768
J. Phys. Chem. 1980, 84, 768-773
Dual Fluorescence and Intramolecular Electronic Energy Transfer in a Bichromophoric Molecule Daphne Getz, Arza Ron,* Mordecai 8. Rubin,” and Shammai Speiser” Department of Chemistty, Technion-Israel Institute of Technology, Haifa, Israel (Received August 16, 1979)
The bichromophoric molecule containing phenanthrene and a-diketone moieties connected by two chains of five methylene groups has been studied. Most spectroscopic properties of this molecule are described by a superposition of those of its constituent chromophores. Unique for the combined molecule is the fact that energy absorbed by the phenanthrene chromophore is transferred in part to the a-diketone and both chromophores emit their characteristic fluorescence spectra. An extensive study was made of the intramolecular energy transfer process in solution as a function of the excitation frequency and of the sample temperature. It was clearly demonstrated that energy is transferred very efficiently to the a-diketone moiety from a thermally activated state of the bichromophoric molecule. The rate of this energy transfer is comparable to the relaxation rate of the activated state. In contrast, the transfer process from the ground vibrational level of the first excited state of the phenanthrene moiety is rather slow (-lo7 d).
Introduction Interaction between excited and ground states of bichromophoric molecules has been a subject of considerable interest-l It is manifested in chemical reactions,l in complex f o r m a t i ~ n ,and ~ > ~in photophysical processes such as electronic energy t r a n ~ f e r .The ~ necessary condition for an energy transfer (ET) process is that the two moieties, the donor D and the acceptor A, satisfy the spectral prerequisite8 for an energy transfer of the type D* + A + A * + D (1) where the asterisk denotes an excited electronic state. Intramolecular energy transfer (intra-ET) can occur whenever two separated chromophores are incorporated in a single molecule. In such cases, control of the spatial relationship between donor and acceptor groups exists without the randomness characteristic of intermolecular interactions. Furthermore, intra-ET can be observed in rigid or viscous media where encounters between separated molecules leading to energy transfer are not possible. Intra-ET also has significant implications in biological systems6 and in dye laser operation.’ The probable mechanism involved in intra-ET is an exchange interaction of the Dexter type,8 while long-range (Forster) transfer5 is expected to be much less important. The first investigations of intra-ET were reported by Weberg-I1 and by Schnepp and Levy.12 Schnepp and Levy12 observed anthracene fluorescence irrespective of exciting wavelength in compounds containing naphthalene and anthracene moieties joined by a variable number of CH2groups, The quantum yield was independent of the number of methylene groups separating the two chromophores. Later workers4examined similar systems involving isolated chromophores. In some cases, the donor and acceptor were attached to a rigid system so that their spatial relationship was known to a considerable degree (unfortunately, geometrical variations in such rigid systems were not feasible). The occurence of intra-ET could be readily evaluated from knowledge of excitation and emission spectra of each moiety alone and comparison with spectra of the bichromophoric species. In all of these cases, complete quenching of donor fluorescence was observed with concomitant emission solely from the acceptor. This was true even for donor chromophores with high fluorescence quantum yields. However, observed yields of emission from A* were less 0022-3654/80/2084-0768$01 .OO/O
than the theoretical maximum, implying that D* in the bichromophoric molecule decayed nonradiatively to the ground electronic state through channels unavailable to the separate D chromophore. Residual D* emission might be anticipated in systems where nonradiative decay is not enhanced in the bichromophoric molecule and/or where intra-ET is similar in rate to fluorescence decay of D*. Preliminary studies with 1,8-(6’,7/-dioxododecamethy1ene)phenanthrenel3(I) indicated that this molecule is an ideal candidate for elucidating mechanisms of the intra-ET process. This molecule is composed of two chromophores, phenanthrene and a-diketone, connected by two chains of five methylene groups. As will be shown below, most spectroscopic properties of I can be described by a superposition of those of its constituent moieties. As a model for the separated disubstituted phenanthrene in I, 1,8-bis(5’-(carbomethoxy)pentyl)phenanthrene(11)was chosen while biacetyl(II1) served as a model for the dione. The structures of I and the model compounds I1 and I11 are shown in Figure 1. In this paper we demonstrate that energy is transferred very efficiently to the a-diketone moiety from a thermally activated state of I. This state might be a vibronically excited state of the phenanthrene chromophore or some specific conformer(s) of the molecule having different relative orientation of the two chromophores or of the two carbonyl groups relative to one another.
Experimental Section A laser fluorometer based on nitrogen laser excitation (Molectron UV 400) or on nitrogen laser-pumped dye laser excitation (Molectron DL 200) was employed. The details of the system are given e1~ewhere.l~ Picosecond laser pulses at 353 nm (mode-locked tripled Nd laser) together with a 2-ns temporal resolution detection system were occasionally used. This system was kindly put at our disposal by Dr. Dan Huppert of Tel-Aviv University. In addition, continuous wave corrected fluorometry measurements were performed (at Tel Aviv University) with a Perkin-Elmer MPF-4 fluorescence spectrophotometer. Cary 14 and 15 spectrophotometers were used for absorption measurements. Syntheses of I and I1 will be described ~eparate1y.l~2-Methyltetrahydrofuran (2MeTHF) (Fluka pur) was refluxed over sodium metal and distilled immediately prior to sample preparation. De0 1980 American Chemical Society
Electronic Energy Transfer in a Bichromophoric Molecule
I
The Journal of Physical Chemistry, Vol. 84, No. 7, 1980 769
II
293 K
CH3COCOCH3
IIi 1,8-(6',7'-Dioxododecamethylene)phenanthrene (I), 1,8bis(5'-(carbomisthoxy)pentyl)phenanthrene (11), and biacetyl (111). Figure 1.
1 0 ;
350
400
500
450
550
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x Inmi Figure 4. Fluorescence spectra of I1 (5 X lo-' M in 2-MeTHF) excited by a nitrogen laser at 337 nm. 10;
BIACETYL
I
Y
0
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104
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450 nm
1
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200
300
400
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2 (nm) Figure 2. Absorption spectra of I, 11, and biacetyl (in acetonitrile).
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Figure 5. Fluorescence spectra of biacetyl (5 X lo-' M in 2-MeTHF) excited by a dye laser at 450 nm.
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Figure 3. Dual fluorescence spectra of I (5 X lo-' M in 2-MeTHF) excited by a nitrogen laser at 337 nm.
k, r = 22 "5
gassing of s,amples was accomplished by three freezepump--thaw cycles. Biacetyl (Fluka pur) was vacuum distilled prior to use.
Results and Discussion S u m m a r y of Preliminary R e ~ u 1 t s . l ~The absorption spectra of I, 11, and biacetyl are shown in Figure 2, where it can be seen that I is a good candidate for study of intra-ET between the phenanthrene and a-diketone moieties. The observed red shift for the absorption band of the dione moiety in I is similar to that observed in other cyclic a-diketones. Figure 3 shLows the fluorescence spectra of I and lifetimes obtained at 293 and at 77 K by excitation with a nitrogen
450
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400
500A i n m550 )
600
650
-.
Figure 6. Fluorescence spectra of 1(5 X lo-' M in 2-MeTHF) excited by a low power dye laser at 450 nm.
laser (337 nm). These can be compared with the nitrogen
laser excited fluorescence spectra of I1 at these temperatures (Figure 4), the dye laser excited (at 450 nm) fluorescence spectra of biacetyl (Figure 5 ) and of I (Figure 6), and the nitrogen laser excited fluorescence spectra of an equimolar mixture of I1 and biacetyl (Figure 7). Comparison of Figure 3 with Figures 4-7 clearly indicates that the fluorescence of I consists of dual emission f r o m the Phenanthrene (350-470 nm) and a-diketone (470-600
770
The Journal of Physical Chemistty, Vol. 84, No. 7, 1980
Getz et ai.
50
IO
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L O S W Intensity (Relative
A
I"rn1
Figure 7. Nitrogen laser excited fluorescence form an equimolar mixture of I1 and biacetyi (5 X IO-* M in 2-MeTHF).
units)
Flgure 9. Fluorescence intensity of the phenanthrene moiety in 1 ( 5 X IO-' M in 2-MeTHF, detected at 380 nm) as a function of the exctting dye laser intensity at 450 nm.
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Flgure 8. Fluorescencespectra of 1(5 X IO-* M in 2-MeTHF) excited by a high power dye laser at 450 nm. The biacetyi moiety emission band (shown in Figure 6) is filtered out.
nmi moieties. The absorption and fluorescence spectra and lifetimes of I1 are identical with published data for phenanthrene i t ~ e 1 f . lThe ~ ~ ~results ~ shown in Figure 7 demonstrate intermolecular energy transfer (inter-ET) between D* and A in fluid solution a t 298 K and its absence in a glass at 77 K. This observation is in agreement with earlier results by Dubois and Coxlg and shows that inter-ET occurs solely via diffusion and collisions involving exchange (Dexter) interactionsa8 The D and A moieties in I are separated by about 6 8, (according to Dreiding models) so that intra-ET by the Dexter mechanism8 is plausible. Recently, Kaplan and Jortner20 observed inter-ET from highly excited electronic states. An analogous process in I would be intra-ET from higher excited states of the a-diketone chromophore to the phenanthrene. Excitation of I a t 450 nm,with high dye laser intensity results, in addition to fluorescence from the a-diketone moiety (obtained also at low pump intensities, cf. Figure 6), in fluorescence from the phenanthrene moiety (Figure 8). Its quadratic dependence on the excitation intensity, saturating a t high intensities (Figure 9), is indicative of a consecutive two-photon absorption process.21 Thus the a-diketone reaches highly excited states via consecutive two-photon excitation and transfers part of the excitation to the phenanthrene moiety, reversing the normal donor-acceptor relationship. A t still higher pump powers simultaneous absorption of two 450-nm photons by I1 alone
Flgure 10. Fluorescence intensity of I1 (5 X IO-* M in 2-MeTHF, detected at 380 nm) as a function of the exciting dye laser Intensity at 450 nm.
is possible, resulting in nonsaturating quadratic intensity dependence of its fluorescence (Figure 10). For the donor chromophore (the phenanthrene moiety in I), the excited state lifetime can be written as 1/7f = 1/70 ~ D A (2) where r0 is the fluorescence lifetime of a separated D (phenanthrene) molecule and kDA is the intra-ET rate constant. The values for ro and rf can be read from Figures 4 and 3, respectively. The obtained values kDA = 3.7 X lo7 at 293 K and hD* = 7 X lo6 s-' at 77 K suggest a rather slow energy transfer process which amounts to lifetime shortening by a factor of 3. However, measurements of the relative fluorescence quantum yields (4) of I and I1 at room temperature (Figure 11) show that the 4 of I is 12fold smaller than that of 11, consistent with a much larger hDA. Indeed, picosecond laser excitation of I (at 353 nm) is followed by a fast rise (
