Photoresponsivity

Applications. Most of the stilbene-based polyquiaolines display photoresponsive (98) and photomechanical effects as manifested by a contraction ia polymer film samples upon kradiation. 

Amorphous (vitreous) selenium, vacuum-deposited on an aluminum substrate such as a dmm or a plate, was the first photoconductor commercially used in xerography (6). It is highly photosensitive, but only to blue light (2). Its light absorption falls off rather rapidly above 550 nm. Because of the lack of photoresponse in the red or near infrared regions, selenium photoreceptors caimot be used in laser printers having He—Ne lasers (632.8 nm), or soHd-state lasers (680—830 nm). 

We have noted above that benzocrowns may be nitrated quite readily. This approach was used in the formation of a photoresponsive bis-crown (see Sect. 3.8) wherein the nitrobenzo crowns reductively dimerize to the corresponding azobenzene. Kikukawa, Nagira and Matsuda have utilized 4-nitrobenzo-15-crown-5 in a somewhat different way during the synthesis of 4 -vinylbenzo-l 5-crown-5Nitration is effected using nitric acid in a mbcture of chloroform and acetic acid. 

Yoshino and co-workers also reported the optical response of a heterojunclion device comprising a P30T and C60 bilayer [90]. The photoresponse of these devices shows a broad excitation profile ranging from 750 nm into the UV. 

The spectral dependence of the photoresponse of these bilayer heterojunction devices, illuminated from the 1TO side, is displayed in Figure 15-22. The onset of photocurrent at hv— 1.7 cV follows the absorption of the fullerene, indicating a symmetric hole transfer from the excited fullerene to the MEH-PPV. The minimum in the photocurrent at /iv=2.5 eV corresponds to the photon energy of maximum absorption of MEH-PPV. The MEH-PPV layer, therefore, acts as a filter, which reduces the number of photons reaching the MEH-PPV/C()0 interlace. Thus, the thickness of the MEH-PPV layer determines the anlibatic spectral be-... 

Photoresponsive polyelectrolytes tethered with a photochemical functional group were first reported in 1964 by Lovrien and Waddington [24] who prepared copolymers of iV-azobenzeneacrylamide and acrylic or methacrylic acid (1). 

Photochemical cage 80, 82 Photon-correlation spectroscopy 141 Photoresponsive polyelectrolytes 54... 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

Salvador P, Chaparro AM, Mir A (1996) Digital imaging of the effect of photoetching on the photoresponse of n-type tungsten diselenide and molybdenum diselenide single crystal electrodes. J Phys Chem 100 760-768... 

Kongkanand A, Tvrdy K, Takechi K, Kuno M, Kamat PV (2008) Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-Ti02 architecture. J Am Chem Soc 130 4007-4015... 

Masahiro Irie received his B.S. and M.S. degrees from Kyoto University and his Ph.D. in radiation chemistry from Osaka University. He joined Hokkaido University as a research associate in 1968 and started his research on photochemistry. In 1973 he moved to Osaka University and developed various types of photoresponsive polymers. In 1988 he was appointed Professor at Kyushu University. In the middle of the 1980 s he invented a new class of photochromic molecules - diaryl-ethenes - which undergo thermally irreversible and fatigue resistant photochromic reactions. He is currently interested in developing singlecrystalline photochromism of the diarylethene derivatives. 

Besides cTph and the photoresponse, the quantum-efficiency-mobility-lifetime product, r]gfj.r, is used as a figure of merit. Usually this product is measured at a wavelength of 600 nm, and typical valuesof (r/i /ur)6oo are lO cm A/ or higher. 

Consequently the photoresponse tTph/deposition rate as about lO exp(Frf). Activation energies amounted typically to 0.7-1.0 eV. From thermally stimulated conductivity (TSC) measurements [489-492] a midgap density of states (DOS) of 1.5 x lO cm eV is determined. The product/zr at 300 K is 9 X 10 cm V . Both DOS and /rr are independent of frequency. 

The cationie dye was assoeiated with the anion (7,8,9,10,11,12 Brg-l-CBnHg) in order to dissolve it in the organie phase. The polarizable window available for photoinduced electron transfer in this system extended over 100 mV for the eonditions specified in Fig. 13. The photoeurrent responses were measured under ehopped light and lock-in detection at 8.4 Hz. Figure 13(b) shows that some photoresponses oeeur only in the presence of the dye speeies, whieh the authors attributed to the transfer of Ru(bpy)3" to the aqueous phase as a result of interfaeial polarization induced by the ehopped light [130]. Upon addition of in the aqueous phase, an inerease in the amplitude of the photo-... 

FIG. 13 Cyclic voltammogram (a) and potential dependence of the photoresponses (b)-(c) to chopped illumination and lock-in detection associated with the photoreaction in Eq. (40). The CV shows that the polarizable window extended to less than 100 mV. The photocurrent measurements carried out were done in the presence (trace 3) and absence (trace 2) of the redox quencher in the organic phase. (Reprinted with permission from Ref 48. Copyright 1989 American Chemical Society.)... 

Another point subject to criticism from this work is the apparent absence of DC photocurrents upon constant illumination. Although the responses in Fig. 13(c) appear effectively in phase at the chopping frequency, it is possible that slow photoinduced perturbation of accumulated charge at the interface can be connected with these photoresponses, e.g., photoionization of the dye [72]. In this respect, clearer evidence of photo-induced ET was introduced by Brown et al. for the reaction [49]... 

Within the potential range where Ru(bpy)3 remains in the aqueous phase, photocurrent responses are clearly observed with a slow rising time of the order of 10 s as shown in Fig. 14(a). According to the convention employed by these authors, positive currents correspond to the transfer of a negative charge from water to DCE. No photoresponses were observed in the absence of either the dye in the aqueous phase or TCNQ in DCE. Further analysis of the interfacial behavior of the product TCNQ revealed that the ion transfer occurred outside of the polarizable window [cf. Fig. 14(d)], confirming that these photoresponses are not affected by coupled ion-transfer processes. An earlier report also showed photoeffects for the photoreduction of the viologen under similar conditions [131]. 

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