Chapter VII Dynamics of Luminescence
One of the most extraordinary things regarding luminescence in general is the small amount of material necessary to cause a visible emission of light. To take an extreme case, the flash of light resulting from the impact on ZnS of a single α particle, a helium atom, is visible to the naked eye. Addition of one part in a million of some heavy metal to pure CaS will confer phosphorescent properties on the latter. We are forced to believe that the heavy metal enters into some reaction during illumination which is reversed with light emission after illumination and a very small amount of heavy metal is necessary. Pyrogallol in water, 1:5,000,000 (m/512,000), can be oxidized with light production by K4Fe(CN)6 and H2O2 (Harvey, 1917) and m/100 pyrogallol + H2O2 will give a visible light with colloidal platinum in 1:250,000 concentration (Goss, 1917).
Luciferin and luciferase from Cypridina will also luminesce in exceedingly small concentration. If one grinds a single Cypridina in a mortar with water and dilutes the extract to 25,600 c.c., light can be observed if luciferin is added to this dilute luciferase solution. By determining the volume of the luminous gland of Cypridina and even assuming that this volume is all luciferase, one can calculate that one part of luciferase in 1,700,000,000 parts of water will give light when luciferin is added. Likewise, a similar dilution of luciferin will give visible light when luciferase is added.
The sensitivity of our eye is largely responsible for the detection of so small an energy change. As we have seen, recent determinations have proved that the dark adapted eye can detect 18 × 10-10 ergs per second. From the heat of complete oxidation of pyrogallol it is possible to calculate the amount of pyrogallol necessary to give 18 × 10-10 ergs if completely oxidized. This quantity is infinitesimally small. When pyrogallol is oxidized by K4Fe(CN)6 and H2O2, it is not completely oxidized and probably only a small amount of the energy is converted into light; otherwise we should be able to see the luminescence of a very much weaker concentration of pyrogallol. As the reaction luciferin ⇆ oxyluciferin is so easily reversible, very little energy must be liberated, and, as experiments indicate, very little heat, if any, accompanies light production. Even though this be true, it is still possible for a very small amount of luciferin to produce a very large amount of light.
A very small amount of luciferase only is necessary because it behaves as an enzyme and follows the general rule that catalysts act in minute concentrations.
On the assumption that luciferase is an enzyme, an organic catalyst oxidizing luciferin with light production, we may appropriately inquire into the relation between the concentration of luciferin and luciferase and intensity and duration of luminescence. Oxygen tension, hydrogen ion concentration and temperature must be maintained constant as these all affect both intensity and duration of luminescence. Before considering luciferin and luciferase, however, let us study a few well-known chemiluminescent oxidations with special reference to concentration of reacting substances and temperature.
The effect of temperature on luminescence is of special interest because it gives us a means of analysis for determining if the luminescence depends on reaction velocity. We know that photochemical reactions are very little affected by temperature because the reaction is dependent on the absorption of light, a physical process, and this increases only a small per cent. for a rise of temperature of 10° C. To put it in the usual way, its temperature coefficient (Q10) for a 10° interval is usually less than 1.1. On the other hand, we should expect photogenic reactions, in which some of the chemical energy is converted into radiant energy, to give off much more light the greater the reaction velocity. As reaction velocity increases so rapidly with temperature (Q10 = 2 to 3), luminescence intensity should rapidly increase with increase in temperature.
Trautz (1905), from his extensive study of the chemiluminescence of phenol and aldehyde compounds came to the conclusion that luminescence intensity was proportional to reaction velocity. He based his conclusions largely on the effects of temperature and concentration of reacting substances and went so far as to declare that any reaction would produce luminescence if the reaction velocity were sufficiently increased. It is quite true that increasing the temperature does increase the intensity of chemiluminescence, but this is only within certain limits. As we raise the temperature, chemiluminescence becomes more intense but we soon reach a temperature for maximum luminescence and above this the intensity diminishes. This is especially well seen in the action of various oxidizers on pyrogallol and H2O2 recorded in Table 10. At 100° C. practically no light is produced by many oxidizers which are themselves unaffected at 100°. If we are to connect reaction velocity with intensity of luminescence we must conclude that the evolution of light is dependent rather on an optimum than a maximum reaction velocity.
TABLE 10
Temperature and Light Production. The Oxidizer is Mixed with an Equal Amount of M/100 Pyrogallol + 3 per cent. H2O2
| Oxidizer | Temperatures | ||||
|---|---|---|---|---|---|
| 0-2° | 20° | 50° | 75° | 98-100° | |
| Turnip juice | Faint | Good | Good | Bright | Negative. |
| 1 per cent. blood extract | Faint | Fair | Good | Fair. | |
| m/20 K4Fe(CN)6 | Negative | Good | Bright | Good. | |
| m/100 KMnO4 | Fair | Good | Bright | Bright | Faint flash. |
| m/50 K2Cr2O7 | Negative | Fair | Faint | Fair | Negative. |
| m/100 CrO3 | Negative | Good | Bright | Bright | Faint. |
| m/10 KCr alum | Negative | Faint | Faint | Faint | Negative. |
| m/10 NH4Fe alum | Negative | Faint | Faint | Faint | Very faint. |
| MnO2 | Negative | Fair | Fair | Fair | Negative. |
| NaClO | Bright flash | Bright flash | Bright flash | Fair flash. | |
Quite a number of instances are known in which increasing the mass of reacting substances leads not to an increase but to an actual cessation of luminescence. This fact does not confirm the theory that reaction velocity is a determining factor in luminescence. The conditions for the luminescence of white phosphorus are most interesting and unusual. (See van’t Hoff, 1895; Ewan, 1895; Centnerszwer, 1895; Russell,1903; Scharff, 1908.) Phosphorus will only begin to luminesce at a certain small pressure of oxygen. This “minimum luminescence pressure” of oxygen is very low, so low that earlier observers, failing to remove traces of oxygen, thought that luminescence might occur in absence of oxygen. Curiously enough there is also a “maximum luminescence pressure” of oxygen above which no luminescence occurs. Phosphorus will not luminesce in pure oxygen. Between the minimum and maximum is an “optimum luminescence pressure” where luminescence of the phosphorus is brightest. The exact values of these pressures vary with degree of water vapor present and with temperature. According to Abegg’s Handbuch der anorganischen Chemie, the maximum luminescence pressure with water vapor present, is 320 mm. Hg at 0° and increases 13.19 mm. Hg for each degree rise in temperature. This means that for a definite temperature, say, 20°, phosphorus will not luminesce with an oxygen pressure of 583 mm. Hg, but will luminesce with pressures under this. If, however, we raise the temperature, luminescence will occur with an oxygen pressure of 583 mm. Hg.
A somewhat analogous case is presented by the oxidation of pyrogallol solution in contact with ozone, except that in this reaction too high a concentration of pyrogallol will hinder the oxidation. I have not studied the effect of varying concentrations of ozone. If oxygen, passed through an ozonizer (the silent electric discharge tube), is bubbled through m/100 pyrogallol, no luminescence occurs at 0°, a fair luminescence at 20°, a good luminescence at 50°, and a bright luminescence at the boiling point. If the pyrogallol is of m concentration, no luminescence occurs at 0° or 20°, a fair luminescence at 50°, and a bright luminescence at the boiling point. For a definite temperature, say 20°, no light appears if the pyrogallol is of m concentration, but if we raise the temperature, luminescence can occur. The similarity to phosphorus is obvious. Thus the “maximum luminescence pressure” of pyrogallol increases with increase of temperature.
We have already seen that pyrogallol can also be oxidized, if H2O2 is present, by a great variety of substances, such as peroxidases of potato or turnip juice, hæmoglobin, KMnO4, K4Fe(CN)6, CrO3, MnO2, hypochlorites and hypobromites, or colloidal Pt and Ag. For convenience we may collectively speak of these as oxidizers. They are recorded in Table 13. No light occurs if H2O2is absent. In the case of some of these oxidizers pyrogallol will luminesce in dilute concentrations but not in strong. Also, dilute pyrogallol will luminesce with a dilute solution of oxidizer but not with a concentrated solution of oxidizer. The effect of rise in temperature in these cases also is to increase the “maximum luminescence concentration” of pyrogallol and the “maximum luminescence concentration” of oxidizer. Table 11 shows this effect of temperature with K4Fe(CN)6 and varying concentrations of pyrogallol, and Table 12 shows the effect of temperature with pyrogallol and varying concentrations of K4Fe(CN)6. Table 10 shows the relation between temperature and intensity of luminescence with pyrogallol and various oxidizers. The terms faint, fair, good, and bright are purely relative designations of brightness as estimated by the eye, for accurate measurements of weak intensities are very difficult to make.
From Table 10 it should be noted that the intensity of luminescence of pyrogallol oxidized with most oxidizers is actually less at the boiling point, a fact which I have repeatedly verified. Let us now see how these facts are to be explained. If we assume that luminescence is dependent on reaction velocity, the intensity of luminescence should increase with increasing temperature. Up to a certain limit this is what we find, but at temperatures above this limit the intensity of luminescence actually decreases. The duration of luminescence also decreases. There is an optimum temperature for luminescence in many cases and we can only conclude that luminescence depends not on a very rapid reaction velocity but on a certain definite reaction velocity. Assuming that this is true, how can we account for the anomalous fact that in high concentrations of oxygen, phosphorus will not luminesce or that in high concentrations of pyrogallol, there is no luminescence in presence of ozone or of oxidizer and H2O2. Of course with high active mass of oxygen (in case of phosphorous luminescence) or of pyrogallol (in case of pyrogallol luminescence) the reaction velocity must be greater than the optimum. If that is the case, then lowering the temperature should reduce the reaction velocity to the optimum and light should appear. However, as we have seen, not lowering but raising the temperature causes luminescence with high oxygen concentration or high pyrogallol concentration.
TABLE 11
Temperature, Concentration of Pyrogallol, and Light Production. An Equal Amount of m/20 K4Fe(CN)6 is Mixed with Pyrogallol + 3 per Cent H2O2
| Concentration of pyrogallol (after mixing) | Temperatures | ||||||
|---|---|---|---|---|---|---|---|
| 0-2° | 10° | 20° | 30° | 50° | 75° | 98-100° | |
| m/4 | Negative | Negative | Good | Very faint | Faint | Fair | Faint |
| m/40 | Negative | Faint | Faint | Faint | Good | Bright | Good |
| m/400 | Faint | Fair | Good | Good | Good | Bright | Bright flash |
| m/4,000 | Bright | Bright | Bright | Bright | Bright flash | Fair flash | Negative |
TABLE 12
Temperature, Concentration of Ferrocyanide and Light Production. An Equal Amount of K4Fe(CN)6 is Mixed with m/100 Pyrogallol + 3 Per Cent H2O2
| Concentration of K4Fe(CN)6 exposed to light (after mixing) | Temperatures | ||||||
|---|---|---|---|---|---|---|---|
| 0-2° | 10° | 20° | 30° | 50° | 75° | 98-100° | |
| Half saturated at 20° C | Negative | Faint | Fair | Fair | Good | Good | Faint flash |
| One-sixth saturated at 20° C | Very faint | Fair | Good | Good | Bright | Very bright | Good flash |