1090
J O H S SPCSCE .LSD 33. H. C-iRROLL
DESESSITIZhTIOS R T SESSITIZISG DYES' J O H S SPEXCE
ASD
B. II CARROLL
Iiodai, Research L n b o i a f o i i c s , Rociiesfer 4>S e w Yord
Received J a n u a r y BO, 1948 ISTRODCCTIOS
Early investigation, of optical sensitization disclosed an optimum concentration for any combination of dye and emulsion. It vas further evident that dyed emulsions generally did not show an increase in white-light speed proportional to the increase in energy absorbed. The esistence of desensitization by optically Yensitizing dyes has been recognized, and more recently has been proved beyond dispute. Howver, its importance appears to have been generally underestimated, and its connection with the existence of an optimum concentration has never been established. Eder (3) attributed the loss of sensitivity produced by excessive amounts of dye to filter action by dye which is not adsorbed by the grains. The effect would increase with the concentration of the dye, as is actually observed. This view, which was supported hy another pioneer in the field, von Hub1 (6), appears quite plausible, and has some basis in fact for emulsions containing n-eakly adsorbed dyes, such as the eosins. It has been Tvidely accepted in textbooks. However. other early workers recognized the existence of true desensitization, Konig and Luppo-Cramer (8, 10) realized that when optical sensitization fails to increase white-light speed, there must be a loss of sensitivity in the region of silver halide absorption, to compensate for the increase of sensitivity to the longer Tvave lengths. The first definite evidence vias supplied by Heisenberg (5), who showed that pinacyanol reduced the sensitivity of silver halide to radiation which it did not absorb. Further data on desensitizing by a number of sensitizing dyes were obtained by Breido and Gorokhovski (I), who measured changes of sensitivity at a number of waye length5 in the ultraviolet region. Hov-ever, Dieterle (2), in a short article on the effect of sensitizing dyes on natural sensitivity, expressed the opinion that desensitizing -\vas no longer of practical importance. While admitting the pohhihility of true deseniitization, he considered that impurities in the dye and filter action are the ii-ual causes of loss in natural sensitivity. It is shonn that desenkitization can tie readily measured in cases Tvhere these esplanations are inadequate; it is even tl factor to be considered when white-light speed is greatly increased by optical sensitizr'i t'ion. Earlier data from the,e Laboratories had indicated that desensitization by practically useful dyes \\-as associated primarily with the use of excessive concentrations. Leermakers, Carroll, and Statid (9) have given esamples of the change in sensitivity to light transmitted by the standard tricolor filters, corresponding t o varying concentrations of dye. Sensitivity to esposure through the red and green filters pas& through well-defined masima as the concentration was increased; 5ensitivity to light tiansmitted by the blue filter remained practically I
C'oniiuuniration So. 1174 f r o m t h e Iiodak IIrwmeh 1,aboratories.
DESESSITIZATIOS BY SENSITIZING DYES
1091
constant for all concentxations u p to the value corresponding to the red and green maxima, and then decreased rapidly. This implied that desensitization increased sharply when the optimum sensitizing concentration was esceeded. It is nondemonstrated that, while the data are correct, the method of measurement included compensat'ing effects which concealed the continuous wiriation of desensitization Jvith concentration of sensitizing dye. Optical sensitization may be espressed in a number of ways. -1,useful measure may lie obtained by esposure to a continuous spectrum through a filter which absorbs only the radiation to n-hich the silver halide is sensitive in the absence of dye; results measured in this Tvay are called "total" sensitization. For the folloiving comparison of dyes, sensitization is expressed in terms of 1,'ElllaXI., n-here E,,,:,,,is the energy of wave length corresponding to the maximum of spectral sensitivity required to produce a developed density of 1.0 (or in some cases, 0.5). I t is shon-n that the sele'ction of narron- spectral regions other than those of the absorption maxima does not affect, the results. Ilesensitization may be measured by the changes in Eicoo, which is t8hecorresponding energy of incident light' of wive length 400 mp required to produce the same dewloped density. This 11-ai-elength vas chosen because it is not appreciably absorbed by any of the dyes used in these esperiments. The ratio of Eao for the unsensitizecl emulsion to E4,, of the sensitized is a direct measure of the desensitization Tvhich the dye has produced. Both E , and E4",were measured in the Physics Department of these Laboratories by exposure with a monochromatic sensitometer, using a slit nidth of 7.3 mp (4). The observed optical sensitization in any given emulsion is the resultant uf three factors: (1) the radiant' energy absorbed b y the dye, ( 2 ) the efficiency of energy transfer from the dye to the silver halide grains, and ( 3 ) desensitization by the (lye. The effectiveness of a dye may be analyzed readily in terms of . Ea:", and the corresponding absorptions. Total Sensitization, n-hilc 1y measured, may be computed only by a complicated integration of absor~ition :mtl sensitivity over the n-hole region of optical sensitization. AU)sorptioii,-4,is expressed as the percentage of light of the selected wave length which is absorbed by the dyed emulsion layer. I t is equal to 100 - (X Y'), \\-here R and T are dfluse reflectance and transmittance of the dyed emulsion layers. Aisin previous communications, these quantities \'\-eremeasured in the Physics Department of these Laboratories by means of the automatic pliotoelectric spectrophotometer, In regions vhere there is no absorption, the sum of reflectance and transmittance varies from 96 to 101 per cent, so t'he error in .-I is believed to he not more than a fei\- per cent. The efficiency of energy transfer from the excited dye to the silver tialitle grains cannot be measured in alisolute terms by photographic measurements. Hoivever, by use of E,,,,,,, Edo0,and the corresponding values of A , it is possible to cdculate the desensitization and the relative quantum efficiency of the sensitized photographic process compared to the process in the region of natural ahsorption liy the silver halide. The relative quantum efficiency is the l m t
+
1092
J O H S SPESCE S X D B. H. CARROLL
available measure of the efficiency of transfer of energy from the dye to the silver halide grains. The quantum efficiency, 4) is defined as (molecules reacting)/ (quanta absorbed). In photographic measurements the density, D , is proportional to the amount of silver developed, with sufficient accuracy for our purposes. Therefore, for the total photographic process,
where K is a constant; and the relative quantum efficiency, +P
=
d(h-sens.rn ix. d’(Li30rn~)
n-here +,,, and +.a0 arc derived from E,,,,, and E400 and the corresponding values of absorption. + r never esceeds 1.0. In all the following experiments, 4r was found to be independent of dye concentration. It is, therefore, not affected by the desensitization, vhich varies with the concentration. This simplifies the comparison of dyes. COJlPARISOS OF DTI:S FOR DCSCSSITIZAiTIOS .%SD HEL.ITIVE
QUrlSTCJf
EFFICIESCT
To illustrate the analysis of optical sensitization, tivo dyes of similar structure and similar distribution of optical sensitization have been compared. These are 3 3’-diethylthiacarbocyanine iodide )
C-CH=CH=CH-C CZH,I
CZH5 Dye 1
and 3 3’-diethyl-4,4’-diphenylthiazolocarbocyanineiodide. )
C2Hb I
C2H6 Dye 2
Both dyes have their maximum sensitivity at 590 mp and secondary maxima a t about 540 mp. Seither has appreciable absorption at -1.00 mp. Sensitization by both dyes is measured by 1/Ejgo and l/EjQ. Desensitization is measurable by the ratio of EQafor the sensitized emulsion to for the unsensitized; since the latter is a constant for each group of experiments, E400may be used as a direct expression of desensitization. Table 1 gives the data for a series of concentra-
1093
DESESSITIZSTIOS BY SESSITIZISG DYES
tions of both dyes in the same emulsion. The relations of l/ESQ, 1/E5g0, A ,
c&, and EMOto dye concentrations are shown in figures 1, 2, 3, and -1. TABLE 1 Comparison of sensitization b y d y e s 1 and 2 CONCENTRATION OF I
DYE
I
x
A
i r
Dye 1 n i p .ililer
20
30
40
60
per c e d
400
5.15
--
400 510 590
3.40 0.85 1.13
75.0 34.5 34.0
400 540 590
3 . 40 1.26 1.23
46.5 42.0
0.44 0.43
400 540 590
3.15 1.23 1.38
74.5 53.0 47.5
0.41 0.47
400 540 590
2 , 70 1.26 1.32
i4.5 61.5 54.0
0.42 0.46
74.5 27.5 25.0
0.92 0.98
0.405 0 230 0 230
75.0 37.5 33.0
0.81 0.87
I
0.345 0.265 0.263
74.5 43.5 39.5
I
0.295 0,250 0.230
75.0 52.0 47.0
tn P
10.0
0.40 0.49
-_ 10.0
Dyc 2
400 20
5.15 I
400 540 590
0.535 0.245 0.265
I
I
30
40
60
400 540 590 400 540 590 400 540 590
I
'
0.95 0.95
0.91 0.85
I n terms of total sensitization, dye 1 is about six times stronger than dye 2 , but the spectral distribution of sensitivity n-hich they produce is nearly the same.
Examination of the data in table 1 shows that these results may be explained in the following terms : ( I ) The absorptions are quite similar; that of dye 1 is greater by 10 per cent, which is a small difference when compared t o the difference in effective sensitivities.
-I E
x
0
xDye2
--- X -
590pp
20 40 60 Concentration of Dye Milligrams /Liter
FIG 1 (’omparison of tlic optical sensitizations of dyes 1 arid 2 at different coiiceiitrations
IO(
Yo A
c
Dye I
0
0
x
xDye2
- =540pt~ --- h=590pp
I
20 40 60 Concentration of Dye Milligrams/Liter
FIG.2 . Comparison of the a h o r p t i o n s of dyes 1 and 2 at 540 nip and590 conccrit rat ions.
nip
at diffcreiit
( 2 ) The relative quuntuni efficiencies of both d>-es are nearly coiibtaiil over the range of concentrations Tvhich \\-ere used, and the efficiency of dye 2, the weaker sensitizer, iz the higher. In eitrh cube, the efhciency of the dye i* the same a t 590 nip and at 540 mp. ( 3 ) Desensitization by dye 2 is much the greater; it is actually of the m n e order a q that of pinalrryptol green. It is the desensitization which is responsible for the great difference between the dyes. It was not expected that a dye show-
1095
DESCSSITIXITIO?; BY SESSITIZISG DYES
ing strong desensitization should have a high efficiency of energy transfer, hut the data available indicate that there is no inherent connection between these properties.? 4.0
3.0a
4
/
L
\
/
0
s
w
2’
-3
Dye 2
3’
0
2.01.0
2’ /
/
-
/ /
/
/ n
c
-
Dye I 0
~
Concentration of Dye Milligrams/Liter FIG.3. Comparison of the descnsitizing effects of dyes 1 and 2 at diffcrcnt concentrations
I .o 0.8
0.6
Dye I
o
o
x
xDye2
-X = 5 4 O p p - - - X=59Opp
0, 0.4
0.2
GoncenGa-tion of Dye MilIigrams/Lier FIG.4 Comparison of t h r I d a t i v c quantum eficicncies of dyes 1 and 2 at different clonccntrat ions,
Dyes may be found with almost any combination of absorption, efficiency of transfer, and desensitization. The best sensitizers have high efficiency of energy *This is particularly ~ c l shown l by the photoconductivity of photographic emulsions containing dye-desensitizers (W. West and B. H. Carroll J. Cbem. Phys. 16, 539 (1947)). I n thcse elperiments the primary act, involving the production of mobile electrons in the silver halide crystal, is isolated from the subsequent photographic phenomena involving the formation and stabilitj, of the latent image and its development t o a visible image, and it has been shown t h a t manv strongly desensitizing dyes do not inhibit the primary production of electrons by light Soine, indeed, are excellent optical sensitizcrs for the photoconluctive process.
1096
J O H S SPESCE .LXD 13. H. CARROLL
transfer, high abm-ption, and little clesenbitization, but thih is quite rare, and there is a considerable range of properties among commercially u5eful dves. Illustrative values are given in table 2 foi dye- at optimum concentra t'ions.
EFFECTOF DESEASITIZ ITIOX
OX THI:
OPTIMT-JIDYI, CONCESTIILTIOS
-1s the concentration of sensitizing dye added t o an eniiilsion is increased, the optical sensitization (espressed either a%1/ E,,,., or total sensitization) pssbes through :i maximum. The concentration corresponding t o this maximum i- referred to as the sensitizing optimum. It i> dependent 011 hoth emulsion type and dye. Leermaker5, ('airoll, and S t a i d (9) found that, foi ;I given dye, the amount adsorbed per unit aiea of silver halide ciiifaccl a t the m,i-,imum sensitization T\ :tz reasonably constant in beven emulbion, of different grain iize.. The optimum corresponded to coverage of the giain surface by a munon~olecularlayer of dye adsorbed flat on the .urface; if the tlyc adwrbrcl cdgc-on, as indicated l)> tlic 14
T\nr,r. P i o p c i izcs of
40 67 i4
64 55 BS
so
37.5 36
10 11 I
50 ~
I
~
'
0.77 O.T8 0.44 0.1s 0.64
i
2
e p i eseuftrtiee d i i t s
Excellent scnsitizrr Goodscxisitizer ; also usi,d as superscsxisitizci ljclectivc sensitizcsr for r e d ; used in color processes Early type of dyc:; 110 longer in use 0nlvmoderatrsc'n.itizcr. hecause of Weak absorption
data of Sheppartl, Lnmbeit, and K:ilker (11>,the grain >iiiface is not completely covered. They concluded that the optiniilm \vas connerted n i t h the degree of saturation of the grain wrface, :i coiic~lusonthat TT 'I> .upported by the rapid increase in unadsorbed dye \\ heii the cwnrentration \\ &> increased past n point near the optimum. However, the ieduction in the -ensitizing optimum on changing trom caarbocyanines to a dic ailiocyanine I\ a b greater than that coniputed from the increase in surface col eiage. It has nom been found that the optiniiini concentration is determined in the main by absorption and desensitization the approximate agreement of sensitizing optimum and surface satui ation io1 .ome cyanine\ :ind carbocyanines ariye-, from a similar contribution of these inctor-. Optical sensitization may he expressed liy the f o l l o iiig ~ ~ ioimula : ~
in \\ hich A and 4, have the meaning3 all eady assigned, K i-, a constant, and Dr ib the desensitization. Since the espeiiinental evidence she\\ i that +r is sub~taiitially *
109;
DI~~HESSITIZATIOS BY PESSITIZISG DYES
constant, the optimuin sensitization corresponds to the maximum of j’(A) f(De) ; \,aria exponentially Irith the amount of dye, the rate of change becoming less and lwh as the concentration increases. Since De increases n-it11 concentration at a more uniform rate, A/De passes through a maximum. The effect of desenqitization on the optimum concentration is rleaily i!liihtrated T,IBT,E 3
0 1 2 3
10 13 30 40
10 15
30 10
6 3 6 !I 14,s 5.15 3 .3 1.15 T o o Ion t o iiicasiire
25.0 :u.0 42.0 59.0 69.0 1 0 .0 79.0
31.0 37.0 34.0 30.0
56.5 67.5 72.0 76.0
-.
-1.55 4.55 4 . 5t5 ti.17 13.5 1.5, 20 56.0 220.0
-
0 24 0.23 0.29 0 32
--
.
-
1098
J O H S SPESCE A S D B. H. CARROLL
Table 3 shows l:E,,,, E400, and absorption for a series of concentrations of each of the dyes in the same emulsion. Both of these dyes sensitize with sharp maxima at 640 mk. The spectral absorptions of the silver halide pluq dyes 3 2nd !
t
011
400
I
'
"
'
'
l
1
l
500
'
l
r
l
600
Wavelength pp
FIG.5 . Absorptions of dye 3 absorption cstends t o 510 niH).
II
(0) and
dye 4 (x) adsorbed to silver halide (silver halide
Y'
I
20
4 )
4 ale given in figure 3 , and the relation between concentration and -I,,,, in figure
G . The similarity of the dyes in these characteristics is evident. However, dye 3 has its optimum concentration at 5 mg. per liter of emulsion 2nd dye 4 at 15 mg. per liter, so that the absorption of dye 3 at its optimum is less
Concentration o f Dye Mi I ligroms / Li ter FIG.7 . Plots of ~ 1 ~ ~ 1 : s i t i z a t i o n - aEt 100 inp-against concentratiorl o f ILJ-C 3 ).P 4 ( x ) .
i o ) a l l ( ] crf
Calculated
A D
-
a
E 0 d. (D
I1
x -IW
tian that of dye 4. In addition t o t l i k , the relatii-e cluantiim efficiency ut (lye 3 lower, SO that its effectiT-e sensitivity is further limited. Seither of thebe ictors, hov ever, explains a ratio of three to one in the optimiim conccntr:ttions
1100
J O H S SPENCE rlSD U . H. CARROLL
of dyes of similar structure and molecular size. The relative desensitizations by the two dyes which are plotted in figure 7 account for this difference. L-sing E400as the measure of De, the ratios of d 'De for the two dyes have been plotted in figure 8, against concentration as alxcissas. In figure 9, l/E,,,, has been plotted against concentration. It is evident that the masima of the curves for each dye agree within the limits of error. and that the optimum concentrat'ions are determined by the relation between absorption and desensitization. It is weU known in practice that the optimum concentration of infrared sensiI
9 7 5
3 I
8
6 4
2 1 I
I
O,
I
I
I
I
1.6
I
I
I
I
1
3.2
1
4 3
Concentrolion o f Dye Milligrams/Liter
FIG.10. Plots of 1 E (A
=
400 mpi and 1,'E (X = SO0 mp) of dye 5
tizers is low. JYhile the tendency to callbe fog limits the useful concentration in some cases, it is normally the desensitization which is the controlling factor. This is illustrated by the data for dye 5 , a tricarbocyanine, which are given in table 4 a i d figure 10:
GS\
C-CH=CH-CH=CH-C€I=CH-CH=C
\K/
's"O
\N/ CzH, CH3
Hac C2H, I
Dye 5
-1bsorption coiild not be mea>iircd Trith iufficient accL1rac.y to calculate _I De, since it is less than 10 per cent at the optimum, hut it is el-ident that the desensitization, mea-ured by the change in l / E ~ oesplains , the loss in seniitization past 0.10 mg. per liter. The molecular area of dye 3 is only one-fifth greater than that uf :L caibocyanine, bo that the change in surface saturation has a relatively T,IBLE 4
0.00 0.01 0.02 0.05 0.10 0.20 0.50 1 .OO 2.00 5.00
0 13 16 0 12.0 76 0 50.0 ?a.0 lS.0
10.50 7.25 6.91 5.75 5.36 0.91 1.95 1.38 0.54 0.19
0
I
0
I
I
I
IO 20 30 Concentration of Dye Milligroms/Liter
'
FIG.11. Plots of inertia spceds exposure through blue (KO.47) Wratten filter against ncreasiiig concentration of dye 6. 0 , Lcerniakeiq, Carroll, and Ftaud; x, this paper.
mall effect on the optimum. Computing areas in 'I2.by the methods of Huggins 7), the computed areas are as follows:
The data already given n d \ e it (’1 iclcnt t h a t tlc,en.itiz~~tion1- ;Lniajoi iucetor in sensitization :it concentiatioris of dye. ell I~elo\ioptimum. It m a y not tw cletccted by esposurei throiigli a blue filter bec.au\e of ;wnpenswting vnsitiz:ition in the bliic-green region tran+mitted by the hlter . The piibli4id reqilty of Leermalcers, Carroll. and Stnud (9) shon et1 that the >peed through n hlue filter wus practically unchanged until the sen4tizing optimum \vas reached ‘1’he.e 11 ere repeated wing the .nmc dye, 3 3’-diethyl-9-mctliyl-4,5,4’,+j’-dibenzothiacurbocyanine chloride (dye G ) in another batch of the hanie emulsion. The results of expowre through :I Kratten S o . A i filter are plotted in figure 1 1 011 u linear scale, along n-ith the data of Leermnliers, C’nrroll, and Staiid (\ of a given semitizing dye is the resultant of itb spectral absorption, eficiency of transfer of adsorbed energy to the silver halide, and desensitization of the emulsion. Changes in the huetiire of the sensitizing dyes appear to vary these properties independently. 2 . Eficiency of energy transfer is practically independent of dye concentration. 3 . Desensitization is an important factor in sensitization even with the best sensitizing dye-es. It is readily measured by the change in sensitivity at 400 mp, Hrcuuse desensitization and absorption increase with concentration by different functions, the ratio nbsorption/desensitization passes through a masimum, and thi.; has been shown to correspond with the maximum of sensitization. There is (
COJIJIUXICATIOKS TO THE EDITOR THE OSID,ITIOS OF FERROCS SULFITE IS AIR Samples of ferrous sulfite were obtained by mixing ferrous sulfate and sodium d h t e aolutions in a hydrogen atmosphere; they were creamy i r h i t e precipitates ind ivere nashed with air-free itater and alcohol and then dried in a vacuiim lesiccator over sulfuric acid. These precipitates became more or less dark on he surface, owing to unavoidable short contact I\ ith the air; nhen dry, they were :ream to various shades of bra\\ 11. L-nder the microscope they were seen to con,ist of colorless or pale greenish yellon- crystals n ith a coat of yellow-bronn xidized material; this latter n-as sometimes of a cauliflon er-like growth from he crystals. T h e n dry, the samples were exposed to air in a large desiccator containing calcium chloride which was opened only occasionally; or they 11 ere txposed to the air in a balance case containing some sulfuric acid, but this rase vas opened more often to the air. These samples \\-ere analyzed from time to imp over a period of years. The per cent of ferrous sulfite fell regularly and coninuously, often slowly at the rate of about 0.4 per cent per year, but sometimes nuch more quickly, even up to 2; or 50 per cent per year. The purer and the Irier the original sample, the slower is the subsequent oxidation. If the original ample is moist or contains much disseminated oxidation product (iron hydrosh?), then the subsequent oxidation is more rapid. The analysis of the ferrous sulfite presents some difficulties. By placing a eighed sample in an excess of standard potassium dichromate solution plus ydrochloric acid, the oxidation is rapidly completed to ferric sulfate, and the
