This was the topic of my diploma thesis at 3.Physikalisches Institut at Stuttgart University in the group of Dr. Helmut Port.
Back then, in 2000, hopes were high on the future of optical data storage. Throughout the 90's, the CD-ROM had clearly dominated software distribution, with a capacity that was absolutely staggering at first and still surpassing most standard hard disc drives (HDD) at the end of the decade. The CD-RW had just been introduced and people expected 1) optical data storage getting three-dimensional soon and 2) magnetic storage density reaching physical limits even sooner. Both assumptions were wrong: the DVD got a second layer, but that's it, and HDD capacity kept increasing steadily for another 20 (at the time of writing this) years.
In a similar way, the persistence of Moore's Law was underestimated by far at the time. Transistor size was down to 180nm and for sure physical limits had to be reached within a few years. The prospect of molecular transistors seemed the ultimate goal. However, we simply could not imagine that semiconductor technology would enable transisors with a footprint even smaller than our organic molecules. So, the entire field of photochromism seems out of fashion these days, with the main application being, as it was back then, sun glasses...
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, at the time of this work they were also considered candidates for data storage or as switching units for molecular electronics. A very important property for these applications would have been the speed of the isomer change.
Transient Absorption Spectroscopy
Absorption spectroscopy reveals the isomerization state of the molecules, as the broad absorption band of the C isomer gives the sample a deep red color. Transient absorption spectroscopy was used in order to measure the picosecond scale isomerization time: Short (~100 femtoseconds) laser pulses were split, one of them converted by frequency doubling or a parametric amplifier into a monochromatic excitation pulse, the other sent over a delay line and transformed into white light before hitting the sample a little later, enabling to take a full absorption spectrum.
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.
Text and Figures (c) 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)