J. Phys. Chem. 1993,97, 11137-1 1142
11137
Intense Phosphorescence Triggered by Alcohols upon Formation of a Cyclodextrin Ternary Complex Adrian Ponce, Peter A. Wong,**tJeremy J. Way, and Daniel G. Nocera.4 Department of Chemistry and the Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 Received: June I O , 1993; In Final Form: August 16, 1993'
Intense phosphorescence is observed when alcohols (ROH) are introduced to aqueous solutions containing 1-bromonaphthalene (1-BrNp) and a glucosyl-modified cyclodextrin (G@-CD). Steady-state and time-resolved luminescence measurements and equilibrium constant data are consistent with phosphorescence arising from l-BrNp as part of a l-BrNpG@-CD*ROHternary complex. The association of the l-BrNp to Gj3-CD is increased in the presence of the alcohols (K = 800 M-l in the absence of ROH and K = 1900-3400 M-l in the presence of ROH with the exception of cyclohexanol where K = 760 M-1). However the phosphorescence quantum yields show no obvious correlation with the apparent formation constants of the ternary complex. For instance, the ternary complex of cyclohexanol exhibits the highest phosphorescence quantum yield (& = 0.035) despite possessing the smallest formation constant. Stern-Volmer analysis shows that the phosphorescence enhancement induced by alcohol is related to its effectiveness in shielding photoexcited l-BrNp from oxygen. The rate constants for oxygen quenching decrease generally as the bulkiness of the alcohol increases. Accordingly, tert-butyl alcohol and cyclohexanol give rise to the smallest oxygen quenching rate constants and the highest emission quantum yields. The ability of alcohols to trigger an intense luminescence response is a first step in the development of an optical scheme to detect alcohols. The advantage of strategies using a l-BrNpGj3CD-ROH ternary complex is that alcohol detection occurs by the appearance of bright green phosphorescence relative to a photonically silent background.
Introduction The perturbation of the excited-state properties of a photoluminescent center (PLC) by an exogenous substrate (S)is a basis for the design of optical sensing schemes. Typically luminescence is quenched by the presence of substrate thereby leading to attenuated emission lifetimes and intensities of the PLC.14 Consequently, schemeswhose functions are derived from quenching mechanisms are hampered by the necessity to detect small differences in luminescence intensity relative to a high background. To this end, we are interested indevelopingschemes in which luminescence from a PLC is triggered by the molecular recognition of S. One approach to achieving such chemistry is predicated on the absorption-energy transfer-emission (AETE) process, which has elegantly been elaborated in recent years by Balzani and Lehn.5 Here, light absorbed by S to produce its excited state S*is channeled to the PLC to produce electronically excited PLC*, which in turn relaxes with the emission of a photon. Because the rate constant for energy transfer exhibits a 1 / P (Foster) or e- (Dexter) distance dependence,6p7efficient AETE processes demand that the light-harvesting substrate (S)be juxtaposed to the PLC. By virtue of the nature of S,two AETE schemes are generated. For those S that are ligands, AETE is established upon coordination of S to the photoluminescent center, which is typically a metal ion. For instance, we have shown the luminescence efficiency of Tb3+C2.2.1 (C2.2.1 is the cryptand 4,7,13,16,21-pentaoxa-1,lO-diazabicyclo[8.8.5]tricosane) to increase by upward of 3 orders of magnitude upon coordination of chelating substrates to the encapsulated Tb3+ion.8 For those LHCs that are not ligands, the short distances needed for AETE may be imposed by the spatial constraints of a supramolecule. We have substituted one arm of the 2.2.1 cryptand with a cyclodextrin (CD) to form a supramolecular assembly, which Current address: Departmentof Chemistry,AndrewsUniversity,Berrien Springs, Michigan 49104. t Alfred P. Sloan Fellow and NSF Presidential Young Investigator. Abstract published in Advance ACS Abstracts, October 1, 1993.
0022-365419312097-11137$04.00/0
effectively amounts to a molecular cup juxtaposed to a molecular swing9 or basketalo The PLC is a Eu3+ or Tb3+ ion residing in the appended aza crown swing or basket. The AETE process is initiated upon inclusion of various S (e.g., benzene) within the CD C U P . ~ J ~ But how can a substrate, which does not lend itself to AETE, be detected? An obvious limitation of AETE schemes is their inability to incorporate substrates that are poor light harvesters. For instance, great interest has been directed toward thedetection of alcohols.12-18 Yet their inability to coordinate metal ions and absorb ultraviolet or visible light obviates an AETE approach. Our interest in elaboratinga method for the detection of alcohols by a triggered luminescent response led us to become interested in the properties of lumophores included within CD cups. It is well-known that the nonradiative decay processes of lumophores are significantly attenuated, and hence luminescence intensity increased, in the protected microenvironment of a CD Many investigations have focused on the fluorescence of CDincluded polyaromatic hydrocarbons or azo dyes, and the effect of alcohol on fluorescence.23 The perturbation by the alcohol on the fluorescence properties has been attributed to the alcohol increasing the associationof fluorophore within the CD modifying CD fluorophore interaction~,2~.*~+~~ and/or physically shielding fluorophore from quenchers present in bulk solution.29 We became interested in integrating the concepts of roomtemperature phosphorescence of lumophores in CD with that of ternary complex formationamong aromatic hydrocarbons, CDs, and alcohols to design a scheme for the luminescent detection of alcohols. Specifically, we were intrigued by Turro's investigation of host-guest inclusion complexes between CD and bromonaphthalene appended with detergent chains.31bJ4 Although the cationic phosphorescent probes are quenched efficiently by CO(NH&~+in aqueous solution, their inclusion within the CD cup preserves their intense phosphorescence. The bromonaphthyl moiety is protected from quencher by the coiled hydrocarbon chain within the CD cavity. We wondered if the protectivefunction of the long detergent chain could be assumed 0 1993 American Chemical Society
Ponce et al.
11138 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
by alcohols in the formation of a complex among bromonaphthalene, 8-CD, and alcohol. In this case, the possibility exists for alcohols to trigger the intense, green, room-temperature phosphorescence of bromonaphthalene,which otherwiseis efficiently quenched by oxygen. Described herein are investigations showing that the green phosphorescenceof 1-bromonaphthalene (1-BrNp) residing within the hydrophobic cup of a modified CD is enhanced significantly upon the formation of a CD/ 1-BrNp/alcohol ternary complex. For our investigations, we have chosen a j3-CD derivatized with a glucosyl attached to the primary rim of the cyclodextrin ring through a 1-6 linkage. The addition of the glucose to the rim of the cup increases the solubility of the (3-CD in water by 40fold,35 and ensured that the ternary complexes formed in our studiesremained solublein aqueous solution at pH 5-7. Moreover, glucosyl substitution only marginally attenuates the binding of guests within the CD cup.36 The relationship between the structure of the ternary complex and its photophysical properties is elucidated.
Experimental Section
Glucosyl-j3-cyclodextrin (GPCD) was purchased from Pharmatec. The alcohols, 1-propanol (1-PrOH), isopropanol (2PrOH), 1-butanol(1-BuOH), sec-butylalcohol (2-BuOH), tertbutyl alcohol (t-BuOH), and cyclohexanol (CycOH) were Burdick and Jackson's Distilled-in-Glass grade and were used without further purification. 1-Bromonaphthalene( 1-BrNp) was purchased from Aldrich and was vacuum distilled prior to use. Quantum yields were recorded on a high-resolution emission instrumentconstructed at Michigan State.37 Measurementswere made on aerated aqueous solutions containing enough alcohol to ensure that the quantum yields were independent of the alcohol concentration (i.e., the luminescenceintensitywas at itsasymptotic limit for all alcohols, uide infra). Absolute quantum yields were determined for dilute samples exhibiting absorbances -0.1 at the exciting wavelength of 3 13 nm; the luminescence standard was 2,3-butanedione (q& = 0.05 in hexane at room t e m p e r a t ~ r e ~ ~ ) and appropriate corrections were made for differences in molar absorbtivities and refractive indi~es.~9 The alcohol concentration profiles of the phosphorescence intensity of 1-BrNp were performed on air-saturated aqueoussolutions containing2.5 mM GB-CD and charged with 5 pL of I-BrNp. Alcohol additions were between 2 and 20 pL so that the total volume change for the solution was not significant. All measurements were performed at 22 f 2 OC. For these experiments, a Perkin-Elmer LS-5 fluorometer in the phosphorescence mode (bXc = 313 nm, bet= 493 nm) was used as the emission instrument. A delay time of 0.1 ms and a gate time of 1.0 ms were set to obtain phosphorescencesignals without fluorescence interference. Data were acquired and stored with the Perkin-Elmer Model 3600 Data Station. Oxygen quenching rate constants were determined by using the Stern-Volmer method of luminescence lifetimes.@ Phosphorescence decay lifetimes were recorded on a previously described time-resolved laser instrument with slight modificat i o n ~ . ~Sample ' excitation at 309 nm was obtained by Raman shifting the 355-nm output of a Quanta-Ray DCR-2 NdYAG laser with a low-pressure H2 cell. Phosphorescence from the sample was detected at 530 nm with a Hamamatsu R928 photomultiplier tube, and the decay of the signal was monitored with a Tektronix DSA 602A digitizing oscilloscope, which was triggered by a photodiode. Luminescence lifetimes were calculated from a standard linear regression of the logarithmof intensity vs time delay data. Stern-Volmer experiments were conducted over an oxygen quencher concentration range lO-s-lO-3 M in specially constructed high-vacuum cells, consisting of a 1-cm quartz cuvette attached to a sidearm terminating with a 10-mL round-bottom flask. Oxygen was added to the cell by using
--a8
.->
-ma
I
a
300
400
500
600
700
h/nm Figure 1. Lumincsccncespectraof I-BrNpin aquebussolutionscontaining GB-CD (10-3 M)and G@-CD/r-BuOH (3% v/v). Spectra (a) and (b) show the relative fluorescence of 1-BrNp from G@-CDand G@-CD/ ROH solutions, respectively. Similarly spectra (c) and (d) show the relative phosphorescence of 1-BrNp from G&CD and GB-CDIROH solutions,respectively. The fluorescenceand phosphorescenceintensities are not normalized to each other.
standard high-vacuum procedures. The concentrationof oxygen dissolved in water, C,,was calculated from42
cg= 1000Xgp/M,(1 - X,)
(1)
xg = PgIKIi
(2)
wherep is thesolution density (g/mL), M Iis themolecular weight of water, X, is the mole fraction solubility of oxygen in water, and KH is the Henry's law constant for oxygen in water. Two techniques were used in determining the association constants for the various equilibria of the system. Equilibrium constants involving the association of 1-BrNp to Gj3-CD were determined by monitoring the absorbance of 1-BrNp, on a Cary 2300 absorption spectrometer, as a function of Gj3-CD added. This resulted in an increase in absorbancethat is due to the binding of 1-BrNp to the cyclodextrin cup. The binding of alcohol to 1-BrNpGO-CD was determined by employing the standard Benesi-Hildebrand analysis43s" of the change in fluorescence of 1-BrNp upon alcohol a d d i t i 0 n . 4 ~ ~
Results Steady-State Phosphorescence. The fluorescence of 1-BrNp in aerated water is easily observed at -340 nm despite the compound's poor solubility in aqueous solution. Addition of Gj3CD to the aqueous solution of 1-BrNp causes this fluorescence to decrease slightly with relative intensity changes in the components of thevibrational fine structure. Changesin FranckCondon factors have been observed previously in the fluorescence spectra of polyaromatichydrocarbons when included in CD, and indeed these types of spectroscopicsignatures have been exploited for investigations of the guest/host chemistry of CD complexes.23,24948As shown in Figure IC,phosphorescence is not observed from solutions of 1-BrNp and Gj3-CD or 1-BrNp and alcohol despite the presence of the heavy atom on the naphthalene. However, the characteristic green phosphorescence of 1-BrNp is readily apparent upon its addition to solutions of Gj3-CD and selected alcohols. Figure Id shows the vibrationally structured band of 1-BrNp phosphorescence upon addition of t-BuOH to aqueous solutions of 1-BrNp and GO-CD. The intensity of the observed phosphorescenceexhibitsa marked dependence on the nature and concentration of alcohol. Figure 2 shows the dependence of the integrated phosphorescence intensities of aqueous solutions saturated with 1-BrNp for varying concentrations of 1-PrOH, 2-PrOH, 1-BuOH, 2-BuOH, t.BuOH, and CycOH. In each case, the emission intensity increases monotonically with increasing alcohol concentration to an
Formation of Cyclodextrin Ternary Complex
0
0.1
0.2 0.3 [ROH] I M
0.4
The Journal of Physical Chemistry, Vol. 97,No. 42, 1993 11139
0.5
TABLE I: Photophysical Data and Oxygen Quenching Rates of I-Bromonaphthalene/Clucosyl &Cyclodextrin/Alcohol Complexes I-BrNpG&CD-ROH & / l W .r/msb &q*(02)/105 M-1 s-1 1.9 0.14 1-BuOH 231 4.1 0.17 228 1-PrOH 80 0.59 45.9 2-PrOH 0.67 39.2 2-BuOH 47 t-BuOH 340 4.6 8.87 350 3.9 1.17 CycOH a Quantumyields for phosphorescence of 1-BrNpin the concentration independent range of alcoholsfor the ternary complex. Phosphorescence lifetimes of 1-BrNp in the concentration independent range of alcohol for theternarycomplex. Theoxygenquenchingrateconstantfor 1-BrNp in water in the absence of GB-CD and alcohol is k, = 5.0 X lo8 M-I s-1.
Figure 2. Dependenceof the integrated phosphorescenceof 1-BrNpfrom
aqueous solutions of Gj3-CD (10-3 M)/ROH as a function of the concentration of the following alcohols: 1-PrOH (0);1-BuOH (a); 2-PrOH (0);2-BuOH (W); 1-BuOH(A);and CycOH (A). The data do not accountfor differencesin bindingof 1-BrNpto G@-CDfor the various alcohol/water mixtures. As a reference, the plateau region for r-BuOH is 6,250 timcs greater than the phosphorescence intensity in the absence of alcohol.
100
5 50
0 2
0
4
6 [02]/ 10 -4 M
0
10
Figure 4. Stern-Volmer plots of the quenching reaction between oxygen and 1-BrNpin aqueous solutions of Gj3-CD (10-3 M) and 1-PrOH (a), 1-BuOH (0),2-PrOH (W), 2-BuOH (0),t-BuOH (A),and CycOH (A) at concentrations in the plateau region of Figure 2.
450
500
550 600 X I nm
650
700
Figure 3. Profiles of phosphorescence spectra of 1-BrNp in aqueous M) and (a) CycOH, (b) t-BuOH, (c) 2-PrOH, solutions of Gb-CD (le3 (d) 2-BuOH, (e) 1-PrOH, and (f) 1-BuOH. Spectra were normalized
for 1-BrNp absorbance and were collected in the concentration independent regime of the alcohol. The spectrum for CycOH is offset from that of t-BuOH for clarity.
asymptotic limit, which differs from one alcohol to another. However, as displayed in Figure 3, the phosphorescence profile is invariant for the different alcohols. The enhancement in the intensity of phosphorescence may be significant, as is the case for f-BuOH which attains a limiting value that is 6250 greater than that observed in its absence. The increase of the phosphorescence intensity for the different alcohols can be quantified by measuring absolute quantum yields. Table I lists the quantum yields for the various alcohols at concentrations in the asymptotic limit. Similar to results of Figures 2 and 3, t-BuOH and CycOH give the largest enhancements in phosphorescence, which decrease in intensity as the branching at the a-carbon decreases. Theapparent discrepancies between the data in Figure 2 and the quantum yield data listed in Table I arise because, unlike the absolute quantum yield measurements, the experiments described by Figure 2 do not account for differences in 1-BrNpconcentrations for the various alcohol/water mixtures. When these corrections are applied to Figure 2, limiting intensities scale precisely with the quantum yield data. Oxygen Quenching. The trends in the alcohol dependence of the phosphorescence intensity are also reflected by time-resolved lifetimes. The lifetime of 1-BrNpphosphorescence is attenuated significantly by oxygen even when the lumophore is included
within the cup of GB-CD. However, the phosphorescence decay rate is significantly perturbed by alcohols, decreasing with the increasing concentration of a given alcohol. Observed rate constants (kqob) for 02 quenching, were determined by SternVolmer analysis of the lifetime quenching data:
= 1+ T , ~ : ~ [ Q ] (3) where TO and T refer to the excited-state lifetime in the absence and presence of oxygen at concentration [02], respectively. Oxygen concentrations were determined according to Henry’s law. In all cases the decay curves from which T was determined were monoexponential. Figure 4 shows the Stern-Volmer plots for oxygen quenching of l-BrNp/G@-CD solutions at alcohol concentrations in the plateau region of Figure 2; Stern-Volmer plots are linear over the entire 0 2 concentration range 10-~-10-3 M and intercepts are unity as predicted by eq 3. The calculated quenching rate constants are listed in Table I. Association Constants. The formation of a 1-BrNp/G&CD/ ROH complex is described by several equilibria: T ~ / T
KI
1-BrNp
+ GB-CD * 1-BrNpGj3-CD
(4)
K2
1-BrNpGB-CD + ROH * 1-BrNp.GB-CD-ROH ( 5 ) K3
GB-CD + ROH * GB-CDmROH
+
4
(6)
1-BrNp GP-CD-ROH* 1-BrNpGB-CD-ROH (7) where K, is the appropriate equilibrium constant. It is wellknown that naphthalene and its derivatives form 1:l complexes with 8-CD and that, in the presence of alcohols, a 1:l:l ternary complex is formed.27a.31a,49-54 We have measured the equilibrium constants for eqs 4-7 by independent methods.
11140 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 2
TABLE Ik Equilibrium Constants for Complexation in l-Bromonaphthalene/GIucosyl &Cyclodextrin/Alcohool Complexes 1-BrNpGB-CD-ROH KzIM-1 K3IM-I E K,(exp)/M-l 36 13 3205 1-BuOH 1-PrOH 18 4.5 1950 2-PrOH 12 3.O 3370 25 12 2-BuOH 2320 67 38 2990 ?-BuOH 128 400 760 CycOH
N
. 0 7
: 1-
Ponce et al.
1.5
I
0
10
4
I
20 1
[t-BuOH]
’
-‘
30
I
K,(calc)/M-l 2215 3200 3200 1610 1410 260
a Calculated from the binding constants of alcohol to d-CD56 assuming a 20% reduction in the binding constant for glucosyl modified @-CD. Calculated from eq 10 with an equilibrium binding for 1-BrNp to G@CD, KI 800 M-’.
40
Figure 5. Benesi-Hildebrand plot for the 1-BrNp/G@-CD/t-BuOH system.
TABLE IIk Rate Constants in the Scheme That Describe the Photophysics of l-Bromonapbthalene/GIcosyl &Cyclodextrin/Alcohol Systems 1-BrNpGB-ROH kl/s-l kd/SW1 a kJM-1 s-l ~
The equilibrium described by K1 was measured by optically monitoring the absorbance of 1-BrNp at 286 nm as a function of GB-CD concentration. A plot of A( 1-BrNp)/Ao(l-BrNp) vs [CD-1-BrNpGB-CD] is linear with slope = K1 = 800 M-1. This measured association constant for binding of 1-BrNp to GB-CD is 20% less than that for l-BrNp/@-CD(K1= 1000 M-I 3oa). This result is consistent with Yamamoto et ale’sstudies on G@-CD showing that on average the appended glucosyl attenuates binding to the cyclodextrin cup by -25%.’5 The equilibrium constant K2 can be expressed in terms of K1 as follows: K, =
[ I-BrNpG/3-CD-ROH] K, [ 1-BrNp] [GO-CD][ROH]
1-BuOH
1-PrOH 2-PrOH 2-BuOH t-BuOH CycOH
168 190 129 247 136 126
2.0 1.4 3.9 15 4.4 7.9
~
6400 2700 13000 35000 13000 6000
a Determined from a fit of eq 11 with kqlgiven in Table I as kq”(0z), ko = 6452 s-l, and kqo= 5.0 X lo* M-’ s-l. Calculated from KA= k./kd where KAis the association constant of the complex and is given by K4.
depends significantly on the nature of the alcohol. The photophysics of the I-BrNp phosphorescence included in Go-CD dissolved in water/alcohol mixtures is described by the following scheme:
(8)
As Hamai has shown,2’a when the concentration of the alcohol is much greater than that of the complex, which is one of our experimental conditions, the following equation results for the ternary complex:
-=1 p‘
-10
1 (K,K[ROH][GB-CDIO)
1 + K[GB-CD]~ (9)
where 1, is the phosphorescence intensity, K is the quantum yield and instrumental factor combined, and the subscript refers to the respective initial concentrations. A double-reciprocal plot of 1/Ip -10 vs 1/ [ROH] yields K2 from the ratio of the intercept to slope. Figure 5 shows a representative double-reciprocal plot for the GB-CD/l-BrNp/Z-BuOH system. For all systems, a linear relationship is observed with correlation coefficients 20.99.A summary of the Kz’s calculated from the Benesi-Hildebrand analysis is listed in Table 11. The important equilibriumin regard to the photophysics of the l-BrNp/GB-CD/ROH system is given by eq 7. The equilibrium constant K4 may be determined from the relation K4 = KlK2IK3 (10) Table I1 lists the values of K4 calculated from eq 10, where K3 was assumed to WO% reduced from that measured by Matsui and Mochida for t G association of the same alcohols to 8-CD.56 Alternatively,K4 may be experimentallydetermined by monitoring the optical change in the 1-BrNp upon addition of GB-CD to alcohol/water solutions of the chromophore. Because the alcohol concentrations are in large excess with respect to those for GBCD, the concentrationsof free GB-CD and therefore 1-BrNpGj3CD are negligible. Consequently a measurement of association of 1-BrNp to GB-CD in the alcohol/water mixtures is effectively a measure of K4. As Table I1 shows, the experimental values of K4 are in good agreement with those calculated from eq 10. Photophysics of the Complex. The intensity and lifetime of the phosphorescence of electronicallyexcited 1-BrNp (* 1-BrNp)
where ka and kd are the second-order association and first-order dissociation rate constants of complex, respectively, ko and kl are the excited-state decay rate constants of the phosphorescence of 1-BrNp in water and within the Go-CD, and kqo and kql are the bimolecular oxygen quenching rate constants of 1-BrNp in water and within the Gj3-CD, respectively. In this scheme, the presence of Gp-CD and 1-BrNpGB-CDwas ignored because, as mentioned above, their concentrationsare negligible at the alcohol concentrations used in our studies. As originally shown by Almgren et al. for micelles,” and later popularized by Turro for cyclodextrins,3l~’~ the observed decay rate constant, k,k, can be expressed under steady-state conditions as follows:
kakd
[GB-CD1
ko + kq,[021 + k,[GB-CDI
(11)
The rate constants ko and kl are simply determined from phosphorescence lifetime measurements of 1-BrNp, in deaerated solutions, in the absence and presence of GB-CD-ROH, respectively (Table 111); kq0 and kql are deduced from Stern-Volmer kinetics measurements (Table I). In addition, if we make the assumption that the equilibria for *I-BrNp parallels that of 1-BrNp, then k, may be substituted by KAkd (KA= ka/kd) in eq 11. Thus a fit of 70b vs [O,] to eq 11 at a given concentration of G@-CDwill yield kd for each of the 1-BrNpGB-CD-ROH ternarycomplexes. An exemplary fit of eq 11 to the lifetime data of the ternary complex 1-BrNpGBCD-t-BuOHis shown in Figure 6; similar fits were obtained for the remaining alcohols. From
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11141
Formation of Cyclodextrin Ternary Complex
quantum yields of the different ternary complexes increase with the branching at the a-carbon (Le., l o < 2O < 3O). The role of alcohol branching in determining the photophysical propertiesof the ternary complex becomes apparent when the oxygen quenching rate constants are considered. To a first approximation, the highest quantum yields for phosphorescence are observed for those ternary complexes with the smallest oxygen quenching rate constants. Thus k,(OZ) for the CycOH and t-BuOH systems is lo2 less than that of 1-PrOH and 1-BuOH and the quantum yields of the former are 102greater. Similarlythe intermediate oxygen quenching rate constants are reflected by their intermediate emission quantum yields. For equivalent branching, the quantum yield of phosphorescence simply reflects the formation constant of the ternary complex for a given alcohol. For instance, the higher quantum yield of 2-PrOH with respect to 2-BuOH is reflected by the greater formation constant for the ternary complex of the former alcohol. The only anomaly to the above analysis is CycOH,whoseoxygen quenching rate constant is commensurate with that of t-BuOH. But similar behavior of these alcohols in CD ternary complexes has been observed for numerous other ternary where steric factors are dominating. Thus these data clearly show that the phosphorescence enhancement induced by alcohol is related to alcohol's ability to shield * 1-BrNp from oxygen. Whereas the alcohol's effectiveness in protecting the CD-included 1-BrNp from the external environment depends on the binding of the alcohol to the CD cup and to its steric bulk, it is the latter factor that is predominant in determining the phosphorescence properties of the 1-BrNpGj3CD-ROH complex. A consequence of our observations is that the photophysical properties of l-BrNp.Gj3-CD.ROH ternary complexes may be exploited in the design of optical sensing schemes for alcohols. The approach described herein complements the recent work of Hamasaki et al., who have observed spectral shifts in emission when aliphatic alcohols complex a-cyclodextrin appended with p(dimethy1amino)benzoyl (DMAB).M In this system the relative intensities of the normal planar and twisted intramolecularcharge transfer fluorescence of DMAB is perturbed upon complexation of alcohol to the branched cyclodextrin. The advantage of the strategy developed for
[email protected] complexes is that the detection of alcohols occurs with the appearance of bright green phosphorescence relative to a photonically silent background, and therefore intrinsically higher detection limits might be achieved with lumophore.Gj3-CDSROH systems. Of course, the development of a practical scheme to optically detect alcohols will necessitate the discrimination of alcoholswith respect to other substrates that are known to associate with CD guesthost complexes.61 Thus investigations of the room-temperature phosphorescence from ternary complexes of CDs and lumophores with other substrates is warranted.
-
0
1 0
2
4
6
8
10
[O,] / 10 -4 M Figure 6. Plot of 7 h vs [OZ] for the 1-BrNplGfl-CDlt-BuOHsystem. The solid line is a fit of the data to eq 11.
kd, k, may be obtained from the associationconstant of the complex KA,which as mentioned above is simply K4. The values for k, and kd for each of the ternary complexes are listed in Table 111. The entrance/exit kinetics of 1-BrNp to and from the Gj3-CD cup are significantly attenuated (102-103) with respect to guesthost complexes of unmodified j3-CD~.3~#5295* Thus we find that it is here, in the kinetics for substrate association and dissociation, that the effect of the appended glucosyl on binding appears. However, because the retardation in rates for 1-BrNp entering and exiting the cup are similar, the overall equilibrium constant for association is comparable to unmodified j3-CDs (vide supra).
Discussion The bright green phosphorescence of 1-BrNp included in a cyclodextrin is "turned on" in the presenceof alcohols. The effect is dramatic with phosphorescenceenhancementsapproaching lo4 for selected alcohols. Although the alcohol plays several roles in the chemistry of the ternary complex, the two predominate functions of the alcohol is its influence on the binding of 1-BrNp to the CD cup and on the bimolecular reactivity of 1-BrNp included in the CD cup. The K4)s in Table I1 reveal that alcohol increases the binding of the lumophore to the cyclodextrin, which is an effect that is well documented for a variety of systems. Indeed, the trends in K4along the series for straight (1-PrOH < 1-BuOH) and branched chainalcohols (CycOH < 2-BuOH < t-BuOH < 2-PrOH) parallel the apparent formation constants for other polyaromatics and 8-CD. For instance, Warner et al. have recently reported that apparent formation constants for acridine/b-CD in the presence of branched alcohols increase along the same series of alcohols.23 For the case of straight-chain alcohols, however, the binding constant is observed to decrease monotonically with increasing length of the alcohol. But as these authors have emphasized, the straight-chain alcohols show a propensity to enter the CD cup and their co-inclusion facilitates formation of a stronger complex by filling the void space. Consequently the formation constant of a given guest maximizes for different alcohol chain lengths depending on the size of the guest. Indeed in our studies, the ternary complex formed with 1-BuOH is more stable than 1-PrOH, a result that is also observed for the spatially more similar guest acenaphthene with j3-CD.Z7a Notwithstanding, the important issue here is that the large variation in the emission quantum yields of the ternary complexes cannot be attributed to the alcohol causing stronger association of 1-BrNp to the cyclodextrin. This is highlighted by the ternary complex of 1-PrOH, which exhibits the lowest quantum yield despiteshowing one of the highest formations constants, whereas 1-CycOH engenders the highest quantum yield and yet features the smallest formation constant. The phosphorescence intensity correlates more closely with the bulkiness of the alcohol. With the exception of CycOH, the
-
Acknowledgment. We thank Jeffrey Zaleski for help in collecting time-resolved laser kinetics data, Steven Gottke at Andrews University for obtaining preliminary steady-state phosphorescence data, and Zoe Pikramenou for helpful discussions. Financial support provided by the Ford Motor Co. is gratefully acknowledged. Summer support for A.P. and P.A.W. were provided by an NSF Research Experiences for the Undergraduate in Chemistry (CHE-9200701) and an Academic Affiliate Program of the Center for Fundamentals Materials Research at Michigan State University, respectively. References and Notes (1) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F. Coord. Chem. Rev. 1975, 15, 321. (2) Demas, J. N. ExcitedStcrteLifetimeMecrsurements; Academic: New York, 1983; Chapter 3. (3) Kavarnos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401.
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