Photochromic molecules switch reversibly between two stable isomers when irradiated with light. The Thiophene-Fulgide in the scheme to the left can exist either as the colorless E isomer, or as C, which has a dark wine red color (look at the absorption band in the diagram). The carbon ring in the center of the molecule is open in the E isomer. When irradiated with UV light the ring closes and the molecule switches into the C state. Irradiation with visible light (preferentially green) opens the ring again, swiching the Fulgide back into the E isomer.
Molecules with this ability are already used in glasses that become dark during the day and light up when the sun is gone. However, it has also been considered to employ them for data storage or as switching units for molecular electronics in the future. A very important property for these applications would be the speed of the isomer change..
By means of absorption spectroscopy it is rather simple to
determine the isomerization state of the molecules. In fact, the broad
absorption band of the C isomer gives the sample a deep red color that
is apparent to the naked eye. However, determining the speed of the
isomerization is much more challenging to measure, since it happens on
the scale of picoseconds.Our method of choice is transient absorption spectroscopy. This technique splits one short (~100 femtoseconds) laser pulse into two. By means of elaborate optics, one of these pulses is delayed and transformed into white light, while the other one remains monochrome. The latter is used to excite the molecules in the sample, while the white pulse, following centimeters or millimeters behind, probes the absorption spectrum only picoseconds (or fractions of) later.
Given the neccessary
long term stability, the critical parameter when using the molecules
for data storage is the switching speed. For the fulgides this had been
measured for the closure of the ring (E to C) already, but not for the
opening (C to E). One result of my diploma thesis was that this happens
within 1-1.5 picoseconds.The diagram on the left shows the bleaching of the C absorption band after an UV photon excites the molecule at t=0.
Photochromic
molecules can be used to switch intramolecular energy or charge
transfers. In the figure we see a thiophene fulgide acting as a switch
(S) between an anthracene unit (donor D) and a coumarine
molecule (acceptor A). If the fulgide is an E isomer, excitation
of the anthracene leads to fluorescence of the coumarine: The
excitation energy has been transfered from the donor to the acceptor.
However, if the fulgide switch is a C isomer, this does not happen. Other systems show the same effect for a charge transfer.
The big vision is, that similar molecules could replace one day the field effect transistors in electronics, when the currently used MOSFETs cannot be reduced in size anymore due to quantum effects.
Ingo Ramsteiner:
Transiente Absorption zum Optischen Schalten Intramolekularer Transferprozesse, Diplomarbeit, Universität Stuttgart 2000 (German).
I.B. Ramsteiner, A. Hartschuh, H. Port:
Relaxation pathways and fs dynamics in a photoswitchable intramolecular D -> A energy transfer system, Chem.Phys.Lett. 343 (1-2) 83-90 (2001)
A. Hartschuh, I.B. Ramsteiner, H. Port, J.M. Endtner, F. Effenberger:
Photocontrol on ultrafast excited state transfer processes, J.Lum. 108 (1-4) (2004)
H. Port, P. Gärtner, M. Hennrich, I. Ramsteiner, T. Schock:
Ultrafast photochromic reactions of fulgide photoswitches, Mol.Cryst.Liq. Cryst. 430 15 (2005)