J. Phys. Chem. 1991, 95, 5496-5502
54%
Surface Reactlon and Pore Mttudon in Flow-Tube Reactors Leon F. Keyser,* Steven B. Moore, and Ming-Tam Leu Earth and Space Sciences Division, Jet Propulsion Laboratory, California Imtitute of Technology, Pasadena, California 91109 (Received: October 9, 1990: In Final Form: January 2, 1991)
The interaction of gas diffusion with surface reaction in porous solids is discussed and applied specifically to heterogeneous rate measurements in flow-tube reactors. External diffusion to the outer surface of a reactive solid, internal diffusion within the pores, surface reaction, and laminar flow are considered. A procedure is developed to correctobserved surface rate constants for the interaction of these processes. Measured surface areas and bulk densities are used to construct a semiempirical model for porous diffusion in vapor-formed HN03-H20 ices which are used to simulate polar stratospheric cloud surfaces. The model is tested experimentally by varying the thickness of these ices from about 15 to 120 fim. The results are consistent with the model predictions and show that the HN03-H20 ices used are highly porous, and the internal surface must be considend in calculating kinetic parameters from observed loss rates. The best fit of the data yields a tortuosity factor of 3.3 f 1.1 for the ice substrates.
Introduction As part of a study of heterogeneous chemistry involving polar stratospheric cloud particles, an experimental measurement*"of reaction probabilitiesand sticking coeff!cients on HN03-H20 ices has recently been completed. Because of the nature of the ice substrates, observed loss rates can be greatly affected by the interaction of surface reaction with diffusion and flow dynamics. In the present paper these processes are considered as they apply to flow-tube heterogeneous reaction rate studies, and procedures are developed to correct observed surface loss rates as needed. Surface reactions in general comprise several stepwise and parallel processes. The following steps can occur during a heterogeneous reaction and are summarized schematically in Figure 1: (1) diffusive mass transport of reactants to the exterior surface of the solid (external diffusion); (2) adsorption-desorption of reactants at the surface; (3) diffusive mass transport of reactants within the interior of the porous solid (internal diffusion); (4) reaction on the surface or within the solid to form products; ( 5 ) adsorption-desorption of products at the surface; (6) external diffusion of products away from the surface; (7) internal diffusion of products toward exterior of solid; (8) bulk, solid-state diffusion of reactants to interior of solid; (9) solid-state diffusion of products; (10) surface diffusion. The complete mechanism is highly complex, and in order to apply it some approximations are necessary. Generally the rates of some of the processes are much slower or faster than others, and this may be used as a basis for simplifying the mechanism. With respect to other pnxxsses in series, adsorption and desorption are usually rapid enough not to limit the reactivity of the surface, and we ignore them as a first approximation. However, it should be noted that the flow-tube experiments yield only an overall loss rate of labile species from the gas phase and, thus, cannot determine whether the rate-limiting process is adsorption or surface reaction. If adsorption is slow enough to become rate limiting, the quantity measured would be an adsorption or sticking rate rather than a surface reaction rate. The same correction procedure can be used as long as the system reaches steady state in a time short compared to the time of the experiment. Compared to gas-phase diffusion, the parallel steps of solid-state and surface diffusion of reactants are slow and need not be considered. At the low concentrations used, we can also ignore the effects of product diffusion in the gas phase. Product diffusion in the solid or on the surface is also neglected as a first approximation, but these processes or slow product desorption can affect surface deactivation (see below and ref 2). In addition, the rates of all homogeneous reactions are assumed to be negligible. The simplified mechanism consists, then, of steps 1, 3, and 4. In the sections which follow, these processes and their interaction with the fluid dynamics of a flow reactor are discussed in detail. *To whom correspondence should be addreseed.
0022-3654191 /2095-5496502.50/0
External Diffusion Under most experimental conditions used in flow reactors, observed surface decay rates are affected by the finite rate of diffusive mass transfer to the external surface and its interaction with laminar flow dynamics. Since this has been discussed in detail earlierw and the correction procedure has been validated experimentally,* only a brief summary is presented here. As a first approximation we ignore laminar flow and assume plug flow conditions so that the flow velocity, v, is independent of axial and radial position. We also ignore the effects of molecular diffusion and assume that the concentration of species C is independent of radial position. Under these conditions, the loss rate due to an irreversible first-order surface reaction in a reactor of volume, V, with uniform cross section may be written d[CI k,'(W[CIS, (1) dz V where k,'(obs), the observed surface rate constant in units of cm s-I, is defined as the concentration of C reacted per unit time per unit surface to volume ratio. In eq 1 reaction time has been converted to axial distance, z, by using the relation z = ut, which holds under the plug-flow conditions assumed. S,is the total reactive surface area which for nonporous solids is usually taken as the exterior or geometric surface area. For a tubular reactor, S,/V = 2/r0, where ro is the reactor radius. If we define the quantity k,(obs) 5 2k,'(obs)/ro with units of s-', eq 1 becomes d In [C] k,(obs) = -0- dz -0-
Equation 2 shows that under plug-flow conditions the surface rate constant can be obtained directly from the observed decay of C vs reactor distance, z. When laminar flow and diffusion are considered, the surface rate constant becomes6
2
ksg = -(2DS/ro2)( 2nBn/ n-0
2 Bn)
n-0
(3)
where D is the gas-phase diffusion coefficient of the reactive species. h e surface rate constant corrected for external diffusion, k,, can be evaluated directly from eq 3 since the coefficients Bm (1) Moore, S. B.; Keyser, L. F.; Leu, M.-T.; Turco, R. P.; Smith, R. H. Narure 1990,345,333. (2) Leu, M.-T.; Moon, S. B.; Keyaer, L. F. J. Phys. Chem., submitted for publication. (3) Kaufman, F. Prog. Reacr. Klnrr. 1961, f, 1 . (4) Walker, R. E. Phys. Fluids 1961, I , 1211. (5) Howard, C. J. J. Phys. Chem. 1979,83,3. (6)Brown, R. L. J. Res. Narl. Bur. Stand. (US.)1978, 83, 1. (7) Kaufman. F. J. Phvs. Chem. 1984.88. 4909. (Sj Keyser, L. F. J. Pip. Chem. 198i, 88,4750.
63 1991 American Chemical Societv
Surface Reaction and Pore Diffusion
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5497 external surface and any internal surfaces that can be reached by gaseous diffusion from the exterior of the solid. Under plug-flow conditions, eq 1 becomes -0-
I
(4) dz In eq 4, k,' is the true surface rate constant; Seis the external surface area; Si is the internal area, usually taken as the area measured by gas-adsorption experiments (the Bnmauer, Emmett, and Teller (BET) surface area); and q is the effectiveness factor which is discussed below. For a tubular reactor of radius ro and length L with a solid substrate of thickness h deposited on its interior surface, the surface areas are Se = 2*(r0
- h)L( 1 - 0)a = 2rr& 1 - 0)a
(5)
Si= r[ro2- (ro- h)2]L~&a= 2rrohLp&,
(6)
where 8 is the porosity (ratio of void to solid volume) of the solid, u is the roughness factor of the surface (ratio of actual external area to geometric area), Pb is the bulk density of the substrate, and Sgis the BET specific surface area of the substrate. The approximations are made for ro >> h which is generally the case. With these substitutions eq 4 becomes ro - h Radius
Figwe 1. Diagram of a tubular reactor of radius, r, with a reactive solid
of thickness, h, located on its inner surface. For clarity, the size of h relative to ro has been greatly exaggerated. The drawing shows several steps that can occur during a heterogeneous reaction involving a gasphase reactant and a porous substrate. R,, R,, and Ri represent reactants in the gas phase, adsorbed on surface, or located within the solid, respectively; PI.PI, and Pi represent products in similar locations. The proc*rscs are numbered to correspond with descriptions given in the text. The relative concentration profiles show the change in reactant with radial distance for an unreactive sutstrate (y = 0.001, r) = 0.65) and for a reactive one (y = 0.1, r) = 0.072). The parameter y is the reaction probability per collision with the surface, and r) is the effectiveness factor or fraction of the porous solid that participates in the reaction. In the gaseous core of the reactor, 0 to (ro - h), diffusive mass transfer and reaction at the external surface interact with laminar flow dynamics. Within the porous solid, surface reaction interacts with modified Knudscn diffusion. The relative concentrations were calculated by using the parameters given in Table I under the following conditions: ro = 0.88 cm, p = 0.4 Torr, u = lo00 cm/s, Dl = 425 cm2/s, T = 196 K, and reacting species is CION02 (MW = 97 g/mol).
are functions of known quantities: Da and the observed surface rate constant, k,(obs), obtained from eq 2. For these calculations, a computer program was used to evaluate the sums in eq 3 out to terms that were less than 1 X of the total sum; usually less than 20 terms were required.
I n t d Diffusion When surface reactions occur in porous solids such as the ices studied here, gas-phase diffusion within the solid must be considered along with diffusive mass transport to the external surface. This is discussed here first in general terms; the results are then applied to the case of HN03-H20 ices and tested experimentally. hter8ction of Surface R e a c t h and Diffupion in Pomw Solids. The thcorye15 of internal diffusion and surface reaction in porous catalysts is well-known, but its application to flow-reactor studies such as those considered here has not been given in detail before. For porous materials, the reactive surface consists of both the (9) Thiele, E. W. Ind. Eng. Chem. 1939, 31,916. (10) Wheeler, A. Adv. Card. 1951, 3, 249. (1 1) Wheeler, A. In Catalysis. Fundamental Principles; Emmett, P. H., Ed.; Rcinhold New York, 1955; Vol. 2, pp 105-165. (1 2) Clark, A. The Theory of Adsorptlon and Catalysis; Academic: New York, 1970; Chapter 12. (13) Carberr J. J. Chemical and Catalytic Reaction Engineering, Maraw-Hill: York, 1976. (14) A h , R. The Mathematical Theory of Dl/jusion and Reaction in Permeable Catalysts; Clarendon: Oxford, 1975; Vol. 1. (15) Zielinrki, J. M.; Petemen, E. E. AIChE J. 1987, 33, 1993.
kw
k,(obs) = k,[(l - O)a
+ qh&]
(7)
where k, = 2k,'/ro and hp&# = Si/&,the ratio of the internal to external surface area. For most surfaces a a ZL0J1and for the acid ices studied here, 0 a 0.5 (see below); thus, (1 - O)a a 1 and eq 7 becomes k,(obs) = k,(l
+ ThPdg)
(8)
The effectiveness factor, q, of a porous solid depends primarily on the relative rates of surface reaction and gas-phase diffusion; it is defined as the ratio of the observed diffusion-limited reaction rate to the rate that would be observed if diffusion were extremely fast. The effectiveness can also be thought of as the fraction of internal surface which takes part in the reaction. The variation of effectiveness with surface reactivity is illustrated in Figure 1, which gives concentration profiles for the reaction of CION02 with HN03-H20 ice under typical conditions used in flow reactors. In the central core of the reactor, the profile is determined by laminar flow, diffusion to the external surface and reaction at the surface. Within the porous solid, the cantrolling proccsa become Knudsen diffusion and surface reaction. The rapid falloff in concentration within a reactive solid (y = 0.1) results in a low effectiveness factor of 0.072. In the less reactive solid (y = 0.001), diffusion competes with surface reaction and a relatively high effectiveness factor of 0.65 results. An expression for the effectiveness factor can be obtained by ignoring the substrate curvature since h 2. In this case the rate constant expression becomes independent of h, and the observed rate constants should approach a limiting value as the substrate thickness is increased. This occurs obviously because the amount of internal surface is proportional to h, but its effectiveness is proportional to h-l. To test the pore diffusion model, rate constants for the reaction of CION02 with H 2 0 (eq 25) were determined on HN03-H20 ice substrates which CION02 + H20 HOC1 + HNO, (25)
-
varied in thickness from about 15 to 120 pm. Thickness was controlled by condensing mixtures of H 2 0 and HNO, vapors for varying amounts of time on the cold inner surface of a flow tube, only one or two thicknesses were used per run. Details of the deposition procedure wed to form the substrates, and the methods used to obtain rate data arc given (19) Motz,
H.;Wire, H.J . Chem. Phys, 1960,32, 1893.
I
I
5 10 Reaction Time (ms)
15
Figure 3. Observed C10N02 decay rates for two thicknesses of HN03-H20 ice: 0, 13 pm; A, 109 pm.
The results are summarized in Table 111. Observed CIONOl decay rates are shown in Figure 3 for substrate thicknesses of 13 and 109 pm; the abrupt change in slope occurs at the point where the substrate thickness increases. A nonlinear least-squares fit of all the data in Table 111 to eq 11 is shown in Figure 4. This was calculated by allowing T and k, to vary and by using the values given in Table I for the other HN03-H20 ice parameters. Discussion Validation of Internal Pore Diffusion Model. The observed increase in reactivity with thickness (see Figure 4 ) show that the HN03-H20 ice substrates are indeed porous and that some portion of the internal surface is available for reaction. A nonporous solid with no solid-state diffusion should exhibit little or
5500 The Journal of Physical Chemistry, Vol. 95, No. 14,I99'I
4~~
0'
u 25
50
75
100
125
h (W
Figure 4. Plot of surface rate constants corrected for external diffusion, k, vs substrate thickness, h. The line through the data is a nonlinear least-squaresfittoeqll. ThebestfitgivesT=3.3* ].land&,= 15.5 & 2.7 S-I. The errors given arc one standard deviation obtained from the least-squares analysis.
no variation in reactivity with thickness since only the external surface is available. Small changes in surface roughness are possible, but this cannot account for the large magnitude of the observed effect. By using the porediffusion model summarized by eqs 9-1 1 and 13, we obtained the true surface rate constants, k,, shown in column 5 of Table 111. The standard deviation of the values is less than 25%of the mean for a variation in thickness over nearly an order of magnitude. For these samples the correction factors (column 3 divided by column 5 ) vary from about 5 to 19. The good agreement among the corrected k, values for such a large range of sample thicknesses and corrections verifies that the diffusion model can be applied to the ices studied. Effect of kctivation. If the substrate were nonporous with a finite rate of solid-state diffusion, deactivation (poisoning) of the substrate by an increase in the solid-state concentration of product HNO, could cause thin samples to appear less reactive than thick samples if the diffusion of HNO, is rapid enough to significantlypermeate the former. Estimates of diffusion distances were made by using a simple random-walk model in which the distance is given by (2D,t)'12 where 0, is the solid-state diffusion coefficient. The results show that, in order to saturate a 15-pm substrate in 10 min (the maximum contact time of the present experiments), 0, would have to be at least 1 X 10-8 cm2s-', which cm2 s-I is several orders of magnitude larger than 1 X estimated for HNO, diffusion in H 2 0 ice at 196 K. The latter value was estimated by using the measured value20 of 4 X lo4 cm2s-' for HN03 diffusion in H 2 0 ice at 258 K and an activation energyz1of 0.6 eV for H 2 0diffusion in H 2 0 ice. The tarnishing reaction22is another and perhaps a more realistic model for reaction and diffusion in nonporous solids. It considers the diffusion of a dissolved gas through an inert film to a reactive surface. Estimates based on the conditions used in the present experiments show that, in this case also, 0, would have to be very much larger cm2s-' in order to saturate than the estimated value of 1 X the thin substrates. Thus, if diffusion of HNO, in HN03-H20 ice occurs at even remotely the same rate as HNO, in H 2 0 ice, deactivation should not interfere with the thickness variation measurements. It should be noted that pore mouth poisoning is not important for the present experiments because y = 1 X lo-' (Table 111) and, therefore, C10N02penetrates most of the solid. Uniform poisoning of the pore walls is also unlikely to affect the thin samples more rapidly than the thick ones over the time of these experiments because solid-state diffusion is too slow to saturate the solid if the average granule size is about 10 pm. The (20)Bamaal, D.;Slotfeldt-Ellin n, D.J . Phys. Chem. 1983,87,4321. (21)Hobbs, P.V. Ice Phydcs; &endon: Oxford, 1974; 384. (22)Crank, J. The Morhemuflcso/D!flus/on;Clarendm: &ford. 1975; p 305.
Keyser et al. linear decay curves for a given thickness (see Figure 3) are additional evidence that deactivation is not important in these experiments. Tortuosity Factor, T. Although it has a basis in theory, the tortuosity factor in practice remains a fitting parameter because most porous solids are not characterized well enough to reliably calculate it. In order to make the correction for internal diffusion, 7 can be determined semiempirically as described here; or the effective diffusion coefficient itself can be measured, thereby avoiding the need to evaluate 7 . The best fit of the data in Table I11 and Figure 4 yields T = 3.3 f 1.1, where the error given is one standard deviation obtained from a nonlinear least-squares analysis. The value 3.3 lies within the range of 2-6 observed for most s0lids.'*J3 Measurements*% of diffusion rates in unconsolidated powders and granules, which are expected to be similar to the vapordeposited ices of the present study, have shown that T is roughly inversely proportional to the porosity of these solids; at a porosity of 0.5, the observed value of 7 is 1.7. This agrees well with a simple modellQ1'J8of the porous structure which consists of randomly oriented cylinders of uniform cross section for which the calculated tortuosity factor is 2. Other m o d e P m of porous solids predict values for 7 which are generally in the range 1-8. The magnitude of the internal diffusion correction is not very sensitive to the value of T used. For example, at 7 = 2, the average k, is only 18%less than the value obtained by setting T at the best-fit value of 3.3; with 7 = 4, k, is 1096 larger. For the internal diffusion corrections in HN03-H20 ices discussed here and elsewhere? a value of 3.3 was used for 7 . Shnple cubic Pore ModeL If, as an alternative to the cylindrical pore model used above, HN03-H20 ices are modeled by simple cubic packing of hard spheres, the bulk density can be calculated from the geometry and is given by pb
= ~ p t / 6= 0.85
(26)
and from eq 19, we have for the porosity 0 = (1
- ~ / 6 )= 0.48
(27)
These results are remarkablely close to the observed values of 0.82 and 0.49, respectively. Other possible simple models for the vapor-deposited ices such as cubic-close-packed or hexagonalclose-packed spheres have porosities)' of 0.26 and are not suitable approximations. To estimate a working pore radius for the simple cubic model, we use the radius of the largest circle that can be inscribed within the interstices among the packed spheres rp = d(2'I2 - 1 )
(28)
If we take d = 8.4 pm (eq 15), rp = 3.5 pm, which is close to the value of 2.7 pm obtained for the cylindrical pore model used above (eq 20). Use of the parameters obtained from the cubic-packed sphere model to make the internal diffusion corrections in Table I11 results in an average value for k, of 14.5 f 2.4 s-', which is within 10% of 15.9 s-l obtained from the cylindrical pore model. Thus, the internal diffusion corrections are not extremely sensitive to the details of the pore model used to approximate the HN0,-H20 ices. Correction Factom. Correction for external diffusion increases the value of the surface rate constant while correction for internal diffusion decreases it. As shown in Table 11, the size of these corrections depends strongly on the reactivity of the substrate. For a relatively unreactive solid (7,< 1 X lom3),the extemal correction generally is small ( 0.1) which has a low effectiveness factor and, thus, a small internal correction; but in this case, the observed decay rate is limited by external diffusion and the d a t e d correction is large. This is illustrated in Figure 5 where the correction factors due to external and internal diffusion and the net corrections are plotted vs y, for typical flow conditions. Net corrections are less than a factor of 5 for y, between about 0.01 and 0.4.
Acknowledgment. This research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
(A31
(Cp(ri,Z))rph
The net flux to the surface due to external diffusion must equal the sum of reaction on the external surface plus the net flux into the pore mouths
where O is the porosity of the solid and u its surface roughness. To simplify the solution, we also rquire that no reaction occur at the pore ends
De(dCp/dri),,-o= 0
(A51
The solutions must also give a finite concentration profile at z = 0; they must remain finite as z approaches infinity, and they must have cylindrical symmetry, that is, aC/& = 0 at r = 0 for all z. By use of the dimensionless parameters R = r/(ro- h), Z = Z/(ro - h), Ri = ri/h, D,' = D8/2~(ro - h), 0,' D,/Zvh, K, (1 - O)uk,'/u, and # = p,,Sgk,'h2/De,eqs AI-A5 become
Appedx. Interaction of Laminar Flow .ad Diffusion with Surface Reactioa h a Porole Solid A more detailed derivation of q 11 is presented here. The problem comprises (1) the interaction of surface reaction, flow dynamics, and diffusive mass transfer to the external surface (external diffusion) and (2) the interaction of surface reaction with gas-phase diffusion within the porous solid (internal diffusion). The treatment of external diffusion closely follows that prtsentecl earlier by Brown? Internal diffusion has been d i d earlier,lbl' but it has not been applied with simultaneous external
iaC)
-K,C(l,Z)/2
D8
R-1
- Di(
"') aRI
(A9) Ri-I
The solution to q A6 can be written as a sum of exponentials
J. Phys. Chem. 1991, 95, 5502-5509
5502
following expression for the concentration within the pores of the solid where the K,+ are dimensionless parameters which describe the experimentally observed decay. Generally KI*is much smaller than the other parameters in the series, and a short distance downstream of the flow-reactor entrance the solution may be taken as the first term in eq A1 16 v 8 Omitting the subscript 1, we have C(R,Z) = Ag(R) exp(-K*Z) (A121 For an experimentally observed exponential decay, P in eq A12 may be equated to ks(obs)ro/v. Substituting (A12) into (A6) leads to the following solution for the concentration in the gas-phase core of the reactor g(R) = ?B,Jt’” C ( R 2 ) = CS(Z)(gB,P”/ n=O kBn) n=O
(A14)
where C,(Z) is the concentration at the solid surface, i.e. C,(Z) = C( 1,Z)and the coefficients, B,, are functions of P and D l . The solution to (A7) can be written
+ bz exp(-4Ri)
(A151
(A161
By substituting into boundary condition (A9), we can obtain an expression for K,: m
m
-Di( E2nBn/XBB,)= K,/2 ne0
n=0
+ D l 4 tanh (4)
(A17)
By evaluating the parameters D l , K,, and 0,’ in terms of dimensional quantities and by noting that k,’ = r&,/2, ro - h = r,,, and D&/h = hpGgk,’/4, we arrive at the following expression 0)
-(2Dg/roz)(
m
E 2nBn/ n=O E Bn) = n=O kJ(1 - @J + h P 8 g tanh w / 4 1 (A181
(A13)
n-0
Cp(Ri,Z) = bi W 4 R i )
Cp(Ri,Z) = Cs(Z) cash (4Ri)/mh (4)
The left-hand side of eq A18 is exactly the solution obtained by Brown6 for the interaction of laminar flow and external diffusion in the absence of reaction within a porous substrate. In the present study, this solution is labeled k, and given above as eq 3. The quantity tanh (4)/4 is the effectiveness factor, 7, which is the ratio of the actual reaction rate in the pores to the maximum rate with very rapid d i f f ~ s i o n . ~With ’ ~ these substitutions and noting that for HNOS-HzO ices (1 - O)u = 1, we obtain eq 11.
ks, = kS(1 + 7hP8g)
Using the boundary conditions (A8) and (AlO), we obtain the
(1 1)
Photochemical Ring-Opening Reaction of Indoiinospiropyrans Studied by Subpicosecond Transient Absorption Niko P. Emsting* and Thomas Arthen-Engeland Max- Planck-Institut ftlr biophysikalische Chemie, Abteilung Luserphysik, D-3400 Gdttingen, Posrfach 2841, Federal Republic of Germany (Received: October 31, 1990)
The photochemical ring-opening reaction of spiropyrans BIPS (1 ’,3’,3’-trimethylspiro[2H- l-benzopyran-2,2’-indoline]), its naphthopyran analogue naphtho-BIB, and 6-nitro-BIB in n-pentane to the corresponding merocyanines has been followed by transient absorption spectroscopy. A broad absorption ca. 1 ps after UV excitation, covering the entire measurement range 380-680 nm, is assigned to transitions from the excited spiro compound. In the case of BIPS and naphtho-BIB, the merocyanine absorption bands rise with time constants of 0.9 and 1.4 ps, respectively. The transient spectra are compared with spectra taken several microseconds after excitation and with the spectra obtained when the spiropyrans are irradiated at low temperature in an argon matrix. There is evidence that a distribution of isomers is already established 10 ps after UV excitation of the spiroform. The reaction paths leading from excited BIPS to ground-state merocyanine isomers are classified according to their steric requirements. The internal molecular temperature after fast internal conversion is estimated to be ca. 900 K. We suggest that, at this temperature, a primary merocyanine product in its electronic ground state may isomerize further on the picosecond time scale, resulting in the observed distribution of isomers.
1. Introduction
Spiropyrans consist of a 2H-pyran moiety and a second moiety that are held orthogonal by a common spiro carbon atom. The r-electron systems of both parts of the molecule do not interact significantly, so that the absorption spectrum of the spiropyran resembles the sum of the spectra for the two parts of the molecules.’ Hence the spiropyran is generally colorless. For example, Figure 1 shows (onthe left-hand side) the structure and absorption spectrum of 1’,3’,3’-trimethylspiro[2H-1-benzopyran-2,2’-indoline] (BIPS). This compound is the parent of a great number of derivatives. In 1952, Fischer and Hirschberg reported that UV excitation of BIPS (and of other spiropyrans) leads to a heterolytic cleavage of the bond between the spiro carbon atom and the oxygen atom.* ~~
~
(1) Tyer, N. W., Jr.; Becker, R. S. J . Am. Chcm. Soc. 1970, 92, 1289.
(2) Fischer, E.; Hirshberg, Y. J . Chcm. Soc. 1952, 4522.
OO22-3654/9 1/2095-5502$02.50/0
The molbcule can now unfold, the two parts of the molecule are allowed to rotate relative to each other, and thus an extended, more planar configuration is attained. Now the r-electrons conjugate across the entire structure (which may be classed among the merocyanine dyes) resulting in an intense absorption in the range 500-600 nm. The merocyanine form of BIPS and its visible absorption spectrum are also shown in Figure 1. In a subsequent thermal back reaction, the photochemically induced color usually fades on a time scale of seconds to minutes (at room temperature), as the merocyanine form reverts to the thermodynamically more stable spiroform of the molecule. The photochromism of spiropyrans has been studied intensely.’ (A comprehensive review4 covers the literature until 1969, so that (3) (a) Hciligman-Rim, R.; Hinhberg, Y.; Fircher, E. J . Phys. Clrcm. 1%2,60,2465,2470. (b) Bercovici, T.; Heiligman-Rim,R.; Fwher, E. Mol. Photochcm. 1969, I , 23.
Q 1991 American Chemical Society
