56 PHOTOCHEMICAL PRINCIPLES 



The exciton itself, being neutral, does not contribute to the photocon- 

 ductivity. 



2. Semiconductivity. At room temperature, the thermal energy is 

 normally insufficient to achieve a very large dark population of the 

 conduction band in most crystals. However, as the temperature is 

 raised, more and more electrons are excited into the conduction band 

 in accordance with Boltzmann's law. Thus, many crystals that are 

 insulators at low temperatures exhibit an increasing conductivity as a 

 function of temperature. 



3. Luminescence. There are, in general, four main mechanisms of 

 luminescence in organic crystalline semiconductors, not all of which 

 need be operative simultaneously. These are: (a) direct decay of the 

 exciton (fluorescence or phosphorescence), (b) recombination and 

 radiative decay of the electron and hole subsequent to ionization but 

 prior to trapping, (c) excitation of the trapped electron and/or hole 

 into the conduction band followed by recombination and radiative 

 decay, and (d) transfer of the excitation energy to a fluorescent im- 

 purity in the crystalline lattice (sensitized fluorescence). 



Luminescence processes a, b, and c will all lead to emissions of the 

 same wavelength but with different time constants and temperature 

 dependencies. Process a will be relatively temperature independent; 

 process b may or may not exhibit a temperature coefficient depending 

 upon the actual mechanism of ionization; process c will have a very 

 definite temperature dependence as a function of the depth of the traps. 



4. Thermoluminescence. If the trap depths are such that, at a 

 given temperature, the excitation of the trapped electron or hole into 

 the conduction band does not proceed at a measurable rate, irradiation 

 followed by an increase in temperature will lead to luminescence. Un- 

 der such conditions, the luminescence vs. temperature curve of the 

 crystal will exhibit peaks corresponding to the various trap depths. 



A typical example of an energy transfer process in molecular crys- 

 tals is given by the study of Bowen and co-workers (1949) on anthra- 

 cene crystals. They found that the presence of 0.1% of naphthacene 

 in anthracene almost completely quenches the blue-violet fluorescence 

 of anthracene and replaces it with the yellow-green emission of 

 naphthacene. The quantum yield of this process is only slightly less 

 than that of the fluorescence of pure anthracene. Similarly, traces of 



