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Electronic energy transfer in supersonic jet expanded naphthalene-(CH2)n-anthracene bichromophoric molecules
13 December 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 263 (1996) 441-448
Electronic energy transfer in supersonic jet expanded naphthalene-( CH 2) n-anthracene bichromophoric molecules Gershon Rosenblum, David Grosswasser, Frank Schael, Mordecai B. Rubin, Shammai Speiser * Department of Chemistry, Technion-lsrael Institute of Technology, Haifa 32000, Israel
Received 19 September 1996
Abstract A study of intramolecular electronic energy transfer (Intra-EET) has provided evidence, for the first time, of a dramatic difference in Intra-EET efficiency results obtained under jet cooled and room temperature solution conditions. This was observed with anthracene-(CH2),-naphthalene bichromophoric molecules, A1N (n = 1) and A3N (n = 3). The rich fluorescence excitation spectrum of the naphthalene moiety in AIN indicates an inefficient EET process whose rate constant is substantially slower than that of the naphthalene moiety fluorescence. It was shown that the EET rate depends on a specific vibronic excitation that affects the molecular conformation, and was found to be at least two orders of magnitude slower in AIN molecule compared to A3N.
1. Introduction Interaction between excited and ground states of two molecules involving electronic energy transfer (EET) has been the subject of considerable interest [1]. This process plays a key role in chemistry, biology and physics and is well documented and summarized. However, some basic problems in molecular photophysics which involve short range interactions manifested in intramolecular EET (IntraEET) are still the subject of current investigations. The first observation of short range Intra-EET was reported by Schnepp and Levy for the naphthalene-(CH2)n-anthracene system [2]. Later, many other groups examined other bichromophoric sys-
* Corresponding author.
tems [1]. The occurrence of Intra-EET could be readily evaluated from knowledge of excitation and emission spectra of each moiety alone and by comparison with the corresponding spectra of the bichromophoric molecules. The basic Intra-EET process can be described by kEr D*-B-A ~ D-B-A* ,
(1)
where the excitation energy is transferred from an excited donor D* chromophore to a ground state acceptor moiety A, resulting in quenching of D* fluorescence and excitation of A. B denotes a molecular spacer bridge connecting the two chromophores which may play a role in promoting the transfer process. In most cases the Intra-EET rate constant, kET , is attributed to two possible contributions. The first is the long range Coulombic contribution which was formulated by FSrster in terms of dipole-dipole
000%2614/96/$12.00 Copyright© 1996Elsevier Science B.V. All fights reserved. PI! S0009-2614(96)01 229-8
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G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
interaction [3]. The second contribution to EET is the short range exchange interaction, as formulated by Dexter [4]. The rate of dipole-dipole induced EET decreases as R -6 whereas that of the exchange induced process decreases as e x p ( - 2R/L), R being the interchromophore distance and L being the average van der Waals radius for the overlapping orbitals. Until recently most reported studies of Intra-EET were performed in solutions where solvent effects cannot be ruled out and where complete vibrational relaxation of donor and acceptor excited electronic states precedes the EET event. In general the prospects for investigating Intra-EET in a bichromophoric molecule from a single excited donor vibronic state, under collisionless conditions in the low pressure jet, are not very high. Yet the unique conditions in a supersonic jet expansion offers a way of studying these processes in some detail. Although supersonic jet spectroscopy has been widely used for investigating molecular radiative and nonradiative processes, very few studies of EET under these conditions have been reported. The first reported supersonic jet study of IntraEET was made by Ito and coworkers [5]. They measured Intra-EET between o-xylene or m-xylene connected by a chain of three methylene groups to p-xylene, in the bichromophoric compounds 1-(otolyl)-3-(p-tolyl)propane and 1-(m-tolyl)-3-(ptolyl)propane, respectively. A more systematic investigation of Intra-EET in isolated bichromophoric molecules was undertaken by Levy and coworkers [6-9]. Several bichromophoric molecules containing two aromatic moieties connected by an aliphatic spacer bridge were studied under jet-cooled conditions. The measurements included excitation and dispersed fluorescence spectra of the bichromophoric molecules, as well as the corresponding spectra of the individual chromophores. In addition, the possible conformations of the molecules were determined using a variety of structure determination techniques. Intra-EET was inferred from spectra that revealed acceptor emission features upon excitation of a donor chromophore. The results were interpreted in terms of existing EET theories and the general theory of radiationless transitions in isolated molecules. We have chosen to study the naphthalene(CHE)n-anthracene bichromophoric molecules (n--
1 and 3) investigated by Schnepp and Levy in their pioneering study of Intra-EET, namely 9-((1-naphthyl)-methyl)-anthracene (A1N) and 9-(3-(1-naphthyl)-propyl)-anthracene (A3N) [2]. The 1962 work showed that the only fluorescence observed in solution came from the anthracene moiety and had spectral characteristics of anthracene emission which indicates that the excited naphthalene moiety had transferred its excitation energy completely to the anthracene moiety. This high efficiency EET process is partly due to the good spectral overlap between the naphthalene emission and anthracene absorption spectra. The spectral features of these spectra are relatively broad due to many different conformations of the floppy molecules, and the fact that interaction with the solvent caused complete relaxation of the initially excited vibrational states of the molecules prior to the EET process. The situation is expected to be different when the molecules are under supersonic jet cooling conditions. A large majority of the molecules are in their ground vibrational state and in their most stable conformation. The spectra of the molecules consist of very sharp lines, and the spectral overlap can be poor for some lines, leading to a relatively slow EET process and to dual fluorescence from the excited bichromophoric molecules, especially for those conformations where either dipole-dipole interaction is weak or orbital overlap is rather poor. For other conformations and spectral lines Intra-EET may still be very efficient as in solution at room temperature. Although existing EET theories applicable for solution conditions can be extended to the isolated molecule case, no good estimates of the relative efficiencies of the Coulomb and exchange mechanisms are available for jet-cooled molecules [10]. We expect that the exchange interaction is more important for the A n N bichromophoric system under jet conditions. In this case the rate should depend strongly on the overlap of the ~ electronic systems of the two closely situated moieties in the molecule. If Coulomb interaction is more important, the crucial geometrical factor is the angle between the transition dipole moments of the two chromophores. Since the molecules are in their ground vibrational states, belonging to their energetically most stable conformation, their geometries should strongly affect the EET rate.
G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441---448
2. Experimental The bichromophoric compounds were synthesized following the literature procedure [11]. The molecules were seeded in a supersonic jet using a pulsed nozzle apparatus described elsewhere [12]. Briefly, 6 atm helium was passed over a heated cell containing liquid organic sample. The temperature was chosen so as to produce an organic vapor pressure of about 0.2 Tort. The A1N sample was heated typically to 198°C, and the A3N sample was heated to 238°C. The gas containing an organic vapor was expanded through a 1.1 mm orifice to an evacuated chamber. The pressure in the vacuum chamber was typically 2 × 10 -4 Tort. The jet was crossed with a laser beam ( < 1 mm spot size) 33 mm down stream. About 10% of the resulting fluorescence was collected by a lens to a photomultiplier. The laser beam ( - - 5 mW) was produced by frequency doubling or mixing an Nd:YAG (Continuum) second harmonic pumped dye laser. The resulting laser bandwidth was about 0.2 cm -1. The experimental uncertainties of absolute frequencies are + 2 c m - J and those of relative frequencies are 0.2 c m - t.
3. Results and discussion
3.1. Calculation of the molecular geometry Calculations were performed to estimate the ground state geometries of the molecules A1N and A3N. We used both MM2 [13] and AMBER [14,15] molecular force field methods. The two methods produced practically the same results. For the A3N the calculated ground state geometry is that of an anthracene-naphthalene sandwich. The two chromophores do not lose their planarity and lie in almost parallel planes. The average interplanar distance is about 3.6 A, almost the same as in the anthracene-naphthalene cluster (3.57 ,~ [16]). The molecule is very floppy - the potential function around the minimum is 'flat'. The flexible aliphatic chain allows the two chromophores to 'slide' easily with respect to one another. The energetically most stable geometry lacks any symmetry, the potential is strongly anharmonic and the normal modes are mixed
443
even at lowest interchromophoric vibrational energies. This A3N geometry seems extremely favourable for fast EET. The w-orbital interchromophoric overlap is most pronounced in the sandwich orientation. The anthracene S t ~ S O transition dipole moment is parallel to the short in-plane axis of the chromophore. The naphthalene S, ~, S O transition dipole moment is small as the transition is forbidden. Its direction depends strongly on the specific vibronic excitation and on the substitutions that break the symmetry while the transition dipole moments stays in plane. This means that, for most vibronic transitions, the direction of the transition dipole moment in the naphthalene chromophore is close to that in the anthracene chromophore. We conclude that the obtained geometry is optimal both for Coulomb and for exchange mechanisms of EET. The calculated geometry of A1N is quite different. The single methylene group bridge is not flexible enough to allow the two chromophores to come into close proximity. The angle between the two chromophore planes is close to 90 °. The anthracene w-system is perturbed by naphthalene hydrogens, but there are no anthracene atoms in the vicinity of the naphthalene "rr-system. The w-orbital overlap is very poor. The anthracene plane short axis is at a large angle with the naphthalene plane (about 65 °) so that any interaction with an in-plane naphthalene transition dipole moment tends to be weak. The geometry possesses no symmetry but it is close to a C s geometry. Some sort of a double well potential vibrational behavior can be expected in this molecule although the fifteen interchromophoric modes are mixed. There is a strong van der Waals interaction between the chromophores. The energy gain of positioning the two chromophores in a sandwich conformation is about 7 kcal/mol [16]. This means that whenever the bridge is long and flexible enough to allow the chromophore planes to be more or less parallel and at about 3.6 A between them, the sandwich geometry is the most stable. This is apparently the case for A3N. In A1N the aliphatic bridge is too short to allow for the needed flexibility, the structure is rigid and the chromophores are far apart. The A2N case would be intermediate: any sandwich structure would require a considerable strain on the aliphatic
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G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
angles and some distortion of the chromophore planes. This will be the subject of future studies.
3.2. Excitation of the anthracene chromophore The fluorescence excitation spectrum of the anthracene moiety S] ,,-- S O transition of A3N at the origin of its spectral region is presented in Fig. 1. The 0 - 0 transition is at 26704.4 cm- l, which is red shifted by 990 c m - l with respect to the bare anthracene origin. This shift is typical of substitution at position 9 combined with van der Waals interaction between naphthalene and anthracene. It can be compared to the 9-methylanthracene shift value of 756 c m - l [17] and to the anthracene-naphthalene cluster value of 556 c m - l [18]. There are some hot bands to the red of the 00° band. Their positions, spacings and intensities suggest that they correspond to molecules possessing one or two quanta of the lowest energy interchromophore vibrational modes prior to the laser excitation. There is a complex structure in the region of 0-100 cm -1 energy excess above the 0 ° transition. No constant step progressions can be identified here. The dominant distances between close lines are 3, 5 and 10 cm-1. This manifests the extremely low frequencies of the large amplitude interchromophore movements that are excited in the molecule (in the sandwich type aromatic clusters the corresponding lowest observed frequencies are usually 10-30 cm- ] [12,18-22]). The intensities of the bands drops quickly with increasing excitation energy. Above
0
/k _ .~_ 26680 26700
20720
28740
26760
26780
Frequency, crrr I
Fig. 1. Fluorescence excitation spectrum of A3N at the origin region of the anthracene chromophore.
v.4: 44cm "t
•~' 80
6o
B~
12~
~= 73crrr~ Vc=141cm-1 vo=149crrrl ~=253cm-t vt2=392cm,f
A'C~ B~C~ l ~ 2O
I I
26800
C~p'* A~,D~/c2
I I ,
/ I
26900
,
I
'2~At°
12~A~B~/
i° i
I
i
27000 27100 27200 Frequency, crn-I
27300
Fig. 2. Fluorescence excitation spectrum of the anthracene chromophore of A1N. The relative intensities of the 0o° band and of the A]o band are 311 and 115 respectively.
100 cm -1 excess energy the different vibrational modes are completely mixed, the Franck-Condon factors are very low, and the spectrum represents a low intensity quasi-continuum. This fast decrease of the spectrum intensity indicates that the S~ geometry of the molecule is close to the S O geometry. The excitation spectrum of the A1N molecule, at the spectral region of the origin of the anthracene moiety SI ~ S O transition is presented in Fig. 2. The red shift of the origin transition (26798.9 cm- ] ) with respect to the bare anthracene molecule is 896 cm- ~, smaller than in A3N. This reflects the smaller perturbation of the anthracene electronic system by the naphthalene moiety in A1N. Many sharp vibronic bands can be identified in the spectrum. The distances between the bands are generally larger than in the A3N spectrum indicating that the molecule is much more rigid. Of the 26 most intense bands, 23 can be positively attributed to combinations of only six different vibrations. One of these vibrations is the well-documented 12ag (lowest frequency) vibration of anthracene chromophore [23], and the other five (denoted as A, B, C, D, E) are tentatively assigned as interchromophore vibrations. Their frequencies correspond to high-mass pseudorotation and bending modes. They are clearly higher than the usual interchromophoric frequencies in aromatic clusters. All second overtones in the spectrum have much lower intensities than the corresponding principal
G. Rosenblum et a l . / Chemical Physics Letters 263 (1996) 441-448
bands. Third overtones were not detected. The mixed excitation frequencies (e.g. Al0 B l) correspond to the sums of the principal frequencies within + 0.3 cm- 1. These facts demonstrate that the anharmonicity of the vibrational potential is low, the modes are not mixed and the excited state geometry is close to that of the ground state.
3.3. Excitation of the naphthalene chromophore Excitation spectra of the naphthalene chromophore were compared to that of l-methylnaphthalene (1-MN). This spectrum is shown in Fig. 3a, it is of better quality than the previously reported one [24]. The fluorescence excitation spectrum of A1N at the spectral region of the naphthalene moiety origin of the S I *-- S O transition is presented in Fig. 3b. The band is red shifted by 242.5 cm-1 compared to the bare naphthalene transition. This value is remarkably close to the 1-MN shift (246.5 cm -1 , Fig. 3a, [2427]). This leads to the conclusion that the anthracene moiety presence hardly perturbs the naphthalene electronic system. This is in contrast with the A3N situation (a 622 cm-1 shift, see below). Analogs of seven of the nine 1-MN bands can be definitely
: ~,2B
~
.
.o,1t. 31800
, b 32000
32200
32400
Frequency, crnFig. 3. (a) Fluorescence excitation spectrum of I-MN. The intensity of the ~ band is - 5 . 2 times higher than the 0o° band intensity. (b) Fluorescence excitation spectrum of the naphthalene chromophore of A1N. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes in ns. All recorded fluorescence decay curves were monoexponential within the experimental uncertainty.
445
identified in the spectrum. These AIN bands are shifted not more than 2 cm-1 from the corresponding 1-MN bands. There are low frequency progressions in the spectrum indicating excitation of some interchromophore motion in the molecule. The structure at 320-380 cm-1 above the origin can be due to some rotational activity of the 9-anthryl-methyl substitution around the methylene-naphthalene bond. The molecular conformation can be different in the ground and the excited state, as is the case for 1-MN [25,26]. An energy difference of 300 cm- l is typical for such conformations. The molecule geometry is relatively close to the C s symmetry, and the origin transition structure can be due to some double well potential-like activity. Fluorescence lifetimes were measured at band maximum frequencies. For most bands they are between 230 and 260 ns. All the bands that have analogs in the 1-MN spectrum correspond to lifetime values close to their analogs. This means that the rate constant of naphthalene fluorescence quenching by the anthracene moiety, if it occurs at all, is slower than 1 X 10 6 s - l . There is only one A1N band with a shorter lifetime (128 ns). No observed transitions of 1-MN [24] or naphthalene [24,28] in the 0-1000 cm -1 excess energy region have a lifetime of less than 200 ns. This means that the process responsible for the shortening of the lifetime is not the mere vibrational coupling induced electronic mixing but is due to the anthracene substitution. The quenching rate constant that corresponds to this shorter lifetime value is 5 X 10 6 S- I, a rather slow EET process. This was proved by recording an excitation spectrum while a 399 nm cut-off filter was introduced to remove any naphthalene moiety emission signal. Most of the naphthalene moiety emission in the jet experiments is between 320-370 nm, whereas the anthracene moiety emits between 360-420 nm. To prevent interference with the spectrally overlapping anthracene emission [12] the signal was time-gated and measured in the interval of 25-325 ns after the excitation. The emission lifetime of the anthracene moiety in this spectral region is about 5 ns. The filtered, time-gated signal results neither from naphthalene emission (it is filtered off) nor from regular anthracene emission (it decays too fast). The detected signal is due to slow excitation transfer from
G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
446
248+5ns
.259~5ns 2det>3OOnm
ll
128+10ns l~
~J
159~34ns
,
I
31780
i
I
"l I "
I"
~'rl"
" ~1 y
,
I
,
31790 31800 31810 31820
Frequency, cm-1 Fig. 4. Fluorescence excitation spectrum of the naphthalene chromophore of A1N at the 0 ° region. Upper trace shows the untiltered spectrum while the lower one is the spectrum filtered for only anthracene fluorescence. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes.
naphthalene to anthracene moiety followed by fast subsequent anthracene emission. The only strong band that remains in the filtered spectrum (Fig. 4) is the one with the short lifetime. This lifetime was measured under these conditions with less precision but its value (159 + 34 ns) is equal within experimental error to the value measured without a filter. The presence of this band in the spectrum under the conditions where all the other bands disappear is striking evidence for the intramolecular EET process. This result is in agreement with the observed slow Intra-EET for the less stable conformers of the A - N cluster [12,16]. It is, however, very different from the extremely efficient EET observed at room temperature in solution [2]. There are two weaker bands in the 1-MN spectrum that have no clearly identifiable analogs in the A1N spectrum. This can be due to a comparatively large shift of the corresponding A1N bands. Such a shift would suggest strong coupling of these vibronic excitations to some substituent vibrations whose frequencies can be strongly affected by the changed substitution mass. Another explanation would be a fast quenching of these vibronic excitations by the anthracene moiety. This would mean that the excited vibrations are capable of bringing the two chromophores into a geometrical position favourable for EET.
The excitation spectrum at the origin region of the naphthalene moiety S I ~ - S O transition in the A3N molecule consisted of a broad featureless structure (cf. Fig. 5). At the same excitation energy, the anthracene moiety of the molecule is excited to the vibronic quasi-continuum which is about 5000 cm-1 higher in energy than its electronic origin. The observed signal was extremely weak (100 times lower than that for AIN) and largely independent of the exciting laser wavelength, except for one broad feature around 31398 cm -~ . The decay time was about 250 ns both for the broad band and for the baseline signal. We suppose that this high baseline signal can only be due to excitation of the small portion of molecules at the outskirts of the jet that were not efficiently cooled. The A3N molecule is very floppy, so that even 100 c m - 1 of excess vibrational energy results in complete mixing of all the interchromophore modes and leads to a very high amplitude chaotic intramolecular movements in the molecule. The spectral position of this broad band is close to the expected electronic origin of the naphthalene moiety. This band is probably an envelope of the signals of the hot molecules excited without changing their vibrational energy (such excitations should have the highest Franck-Condon factors). The most interesting result of this experiment is the total absence of sharp bands that can be at-
249~21ns 1
2~t>4OOnm o)
262+28ns I
~1~
290"t:30ns
~4 ~ I
0
i
31390
i
31395
i
31400
i
31405
Frequency, cm-1 Fig. 5. Fluorescence excitation spectrum of the naphthalene chromophore of A3N at the 00° region filtered for only anthracene fluorescence. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes. The maximum signal is 20 times weaker than that from A I N (cf. Fig. 4). No signal was detected in the absence of A3N.
G. Rosenblum et a l . / Chemical Physics Letters 263 (1996) 441-448
tributed to cold A3N molecules. This should be compared to 1-MN case (Fig. 3a). The 1-MN spectrum obtained under similar conditions exhibits quite a number of strong sharp features. The difference between the A3N and the 1-MN spectrum is most probably due to the total quenching of the naphthalene moiety fluorescence by the anthracene chromophore in A3N. This Intra-EET process is completed in less than 20 ns and is followed by fast anthracene emission that could not be detected because of the time gating. We can thus conclude that the Intra-EET process is efficient in the A3N molecule. This result can be compared to our previous studies of the anthracenenaphthalene cluster [12]. Similar quenching of the naphthalene moiety fluorescence was observed in these experiments. The energetically most stable calculated geometry of the cluster is very similar to that of the A3N molecule [16]. The minute amount of excited hot A3N molecules exhibits a long-lived fluorescence lifetime. This means that EET is not as efficient in these molecules as it is in the cold ones. These hot molecules are either vibrating with large amplitudes or trapped in metastable conformations. The optimal sandwich configuration has low statistical weight or is not achieved at all in such molecules. This geometry distortion slows down EET and makes it possible to detect the slow residual fluorescence from the anthracene moiety.
4. Summary We have observed for the first time a dramatic difference between Intra-EET efficiencies in a bichromophoric molecule under jet-cooled and room temperature solution conditions [2]. The rich excitation spectrum of the naphthalene moiety in the A1N molecule indicates an inefficient EET whose rate constant is substantially smaller than that of the naphthalene moiety fluorescence (3 x 10 6 S - I ) . Only for one vibronic transition, the EET rate constant was high enough to be measurable (5 × 10 6 s - 1). This shows that the EET rate depends on a specific vibronic excitation that affects the molecular conformation. We assume that there can be vibronic states with high rates of EET, but our present experimental
447
arrangement allows only measurements of EET rate constants less than 40 x 106 S - l . Two-photon twocolor ionization experiments currently underway in our laboratory will resolve this issue. The EET rate constant was found to be at least two orders of magnitude slower in the A1N molecule compared to the A3N molecule. The geometry differences are thought to be responsible for this difference. This is in accord with our previous studies of the anthracene-naphthalene cluster which gave evidence for two EET rates associated with two cluster isomers [12]. Calculations [16] predict two isomers of the cluster, one of the sandwich type, and the other with the two molecules ~r-electronic systems far apart.
Acknowledgements We are grateful to Dr. Irina Fedatov for preparation of the bichromophoric molecules. This work was supported in part by the Technion V.P.R. Fund for the Promotion of Research at the Technion and by grant number 92-00224 from the US-Israel Binational Science Foundation.
References [1] S. Speiser, Chem. Rev. (in press) and references therein. [2] O. Schnepp and M. Levy,, J. Am. Chem. Soc. 84(1962) 172. [3] Th. F~rster, in: Modern quantum chemisu'y, Vol. 3, ed. O. Sinanoglu (Academic Press, New York, 1968) p. 93. [4] D.L. Dexter, J. Chem. Phys. 2 (1953) 836. [5] T. Ebata, Y. Suzuki, N. Mikami, T. Miyashi and M. Ito, Chem. Phys. Lett. 110 (1984) 597. [6] M. Chattoraj, B. Bal, G.L. Closs and D.H. Levy, J. Phys. Chem. 95 (1992) 9666. [7] M. Chattoraj, B. Panlson, Y. Shi, G.L. Closs and D.H. Levy, J. Phys. Chem. 97 (1993) 13046. [8] M. Chattoraj, D.D. Chung, B. Paulson, G.L. Closs and D.H. Levy, J. Phys. Chem. 98 (1994) 3361. [9] N.A. Van Dantzig, D.H. Levy, C. Vigo and P. Piotrowiak, J. Chem. Phys. 103 (1995) 4894. [10] D.W. Liao, W.D. Cheng, J. Bigman, Y. Karni, S. Speiser and S.H. Lin, J. Chin. Chem. Soc. 42 (1995) 177. [11] P. Rona and U. Feldman, J. Chem. Soc. (1958) 1737. [12] J. Bigman, Y. Karni and S. Speiser, Chem. Phys. 177 (1993) 601. [13] N.L. Allinger, J. Am. Chem. Soc. 99 (1977) 8127.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters 263 (1996) 441-448
Electronic energy transfer in supersonic jet expanded naphthalene-( CH 2) n-anthracene bichromophoric molecules Gershon Rosenblum, David Grosswasser, Frank Schael, Mordecai B. Rubin, Shammai Speiser * Department of Chemistry, Technion-lsrael Institute of Technology, Haifa 32000, Israel
Received 19 September 1996
Abstract A study of intramolecular electronic energy transfer (Intra-EET) has provided evidence, for the first time, of a dramatic difference in Intra-EET efficiency results obtained under jet cooled and room temperature solution conditions. This was observed with anthracene-(CH2),-naphthalene bichromophoric molecules, A1N (n = 1) and A3N (n = 3). The rich fluorescence excitation spectrum of the naphthalene moiety in AIN indicates an inefficient EET process whose rate constant is substantially slower than that of the naphthalene moiety fluorescence. It was shown that the EET rate depends on a specific vibronic excitation that affects the molecular conformation, and was found to be at least two orders of magnitude slower in AIN molecule compared to A3N.
1. Introduction Interaction between excited and ground states of two molecules involving electronic energy transfer (EET) has been the subject of considerable interest [1]. This process plays a key role in chemistry, biology and physics and is well documented and summarized. However, some basic problems in molecular photophysics which involve short range interactions manifested in intramolecular EET (IntraEET) are still the subject of current investigations. The first observation of short range Intra-EET was reported by Schnepp and Levy for the naphthalene-(CH2)n-anthracene system [2]. Later, many other groups examined other bichromophoric sys-
* Corresponding author.
tems [1]. The occurrence of Intra-EET could be readily evaluated from knowledge of excitation and emission spectra of each moiety alone and by comparison with the corresponding spectra of the bichromophoric molecules. The basic Intra-EET process can be described by kEr D*-B-A ~ D-B-A* ,
(1)
where the excitation energy is transferred from an excited donor D* chromophore to a ground state acceptor moiety A, resulting in quenching of D* fluorescence and excitation of A. B denotes a molecular spacer bridge connecting the two chromophores which may play a role in promoting the transfer process. In most cases the Intra-EET rate constant, kET , is attributed to two possible contributions. The first is the long range Coulombic contribution which was formulated by FSrster in terms of dipole-dipole
000%2614/96/$12.00 Copyright© 1996Elsevier Science B.V. All fights reserved. PI! S0009-2614(96)01 229-8
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G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
interaction [3]. The second contribution to EET is the short range exchange interaction, as formulated by Dexter [4]. The rate of dipole-dipole induced EET decreases as R -6 whereas that of the exchange induced process decreases as e x p ( - 2R/L), R being the interchromophore distance and L being the average van der Waals radius for the overlapping orbitals. Until recently most reported studies of Intra-EET were performed in solutions where solvent effects cannot be ruled out and where complete vibrational relaxation of donor and acceptor excited electronic states precedes the EET event. In general the prospects for investigating Intra-EET in a bichromophoric molecule from a single excited donor vibronic state, under collisionless conditions in the low pressure jet, are not very high. Yet the unique conditions in a supersonic jet expansion offers a way of studying these processes in some detail. Although supersonic jet spectroscopy has been widely used for investigating molecular radiative and nonradiative processes, very few studies of EET under these conditions have been reported. The first reported supersonic jet study of IntraEET was made by Ito and coworkers [5]. They measured Intra-EET between o-xylene or m-xylene connected by a chain of three methylene groups to p-xylene, in the bichromophoric compounds 1-(otolyl)-3-(p-tolyl)propane and 1-(m-tolyl)-3-(ptolyl)propane, respectively. A more systematic investigation of Intra-EET in isolated bichromophoric molecules was undertaken by Levy and coworkers [6-9]. Several bichromophoric molecules containing two aromatic moieties connected by an aliphatic spacer bridge were studied under jet-cooled conditions. The measurements included excitation and dispersed fluorescence spectra of the bichromophoric molecules, as well as the corresponding spectra of the individual chromophores. In addition, the possible conformations of the molecules were determined using a variety of structure determination techniques. Intra-EET was inferred from spectra that revealed acceptor emission features upon excitation of a donor chromophore. The results were interpreted in terms of existing EET theories and the general theory of radiationless transitions in isolated molecules. We have chosen to study the naphthalene(CHE)n-anthracene bichromophoric molecules (n--
1 and 3) investigated by Schnepp and Levy in their pioneering study of Intra-EET, namely 9-((1-naphthyl)-methyl)-anthracene (A1N) and 9-(3-(1-naphthyl)-propyl)-anthracene (A3N) [2]. The 1962 work showed that the only fluorescence observed in solution came from the anthracene moiety and had spectral characteristics of anthracene emission which indicates that the excited naphthalene moiety had transferred its excitation energy completely to the anthracene moiety. This high efficiency EET process is partly due to the good spectral overlap between the naphthalene emission and anthracene absorption spectra. The spectral features of these spectra are relatively broad due to many different conformations of the floppy molecules, and the fact that interaction with the solvent caused complete relaxation of the initially excited vibrational states of the molecules prior to the EET process. The situation is expected to be different when the molecules are under supersonic jet cooling conditions. A large majority of the molecules are in their ground vibrational state and in their most stable conformation. The spectra of the molecules consist of very sharp lines, and the spectral overlap can be poor for some lines, leading to a relatively slow EET process and to dual fluorescence from the excited bichromophoric molecules, especially for those conformations where either dipole-dipole interaction is weak or orbital overlap is rather poor. For other conformations and spectral lines Intra-EET may still be very efficient as in solution at room temperature. Although existing EET theories applicable for solution conditions can be extended to the isolated molecule case, no good estimates of the relative efficiencies of the Coulomb and exchange mechanisms are available for jet-cooled molecules [10]. We expect that the exchange interaction is more important for the A n N bichromophoric system under jet conditions. In this case the rate should depend strongly on the overlap of the ~ electronic systems of the two closely situated moieties in the molecule. If Coulomb interaction is more important, the crucial geometrical factor is the angle between the transition dipole moments of the two chromophores. Since the molecules are in their ground vibrational states, belonging to their energetically most stable conformation, their geometries should strongly affect the EET rate.
G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441---448
2. Experimental The bichromophoric compounds were synthesized following the literature procedure [11]. The molecules were seeded in a supersonic jet using a pulsed nozzle apparatus described elsewhere [12]. Briefly, 6 atm helium was passed over a heated cell containing liquid organic sample. The temperature was chosen so as to produce an organic vapor pressure of about 0.2 Tort. The A1N sample was heated typically to 198°C, and the A3N sample was heated to 238°C. The gas containing an organic vapor was expanded through a 1.1 mm orifice to an evacuated chamber. The pressure in the vacuum chamber was typically 2 × 10 -4 Tort. The jet was crossed with a laser beam ( < 1 mm spot size) 33 mm down stream. About 10% of the resulting fluorescence was collected by a lens to a photomultiplier. The laser beam ( - - 5 mW) was produced by frequency doubling or mixing an Nd:YAG (Continuum) second harmonic pumped dye laser. The resulting laser bandwidth was about 0.2 cm -1. The experimental uncertainties of absolute frequencies are + 2 c m - J and those of relative frequencies are 0.2 c m - t.
3. Results and discussion
3.1. Calculation of the molecular geometry Calculations were performed to estimate the ground state geometries of the molecules A1N and A3N. We used both MM2 [13] and AMBER [14,15] molecular force field methods. The two methods produced practically the same results. For the A3N the calculated ground state geometry is that of an anthracene-naphthalene sandwich. The two chromophores do not lose their planarity and lie in almost parallel planes. The average interplanar distance is about 3.6 A, almost the same as in the anthracene-naphthalene cluster (3.57 ,~ [16]). The molecule is very floppy - the potential function around the minimum is 'flat'. The flexible aliphatic chain allows the two chromophores to 'slide' easily with respect to one another. The energetically most stable geometry lacks any symmetry, the potential is strongly anharmonic and the normal modes are mixed
443
even at lowest interchromophoric vibrational energies. This A3N geometry seems extremely favourable for fast EET. The w-orbital interchromophoric overlap is most pronounced in the sandwich orientation. The anthracene S t ~ S O transition dipole moment is parallel to the short in-plane axis of the chromophore. The naphthalene S, ~, S O transition dipole moment is small as the transition is forbidden. Its direction depends strongly on the specific vibronic excitation and on the substitutions that break the symmetry while the transition dipole moments stays in plane. This means that, for most vibronic transitions, the direction of the transition dipole moment in the naphthalene chromophore is close to that in the anthracene chromophore. We conclude that the obtained geometry is optimal both for Coulomb and for exchange mechanisms of EET. The calculated geometry of A1N is quite different. The single methylene group bridge is not flexible enough to allow the two chromophores to come into close proximity. The angle between the two chromophore planes is close to 90 °. The anthracene w-system is perturbed by naphthalene hydrogens, but there are no anthracene atoms in the vicinity of the naphthalene "rr-system. The w-orbital overlap is very poor. The anthracene plane short axis is at a large angle with the naphthalene plane (about 65 °) so that any interaction with an in-plane naphthalene transition dipole moment tends to be weak. The geometry possesses no symmetry but it is close to a C s geometry. Some sort of a double well potential vibrational behavior can be expected in this molecule although the fifteen interchromophoric modes are mixed. There is a strong van der Waals interaction between the chromophores. The energy gain of positioning the two chromophores in a sandwich conformation is about 7 kcal/mol [16]. This means that whenever the bridge is long and flexible enough to allow the chromophore planes to be more or less parallel and at about 3.6 A between them, the sandwich geometry is the most stable. This is apparently the case for A3N. In A1N the aliphatic bridge is too short to allow for the needed flexibility, the structure is rigid and the chromophores are far apart. The A2N case would be intermediate: any sandwich structure would require a considerable strain on the aliphatic
444
G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
angles and some distortion of the chromophore planes. This will be the subject of future studies.
3.2. Excitation of the anthracene chromophore The fluorescence excitation spectrum of the anthracene moiety S] ,,-- S O transition of A3N at the origin of its spectral region is presented in Fig. 1. The 0 - 0 transition is at 26704.4 cm- l, which is red shifted by 990 c m - l with respect to the bare anthracene origin. This shift is typical of substitution at position 9 combined with van der Waals interaction between naphthalene and anthracene. It can be compared to the 9-methylanthracene shift value of 756 c m - l [17] and to the anthracene-naphthalene cluster value of 556 c m - l [18]. There are some hot bands to the red of the 00° band. Their positions, spacings and intensities suggest that they correspond to molecules possessing one or two quanta of the lowest energy interchromophore vibrational modes prior to the laser excitation. There is a complex structure in the region of 0-100 cm -1 energy excess above the 0 ° transition. No constant step progressions can be identified here. The dominant distances between close lines are 3, 5 and 10 cm-1. This manifests the extremely low frequencies of the large amplitude interchromophore movements that are excited in the molecule (in the sandwich type aromatic clusters the corresponding lowest observed frequencies are usually 10-30 cm- ] [12,18-22]). The intensities of the bands drops quickly with increasing excitation energy. Above
0
/k _ .~_ 26680 26700
20720
28740
26760
26780
Frequency, crrr I
Fig. 1. Fluorescence excitation spectrum of A3N at the origin region of the anthracene chromophore.
v.4: 44cm "t
•~' 80
6o
B~
12~
~= 73crrr~ Vc=141cm-1 vo=149crrrl ~=253cm-t vt2=392cm,f
A'C~ B~C~ l ~ 2O
I I
26800
C~p'* A~,D~/c2
I I ,
/ I
26900
,
I
'2~At°
12~A~B~/
i° i
I
i
27000 27100 27200 Frequency, crn-I
27300
Fig. 2. Fluorescence excitation spectrum of the anthracene chromophore of A1N. The relative intensities of the 0o° band and of the A]o band are 311 and 115 respectively.
100 cm -1 excess energy the different vibrational modes are completely mixed, the Franck-Condon factors are very low, and the spectrum represents a low intensity quasi-continuum. This fast decrease of the spectrum intensity indicates that the S~ geometry of the molecule is close to the S O geometry. The excitation spectrum of the A1N molecule, at the spectral region of the origin of the anthracene moiety SI ~ S O transition is presented in Fig. 2. The red shift of the origin transition (26798.9 cm- ] ) with respect to the bare anthracene molecule is 896 cm- ~, smaller than in A3N. This reflects the smaller perturbation of the anthracene electronic system by the naphthalene moiety in A1N. Many sharp vibronic bands can be identified in the spectrum. The distances between the bands are generally larger than in the A3N spectrum indicating that the molecule is much more rigid. Of the 26 most intense bands, 23 can be positively attributed to combinations of only six different vibrations. One of these vibrations is the well-documented 12ag (lowest frequency) vibration of anthracene chromophore [23], and the other five (denoted as A, B, C, D, E) are tentatively assigned as interchromophore vibrations. Their frequencies correspond to high-mass pseudorotation and bending modes. They are clearly higher than the usual interchromophoric frequencies in aromatic clusters. All second overtones in the spectrum have much lower intensities than the corresponding principal
G. Rosenblum et a l . / Chemical Physics Letters 263 (1996) 441-448
bands. Third overtones were not detected. The mixed excitation frequencies (e.g. Al0 B l) correspond to the sums of the principal frequencies within + 0.3 cm- 1. These facts demonstrate that the anharmonicity of the vibrational potential is low, the modes are not mixed and the excited state geometry is close to that of the ground state.
3.3. Excitation of the naphthalene chromophore Excitation spectra of the naphthalene chromophore were compared to that of l-methylnaphthalene (1-MN). This spectrum is shown in Fig. 3a, it is of better quality than the previously reported one [24]. The fluorescence excitation spectrum of A1N at the spectral region of the naphthalene moiety origin of the S I *-- S O transition is presented in Fig. 3b. The band is red shifted by 242.5 cm-1 compared to the bare naphthalene transition. This value is remarkably close to the 1-MN shift (246.5 cm -1 , Fig. 3a, [2427]). This leads to the conclusion that the anthracene moiety presence hardly perturbs the naphthalene electronic system. This is in contrast with the A3N situation (a 622 cm-1 shift, see below). Analogs of seven of the nine 1-MN bands can be definitely
: ~,2B
~
.
.o,1t. 31800
, b 32000
32200
32400
Frequency, crnFig. 3. (a) Fluorescence excitation spectrum of I-MN. The intensity of the ~ band is - 5 . 2 times higher than the 0o° band intensity. (b) Fluorescence excitation spectrum of the naphthalene chromophore of A1N. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes in ns. All recorded fluorescence decay curves were monoexponential within the experimental uncertainty.
445
identified in the spectrum. These AIN bands are shifted not more than 2 cm-1 from the corresponding 1-MN bands. There are low frequency progressions in the spectrum indicating excitation of some interchromophore motion in the molecule. The structure at 320-380 cm-1 above the origin can be due to some rotational activity of the 9-anthryl-methyl substitution around the methylene-naphthalene bond. The molecular conformation can be different in the ground and the excited state, as is the case for 1-MN [25,26]. An energy difference of 300 cm- l is typical for such conformations. The molecule geometry is relatively close to the C s symmetry, and the origin transition structure can be due to some double well potential-like activity. Fluorescence lifetimes were measured at band maximum frequencies. For most bands they are between 230 and 260 ns. All the bands that have analogs in the 1-MN spectrum correspond to lifetime values close to their analogs. This means that the rate constant of naphthalene fluorescence quenching by the anthracene moiety, if it occurs at all, is slower than 1 X 10 6 s - l . There is only one A1N band with a shorter lifetime (128 ns). No observed transitions of 1-MN [24] or naphthalene [24,28] in the 0-1000 cm -1 excess energy region have a lifetime of less than 200 ns. This means that the process responsible for the shortening of the lifetime is not the mere vibrational coupling induced electronic mixing but is due to the anthracene substitution. The quenching rate constant that corresponds to this shorter lifetime value is 5 X 10 6 S- I, a rather slow EET process. This was proved by recording an excitation spectrum while a 399 nm cut-off filter was introduced to remove any naphthalene moiety emission signal. Most of the naphthalene moiety emission in the jet experiments is between 320-370 nm, whereas the anthracene moiety emits between 360-420 nm. To prevent interference with the spectrally overlapping anthracene emission [12] the signal was time-gated and measured in the interval of 25-325 ns after the excitation. The emission lifetime of the anthracene moiety in this spectral region is about 5 ns. The filtered, time-gated signal results neither from naphthalene emission (it is filtered off) nor from regular anthracene emission (it decays too fast). The detected signal is due to slow excitation transfer from
G. Rosenblum et al. / Chemical Physics Letters 263 (1996) 441-448
446
248+5ns
.259~5ns 2det>3OOnm
ll
128+10ns l~
~J
159~34ns
,
I
31780
i
I
"l I "
I"
~'rl"
" ~1 y
,
I
,
31790 31800 31810 31820
Frequency, cm-1 Fig. 4. Fluorescence excitation spectrum of the naphthalene chromophore of A1N at the 0 ° region. Upper trace shows the untiltered spectrum while the lower one is the spectrum filtered for only anthracene fluorescence. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes.
naphthalene to anthracene moiety followed by fast subsequent anthracene emission. The only strong band that remains in the filtered spectrum (Fig. 4) is the one with the short lifetime. This lifetime was measured under these conditions with less precision but its value (159 + 34 ns) is equal within experimental error to the value measured without a filter. The presence of this band in the spectrum under the conditions where all the other bands disappear is striking evidence for the intramolecular EET process. This result is in agreement with the observed slow Intra-EET for the less stable conformers of the A - N cluster [12,16]. It is, however, very different from the extremely efficient EET observed at room temperature in solution [2]. There are two weaker bands in the 1-MN spectrum that have no clearly identifiable analogs in the A1N spectrum. This can be due to a comparatively large shift of the corresponding A1N bands. Such a shift would suggest strong coupling of these vibronic excitations to some substituent vibrations whose frequencies can be strongly affected by the changed substitution mass. Another explanation would be a fast quenching of these vibronic excitations by the anthracene moiety. This would mean that the excited vibrations are capable of bringing the two chromophores into a geometrical position favourable for EET.
The excitation spectrum at the origin region of the naphthalene moiety S I ~ - S O transition in the A3N molecule consisted of a broad featureless structure (cf. Fig. 5). At the same excitation energy, the anthracene moiety of the molecule is excited to the vibronic quasi-continuum which is about 5000 cm-1 higher in energy than its electronic origin. The observed signal was extremely weak (100 times lower than that for AIN) and largely independent of the exciting laser wavelength, except for one broad feature around 31398 cm -~ . The decay time was about 250 ns both for the broad band and for the baseline signal. We suppose that this high baseline signal can only be due to excitation of the small portion of molecules at the outskirts of the jet that were not efficiently cooled. The A3N molecule is very floppy, so that even 100 c m - 1 of excess vibrational energy results in complete mixing of all the interchromophore modes and leads to a very high amplitude chaotic intramolecular movements in the molecule. The spectral position of this broad band is close to the expected electronic origin of the naphthalene moiety. This band is probably an envelope of the signals of the hot molecules excited without changing their vibrational energy (such excitations should have the highest Franck-Condon factors). The most interesting result of this experiment is the total absence of sharp bands that can be at-
249~21ns 1
2~t>4OOnm o)
262+28ns I
~1~
290"t:30ns
~4 ~ I
0
i
31390
i
31395
i
31400
i
31405
Frequency, cm-1 Fig. 5. Fluorescence excitation spectrum of the naphthalene chromophore of A3N at the 00° region filtered for only anthracene fluorescence. The fluorescence was detected between 25 and 300 ns after the excitation pulse maximum. Also shown are the fluorescence lifetimes. The maximum signal is 20 times weaker than that from A I N (cf. Fig. 4). No signal was detected in the absence of A3N.
G. Rosenblum et a l . / Chemical Physics Letters 263 (1996) 441-448
tributed to cold A3N molecules. This should be compared to 1-MN case (Fig. 3a). The 1-MN spectrum obtained under similar conditions exhibits quite a number of strong sharp features. The difference between the A3N and the 1-MN spectrum is most probably due to the total quenching of the naphthalene moiety fluorescence by the anthracene chromophore in A3N. This Intra-EET process is completed in less than 20 ns and is followed by fast anthracene emission that could not be detected because of the time gating. We can thus conclude that the Intra-EET process is efficient in the A3N molecule. This result can be compared to our previous studies of the anthracenenaphthalene cluster [12]. Similar quenching of the naphthalene moiety fluorescence was observed in these experiments. The energetically most stable calculated geometry of the cluster is very similar to that of the A3N molecule [16]. The minute amount of excited hot A3N molecules exhibits a long-lived fluorescence lifetime. This means that EET is not as efficient in these molecules as it is in the cold ones. These hot molecules are either vibrating with large amplitudes or trapped in metastable conformations. The optimal sandwich configuration has low statistical weight or is not achieved at all in such molecules. This geometry distortion slows down EET and makes it possible to detect the slow residual fluorescence from the anthracene moiety.
4. Summary We have observed for the first time a dramatic difference between Intra-EET efficiencies in a bichromophoric molecule under jet-cooled and room temperature solution conditions [2]. The rich excitation spectrum of the naphthalene moiety in the A1N molecule indicates an inefficient EET whose rate constant is substantially smaller than that of the naphthalene moiety fluorescence (3 x 10 6 S - I ) . Only for one vibronic transition, the EET rate constant was high enough to be measurable (5 × 10 6 s - 1). This shows that the EET rate depends on a specific vibronic excitation that affects the molecular conformation. We assume that there can be vibronic states with high rates of EET, but our present experimental
447
arrangement allows only measurements of EET rate constants less than 40 x 106 S - l . Two-photon twocolor ionization experiments currently underway in our laboratory will resolve this issue. The EET rate constant was found to be at least two orders of magnitude slower in the A1N molecule compared to the A3N molecule. The geometry differences are thought to be responsible for this difference. This is in accord with our previous studies of the anthracene-naphthalene cluster which gave evidence for two EET rates associated with two cluster isomers [12]. Calculations [16] predict two isomers of the cluster, one of the sandwich type, and the other with the two molecules ~r-electronic systems far apart.
Acknowledgements We are grateful to Dr. Irina Fedatov for preparation of the bichromophoric molecules. This work was supported in part by the Technion V.P.R. Fund for the Promotion of Research at the Technion and by grant number 92-00224 from the US-Israel Binational Science Foundation.
References [1] S. Speiser, Chem. Rev. (in press) and references therein. [2] O. Schnepp and M. Levy,, J. Am. Chem. Soc. 84(1962) 172. [3] Th. F~rster, in: Modern quantum chemisu'y, Vol. 3, ed. O. Sinanoglu (Academic Press, New York, 1968) p. 93. [4] D.L. Dexter, J. Chem. Phys. 2 (1953) 836. [5] T. Ebata, Y. Suzuki, N. Mikami, T. Miyashi and M. Ito, Chem. Phys. Lett. 110 (1984) 597. [6] M. Chattoraj, B. Bal, G.L. Closs and D.H. Levy, J. Phys. Chem. 95 (1992) 9666. [7] M. Chattoraj, B. Panlson, Y. Shi, G.L. Closs and D.H. Levy, J. Phys. Chem. 97 (1993) 13046. [8] M. Chattoraj, D.D. Chung, B. Paulson, G.L. Closs and D.H. Levy, J. Phys. Chem. 98 (1994) 3361. [9] N.A. Van Dantzig, D.H. Levy, C. Vigo and P. Piotrowiak, J. Chem. Phys. 103 (1995) 4894. [10] D.W. Liao, W.D. Cheng, J. Bigman, Y. Karni, S. Speiser and S.H. Lin, J. Chin. Chem. Soc. 42 (1995) 177. [11] P. Rona and U. Feldman, J. Chem. Soc. (1958) 1737. [12] J. Bigman, Y. Karni and S. Speiser, Chem. Phys. 177 (1993) 601. [13] N.L. Allinger, J. Am. Chem. Soc. 99 (1977) 8127.