, Chem. Phys. Lett., 52, 117 (1980). (7)B. F. Plummer and Z. Y. Al-Saigh, Chem. Phys. Lett., 91, 425 (1982). (8)R. Englman and J. Jortner, Mol. Phys., 18, 145 (1970). (9)B. F.Plummer, M. J. Hopkinson, and J. H. Zoeller, J.Am. Chem. SOC.,101, 6779 (1979). (IO) H. Baba, A. Nakajima, M. Aoi, and K. Chihara, J. Chem. Phys., 55, 2433 (1971). (11)A. Nakajima and H. Baba, Bull. Chem. SOC.Jpn., 43,1967(1970). (12) P. A. Geldof, R. P. H. Rettachmick, and G. J. Hoytink, Chem. Phys. Lett., 4,59 (1969). (13)J. B.Birks, Chem. Phys. Lett., 25, 315 (1974). 0022-365418312087-1579$01.50/0
Cyclopenta[cd]pyrene (CPP) is a yellow hydrocarbon with AEga x 7000 cm-' that can be viewed as an annelated acenaphtkylene or alternatively as a perturbed [ 151annulene. We report that this hydrocarbon produces a readily measurable S2 So fluorescence. Assignment of its excited states and its magnetic circular dichroism (MCD) spectrum is also reported.
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Experimental Section Cyclopenta[cd]pyrene was kindly obtained from Professor R. G. Harvey, the University of Chicago, as a pure product15which was further purified by sublimation under vacuum. No trace of pyrene is observed by spectroscopic techniques and careful GLC analysis on a 4 f t X 0.125 in. column of OV-17 shows no significant impurities at high sensitivities. Absorption spectra were run on a Cary 118 spectrophotometer in dilute solutions of heptane and acetonitrile. Fluorescence and excitation spectra were measured with a Perkin-Elmer MFP-44B fluorescence spectrophotometer, which consists of a 150-W xenon lamp, two grating monochromators, and a photomultiplier detector type R928. A DCSU-2 differential corrected spectra unit was used in connection with the fluorescence spectrophotometer and slit widths of 5 nm (ex) and 10 nm (em) were used to obtain corrected emission and excitation spectra. All the solutions were degassed with nitrogen for 15 min prior to the measurement. Fluorescence lifetimes were measured on 10-4-10* M solutions of CPP by the use of a Nd:YAG laser produced by Quantel (30-40-ps fwhm) with the third harmonic at 353 nm. A Tektronix R7912 transient digitizer and a 4010 videographic terminal are interfaced and programmed by the use of a PDP 11/34 computer system. A R928 photomultiplier tube in which the first 6 of the 10 dynodes were used for amplification was calibrated against a Hamamatsu camera and standard. The tube has a response time of 800 ps. The excited-state lifetime measurements were obtained by deconvolution calculations using software written locally.16 (14)P.A. M.Van Den Bogaardt, R. P. H. Rettschnick, and J. D. W. Van Voorst, Chem. Phys. Lett., 41, 270 (1976). (15)M.Konieczny and R. G. Harvey, J . Org. Chem., 44, 2158 (1979).
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Flgure 1. Cyclopenta[cd]pyrene(CPP): top, MCD (B terms in units of 103@p2/cm-') overlayed with absorption spectrum (-); emission spectrum (- -); excitation spectrum (- - -); bottom, calculated values. Calculated -B values are indicated by the length of the bars (IS] < 2, short; 2 < IBI < 10, medium; 10 < IS1 < 20, long); calculated oscillator strengths are indicated by their grades of thickness. The flags on the bars are drawn to indicate the angle of polarization of the transition with respect to the molecule as oriented in the diagram. 0
Magnetic circular dichroism (MCD) spectra were measured with a Jasco (J500C) in heptane solution. The molecular ellipticity [e], was measured relative to a known potassium ferricyanide s01ution.l~ Molecular orbital calculations were carried out with the PPP SCF CI program as previously described.lsJg All of the modifications of the formalism described in ref 18 and 19 have been retained. The 82 lowest singly excited configurations below 100 eV were used in the calculation. The hydrocarbon solvent was purified by stirring with concentrated H,S04 and subsequently washed with water, dilute sodium hydroxide, and saturated brine solution. The dried hydrocarbon was fractionally distilled through a 1-m column with the heart cut of boiling point range of 1 "C taken. This fraction was then refluxed over sodium under argon for 3 days until impurity fluorescence was minimized. Acetonitrile (spectroscopic grade) was continuously refluxed over calcium oxide under argon until impurity fluorescence was minimized.
Results and Discussion MCD Spectrum of CPP. The absorption and MCD spectra of cyclopenta[cd]pyrene are shown in Figure 1 along with the calculated results. A t least six excited singlet states are characterized in MCD with additional transitions possible but not resolved by the measurement. The first transition in CPP occurs near 19 800 cm-l and (16) D. C. Foyt, Comput. Chem., 5,49 (1981). (17) P. N. Schatz, A. J. McCaffrey, W. Suetaka, G. N. Henning, and A. B. Ritchie, J. Chem. Phys., 45, 722 (1966). (18) J. Michl and S. M. Warnick, J . Am. Chem. SOC.,96,6280 (1974). (19) J. Michl, J . Am. Chem. SOC.,100, 6801, 6812, 6819 (1978).
Flgure 2. Effect upon the molecular orbitals of the [ 15lannulene cation cross-linked with an allyl anion. The radius of the circle is proportional to the magnitude of the A 0 coefficient at each atom.
it is very weak in both absorption and MCD. More intense transitions occur at 25 900,28400,32500,33900, and 36 200 cm-l and their relative positions are based upon MCD analysis since the electronic absorption spectrum shows considerable overlap in the same region. The PPP calculations suggest the presence of additional transitions in the region of 40 500-46 000 cm-' but they are not experimentally resolved. The agreement of the experimental excitation energies, relative intensities, and MCD signs with the values calculated is very good. This is not surprising since the validity of the PPP method has been verified in the extensive studies of Mich1.18-20 A satisfying qualitative understanding of the origin of the B terms was proposed by Michl for a variety of polycyclic hydrocarbons. The polycyclic of interest, in this case CPP, is imagined to be derived from [4N + 2l~electron [nlannulene perimeter. The origin of the B terms for the low-energy L states and higher energy B states is then related to the perimeter model for [4N + 21-electron annulenes. By definition the sign of the B term of a transition is opposite to the sign of its absorption in the MCD spectrum. In Figure 2 the qualitative orbital scheme for CPP is simulated by the union of a 14-electron [15]annulene cation with an internal cross-link of an allyl anion. The original symmetry plane of the [15]annulene is removed by the required distortion of the perimeter to that of the CPP skeleton. For convenience the frontier Huckel MO's are labeled as antisymmetric (a) and symmetric (s) with respect to the vertical symmetry plane before the distortion. The diagram shows the correlation of allyl anion energies with those of the [ 15lannulene during the process (20) E. W. ThuLstmp and J. Michl, J.Am. Chem. Soc., 98,4533 (1976); J. W. Kenney, 111,D. A. Herold, J. Michl, and J. Michl, ibid., 100,6884 (1978).
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Electronic Excited States of Cyclopenta [ cd] pyrene
of creating CPP. The frontier orbitals of the perimeter for LUMO where are labeled +a, +s for HOMO and -a, the plus sign represents a bonding MO and the minus sign represents an antibonding MO. The magnitudes of the coefficients for each MO are obtained from tabular compilations in the literature.21r22 The MO’s labeled +a and -a have inappropriate symmetry for effective interaction with the allyl cross-link. However, the nonbonding orbital of allyl anion (its HOMO) is expected to interact strongly with -s of the annulene while the LUMO of allyl anion is expected to interact strongly with +s. This in-phase interaction perturbs the original degeneracy of the +a, +s annulene orbitals, causing them to split so that the difference in energy IE(-a) - E(-s)) = A(LUM0) > 0. The HOMO of allyl anion inserts itself between the +a, +s, and -a, levels on an orbital energy scale. The result is that the in-phase interaction creates a new occupied MO of lower energy that removes the degeneracy of the original perimeter HOMO’s so that A(HOM0) > 0. The out-ofphase combination creates a new vacant orbital of higher energy than the original degenerate HOMO’s of the perimeter states. Since the energy of the new HOMO of CPP lies well above the energy of the old perimeter states and close to the LUMO called -a, it is not surprising to find that CPP has a low-energy transition characterized by the yellow of the pure hydrocarbon. The qualitative prediction of the magnitude of A(HOM0) compared to A(LUM0) in CPP is not easily assessed. The relative magnitude of A(HOM0) compared to A(LUM0) is used to determine whether CPP is classified as a positive-hard or negativehard chrom~phore.’~Since CPP is a perturbed [4N + 3]-atom perimeter it is expected to be classified as a negative-hard chromophore according to the definitions proposed by Michllg wherein A(HOM0) < A(LUM0) for this class of molecules. Since Huckel MO’s do not include the effect of electron correlation, the qualitative predicted magnitudes for A(HOM0) and A(LUM0) could be reversed from that shown in the MO correlation diagram of Figure 2. The signs of the first four MCD B terms for the L and B transitions of the perimeter states are predicted from the model to be -, +, -, + for a negative-hard chromophore since the larger p+ terms are expected to dominate the smaller p- terms in this case. If the molecule is a positive-hard chromophore the B terms should occur in the order +, -, +, -. The qualitative correlation of the perturbed states of CPP with those of the [15]annulene perimeter will be complicated if extensive configuration interaction occurs in CPP. In the interpretation of the spectrum of 6,7-dihydroacenaphtho[5,6-~d] [1,2,6]thiadia~ine~~ it was proposed that new electronic transitions were introduced by the insertion of an intruder orbital into the [12]annulene perimeter and that these transitions caused complications in the simple model used to interpret the MCD spectrum. It can be expected that similar problems of interpretation will arise in the CPP spectrum. The charged allyl anion cross-link constitutes a large perturbation and intrudes into the perimeter states between the degenerate HOMO and LUMO set. As a result, electronic transitions from the (21) E. Heilbronner and P. A. Straub, “Huckel Molecular Orbitals, HMO”, Springer-Verlag,New York, 1966. (22) The objective of producing rapid, qualitative predictions dictates the use of simple MO’sand their nodal behavior aa obtained from a ready reference source. The use of the more accurate coefficienta from the PPP SCF CI calculation would vitiate any advantage gained, since every molecule studied would have to be treated by a complex computer program.
(23) B. F. Plummer and J. Michl, J. Org. Chem., 47, 1233 (1982).
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LUMo A E = 19800 cm-’
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Flgure 3. Magnitude of the A 0 coefficients in the HOMO and LUMO of CPP as computed by the Pariser-Parr-Pople SCF C I method.
newly created HOMO in CPP are characterized by significant involvement of the allyl cross-link. We propose that the first transition in CPP is uncharacteristic of a perimeter state of a [15]annulene and that this transition occurs at lower energies than the two L states of the [15]annulene. The next two transitions in the spectrum could reasonably be expected to represent contributions from the perimeter states: that is, (+a -a), (+a -s), (+s -a), and (+s -s). Since the B terms in the MCD spectrum of CPP for the first three transitions are -, -, +, we suspect that the second and third transitions are attributable to the perimeter states. These B terms are ascribable to a negative-hard chromophore and agree with the signs calculated for CPP by PPP SCF CI computations. Unfortunately, the nodal properties of the HOMO and LUMO orbitals calculated for CPP from the PPP approach are significantly altered from those of the proposed correlation of states shown in Figure 2. For example, Figure 3 shows the nodal properties of HOMO and LUMO for CPP as obtained from the PPP SCF CI calculation. A comparison with the correlation of orbitals scheme in Figure 2 reinforces the fact that the perimeter perturbation is indeed large, as is expected. The calculated magnitudes of the splitting of the orbital degeneracies are A(HOM0) < A(LUM0). This is in accord with the experimentally determined B terms which also confirm that CPP is a negative-hard chromophore. Investigation of the wave functions calculated by the PPP computation shows that there is significant CI among all the transitions such that the L, and L, perimeter states are mixed with those involving the allyl perturbatioon. The higher energy B1 and Bz states are not readily identifiable in CPP to any perimeter states in a straightforward manner. However, investigation of the PPP wave functions shows that calculated transition seven (38 000 cm-l) contains considerable CI between a perimeter transition and the allyl cross-link and so we tentatively assign this as the B, transition. The three relatively intense high-energy transitions calculated to occur between 43 OOO and 46 000 cm-’ are negative in MCD. The B2transition is presumed to be one of these three. If our tentative assignments are correct, then the -(Bl), +(B2)states as calculated are in accord with the perimeter model prediction and the experimentally determined spectrum of CPP. The extent of the perturbation incorporated into the qualitative model is quite large and it is not surprising that only extensive calculations produce accurate simulations. Nevertheless, the qualitative model does predict that A(HOM0) and A(LUM0) are different from zero. And it is possible to rationalize the low-energy transition as the result of the cross-linking of the [15]annulene perimeter.
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It should be noted that the signs of the first three B terms for CPP are identical with those for acenaphthylene (A), a cross-linked [4N + 31-atom annulene which is a negative-hard chromophore. Because this chromophoric grouping is "hard", annelation of A to produce CPP is predicted to have minimal effects on the perimeter L statesz0and this indeed is true. Likewise, the colors of A and CPP are similar and it could be argued that the socalled lowest energy K bandz0 in A is preserved upon transformation to CPP. Further spectroscopic studies of other annelated derivatives of A are warranted to explore the best qualitative model for the spectroscopic behavior of these compounds. Fluorescence of CPP. Cyclopenta[cd] pyrene fluorescences near 25 000 cm-' when excited into the S2surface at 26000 cm-l. The slightly structured emission (Figure 1) has an absorption envelope similar to that of the absorption band located at 25 900 cm-'. This emission persists when CPP is excited into higher order S, states. The excitation spectrum is in good agreement with the absorption spectrum in the Szregion verifying the origin of this anomalous fluorescence. The excitation spectrum for CPP begins to deviate at high energies as is often observed in those compounds that produce anomalous fluoresThe lifetime of the fluorescence is 3 ns as determined by laser flash photolysis. The fluorescence quantum yield of CPP was measured relative to the azulene fluorescence quantum yield as a standard (4F = 0.031)26and was found to be 3 X The fluorescence quantum yield increased slightly with an increase in excitation energy. This is probably due to the increase in absorptivity resulting in an increase in the number of excited molecules emitting fluorescence. The radiative rate constants were obtained from the first-order exponential decay curve following pulsed-laser excitation at 353 nm (28400 cm-l). The calculated rate constants were obtained by integrating the Sz absorption band from its onset near 25 000 cm-' to a perpendicular cutoff at 29000 cm-' since the MCD curve indicates the onset of S3 at this point. The simplified Strickler-Bergz7equation (SBE) was then used to calculate k,:
St(?)
kF = (2.88 X. 1 0 - 9 ) n 2 ( ? ~ 3 / ? ~ ) d?
From the relationshipz8 of the fluorescence lifetime, radiation rate constant, and quantum yield of fluorescence TF = &/kF, the value of the radiationless rate constant kNR can be approximated by 1 / when ~ 4 F is small. The calculated value of kF = 7.4 x lo7 from SBE results in a calculated value for km = 2.5 X 1O1Os-'. The value of kNR from the flash experiment is 3.3 X lo8 s-l, a value significantly smaller than that derived by the SBE. Such discrepancies among values derived from SBE results and the results of lifetime measurements are known to occur in other compounds that exhibit anomalous fluorescence. Azulene, whose anomalous fluorescence is extensively documented,'+fZ6shows a similar behavior. This disparity is believed to originate in the vibronic coupling of upper excited states with the ground-state A. Olszowski, Chem. Phys. Lett., 73, 256 (1980). W. Leupin and J . Wirz, J. Am. Chem. SOC.,102, 6068 (1980). S. Murata, C. Iwanaga, T. Toda, and H. Kokubun, Ber. Bunsenges. Phys. Chem., 7 6 , 1176 (1972). (27) S. J . Strickler and R. Berg, J. Chem. Phys., 37, 814 (1962). (28) D. 0. Cowan and R. L. Drisko, "Elements of Organic Photochemistry", Plenum Press, New York, 1976.
vibrational manifold (Duchinsky co~pling).~!+~l The naphthothiadiazines also are illustrative of similar disparity in calculated (SBE) and experimental r e ~ u l t sperhaps ,~ for the same reason. Recently, arguments have been advanced that suggest that the SBE does not rigorously apply to emissions from higher excited states.31 Our results lend support to this hypothesis. A perusal of the limited number of published excitation spectra of those compounds exhibiting Sz So or anomalous fluorescenceU~ shows that excitation spectra at higher energies often deviate from the anticipated congruence with the absorption spectra. This is the result also observed for CPP. If vibrational coupling of higher excited states with the ground state is significant, this route for loss of energy could interfere with internal conversion such as S, Sz and S, S1 where S, S1is the typical pathway expected3zfor most compounds. According to the criteria given by Eber et compounds with hE(S2-S1) lower than 10000 cm-' may emit either dual fluorescence or only S1 So fluorescence. The energy gap in CPP between the Szand SI surfaces is about 6900 cm-'. Although we cannot detect S1 So fluorescence from CPP by steady-state fluorimetry, we have detected a second component in the decay curve of CPP after laser excitation. The lifetime of this emission is about 17 ns and it is shown by cutoff filters to occur between 420-700 nm where S1 So fluorescence is expected. Its intensity is very much weaker than the Sz Soemission. We estimate that its quantum yield is about lo*. We have not detected phosphorescence from CPP at 77 K.
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Summary The PPP SCF CI approach gives remarkably good results for the MCD B terms and electronic t r a n ~ i t i o n sof~ ~ CPP. The estimation of the sign of the B terms using qualitative Huckel MO correlation diagrams for union of a [ 15lannulene cation perimeter with a charged allyl anion cross-link is less satisfactory, probably because the perturbation created by a charged anion introduces new energy states that are not characteristic of the simple perimeter model used to interpret the signs of the B terms. The assumption that annelated derivatives of acenaphthylene will exhibit S2 So fluorescence is tentatively verified by the emission behavior of CPP. Further spectroscopic study of other derivatives is warranted.
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Acknowledgment. The support of the Robert A. Welch Foundation is gratefully acknowledged. We thank Dr. P. Horowitz for the use of the MCD instrument. The fluorescence spectrophotometer was purchased through the support of the National Science Foundation (CDP 7924308). The laser work done at the Center for Fast Kinetics Research is supported by the Biotechnology Branch of the Division of Research Resources of NIH (Grant RR00886) and by the University of Texas at Austin. Special thanks go to Dr. J. Downing and Dr. J. Michl for supplying computational support. Registry No. CPP, 27208-37-3. (29) F. Duchinsky, Acta Physicochim. URSS, 7, 551 (1937). (30) B. Sharf, J. Chem. Phys., 64, 441 (1971). (31) G. Eber, S. Schneider, and R. Dorr, Chem. Phys. Lett., 52, 59 (1977). (32) M. R. Topp, H. B. Lin, and K. J. Choi, Chem. Phys., 60,47 (1981). (33) The neglect of double CI in the calculations raises the possibility
that some of the low-energy weak (g) states may be poorly predicted. See, for example, J. A. Bennett and R. R. Birge, J. Chem. Phys., 73, 4234 (1980).
