Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 344 (1995) 151 155
Twisted intramolecular charge transfer of dimethylaminobenzaldehyde in c -cyclodextrin cavity Santi Kundu,
Nitin Chattopadhyay*
Department of Chemistry, Jadavpur University, Jadavpur, Calcutta 700 032, India
First received 5 May 1994; in final form 5 August 1994
Abstract Absorption and steady-state and time-resolved emission studies ofp-N,N-dimethylaminobenzaldehyde (DMABA) in aqueous ~-cyclodextrin (c~CD) solutions are reported. The twisted intramolecular charge transfer (TICT) emission is extremely poor in pure water, but is greatly enhanced upon complexation with c~CD. The cavity size of c~CD is insufficient to encapsulate the entire fluorophore; rather it embeds DMABA only partially, keeping the dimethylamino group open in bulk water. The enhancement in the TICT emission is attributed to the reduced polarity provided by the CD environment.
1. Introduction Cyclodextrins (CDs) have received considerable attention in recent years [1-4]. They are interesting microvessels for appropriately sized molecules and the resulting supramolecules can serve as excellent miniature models of enzyme substrate complexes. Depending on the cavity size, CDs are capable of encapsulating different guest molecules. The reduced polarity and the restricted space provided by the CD cavity markedly influence a number of photophysical/ photochemical pathways [5 8]. Thus twisted intramolecular charge transfer (TICT) emission of a fluorophore, namely dimethylaminobenzonitrile ( D M A B N ) has been shown to be modified when it is encaged within a cyclodextrin cavity [9 12]. Here, we report the effect of a reduction in polarity, as imposed by the c~CD * Corresponding author.
microenvironment in a bulk aqueous medium, on the non-polar as well as the T I C T emission of p-dimethylaminobenzaldehyde (DMABA) [131.
2. Experimental D M A B A (Aldrich) was purified by vacuum sublimation followed by recrystallization from 90% ethanol, c~CD (Aldrich) was used as received. Triply distilled water was used for the preparation of solutions. The solutions were sonicated well so that steady-state equilibrium for the complexation process was achieved. A Shimadzu MPS 2000 absorption spectrophotometer and a Spex Fluorolog were used to record the absorption and emission spectra, respectively. For the time-resolved experiments, the time-correlated single-photon counting technique was adopted. A nanosecond nitrogen
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S. Kundu, N. Chattopadhyay/Journal of Molecular Structure 344 (1995) 151 155
ftashlamp ( E d i n b u r g h Instruments, 199 fluorescence spectrometer) was used as the excitation source for this purpose.
d
t
3. Results and discussion
¢.,,
320
I 340
I 360 380 h (nm)--,-
Fig. 1. Absorption spectra of a 1 x 10 5 M aqueous solution of DMABA containing (a) 0, (b) 5, (c) 15 and (d) 25 mM of c~CD.
T h e a b s o r p t i o n spectra o f a n a q u e o u s s o l u t i o n o f 1 x 10 -5 M D M A B A as a f u n c t i o n o f c~CD are s h o w n in Fig. 1. O n a d d i t i o n o f cxCD, the a b s o r p t i o n m a x i m u m shifts to a s h o r t e r wavelength, i n d i c a t i n g t h a t the p r o b e is experiencing less p o l a r i t y in the C D e n v i r o n m e n t [9]. This has been e s t a b l i s h e d i n d e p e n d e n t l y f r o m the o b s e r v a t i o n o f a blue shift in the a b s o r p t i o n b a n d o f D M A B A as the p o l a r i t y o f the solvent is l o w e r e d (when the solvent was water, m e t h a n o l , c h l o r o f o r m a n d cyclohexane, the m a x i m a were at 358, 342, 337 a n d 325 nm, respectively). Existence o f an isosbestic p o i n t (at 355 nm) indicates the f o r m a t i o n o f a 1:1 c o m p l e x between D M A B A a n d c~CD in a q u e o u s solution. T h e emission s p e c t r a o f a series o f D M A B A solutions at v a r i o u s c~CD c o n c e n t r a t i o n s are s h o w n in Fig. 2. The spectra show a r e m a r k a b l e c h a n g e with the a d d i t i o n o f c~CD. In p u r e water, D M A B A yields a l m o s t a single fluorescence with
f
350
bOO
450 Wavetength (nm)
500
550
600
Fig. 2. Emission spectra of 1 x l0 -5 M aqueous solution of DMABA containing c~CDconcentrations of(a) 0, (b) 5, (c) 10, (d) 15, (e) 20 and (f) 25 mM.
S. Kundu, N. Chattopadhyay/Journal of Molecular Structure 344 (1995) 151-155
a maximum at about 400 nm, ascribed to the non-polar emission, with extremely poor TICT emission (the term "non-polar" is commonly used for the blue edge emission where such dual luminescences do occur and the term reflects that the form responsible for this emission is less polar than the other form and does not necessarily represent its actual dipole moment). This is probably due to the rapid non-radiative decay of the T1CT state to ground and/or low-lying triplet states, very similar to that accepted for the probe D M A B N [9,10]. With the addition of c~CD, both the non-polar (at about 400 nm) and T I C T (at about 500 nm) emissions are enhanced. However, it is clear from Fig. 2 that the rate of enhancement of the T I C T emission is one order of magnitude higher compared with the non-polar one. While the non-polar emission of D M A B A in a host concentration of 25 mM increases by only 50% over that of the pure aqueous solution, the TICT emission is enhanced about seven-fold under the same condition. The increase in the non-polar emission, though small, is too large to be explained by the primitive proposition of dissolution of the fluorophore molecules adsorbed on the walls of the container because of the detergent action of CD [6,8,10]. There was no noticeable increase in absorbance with the addition of c~CD, as observed previously by Nag et al. [9,10]. This indicates that, even if additional dissolution occurs, it is not appreciable. Again, enhanced T I C T emission indicates that the guest molecule is not totally encapsulated within the c~CD cavity. Were it so, one would not expect any T I C T luminescence to result from the inhibition of the twisting of the ring-NMe2 bond
H3C\NTCH3
0~ c ~
H
CH3 H3CW,N
o~C~H
Fig. 3. Twisting motion of t h e - N M e 2 group fo D M A B A in a c~CD environment.
153
because of the space restriction within the small core of c~CD (diameter about 4.5 ~ ) [9 11]. Thus, it seems that part of the molecule remains within the CD cavity while the other part is projected outside to face the polar aqueous environment (Fig. 3). In accounting for the enhancement of the T I C T emission, we do not insist on the proposition that the restriction on the molecular motion imposed by ctCD is responsible for lowering the rates of non-radiative processes and hence increasing fluorescence [14], because, in the present case, twisting about the ring NMe2 bond is not prevented. Because of the generation of charged centres at the two ends of the molecule, the TICT state has an extremely large dipole moment and hence its energy is expected to decrease with an increase in solvent polarity. This has two consequences. First, the lowering in the energy of the TICT state reduces the energy barrier between the F r a n k - C o n d o n (FC) excited state and the TICT state [12,15]. Second, the stabilisation of the TICT state decreases the energy gap between the TICT state and the FC ground state. Thus, in going from higher to lower polarity, the T I C T state energy will increase, resulting in an increase in the activation energy for the non-polar to T I C T transition [12]. Thus, the non-polar to T I C T transition should decrease and the TICT emission should exhibit a blue shift. In the present case, in going from aqueous to cyclodextrin solution, the polarity of the microenvironment of the fluorophore is reduced and, as expected, a gradual blue shift was observed in the T I C T emission on the addition of c~CD, ultimately to 490 nm in 25 mM solution (the corresponding value in water is not precise because of the extremely poor T I C T yield in pure water), as well as an enhancement in the non-polar emission. A similar hypsochromic shift was also observed by Kosower and Dodiuk [16]. A change in the micropolarity around the fluorophore is again reflected in the gradual change in the pattern of the non-polar band in the fluorescence of D M A B A in going from aqueous to 25 m M c~CD solution (see Fig. 2). Although we are convinced that the polarity in the vicinity of the probe is changed on complexation with c~CD, the entire molecule cannot be
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S. Kundu, N. Chattopadhyay/Journal of Molecular Structure 344 (1995) 151 155
Table 1 Life-time values (r) of the non-polar (at 400 nm) and TICT (at 500 nm) species of D M A B A in aqueous solution containing different concentrations of o C D c~CD concentration (mM)
r
at 400 nm, non-polar (ns)
r at 500 nm, TICT (ns)
0.0
5.0
15.0
25.0
0.96
0.98
1.02
1.05
0.91
1.30
1.42
1.89
entrapped within the small cavity, and thus the formation of the T I C T species is not restricted to any great extent. The enormous enhancement of the T I C T emission is, we tentatively suggest, due to a lowering of the non-radiative decay from the T I C T state to the low-lying state(s). As mentioned above, c~CD complexation gives rise to a much less polar microenvironment around the fluorophore (DMABA), resulting in destabilisation of the T I C T state. This increases the energy gap between the T I C T state and FC ground state or the low-lying triplet state(s) (reflected in a blue shift in the emission maxima) and, according to the energy gap law, the increased energy gap reduces the non-radiative decay of the T I C T state resulting in an enhancement in the fluorescence yield. Thus, lowering the polarity of the microenvironment around the probe alone can explain the enhancement of both the emissions. This is corroborated by the results of the time-resolved studies. The extracted life-time values of both non-polar and T I C T species in different c~CD concentrations are reported in Table 1. As explained above, the high-energy emission increases, basically due to the lowering of the non-polar to T I C T transition. The other band is enhanced because of the reduction of non-radiative transitions from the T I C T state to the lower levels because of the energy-gap restriction imposed by the less polar c~CD environment. This would lead to an increase in the radiative life-time of both emitting species. Although the decays were very rapid and of the order of the pulse duration of the exciting source, reliable life-time values were extracted from these through deconvolution (the lamp was very stable). Table 1 reveals that
on the addition of c~CD the radiative life-time of both the non-polar and the T I C T species increases and, as expected from the relative enhancement of the two bands (see Fig. 2), the increase in 7- is more for T I C T than for the non-polar species, thus establishing the validity of the proposition for the present fluorophore.
4. Conclusion The cavity diameter of c~CD is insufficient to encapsulate the entire D M A B A molecule. Thus it cannot prevent the formation of the T I C T state. We have established, at least qualitatively, that the fluorophore ( D M A B A ) is a unique molecule for which both the nonpolar and the T I C T emissions are enhanced because of the effect of the polarity of the microenvironment.
Acknowledgements Thanks are due to Dr. S.C. Bera for his kind interest in the present work. Financial support from C.S.I.R., Government of India (Project No. 01(1283)/93/EMR-II) is gratefully acknowledged. Thanks are due to the referee for his comments.
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