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Photoacoustic Detection of N a t u r a l Circular D i c h r o i s m i n Crystalline Transition M e t a l Complexes
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RICHARD ALAN PALMER, JOSEPH C. ROARK, and JAMES C. ROBINSON P. M. Gross Chemical Laboratory, Duke University, Durham, NC 27706
The advantages of using crystalline solids (particularly single crystals) for optical absorption and linear dichroism measurements are well known (1). Less widely appreciated perhaps are the advantages (and limitations) of extending this technique to the measurement of optical activity. Although many of the points below apply equally well to magnetically induced "optical activity", the primary emphasis of our work has been on the measurement of "natural" optical activity as a technique for studying conformation and absolute configuration. These data can serve as important bench marks since independent (X-ray) determination of absolute configuration and conforma tion is frequently possible (2). The relatively low dipole strengths of d-d and certain other low lying, highly forbidden electronic transitions makes solid state optical activity measurements particularly applicable to transition metal ion complexes. For intrinsically chiral species that are inert enough to be resolved conventionally, the measurement of natural optical activity in crystals has the same advantages as single crystal absorption measurements. In addition, however, it also affords the opportunity to determine rotational strengths of species which do not exhibit optical activity in solution. There are two classes of such materials: 1) intrinsically a chiral chromophores which crystallize in enantiomorphous space groups, and 2) intrinsically chiral but labile chromophores which spontaneously resolve on crystallization. The measurement of optical activity in single crystals has been studied most extensively by transmission techniques, although recent development of emission methods has also been reported (3,4). Transmission methods are limited to those compounds which can be obtained in suitably large and perfect single crystals. In addition, only in non-biaxial crystals can the complications of linear birefringence be avoided easily. (However, a two-angle method for cancelling these effects has 0-8412-0538-8/80/47-119-375$05.25/0 © 1980 American Chemical Society
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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been r e c e n t l y proposed (5).) G e n e r a l l y , of the two p o s s i b l e experiments, c i r c u l a r dichroism (CD) i s the choice over o p t i c a l r o t a t o r y d i s p e r s i o n (ORD) due to the r e l a t i v e i n s e n s i t i v i t y of the former to c r y s t a l imperfection and s t r a i n . However, the most serious l i m i t a t i o n on s i n g l e c r y s t a l o p t i c a l a c t i v i t y measurement i s high a b s o r p t i v i t y , s i n c e , as i s g e n e r a l l y the case, absorbançe A = eel(=3A) must be ^ 1. (Note that i f ε = 100 ι moT^-cm" and c = 5 mol then the c r y s t a l must be only 0.02 tmi t h i c k f o r A = 1 ) . Unless doping i n t o a c o l o r l e s s host c r y s t a l i s p o s s i b l e , the d i f f i c u l t i e s of c u t t i n g and p o l i s h i n g a c r y s t a l with ε > 50 (of appropriate t h i c k n e s s ) while r e t a i n i n g a macrosized window are obvious. The use of micro equipment i s a v i a b l e approach i n some cases, whereas d i f f u s e transmission spectra of mulls i s another p o s s i b l e s o l u t i o n . However, transmission CD of mulls i s p a r t i c u l a r l y s e n s i t i v e to s c a t t e r i n g d e p o l a r i z a t i o n . For those c h i r a l systems which r a d i a t i v e l y decay a f t e r e l e c t r o n i c e x c i t a t i o n , c i r c u l a r l y p o l a r i z e d luminescence (CPL) ( o p t i c a l a c t i v i t y of the e x c i t e d s t a t e ) and fluorescence detected c i r c u l a r dichroism (FDCD) ( d e t e c t i o n of o p t i c a l a c t i v i t y of the ground s t a t e by measurement of r a d i a t i v e decay) are useful probes of conforma t i o n i n the s o l i d s t a t e (3,4). The recent r e v i v a l of i n t e r e s t i n the photoacoustic e f f e c t i n condensed media (6,7) led us to consider the p o s s i b i l i t y of d e t e c t i n g natural c i r c u l a r dichroism p h o t o a c o u s t i c a l l y . C e r t a i n aspects of the photoacoustic e f f e c t suggest that t h i s technique might be g e n e r a l l y a p p l i c a b l e to a l l c h i r a l s o l i d s regardless of c r y s t a l c l a s s , s i z e or p e r f e c t i o n , or strength of a b s o r p t i o n . Although subsequent t h e o r e t i c a l developments and experimental r e s u l t s have caused us to l i m i t considerably the p r e d i c t e d scope of t h i s method, nevertheless, i t i s pos s i b l e now to say c l e a r l y that the experiment does work and o f f e r s prospects f o r unique r e s u l t s . In t h i s paper we review b r i e f l y the nature of the theory and p r a c t i c e of condensed phase photoacoustic spectroscopy and i t s extension to the measurement of natural c i r c u l a r dichroism, and present i n i t i a l r e s u l t s f o r s i n g l e c r y s t a l s and powders. The Nature of the Condensed Phase Photocacoustic E f f e c t The photoacoustic e f f e c t i n s o l i d s and l i q u i d s was f i r s t described by B e l l almost 100 years ago ( 8 ) . Recent i n t e r e s t i n photoacoustic d e t e c t i o n has centered around the p o s s i b i l i t i e s of applying i t to the measurement of the absorption spectra of h i g h l y absorbing and/or l i g h t s c a t t e r i n g m a t e r i a l s of both physical and biochemical i n t e r e s t ( 7 ) . In the conventional photoacoustic spectroscopy (PAS) experiment a sample i s enclosed i n a small a i r - t i g h t c e l l and i l l u m i n a t e d with intensity-modulated monochromatic l i g h t . Absorption of the r a d i a t i o n followed by n o n - r a d i a t i v e decay r e s u l t s i n a p e r i o d i c heat flow w i t h i n the sample, which upon reaching the sample s u r f a c e , causes a c y c l i c thermal expansion of the l a y e r of gas 1
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Complexes
at the s u r f a c e . This produces pressure pulses i n the gas which are detected by a s e n s i t i v e microphone placed i n the c e l l . A p l o t of the microphone signal as a f u n c t i o n of wavelength λ, i n p r i n c i p l e , represents a n o n - r a d i a t i v e decay e x c i t a t i o n spectrum analogous to the r a d i a t i v e decay e x c i t a t i o n spectrum measured for luminescent m a t e r i a l s . The c h a r a c t e r i s t i c s of the photoacoustic s i g n a l may be summarized b r i e f l y as f o l l o w s : 1. Transmission of the l i g h t i s not necessary; o n l y absorption and (some) n o n - r a d i a t i v e decay. 2 . The strength of the signal i s p r o p o r t i o n a l , not only to the i n t e n s i t y of the i n c i d e n t r a d i a t i o n , but a l s o to i t s energy. (More heat w i l l r e s u l t from a UV t r a n s i t i o n than from an IR t r a n s i t i o n ) . 3. The strength of the signal w i l l a l s o depend i n v e r s e l y on the modulation frequency of the i n c i d e n t r a d i a t i o n . (Short pulses of the same i n t e n s i t y have less power than long ones.) 4. S c a t t e r i n g i s not a serious problem and, i n f a c t , the more surface area i n contact with the energy t r a n s f e r gas, the b e t t e r . That i s , powders should give stronger s i g n a l s than massive pieces of the same material. 5. The s i g n a l has phase φ as well as amplitude q, the phase being r e l a t e d t o the time required f o r the heat to reach the surface and the sound to reach the microphone. The phase w i l l depend on the a b s o r p t i v i t y 3(cm" ), the thermal d i f f u s i v i t y 0 5 (cm s" ) and the n o n - r a d i a t i v e decay l i f e t i m e ( τ ) . 6. The l i n e a r dependence of the signal strength on 3 i s l i m i t e d to regions of the spectrum where 3/a < 1. ( a = ( 2 α / ω ) ϊ / ) (see below). Near and above t h i s l i m i t the v a r i a t i o n of q with 3 approaches zero. The above statements of the nature of the photoacoustic e f f e c t are drawn p r i m a r i l y from the conclusions of the theory developed by Rosencwaig and Gersho (R-G) ( 9 ) , based on a onedimensional thermal p i s t o n model. Since t h i s i s the s t a r t i n g point f o r extending the theory of photoacoustic spectroscopy (PAS) t o i n c l u d e photoacoustic c i r c u l a r dichroism (PACD), a b r i e f survey of the s a l i e n t f a c t o r s and important parameters of that treatment f o l l o w s . According t o the R-G theory the c e n t r a l parameters of the mathematical model of the photoacoustic e f f e c t are: 1) the o p t i c a l absorption c o e f f i c i e n t 3 (cm" ); 2) the o p t i c a l absorp t i o n length yo = 1/3 (cm); 3) the thermal conduction c o e f f i c i e n t a = ( 2 a A ) ) ! / ; 4) the angular frequency of modulation ω (rad s " ) ; 5) the thermal d i f f u s i v i t y a ( c m s ' ) ; and 6) the thermal d i f f u s i o n length μ = ( l / a ) . The signal i s a l s o dependent on various constants which are defined i n the o r i g i n a l paper ( 9 ) , i n c l u d i n g the source i n t e n s i t y . Assuming various 1
2
1
s
2
s
δ
1
2
s
s
1
2
1
s
δ
s
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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l i m i t i n g c o n d i t i o n s of sample thickness and a b s o r p t i v i t y , the R-G theory may be shown t o y i e l d several more s i m p l i f i e d r e s u l t s (see below). An important d i f f e r e n c e between PAS and conventional transmission spectroscopy i s that the PAS s i g n a l depends both on the sample's thermal and o p t i c a l p r o p e r t i e s . Only l i g h t absorbed w i t h i n one thermal d i f f u s i o n length (μ ) of the sur face c o n t r i b u t e s t o the s i g n a l . I f the absorption c o e f f i c i e n t i s large enough so that μ^ < μ , ( o r 3 / a > 1) the PAS signal becomes independent of changes i n 3 . This " s a t u r a t i o n " condi t i o n may be a l l e v i a t e d somewhat by i n c r e a s i n g the modulation frequency ω, (thus e f f e c t i v e l y shortening the thermal d i f f u s i o n length so that μ^ > μ ) . However, since the s i g n a l strength i s a l s o shown to depend on ω ~ (where η = 1.5 f o r 3 / a < 1), the s e n s i t i v i t y of the system l i m i t s the u t i l i t y of t h i s technique f o r avoiding s a t u r a t i o n e f f e c t s . The incidence of s a t u r a t i o n i s a serious impediment t o the q u a n t i t a t i v e a p p l i c a t i o n of PAS. Various sampling techniques such as co-grinding with MgO have been proposed as s o l u t i o n s t o t h i s problem (10). In a d d i t i o n , the use of the phase angle of the s i g n a l (φ) has been shown t o permit q u a n t i t a t i v e determination of 3 f r e e of many of the u n c e r t a i n t i e s of conventional amplitude (q) measurements, even in the region of i n i t i a l s a t u r a t i o n (11). From the r e s u l t s of the R-G theory i t would appear that d e t e c t i o n of c i r c u l a r dichroism p h o t o a c o u s t i c a l l y would have the f o l l o w i n g p o t e n t i a l a p p l i c a t i o n s : 1) C r y s t a l l i n e (or n o n - c r y s t a l l i n e ) powders and t u r b i d suspensions. This p o t e n t i a l r e s u l t s from the lack of depen dence on the d e t e c t i o n of transmitted l i g h t . This would permit the averaging of l i n e a r b i r e f r i n g e n c e e f f e c t s i n b i a x i a l c r y s t a l s , measurements on c r y s t a l s with poor growth character i s t i c s , and on compounds too l a b i l e to grow large c r y s t a l s even though they are r e s o l v a b l e by rapid p r e c i p t i t a t i o n of d i a s t e r eomers. Turbid b i o l o g i c a l specimens, suspensions, g e l s , e t c . , might also be probed f o r o p t i c a l a c t i v i t y by such a technique. 2) S i n g l e c r y s t a l s too t h i c k or too h i g h l y absorbing t o permit s u f f i c i e n t transmission of l i g h t f o r conventional measurements of o p t i c a l a c t i v i t y . It has been the goal of t h i s work t o develop the theory and techniques to t e s t these p r o p o s i t i o n s . Obviously PACD w i l l not be completely immune t o the d e p o l a r i z a t i o n e f f e c t s o f s c a t t e r i n g , and when q no longer v a r i e s with 3 (because of s a t u r a t i o n ) Aq (=q^-q ) must approach zero. The i n i t i a l e v a l u a t i o n of the extent of these problems i s the subject o f t h i s paper. The PACD Experiment Due to the unique way i n which the photoacoustic signal i s generated, there are two p o s s i b l e ways of performing the PACD experiment (Figure 1 ) . The Type 1 experiment involves only c i r c u l a r p o l a r i z a t i o n at modulation frequency ω · I f Η ¥ 3r» δ
5
s
$
η
s
r
0
Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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PALMER ET AL.
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a photoacoustic signal w i l l be generated which i s p r o p o r t i o n a l to Δ 3 . I f β£ = 3 , no photoacoustic signal can r e s u l t because there i s no i n t e n s i t y modulation of the heat d i s t r i b u t i o n i n the s o l i d . This experiment has been a p p l i e d s u c c e s s f u l l y to detect MCD and LD by the photoacoustic technique (12). I f t h e l i g h t i s also i n t e n s i t y modulated at frequency ω ^ » ω , then the Type I I experiment i s obtained. Type I I d i f f e r s from Type I i n that a photoacoustic signal i s a l s o generated at ω ρ . The signal detected at ω then v a r i e s i n i n t e n s i t y at frequency o) when c i r c u l a r dichroism i s present. (One may draw an analogy t o AM r a d i o with ωρ being the c a r r i e r frequency and u) p r o v i d i n g the amplitude modulation.) The PACD signal may be obtained by the f o l l o w i n g sequence: 1) demodulation of t h e signal at ωρ ( p h a s e - s e n s i t i v e d e t e c t i o n (PSD) w i t h a time constant τ « 1 / ω ) , and 2) PSD o f the demodulated signal using ως as the reference frequency. Since the photoacoustic signal ( p r o p o r t i o n a l to 3) can be obtained by low-pass f i l t e r ing the output of PSD#1, and the PACD signal ( p r o p o r t i o n a l to Δ3) i s obtained at the output of PSD#2, the r a t i o of the out puts (QpsD2/QpSDl) should be p r o p o r t i o n a l t o Δβ/β = g. The sign of the CD i s obtained from the phase angle o f the PACD s i g n a l , with (+)CD g i v i n g a phase 180° from that of (-)CD (Figure 2). Since the phase angle i s a f u n c t i o n of the a b s o r p t i v i t y (11), v e c t o r - t r a c k i n g l o c k - i n a m p l i f i e r s must be used, and since the vector magnitude i s always p o s i t i v e , the sign information must be obtained from the phase angle. The experimental phase angle i s a r e l a t i v e q u a n t i t y which i n c l u d e s c o n t r i b u t i o n s from the modulation source, c e l l and microphone responses, and e l e c t r o n i c s . Although i t i s p o s s i b l e t o determine the r e l a t i o n of the phase angle t o the CD sign by measuring the PACD of a known p a i r of enantiomers, t h i s c a l i b r a t i o n may not be c o n s i s t e n t due to the dependence of the phase angle on the above parameters. PAS Theory Using the R-G photoacoustic theory (£), the pressure v a r i a t i o n s i n the c e l l may be w r i t t e n as: r
α
ρ
c
c
0
AP(t) = qcos(o)t-ir/4-)
(1)
where q i s the vector magnitude of the complex s i n u s o i d a l pressure v a r i a t i o n Q = Qi + iQ2* The general form of Q, given by equation 21 of Réf. ( 9 J , i s q u i t e complicated but can be s i m p l i f i e d f o r several special cases. For the p a r t i c u l a r case of o p t i c a l l y and thermally t h i c k samples (R-G Case 2 c , i.e.,
Q
p A S
)
:
Q
filter ' PAS
L R
=
f ( c >1 / ω
ρ'
juifUUlnrdinjlnniUUlnn. * P
S
ω
ρ
D
, Δ0 = f ( « . l / „ " ) Ρ
2
4
«ΡΑ 4->
