Volume 157, number I ,2
28 April 1989
CHEMICAL PHYSICS LETTERS
EFFECT OF CYCLODEXTRINE CAVITY SIZE ON TWISTED INTRAMOLECULAR CHARGE TRANSFER EMISSION: DIMETHYLAMINO BENZONITRILE IN &CYCLODEXTRINE Ashis NAG, Rina DUTTA,
Nitin CHATTOPADHYAY
Department of Physical Chemistry
Indian Association for the Cultivation ofscience,
and Kankan BHATTACHARYYA Jadavpur, Calcutta 700 032, India
Received 7 November 1988; in final form 12 January 1989
Emission spectra of pN,Ndimethylamino benzonitrile (DMABN) in aqueous &cyclodextrine @CD) solutions are reported, In ECD two distinct kinds of emission are observed, one with enhanced nonpolar emission (at 355 nm) and the other exhibiting enhanced TICT emission (at 480 nm). The former is attributed to those DMABN molecules which are totally inside the B-CD cavity while the latter is due to DMABN molecules only partially enclosed in the cavity.
1. Introduction
2. Experimental
In recent years cyclodextrines (CD) have been shown to be interesting microvessels for several molecules and the resulting supramolecules serve as excellent miniature models for enzyme-substrate complexes [ l-31. The reduced polarity and the restricted space provided by the CD cavity markedly influence a number of molecular properties [ 4-8 1. In a recent communication we have shown [ 9 ] that in aqueous solutions the reduced polarity of the cc-CD environment enhances selectively the TICT emission of DMABN while the normal or “nonpolar” emission remains very weak. It was proposed that the DMABN molecule fits snugIy inside the small u-CD cavity (4.5 A) with its dimethylamino part projected into the polar region consisting of hydroxyl groups of CD and /or water molecules. We have now extended this work to study the interaction of DMABN with P-CD, which possesses a bigger cavity (7 A). The entire DMABN molecule can easily fit inside the nonpolar interior of the P-CD cavity. Since TICT phenomenon occurs only in polar media [ 10-l 5 1, the TICT process is expected to be severely suppressed inside the P-CD cavity and hence in the presence of B-CD the nonpolar emission of DMABN should be enhanced at the expense of the TICT emission. This will be shown to be the case in the following sections.
DMABN (Aldrich) was purified by vacuum sublimation followed by recrystallisation. P-CD (Aldrich) was used as received. Absorption and emission spectra were recorded, respectively, with a Cary 17D and a Perkin-Elmer MPF 44B spectrofluoritieter. The single-photon counting apparatus has been described elsewhere [ 16 ] _
3. Results and discussion The absorption spectra of an aqueous solution containing 3 x 1O-’ M DMABN as a function of concentration of P-CD are shown in fig. 1. On addition of @CD the absorbance gradually increases with a slight broadening and shift of the spectra towards the blue. The absence of an isosbestic point rules out the possibility of a single equilibrium involving only one 1: 1 complex. It should be mentioned that similar cyclodextrine complexes with stoichiometries different from 1: 1 have been observed by a number of workers [ 5-7 1. Since B-CD is known to accommodate naphthalene derivatives, it is reasonable to expect multiple occupancy, i.e. the presence of more than one DMABN molecule within the same @CD cavity. The other likely cause of the absence of an isosbestic point may be the formation
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OD
c \
I
I
480 m WAVE LENGTMnml -
260
280 WAVE
360
320 LENGTHWW
560
Fig. 2. Emission spectra of aqueous DMABN solutions containing:(a)O,(b) 5~1O-~,(c)6xlO-~, (d) 1x10-2,(e) 1.5~10-~ and (f) ~x~O-~MP_CD.
-
Fig. 1. Absorption spectra of aqueous DMABN solutions (against j3-CDsolutionas reference) containing: (a) 0, (b) 6x lo-‘, (c) ~xIO-~, (d) 1.5x10-zand (e)ZxlO-*MbCD.
of more than one kind of complex, e.g. one with the DMABN molecule completely inside the P-CD cavity and the other in which a part of the DMABN molecule projects out of the cavity. The increase in absorbance with addition of CD has also been observed earlier [ 6 ] and has been attributed to the dissolution of the guest molecules adsorbed on the walls of the container due to the detergent action of CD. In contrast to the absorption spectra, on addition of P-CD the emission spectra of DMABN undergo a remarkable change (fig. 2). In aqueous solution the emission quantum yield of DMABN is extremely low, presumably because of the large stabilisation of the highly polar TICT state and consequent rapid nonradiative transitions to ground and/or low-lying triplet states [ 93. With addition of B-CD, both the nonpolar and the TICT emission are enhanced. However, the rate of enhancement of the nonpolar emission is found to be greater than that of the TICT emission. As a result, although in aqueous solution 84
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the TICT emission has a slightly higher quantum yield compared to the nonpolar emission, the latter overtakes the former at a P-CD concentration of x 6 X 10m3 M. On further addition of B-CD the nonpolar emission becomes increasingly stronger than the TICT emission. To ascertain whether the enhanced nonpolar and TICT emission are related or not, the excitatibn spectra of DMABN solution containing 2 X lo-’ M @CD were recorded monitoring the nonpolar emission (at 360 nm) and the TICT emission (at 480 nm) (fig. 3). It is evident that the excitation spectrum monitored at 360 nm is slightly blue-shifted compared to that at 480 nm, the excitation peaks being at 295 and 306 nm, respectively. This small but perceptible shift indicates that the enhanced nonpolar and enhanced TICT at such high concentration of B-CD are due to two distinct sets of molecules, with absorption maxima respectively at 295 and 306 nm. At such concentrations of B-CD the absorption spectrum may be considered as a superposition of two spectra with maxima at 295 and 306 nm. As discussed above, the steady-state spectra indi-
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Volume 157. number 1,2
WAVE LENGTHlnm)
--a
Fig. 3. Excitation spectra of aqueous DMABN solution containing 2x lo-’ M &CD monitored at 360 nm (solid) and at 480 nm (dashed).
, 58
ii
lk
288 250 3ie 3ie see 458 588 CHllmElmml
Fig. 4. Decay traces at 355 and 480 nm of an aqueous DMABN solution containing 2 x lo-’ M B-CD. Time interval between the channels is 0.087 ns.
cate that the enhancement of the nonpolar and the TICT emission of DMABN in the presence of B-CD are due to different sets of molecules. To substantiate this further, we have carried out a time-resolved study of the nonpolar (355 run) and the TICT (480 nm) emission at the highest concentration of P_CD (fig. 4). In aqueous solutions, the decay at these wavelengths is too fast [ 9 ] to be resolved in our apparatus (system response 500 ps). The decay becomes much slower on addition of P-CD. If these two
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emissions were due to the same set of DMABN : pCD complexes the decay of the nonpolar emission should match the growth of the TICT emission at short times. However, we could not resolve the rising part of the TICT emission (fig. 4) because the interconversion of the nonpolar and the TICT states is too fast (a few picoseconds) [ 11,121 to be detected. But at long time ( > 1 ns) they should be equilibrated and hence both of them should decay with the same rate constant. However, the actual decay traces were found to be multiexgonential; the initial parts could be fitted to a single exponential with lifetimes of 4 ns for the nonpolar and 2 ns for the TICT emission. The fact that the lifetime of the enhanced nonpolar emission is not equal but actually longer than that of the enhanced TICT emission excludes a parental relation between these two emissions. Thus, the time-resolved studies also suggest that the enhanced nonpolar and enhanced TICT emission in P_ CD originate from molecules in different environments. At such high concentrations of P-CD, one is observing emission of DMABN in essentially three different environments, some in pure water and the rest in two distinct environments inside P-CD, one enhancing the nonpolar emission and the other exhibiting enhanced TICT emission. It is difficult to extract meaningful rate constants in such multicomponent systems [ 171. Therefore no quantitative analysis of the decay curves were attempted beyond the qualitative conclusion that the enhanced nonpolar and TICT emissions are unreIated. Having established that in the presence of @CD there are broadly two sets of complexes, one exhibiting enhanced TICT emission with an excitation peak at 306 nm and another exhibiting enhanced nonpolar emission with an excitation maximum at 295 nm, we will not try to understand the environment of these two sets of molecules and the cause of the enhancements. In the case of the TICT emission a blue-shift of the emission maximum from 500 nm in water to 480 nm at the highest concentration of P-CD is observed. The blue-shift indicates that the molecules are in a less polar environment than water solvated. so that the TICT state is not suffkiently This insufficient solvation of the TICT state increases the energy gap between the TICT state and the low-lying triplets and/or ground state. As a result, because of the energy gap law [ 181, the rate of nonradiative transitions from the TICT state de85
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creases on complexation with BCD. The reduction of the nonradiative transition rates is the cause of the enhanced TICT emission as discussed in our earlier works [9,19] and also by Seliskar and Brand [ 201. It may be recalled [ 91 that in the presence of a-CD in which the DMARN molecule is partially enclosed with one part projecting into the polar region of hydroxyl groups and water molecules, the TICT emission shows a marked enhancement presumably because of the reasons discussed above. The more interesting point is that the excitation and absorption maximum at the highest u-CD concentration is at 305 nm, which is very close to the maximum of the excitation spectra monitoring the TICT emission at the highest B-CD concentration. Thus we conclude that the enhanced TICT emission in the presence of P_CD is due to those DMABN molecules which are partially encaged in the P-CD cavity with one part projecting out into the polar environment. The most increasing observation in this work is the enhancement of the nonpolar emission. It has been demonstrated very convincingly by Hicks et al. [ 11,121 that the energy barrier for the TICT process increases ivith decreasing polarity of the media. In nonpolar media the barrier is so high that emission from the nonpolar state predominates over that from the TICT state. Since the interior of the CD cavity is essentially nonpolar it is reasonable to expect nonpolar emission if the DMABN molecule is entirely inside the cavity. In view of this and the large size of the P-cavity we conclude that the enhanced nonpolar emission is due to those DMABN molecules which are totally encapsulated in the P_CD cavity. It is also interesting to note that the polarity of the CD cavity has a greater influence on the TICT process than the restrictions imposed by the finite size of the cavity. The P-CD cavity is larger than the a-CD cavity. Hence, the rotation of the dimethylamino group (associated with the TICT process) is expected to be much more free in P_CD. However the TICT process is still severly suppressed in l&CD. This once again confirms the contention that the TICT phenomenon for DMABN is dependent more on the polarity of the media than the restrictions imposed on the torsional motion (e.g. by viscosity etc. ) [ 1 1- 13 ] _
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4. Conclusion This work demonstrates that when the size of the cyclodextrine cavity is large enough to enclose the DMABN molecule totally the TICT process is severely inhibited. This is consistent with earlier speo ulations [ 10-I 51 that the TICT process is polarity controlled. Acknowledgement Thanks are due to Professor Mihir Chowdhury his kind interest in the present work.
for
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