Molecular structure control of intramolecular electronic energy transfer

~ . ~

journal of MOLECULAR

LIQUIDS ELSEVIER

Journal of Molecular Liquids 86 (2000) 25-35

www.elsevier.nl/locate/molliq

Molecular structure control of intramolecular electronic energy transfer Shammai Speiser and Frank Schael* Department of Chemistry, Technion- Israel Institute of Technology, Haifa 32000, Israel We show that the rate of through-bond-mediated intramolecular singlet-singlet electronic energy transfer (EET) in the rigid novel 10-bond DMN-bridge-dione 2(10) is five orders of magnitude faster than that predicted for 1(10) DMN monoketone and observed in the steroid systems exhibiting comparable interchromophore separations of 12.5A. The efficiency of energy transfer (>99.5% calculated based on the measured rate) is also much larger than that observed in flexibly linked systems containing the same donor-acceptor chromophores, for which through space EET via exchange interaction is dominant, thus indicating the possibility of controlling EET by judicious molecular engineering. © 2000 ElsevierScience B.V.All rights reserved. 1. INTRODUCTION Interaction between excited and ground states of two molecular chromophores has been the subject of considerable interest [1]. It is manifested in chemical reactions, in complex formation, and in photophysical processes such as electronic energy transfer (EET) [2-4]. These processes play a key role in chemistry, biology and physics. EET processes occur at distances ranging from 1A to more than 50A, and on time scales from femtoseconds to milliseconds. EET can be observed in the solid state, on surfaces, in solution, in the gas phase and in isolated molecular systems in supersonic jet expansions. Until recent years most studies involved inter-molecular EET (lnter-EET) long-range processes between donor (D) and acceptor (A) chromophores. While these studies are well documented and summarized by numerous reviews [5-7], some basic problems in molecular photophysics which involve short range interactions manifested in intramolecular EET (Inlra-EET) and intramolecular electron transfer (Intra-ELT) processes are still the subject of current investigations [8]. Intra-EET and Intra-ELT can occur whenever two separated chromophores are incorporated in a single molecule. In such eases, control of the spatial relationship between donor and acceptor groups exists without the randomness characteristic of intermolecular interactions. Furthermore, Intra-EET and Intra-ELT can be observed in rigid or viscous media where encounters between separated molecules leading to short range transfer processes are not possible. Thus linked Donor-Accepter biehromophorie molecular systems of the type Donor-bridgeAcceptor (D-B-A)~ in which the bridge is a saturated hydrocarbon moiety, have been developed into a major tool for studying various aspects of electron donor-acceptor interaction over the past decades. Especially the process of photoindueed charge separation has been studied extensively [9], where an electron is transferred from donor to aceeptor upon optical

* Present address: Institut fllr Physikalische und Theoretische Chemic, Friedrich-Alexander Universitat Eflangen-Nllrenberg,D-91058, Erlangen,Germany 0167-7322/00/$ - see front matter © 2000 ElsevierScience B.V. All rights reserved. PII SO167-7322(99) 00121-X

26 excitation of either of the chromophores to create the highly dipolar charge transfer (CT) state D+-bridge-A ". Intra-EET is usually described by D*-B-A ---kEET'-~D-B-A*

(I)

where the excitation energy is transferred from an excited donor D* chromophore moiety to a ground state acceptor moiety A, resulting in quenching of D* fluorescence and sensitization of A. In most cases the intra-EET rate constant kEET, is attributed to two possible contributions. The first is the long range Coulombic contribution which was formulated and by FSrster [10-12] in terms of dipole-dipole interaction. The second contribution to EET is the short range exchange interaction, as formulated by Dexter [13]. The basic features of Dexter-FSrster theory are: a. While for allowed singlet-singlet EET, both dipole-dipole induced (Coulombic) interaction and exchange interaction are nonvanishing; for triplet-triplet EET and for EET involving forbidden singlet-singlet transitions, only exchange interaction contributes to the observed EET process, b. The rate of dipole-dipole induced EET decreases as R "tl whereas that of the exchange induced process decreases as exp(-2R/L), where R is the interchromophore separation and L is the orbital vander-Waals radii, c. The rate of dipole-induced EET depends on the oscillator strengths of D--~D* and A--~A* radiative transitions, however, the rate of exchange induced transfer does not depend on either of the two oscillator strengths. Similar approach is applied when ELT processes are discussed. However, The main problem is to differentiate between all these processes which may act simultaneously and competitively. Sometimes an Intra-EET process may precede ELT, and often bridge-relayed, superexchange-mediatedIntra-EET, Intra-ELT and combinations 0fthe two processes, are observed [14-20]. There are many experiments, in particular photoinduced Intra-ELT studies, that indicate donor and acceptor interactions at D and A separations much larger than the sum of their van der Waals orbital radii [14-19]. The proposed mechanism is a superexehange interaction operating beyond actual orbital overlap region, usually thought to be mediated by electronic through band (TB) coupling of the interehromophore bridge orbitals. Experimental evidence for TB-superexchange interaction promoting bridge mediated intramolecular transfer processes is especially available from studies of photoinduced IntraELT. These studies have shown that the ELT rate is strongly depomoleeular structure, in particular on the inter-chromophore bridge conformation and configuration. A systematic study of these phenomena was done by Verhoeven and coworkers [16, 19]. They have examined Intra-ELT in specially designed bichromopboric molecules, mostly having a rigid interchromophore bridge. The general conclusion that could be made is that the governing mechanism for this process is superexchange involving TB bridge interaction. In some cases the competition between through bond and through space interactions was examined, comparing rigid arid flexible bichromophoric structures. Following their Intra-ELT work [19], Verhoeven and coworkers examined singiet-singlet intra-EET processes a series of bichromophorie molecules, DM1N-n-ketone, l(n), containing rigid polynorbornyl interehromphore bridge spacers, separating the dimethoxynaphthalene donor from the carbonyl acceptor [14-16]. When the two chromophores were connected via 410 t~ bonds in an all trans conformation. Intra-EET was observed even for R=ll.5A where direct orbital overlap is not possible. The measured transfer rate constant was much larger than that calculated from the FOrster model and depended exponentially on the number

27 of interchromophore c bonds. Changing the conformation to cis or gauche resulted in much lower EET yield without, however, a marked change in R for the same number of C-C bonds. It was concluded that Intra-EET was mediated by a TB coupling which was hindered by adding a kink to the all trans six-bond bridge, thus resulting in a much less efficient EET process. Comparison with Intra-ELT results for biehromophoric molecular systems with similar bridge structures led to the conclusion that k ~ = k~r 2 [14, 16]. The main experimental manifestations of TB superexchange interaction in EET and ELT are: a. The R interchromophore distance dependence of the efficiency of the process does not follow either Dexter or Frrster predictions, b. Intra-EET and Intra-ELT are still very efficient at R> I 0A for dipole-dipole forbidden processes, especially in rigidly linked D-B-A molecules. However, it seems that in more flexible structures Intra-EET is still controlled by short-range through-space Dexter type interaction, c. The observed rates depend on the interchromophore bridge conformation for a relatively fixed mutual orientation of the interacting chromophores. d. For trans cr bond bridged structures the transfer rate decreases exponentially with the number of bonds.The major approach for treating TB superexchange controlled short-range Intra-EET is to generalize the theory by including TB, involving any number of bridge relay units in the interaction matrix element. As expected for the TB mechanism, the EET rates display a strong exponential distance dependence of the form shown by eq (1), in which n is the number of bonds in the bridge relay, and [3' is an attenuation coefficient with experimentally determined values in the range of 2 - 2.5 per bond [14-19]. kEET= A exp(-[3'n) = A exp (-[3R)

(2)

In order to determine the role played by the interchromophore bridge we compare here intra-EET between the same two chromophores connected by a rigid bridge for which previous studies indicated a through-space exchange interaction controlled EET for aflexible bridge [21 ].

2. EXPERIMENTAL

The synthesis of the relevant bichromophoric compounds shown in scheme 1 is described elsewhere [20-22], Scheme 1 shows the novel rigid bichromophores. Time correlated single photon counting [23] was used to obtain the rate of EET as previously described [21]. Fluorescence spectra were recorded by a Perkin Elmer LS50 spectmfluorimeter. All samples were deoxygenized by bubbling 15 min purified nitrogen through the solution and were sealed off.

3. RESULTS AND DISCUSSION Earlier studies of intra-EET in semiflexible aromatic compounds, of the type shown in scheme 2, establisehd through-space exchange interaction as the process promoting mechanism [7]. This was now implemented by the study of the DMN bichromophoric compounds of scheme 1.

28

~

o

1(4);m=1

1(6);m=2

M

0

2(61;m"1

2110);m-2

Scheme 1.

1,4-Naph-5,5

P-n,n

(C H 2)n /

n = 4,5,6

O-n,n

"~0

~

"(C H 2)n/"~""0

n = 2,3,4

M=n,n

~

(

C

H :~n"

"~

n = 3,4

Scheme 2. Figure 1 displays the absorption and fluorescence spectra of DMN model compound (I), which does not have the dione moiety, DMN-6-dione (H), and of DMN-10-dione (HI). Compounds H and IH exhibit dual fluorescence spectra upon excitation around 300 nm. The spectra consist of a component very similar to the fluorescence of I and an additional biacetyl-like component between ca. 490 and 550 nm. The absorption and fluorescence spectra of compounds II and HI can be very well described as superposition of the spectra of I and a biacetyl-like chromophore. The fluorescence spectra of compounds H and IH exhibit changes upon irradiation. With increasing irradiation time the relative intensity of the DMN-like emission increases and the

29 dione emission decreases. To minimize degradation during collection of the spectra the irradiation intensity of the spectrometer was attenuated. It was checked that under these conditions no degradation of the compounds takes place in the spectrometer (cf. inset of Figure 2). Although all bulbs and cuvettes used for sample preparation, deoxygenization and storage were covered with aluminium foil, fleshly prepared samples exhibited different degree of degradation (Figure 2). It was concluded that decomposition took place already during preparation of the samples. It was not possible to identify the step in the sample preparation during which the observed decomposition took place. Thus the spectra obtained in different laboratories and the sample preparation procedures should be compared. All samples of II and III prepared in this laboratory showed DMN-like fluorescence. Fluorescence excitation spectra of II and Ill monitored at 350 nm and at 500 nm, where the dione chromophore emits, closely resemble the absorption spectrum of the DMN chromophoric unit (Figure 3). Excitation of the DMN chromophore in II and HI thus results in emission from the dione chromophore, which is a clear evidence for electronic energy transfer (EET) from the excited DMN chromophoric unit to the dione moiety. (The reference compound I exhibit under comparable conditions only negligible intensity in the fluorescence excitation spectrum monitored at 500 nm). Quantitative evaluation of fluorescence quantum yields are complicated by the degradation of the compounds. Attempts to determine fluorescence quantum yields of II in n-hexane excited at 290 nm yielded ~pFDMN~ 0.07 for the fluorescence from the DMN unit (vs. DMN model in n-hexane, ~F = 0.35, XF= 5.4 ns [1]) and dpFDi°ne~ 0.002 for the fluorescence from the dione moiety (vs. Tetracene in toluene, ~bF= 0.17 [2]). dpFDr~ ~ 0.05 was obtained for III. (The samples were freshly prepared and not exposed to direct irradiation prior to measurement.) As a rough estimate for the quenching rate constant of the DMN fluorescence due to the dione substitution, kq > 7x108 s"t and kq > lxl09 s"1 can be obtained for II and III, respectively. The experiment with II was repeated twice with similar results. Because it cannot strictly be excluded that the samples used were already decomposed to a considerable degree (el. inset of Figure 2), neither the relative intensity of the DMN and the dione fluorescence nor the extinction of the samples at the excitation wavelength can be taken as very reliable. The estimation for kq and ~Ft~i°nementioned above should be only regarded as a rough estimate of a lower limit. Investigation of the degradation kinetics and extrapolation to zero time may give more reliable results for the bichromophoric molecules. Time resolved studies of the dione fluorescence of II and IlI should overcome the problems with stationary experiments caused by the degradation and should yield much more reliable results than stationary measurements. On the other hand the occurence of the photodegradation of II and III, which is apparently very efficient, can be interpreted as a result of (probably efficient) EET. The absorption spectra of the bichromophoric molecules show that direct excitation of the dione moiety is negligible. It can thus be concluded that excitation of the DMN chromophore leads to excited dione chromophore which undergo subsequent intersystem crossing and decomposition. This finding can only be explained in terms of EET taking place from the loeaUy excited DMN chromophore to the dione group which then undergoes coupling and other reactions [22] with the consequent switching on of the DMN fluorescence. Monitoring the kinetics of the appearance and decay of the emission of the acceptor following excitation of the donor provides one of the most direct methods of determining the efficiency and rate of energy transfer. The time-resolved fluorescence decay profiles obtained

30 02

50

",~

//\/ \ , o \\1

t

\q\

0 300 S~S~IIVAD~JIIS.t.#~

\\

350

400 Z/nm

- -

DMN[lO]dione

-

DMNt6idione '

-

450

500

.~

55~

Fig. 1. Absorption and uncorrected fluorescence spectra of DMN model, I, DI~l-6-dione, II, and DMN-10-dione, Ill, in n-hexane. Excitation wavelength was 290 nm. The fluorescence spectra were approximately normalized at wavelength of maximum intensity.

20

i~

O- 75 rain

500

.~ 250

0 300

350

400

450

500

Fig. 2. Uncorrected fluorescence spectra of different freshly prepared samples of DNN4dione in n-hexane. Inset: Fluorescence spectra of DNl'q-6-dione in n-hexane recorded at different irradiation times in the fluorimeter. The irradation time was 0-'/5 rain.

31 0.2 DMN[6]dione@500nm . . . . . . . . . . DMN[6]dione@350nm - DMN[10]dione@500nm . . . . . . . . . . DMN[10]dione@350nm

iI

!

\ i

\

iI

"~, 0.1

.e,

R

i L

._= i •

•

• .

0

'

' '

' ' 300

Se~afI [PADDO..V6110.1.1999

a i i

~

'

~

j e

I

" 350

' "~"~' 400

450

)~ / a m

Fig. 3. Uncorrected fluorescence excitation spectra of DMN-6-dione and DMN-10-dione in n-hexane monitored at 350 and 500 nm. R denotes Raleigh peaks. from DMN-6-dione and DMN-10-dione in n-hexane, recorded at 505 nm (acceptor region) following 295 nm excitation (DMN absorption), were measured. A rise time o f - 4 0 ps (which is approaching the time resolution of the present instrumentation) was obtained for DMN-10dione by fixing one of the decay lifetimes in the non-linear least squares, iterative reconvolution analysis [23] to the fluorescence lifetime of the dione (11.6 ns) determined from decays recorded on longer timescales. The observed risetime for the dione emission in DMN-10-dione, corresponds to a rate for EET from DMN to the dione of 2.5 x 1010 s -1. No risetime was resolvable for DMN-6-dione, within the time-resolution of our instrument and the rate is thus estimated to be >1011 s -1. Collection of fluorescence data in these experiments was hampered by a significant reduction in emission count rate throughout the duration of the data acquisition which is attributed to photolysis o f the dione groups as discussed below. If one assumes an energy transfer rate of c a 1013 s "l for the 4-bond analogue, DMN-4dione, 2(4) then an estimate for 13' of only c a 1.0 per bond for the 2(n) series is obtained, which is about one half that found in other systems [14-19]. This diminished [5' value for the 2(n) series, coupled with the sheer speed with which TB-mediated singlet-singlet EET takes place within, DMN-10-dione, 2(10) are quite without precedent. Similar results were obtained for the dimethylbenzene-dione (DMB-6-dione, Table 1). The rate o f EET by either resonance or exchange mechanisms is dependent upon the degree o f overlap o f the donor emission and the acceptor absorption spectra [19]. The spectral overlap integrals for resonance and exchange interactions between the DMN emission and the dione absorption for the system under discussion here are calculated to be 6.6 x 10 -18 cm 3 M "l and 1.16 x 10 -5 cm respectively [19]. These values can be compared with those

32 reported for the l(n) 00MN-n-ketone) compounds in cyelohexane (0.259 x 10-18 cm3 M-1 and 2.26 x 10-6 cm respectively) [19] and to the slow EET in the flexible 0-4,4 and 1,4 Naph 5,5 [7]. If a resonance dipole-dipole mechanism were dominant, the rate of EET for DMN-10-dione is calculated to be of the order of 106-107 s"1 which is far less than observed. The larger exchange overlap integral calculated for the DMN-bridge-diones compared to that of the DMN-bridge-ketone system cannot account for the two orders of magnitude increase in the observed rate of energy transfer in 2(10) compared with, for example, DMN-6-ketone [19] (Table 1). Preliminary CIS/6-31G(d) calculations on the first excited states of model DMNnorbomane, 7-norbornanone, and norbomane-2,3-dione chromophores, all possessing Cs symmetry, reveal that they have 1A', 1A", and 1A' symmetries, respectively. Thus, singletsinglet EET in the series 2(n) is electronically allowed, whereas in l(n) this is electronically forbidden, However, vibronic coupling of the A' (1Lb) with the low lying 1A" (1La) state of DMN should facilitate rapid EET in l(n) [25], so we do not believe that electronic symmetry lies at the heart of the anomalous behaviour of 2(10). A possible cause might lie in interrelay interference effects in the hydrocarbon bridges of l(n) and 2(n). Such effects, which are known to play important roles in determining the magnitude and distance dependence of coupling through saturated hydrocarbon bridges [26], might be constructive in 2(n) but destructive in l(n), owing to the different symmetry of the highest lying lone pair orbital of the oxygen atom(s), which is a' for 2(n) yet a" in l(n). This possibility is being further explored computationally. The comparison between results pertaining to rigid and flexible bichromophores [27, 28] on one hand, and those pertaining to changing of the acceptor's chromophore for rigid molecules on the other hand, is done in Table 1. The 13values depend both on the rigidity of the bridge and on the energy gap between bridge electronic energies and those of D* and A* [29], as probed by comparing EET for monoketone and dione acceptors, as expressed by McConnell's model. McConnell suggested that orbital sites intervening between D and A could facilitate ELT. In his superexchange model, an electron is transferred between degenerate D and A orbitals, aided by the presence of empty (not necessarily degenerate) high-lying bridge orbitals. McConnell's expression for the coupling matrix element is given by

V~J ~,Ea° _ E~, ./k ,.z Eat>- EB,., J

(3)

where H/j is the tunneling integral between orbitals i and j, EAD is the degenerate D and A orbitals energy, EB~is the energy of the/th bridge orbital, and n is the number of B orbitals. From the oxidation potential of I (Eoxla = 1.I eV vs. SCE in acetonitrile [19a]), the reduction potential of biacetyl (Ercala --- -1.3 V vs. SCE in acetonitrile [30]) and the singlet state energy of I (E(S0 = 3.78 eV[19a]), the free energy change without work term for electron transfer (AGET)from the excited DMN chromophoric unit to the dione moiety can be estimated by using eq. (4), AGET=

Eox I / 2 - Erco It2 -

E(S0 = - 1.4 eV

(4)

33 Table 1. Rates of intramolecular singlet-singlet EET in solution.

MOLECULE 1,4-Naph-5,5 0-4,4 DMN-4-ketone DMN-6-ketone DMN-8-ketone DMB-6-dione DMN-6-dione DMN-10-dione

R(A) 5.23 6.00 5.00 7.50 10.0 7.50 7.50 12.5

k EEr(Sq) 6.0x 10s 5.0x 106 1.2× 10l° 1.9x 10s 3. lx 107 > 1011 > 10t 1 2.5x10 ~°

13(A"t) 0.9 0.9 1.2 1.2 1.2 0.6 0.6

Thus electron transfer (ELT) in the DMN/dione bichromophoric system is considerably exergonic. AGET seems to be in aeetonitrile even more exergonic than for the system DMN/dicyanoethylene (AGET= -0.98 eV without work term, of. [19a]). For the latter system connected with a [6J-bond unit, an ET rate constant of kET> 101~ S"l irrespective of the solvent was found in [19a]. Comparison o f l I and III with the respective dicyanoethylene compounds should yield valuable information about possible competition between EET and ET in photoexcited II and lII. However, since the sensitized dione emission in both II and III is identical with that of a dione model system lacking the DMN chromophore, we maintain that EET and not ELT is the dominant transfer mechanism in 2(n). In conclusion we note that we have provided evidence that through bond superexchange is significant in promoting Intra-EET in rigidly bridged bichromophoric compounds at interchromophore separations exceeding 12A. This is not the case in flexibly bridged compounds [7], [27, 28]. We believe that the flexibility results in vibronic coupling of bridge modes resulting in loss of coherence needed for efficient bridge mediated through bond coupling, such as invoked in McCormell's model [29].

ACKNOWLEDGMENTS

This research was supported by the Fund for the Promotion of Research at the Technion.

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