1P-077
Time-resolved spectroscopy of a multi-component supramolecular system(Molecular Spectroscopy Lab., RIKEN) Matthew M. Sartin, Masahisa Osawa, Tahei Tahara

Introduction
We present the time-resolved spectroscopy of a supramolecular system, in order tobetter understand the influence of the motion of one component on another. The study of such
systems can bridge our understanding of the dynamics of small molecules to those of larger,
more complex systems, such as proteins or artificial molecular machines. While it is possible
to observe the initial and final states of switchable supramolecular systems, a detailed study ofthe molecular motions of such systems is much more difficult, since the lack of chromophores
NO2NO2NO2N

O2 NN

often makes their dynamics inaccessible to ultrafast methods. Inorder to examine these dynamics, a model system that undergoesa photo-induced process followed by a measurable secondaryNNOOUVVisNNO

process is needed.
For this purpose, we have chosen a system from theNNONNNNliterature, 1 in which α-cyclodextrin is sterically trapped on asubstituted azobenzene (Figure 1). In the trans state, theO2N

O2N
NO2NO2Figure
1.
Rotaxaneisomerization. The conerepresents cyclodextrin.

cyclodextrin remains on the azobenzene moiety, but upon
photoisomerization, it is pushed out onto the substituents. We canindirectly study the motion of the cyclodextrin by its effect on thedynamics of the azobenzene moiety.

Experimental
The rotaxane was synthesized using a modified literature procedure.1 The starting
material for our method, 4,4’-dihydroxyazobenzene (DHAB), was prepared as describedelsewhere, 2 and dibromoethoxyazobenzene was synthesized directly by reacting DHAB with
CH2Br2 in acetone with added K2CO3. For femtosecond transient absorption, the sample wasexcited at 350 nm and probed using a white light continuum.
Results
Exciting the π*←π transition at ~350 nm causes trans-cis isomerization of theazobenzene moiety. This change can be monitored by a decrease in the 350 nm absorption
band and a relatively small increase in the absorption at longer wavelength. The movement of

the cyclodextrin is separately observable by comparing the NMR or circular dichroism spectra
before and after irradiation.
Femtosecond transient absorption experiments were used to compare the dynamics ofthe excited state processes of the rotaxane to those of unsubstituted azobenzene. The transientabsorption spectrum of unsubstituted azobenzene isTransient Absorbance Change (1 div. = 0.02)

broad and exhibits three observable decay processes.
A small, short-lived peak around 650 nm is attributed
to the Sn←S2 absorption, which decays within 0.5 ps.
Another peak, at 405 nm, exhibits a two-exponential
decay, with one component assigned to the Sn←S1transition (1 ps in hexane), and the other assigned tovibrational cooling in the ground state (16 ps inhexane). 3 The transient absorption spectra of therotaxane exhibit both features (Figure 2), but thelong-wavelength feature at 650 nm is more
prominent

thaninazobenzene,

and

it

hypsochromically shifts to 625 nm during its 1 ps

400500600700decay. At 405 nm, the major component of theWavelength / nm

rotaxane decays with a time constant of 6 ps, which

Figure 2. Transient absorption spectra ofrotaxane in water from -0.2 to 0.2 ps.

is considerably slower than that observed for
unsubstituted azobenzene.

Conclusions
We have shown the electronic aspects of the time-resolved behavior of a rotaxane
consisting of a cyclodextrin hosting a substituted azobenzene. The potential to correlate thedifferences in the spectroscopic behavior of these molecules to their structural dynamics could
lead to better understanding of how such supramolecular systems operate at the molecularlevel.
1

Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Nakashima, N. J. Am. Chem. Soc. 2005, 127,15891.
2Shunhua L.; Yuefeng Z.; Chunwei Y.; Feiran C.; Jingou X. New J. Chem. 2009, 33, 1462.
3Fujino, T.; Arzhantsev, S. Y.; Tahara, T. Bull. Chem. Soc. Jpn. 2002, 75, 1031.