Methylene blue sensitized dichromated gelatin holograms:a new electron donor for their improved photosensitivity
Jeff Blyth
Tetramethylguanidine (TMG) used as the electron donor in conjunction with methylene blue sensitizeddichromated gelatin (MBDCG) can produce bright reflection holograms recorded at 633 nm at an exposurelevel of 50 mJ/cm 2. Thus DCG hologram recording and some independence from suppliers of proprietaryholographic recording materials is now open to all those with low power He-Ne lasers. The high alkalinity ofthe system allows the unexposed coated material to have an effective lifetime at room temperature of weeksrather than days. Its low dichromate content has an environmental advantage over the usual DCG formula-tions. Replacement of methylene blue by eosin can produce similar photosensitivity to 514 nm. Thesuperiority of TMG over previously reported electron donors used with MBDCG is discussed.
1. IntroductionThe use of methylene blue to enable dichromated
gelatin holograms to be made with red laser light hasbeen reported over a number of years. Changkakoti etal.1 made a recent detailed study of three so-called"external electron donor" amines: ethylenediamine-tetraacetic acid (EDTA); triethanolamine (TEA);N,N-dimethylformamide (DMF). Their results indi-cated that the advantages of adding such compoundsin a dye sensitized dichromated gelatin were not obvi-ous. I decided it would be worth searching for relatedcompounds that might be of greater benefit, with theaim of opening up the use of low power lasers to recordDCG holograms.
The plan used in this research was first to investigatethe photobleaching reaction of methylene blue (MB)in the presence of certain amines but in the absence ofdichromate. It has been known for some time thatEDTA can reduce photoexcited dyes.2 4 The work ofKayser and Young5 was a useful source since they hadinvestigated the reaction times of over twenty amineswith photoexcited MB in flash photolysis experiments.
The effect of red light absorption on MB is to make itan excited molecule with a transitory existence as anoxidant seeking electrons. Certain types of amine canprovide electrons to photoexcited MB via the transito-
ry formation of a charge transfer complex which isbelieved to be produced. 5 The MB is thus turned intoa colorless form known as leucomethylene blue. Thisis a reactive reducing agent able to gradually return tothe colored form of MB by reacting with oxygen fromthe air.
11. Experimental Procedure to Find the Most ReactiveAmines
A stock solution of gelatin (12% wt/vol) with MB(0.04%) was prepared. Amine was slowly added to thestirred solution to give a final amine concentration of2% in solution while the pH was kept close to a value of8.0 by the addition of acetic acid. Under green safe-light, coatings were made with Meyer bars (these arebars closely wound with stainless steel wire; the wiregauge governs the coating thickness). Coating thick-ness was usually -10 ,am after drying in a warm airflow. No dichromate was involved at this stage. Thecoatings were then covered by a simple photographicnegative of some lettering, followed by red filter mate-rial which removed light of a wavelength shorter than600 nm. Exposures -6 in. from a 100-W bulb weremade and the time taken for the exposed area to bleachas monitored by transmitted light was compared. Theimage created by the negative mask was useful to ob-serve how long the bleached area took to recover itscolor at ambient humidity in the dark.
111. Results of Studies on AminesA general characteristic of the amine groups that
cause the fastest photobleaching effect on MB is thatthey contain a nitrogen atom attached to at least twoand preferably three alkyl groups and that these
groups have an electron donating inductive effect onthe nitrogen atom. Kayser and Young5 used flashphotolysis of solutions of MB with an amine and rela-tive molar concentrations, whereas I used continuousillumination of dried gelatin coatings containing theamine based on mass concentrations. The compara-tive ability of amines to photobleach the dye appearedto be of the same order as those obtained by Kayserand Young for the limited number of amines I exam-ined. For example, they found the fastest photob-leaching from using 1,4-diazabicyclo[2.2.2.]octane(DABCO). They found this reacted about nine timesfaster than triethanolamine and about three timesfaster than tri-n-propylamine on a mole for mole basis.In my conditions I found DABCO reacted twice as fastas tri-n-propylamine and six times faster than trieth-anolamine on a gram for gram basis.
I then went on to discover two other amines whicheach reacted faster than DABCO: N,N,N,N-tetra-methylethylenediamine (TMED) and 1,1,3,3-tetra-methylguanidine (TMG). Ethylenediaminetetraace-tic acid (EDTA) as its disodium salt was far less able tocause photobleaching than the above amines.
IV. Effect of Adding Dichromate to the MB-GelatinSystem
Having found which compounds gave the fastestreaction, it was then thought that, by including ammo-nium dichromate in the formulation, it might result inthe reduction of the dichromate to a chromium 3+compound able to form a cross-linking reaction ortanning of the gelatin so that its water solubility wouldbe reduced. The process turned out to be not quitethat simple.
Methylene blue forms an insoluble product belowpH 8 with dichromate ions (Graube 6). Accordingly,dichromate addition was carried out on test formula-tions which had a pH of at least 9. Using the sameexperimental conditions as before, it was found that,with all the amines tested, the addition of as little as0.02% ammonium dichromate to the gelatin solutionseemed to cause the photobleaching reaction to bearrested unless the exposure intensity was very high orthe coating was made very dry. This apparent lack ofphotobleaching was, however, deceptive and the reali-ty of what was happening is now discussed.
Since the exposures were made through a negativemask of some lettering, it was expected that a reliefimage of the lettering might be detectable by treatingthe exposed sample with cold water and then partiallydrying it in an alcohol bath to accentuate the differen-tial swelling between exposed and unexposed areas. Itwas at this stage that a major discovery was made. Atan arbitrarily chosen exposure level, the three aminesfound earlier to react fastest with MB in the absence ofdichromate were now tested with dichromate in theformulation. After water swelling and partially dry-ing in alcohol, no relief image could be detected in thecase of either DABCO or TMED. However in the caseof TMG the relief image was so strong that gentle
washing under warm water left only the exposed letter-ing on the sheet.
More such tests with more amines and higher expo-sure levels showed that doubling the first exposure justmanaged to produce a relief image when tri-n-propyla-mine was the amine chosen. DABCO failed to pro-duce any image until the exposure level was four timesgreater than the original level which gave a good reliefimage with TMG. Even at this level the relief imagedisintegrated when an attempt was made to removeunexposed areas with warm water. TMED, however,failed to allow any relief image to be produced. Fur-thermore, if a formulation was prepared with theamine content made from equal concentrations ofTMG and TMED, this formula also failed to produceany relief image and hence it could be inferred thatTMED spoiled the good ability of TMG to enable reliefimages to be made from photocrosslinked gelatin.
It was found that the ability to form strong reliefimages in red light corresponded with the ability toform high diffraction holograms with a He-Ne laser at633 nm using Denisyuk's single beam method.7 Theclarity and brightness of reflection holograms madeusing a formulation incorporating TMG was quitespectacular.
V. Explanation of the ObservationsThe addition of dichromate to formulations which
gave good photobleach reactions did not in fact arrestthese reactions but only appeared to do so. Probably,bleached MB was being formed but the resulting leu-comethylene blue reduced the dichromate ion virtual-ly instantly. (Strictly speaking, the dichromateCr202- ions in a high pH solution are converted tochromate ions CrO4 which are in the same oxidationstate as dichromate.) DABCO and TMED, in spite ofbeing two of the most reactive amines toward photoex-cited methylene blue, failed to produce formulationswhich would give satisfactory photocross-linking ofthe gelatin in red light. This fits both the observationand explanation given by Changkakoti et al.1 for thecase of EDTA. In their work they found that EDTAconcentrations above 10 g/liter were detrimental tohologram formation. They pointed to the originalwork of Oster and Oster8 and the statement made therethat ". . . the chelating agent (EDTA) must not beadded in such excess as to interfere with the crosslink-ing action of the trivalent chromium which is producedby the leuco-dye."
In spite of its long full name, DABCO could moresimply be called triethylenediamine; TMED is tetra-methylethylenediamine and EDTA is ethylenedia-minetetraacetic acid. Hence we have here the samechemical family based on ethylenediamine and theseare able to form a strong chelate ring structure withany chromium 3+ ions produced by the photoreduction(Fig. 1). Certain groups on the gelatin molecule,which would normally be able to form complexes withchromium 3+ and hence become crosslinked, are sim-ply not able to compete in the presence of these ethy-lenediamines. Tri-n-propylamine, which cannot form
a chelate ring, worked almost half as well as TMG atproducing either relief images or holograms.
Changkakoti et al. also looked at the behavior oftriethanolamine (TEA) as an electron donor, and inthis case its complexing power probably does not ex-ceed that of gelatin. However, it has already beenpointed out that it has been found to have compara-tively weak ability to produce the photobleaching reac-tion.5 In tri-n-propylamine we have in effect threemethyl groups replacing the three hydroxyl groups oftriethanolamine. Hydroxyl groups have an electronwithdrawing inductive effect, and this can reduce theability of triethanolamine to act as an electron donor.Furthermore I found that triethanolamine has the dis-advantage of undergoing a dark reaction (over a 30-min period) when dichromate is added even in quitealkaline conditions. (This is in agreement withGraube 6 and at variance with statements made byChangkakoti et al.1)
VI. Tetramethylguanidine as a Special Electron DonorApart from not being a complexing agent in the same
class as the ethylenediamines, an additional factormay be involved. To find evidence for this the TMGwas substituted with guanidine (as carbonate salt).This worked surprisingly well at enabling holograms tobe produced at exposure levels between two and threetimes longer than those used with TMG in the formu-lation. This put guanidine in the same class of effec-tiveness as tri-n-propylamine but without the mal-odorous nature of the latter. The fact that thisunsubstituted amine works well as an electron donor isunexpected. Primary amines are known to be general-ly ineffective as electron donors.10 3 Possibly the reso-nating structure of guanidine allows easier electrondonation than can occur with monomethylamine, forexample. If this is so, the same resonating structure inTMG may also be important (Fig. 2).
VII. Yield Point Temperature and NoiseNoise in any DCG material is anathema to those
wanting to produce holographic optical elements. Us-ing a simple processing procedure described in theAppendix, it is possible to produce bright noise-freeholograms.
It can be found helpful during processing to keep asimple mechanical model in mind. Gelatin exhibitssome of the classical behavior of an elastic extensionspring. Once it is stretched beyond its elastic limitknown as the yield point, it will not recover its originalunstretched length and remains permanently distort-ed. Increasing the temperature will decrease thestretching force which can be applied without reachingthe critical yield point. If a hologram is recorded insome types of freshly coated gelatin, even water at-25°C can force the unexposed regions to stretch be-
yond the yield point of the material. The final pro-cessed hologram may therefore contain noise createdby permanently distorted fringes.
The art of producing the brightest reflection ortransmission DCG holograms without noise is to pro-cess the hologram in water at a temperature that willswell the material by an amount which takes the gela-tin just up to its yield point. If only a limited area ofthe plate is being exposed for a test, the noise levelshould be judged only within the exposed area. This isbecause the exposed area may have a significantlyhigher yield point temperature, and one can onlyachieve optimum diffraction efficiency by taking theplate up to that temperature even though it will meanthat the unexposed area may then be noisy or milky.The critical yield point temperature will increase withthe age of the plate due to a gradual dark reaction. 1
Vil. Importance of pH ControlIt seems that reducing the pH of gelatin solutions of
these amines with weak acids, such as acetic or carbon-ic, still allows the amines to act as electron donors whenthe pH is above 7. This is fortunate since an apprecia-ble quantity of the free base would irreversibly damagethe gelatin by hydrolysis. On the other hand, neutral-ization of the amines by strong acids might be expectedto turn most of the TMG molecules into positive ionsand render them ineffective as electron donors. In theformulation given in the Appendix, control of the pHvalues is essential. A balance has to be struck betweendamaging the gelatin at high pH values of 10 or moreand precipitating the dye out of solution in the pres-ence of dichromate 6 at pH values below 9. It was
Mv Me M
C= NH -N
Me 'Me M
le MqMeN
-NH *. C-NH
le MeMeFig. 2. Resonating structures of 1,1,3,3-tetra-
found that a complication was produced by the pres-ence of free ammonia released if ammonium dichro-mate was used in these alkaline formulations. Theammonia evaporated from coatings as they dried andleft them too acid and devoid of active dye. Accord-ingly, the slightly alkaline salt potassium chromatewas used instead. The proportion of hexavalent chro-mium is very low compared with the usual DCG formu-lations and it should be noted that substantially in-creasing the level is of no benefit and is generallydetrimental.
IX. Photosensitivity and Relative HumidityIt appears that the photosensitivity of this system is
crucially dependent on a certain degree of residualmoisture in the coating. Some coated plates were leftfor 24 h in small sealed containers to reach equilibriumwith various saturated salt solutions at room tempera-ture. On removing the plates they were immediatelyprotected from the ambient room humidity by glasscover plates before being exposed. It was found thatonly a small drop in photosensitivity occurred when aplate was left in a relative humidity (RH) of 55% com-pared with one that had been left in 66% RH. Howev-er, a plate left at 42% RH had less than half of thephotosensitivity of the one left at 66% RH. A plate at32% RH had lost all photosensitivity for practical pur-poses. This point must be borne in mind by thosewishing to use this system in controlled low humidityenvironments. 9 However, the lower the moisture con-tent of the coating before exposure, the more the re-play wavelength of a reflection hologram shifts to thered.
X. Recording
A. With a Ruby Pulse Laser at 694 nmReflection holograms 4 cm in diameter have been
produced using a 1-J pulse ruby laser at 694 nm (lightat 694 nm is only poorly absorbed by MB). However,the holograms were only obtained by switching off thePockels cell Q-switch. The overall exposure time wasthereby thought to be around 1 ms. With the Q-switchin operation to give a pulse duration of -30 ns, it wouldappear that the photochemical processes involvingMB were not able to operate normally. The dfferencein behavior was proved to be not due to any loweroverall energy being delivered when in the Q-switchedmode.
B. With 514 nm from an Argon-Ion LaserIt has been found that by substituting the dye eosin
Y for methylene blue one can produce bright greenreflection holograms at an exposure level of -50 mJ/cm2 of 514-nm light.
The author is grateful to G. W. Clare for his manyindependent tests of formulations and for practicalhelp.
Appendix: Procedure for Producing DCG Holograms witha He-Ne Laser
1. Preparation of Stock Solutions (Solution Name forReference)
25% TMG, 75-cm3
25-cm3
TMG acetate, 80-cm3
-8-cm 3
deionized water,1,1,3,3-tetra-methylguanidine,
"25% TMG soln.",glacial acetic acidis added slowlywhile stirring welluntil the pH is be-tween 7 and 8,
5% potassium chromate, 5.0-g potassium chro-mate made up to100-cm 3 with de-ionized water.
2. Preparation of 100 cm3 of MBDCG Solution
Twelve grams of a suitable grade of gelatin is stirredinto 80-cm3 cold deionized water. [I used a high clar-ity inert photographic gelatin grade known as limedossein, with a Bloom strength value of 260. (CrodaColloids, Ltd., U.K.) Note that acid-processed gela-tins with isoelectric points above 6 are probably bestavoided in this system.] A water bath is used to heatthe solution to 50°C with regular stirring until dis-solved; this may take 30 min. Maintaining the tem-perature at 50°C, the following stock solutions arestirred in:
TMG acetate, 10 cm 3,5% potassium chromate, 1.0 cm 3,25% TMG, a small quantity is slowly added dropwise
until the pH is between 9.0 and 9.5. Note: if too muchis added, the pH should be immediately brought backto between 9.0 and 9.5 with acetic acid.
0.4% MB, 5.0 cm 3.Note that from this point on moderate green safe-
lighting is required.The formulation is now filtered through a fine nylon
mesh and care is taken to avoid any action which wouldproduce air bubbles.
3. Coating Methods at 500CIf glass plates are clean and warm, conventional
coating methods can be used depending on the thick-ness of finished coating required. To avoid dust be-fore the coating has gelled, my method has been tomake the coating inside a laboratory glove box, largeareas of the walls of the box having been coated with apermanently tacky adhesive several days previously.Thus, dust particles are effectively removed from cir-culation within the glove box. After they have gelledthe plates are removed from the box and dried in atepid air flow. Plates are then left for at least a day at-60% RH before exposing. Low relative humiditylevels will cause loss of photosensitivity.
4. Exposing at 633 nmAn aide memoire involves the number five. A 5-mW
He-Ne laser produces a 5-cm diam bright reflectionhologram with a 5-min exposure.
5. Processing
As processing baths I used cylindrical containers(with lids) which are the right size to allow a 5- X 4-in.plate to rest diagonally upright with the coated sidedown while the liquid is stirred by a small magneticfollower. The processing times are based on a coatingthickness not exceeding 10 Am.
BathNo.
1 1% solution of sodium dith-ionite (Na2S204) (This re-moves both dye and chro-mate ions.)
Rinse in cold running water.2 Warm water at the yield
point temperature as ex-plained in the text. Thispoint is crucial to get the bestdiffraction efficiency with-out noise in the image area.(Any nonimage area shouldbe hazy in the finished holo-gram.)
3 Isopropyl alcohol, 90%4 Isopropyl alcohol, 100%
Temp Time('C) (min)
15-20 2
25? 120 220 5-10
The alcohol is blown off the plates in a warm dry airflow. A finished result that is completely free of hazein the nonimage area probably needs reprocessing at ahigher temperature in bath 2 to develop the image tomaximum diffraction efficiency. A completely dryimage that is milky in the image area cannot be re-stored and another one must be made with a lowertemperature in bath 2. Note that images that are not
completely dry often have temporary levels of noisewhich will substantially diminish when they are thor-oughly dry.
The finished holograms are then left in a warmer at,70°C for several hours to obtain long term stability.Finally, readers should note that increasing the con-
centration of TMG acetate will increase the photosen-sitivity but will also cause a blue shift in the replaywavelengths.
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3. G. Oster and N. Wotherspoon, "Photoreduction of MethyleneBlue by Ethylenediaminetetraacetic Acid," J. Am. Chem. Soc.79, 4836-4838 (1957).
4. R. Bonneau, P. Fornier de Violet, and J. Joussot-Dubien,"Mechanism of Photoreduction of Thiazine Dyes by EDTAStudied by Flash Photolysis," Photochem. Photobiol. 19, 129-132 (1974).
5. R. Kayser and R. Young, "The Photoreduction of MethyleneBlue by Amines," Photochem. Photobiol. 24, 395-401 (1976).
6. A. Graube, "Holograms Recorded in Red Light in Dye Sensi-tized Dichromated Gelatin," Opt. Commun. 8, 251-253 (1973).
7. Y. Denisyuk, "On the Reproduction of the Optical Properties ofan Object by the Wave Field of its Scattered Radiation," Optics& Spectroscopy 18 152-157 (1963).
8. G. K. Oster and G. Oster, "Photomechanical Modifications ofHigh Polymers by Visible Light," J. Polymer Sci. 48, 321-000(1960).
9. T. Kubota, T. Ose, M. Sasaki, and K. Honda, "Hologram Forma-tion with Red Light in Methylene Blue Sensitized DichromatedGelatin," Appl. Opt. 15, 556-558 (1976).
10. H. Obata, K. Kogasaka, and M. Koizumi, "Photochemical Reac-tions Between Methylene Blue and Tri, Di and Mono Methyla-mines," Bull. Chem. Soc. Jpn. 30, 136-147 (1957).
11. B. J. Chang and C. D. Leonard, "Dichromated Gelatin for theFabrication of Holographic Optical Elements," Appl. Opt. 18,2407-2417 (1979).
