On the Causes of Altered Photophysics of ... - ACS Publications

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On the Causes of Altered Photophysics of Luminescent Metal Complexes Embedded in Polymer Hosts Samuel A. Moore,† Steven M. Frazier,† Morgan S. Sibbald,† Benjamin A. DeGraff,*,† and James N. Demas‡ † ‡

Department of Chemistry, James Madison University, Harrisonburg, Virginia 22807, United States Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, United States

bS Supporting Information ABSTRACT:

A suite of luminescent Re(I) complexes has been prepared whose emissive properties are responsive to the probe’s local environment. These complexes were embedded in a series of chemically similar polymers whose room temperature rigidity varied over a significant range. It is shown that the degree of local rigidity experienced by the embedded complexes significantly alters the observed emission in terms of both spectra and lifetime. Time resolved emission measurements show that the spectral shifts and lifetime complexity are correlated and track well the polymers’ Tg within the series. Fluorescence confocal microscopy did not show the presence of discrete domains, and thus, the environmental features responsible for the altered photophysics must be submicrometer in size.

’ INTRODUCTION Interest in luminescent metal ion complexes as molecular reporters continues to grow. Use of these materials in chemosensors is well reported, and several good reviews of this type of application are available.1 The salient features of these materials include (1) excitation by inexpensive light sources, (2) large emission Stokes shifts for ease of excitation/emission separation, (3) relatively long lifetimes that allow for discrimination against background fluorescence, and (4) remarkable tuning of photophysical properties via structural modification. To fabricate a working sensor usually requires that these dyes be supported in some fashion. The support matrix can be strictly passive or can act as a partial gatekeeper for the desired analyte. Typically, these supports have been either porous gels or polymers.2 Various ways of anchoring the dye to the support have been examined, ranging from simple occlusion embedding to chemical tethering. Unfortunately, the interaction between the dye and support often introduces additional complexity to the dye’s photophysical response. While emission color and intensity changes are often seen, the more troublesome alteration is a change from simple exponential intensity decay to more complex decays that characterize the dye’s lifetime. There are several “work arounds” that have been used to deal with this problem. One can use the sor 2011 American Chemical Society

called “two site model”, which is an empirically useful approach that simply models the dye as existing in two types of sites in the support matrix.3 For more complex decays, a series of exponential terms can be used successfully.4 One can also dodge the problem by measuring the lifetimes by phase shift methods in which only a single composite lifetime is returned when the measurement is made at a single frequency.5 In addition, other models have been proposed to model lifetime data from supported dyes.6 These have met with some success in that the data can be fit to the model’s equations. The problem remains a reasonable physical explanation of the model. There seems to be general agreement that system heterogeneity is the cause of many of the changes in the dye photophysics observed when luminescent metal complexes are placed in a support matrix. We, somewhat arbitrarily, set 1 μm as the boundary between microheterogeneity and nanoheterogeneity. Features such as dye or support crystallites, cracks and fissures, and phase boundaries fall into the micro regime. These features can usually be seen with a good fluorescence microscope and Received: April 19, 2011 Revised: June 21, 2011 Published: June 23, 2011 9567

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Langmuir several studies have been reported in which these features have been observed and played a significant role in the observed altered photophysics.7 However, our recent work has provided evidence that even when micro-heterogeneous features are absent as tested via fluorescence microscopy, the photophysics of many luminescent metal complexes are altered by inclusion in a polymer support. To our knowledge, there has been little work on exploring the possible dye
polymer interactions that could give rise to the observed alterations in the luminescent response of the complexes.8 We set out to systematically evaluate some of the possible dye
polymer parameters that could cause the observed variation in luminescent behavior. We chose as the dye a series of Re(I) complexes of the general form Re(I)(L2)(CO)3X in which L2 is a 2,20 -bipyridine or 1,10-phenanthroline derivative and X is either CN
or a pyridine derivative. Re(I) complexes are wellknown to show the impact of both environmental rigidity and polarity in their emission characteristics.9 Various members of the acrylate family of polymers were used for this study as one can obtain a significant variation in both Tg and hydrophobicity using a common backbone, [
CH2
CR
(CO2R0 )
]n.These polymers are well characterized, available in very pure form, and give cast films with excellent optical properties. While this study provides some basic insights into the various interactions that are the cause of the observed altered photophysics, a unified model of dye
polymer heterogeneity is still a work in progress.

’ EXPERIMENTAL SECTION Materials. All organic solvents, 4-methyl pyridine, and pyridine were HPLC grade or better and obtained from Fisher Scientific. 1-Iodoheptane, butyl lithium (2.5 M in hexane), and diisopropylamine were obtained from Aldrich Chemical as was Re(CO)5Cl. The ligands 2,2-bipyridine (bpy), 1,10-phenanthroline (phen), 4-methyl-1,10-phenanthroline (4-Me phen), and 3,4,7,8-tetramethyl-1,10-phenanthroline (Me4 phen) were obtained from GFS Chemical and checked via GLC for purity (>99%). Anhydrous AgClO4 was also obtained from GFS Chemcial. The polymers poly(methyl acrylate), PMA, Mw ∼ 40 000 Da; poly(ethyl acrylate), PEA, Mw ∼ 95 000 Da; poly(butyl acrylate), PBA, Mw ∼ 99 000 Da; poly(methyl methacrylate), PMMA, Mw ∼ 123 000 Dd; isotatic poly(methyl methacrylate); poly(ethyl methacrylate), PEMA, Mw ∼ 850 000 Da; poly(n-butyl methacrylate); PnBMA, Mw ∼ 337 000 Da; poly(isobutyl methacrylate), PiBMA, Mw ∼ 130 000 Da; and poly(4-vinylpyridine), Mw ∼ 60 000 Da were all obtained from Aldrich Chemical. All the foregoing were used as received. Ligand Synthesis. Two ligands, 4-n-octylpyridine (octylpyr) and 4-n-octyl-1,10-phenanthroline (4-octyl phen) were sythesized using standard butyl lithium/diisopropyl amine procedures.10 In a typical procedure 10 mmol of butyl lithium (2.5 M in hexane) were added to 100 mL of dry tetrahydrofuran (THF) containing 10 mmol of dry diisopropyl amine at 0 °C and stirred for 20 min under Ar. To this cold mixture was then added via syringe 10 mmol of either 4-methylpyridine or 4-methyl-1,10-phenanthroline dissolved in 30 mL of dry THF. This addition was done over a 10
15 min period during which the mixture became blue/black. This solution was allowed to stir at ice-water temperature for 1 h under Ar. To this solution was then added dropwise via an addition funnel 10 mmol of 1-iodoheptane in 25 mL of dry THF. This addition took ∼30 min, and the solution color changed from blue/ black to straw yellow. At the end of the addition, the stirred solution was allowed to warm slowly to room temperature over a 3
4 h period. Standard workup resulted in either an oil (4-n-octylpyridine) or a waxy solid (4-n-octyl-1,10-phenanthroline), which was further purified by

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Scheme 1

column chromatography using alumina as the support with elution by CH2Cl2 containing an increasing percentage of CH3CN. Typical yields were ∼70% for octylpyr and 65% for 4-octyl phen. The structure of each was verified with 1H and 13C NMR (see the Supporting Information). Complex Synthesis. The Re(I) complexes were synthesized using Re(CO)5Cl as a starting material using a procedure reported earlier.11 Scheme 1 illustrates the various ligands and complexes used. WARNING: Perchlorates are potentially explosive and due care is required. Preparation of Re(L2)(CO3)Cl Complexes. Typically, 100 mg of Re(CO)5Cl and a 10% molar excess of the appropriate ligand were placed in a 25 mL round-bottom flask with ∼8
10 mL of toluene. This mixture was heated at reflux until TLC (alumina plates, CH2Cl2/ CH3CN (80%/20%)) showed no remaining Re(CO)5Cl, usually between 1.5 and 2.5 h. The mixture was then transferred to a beaker, and the solvent allowed to evaporate. The resulting yellow solid was washed several times with diethylether to remove the excess ligand and collected via suction filtration. Yields were >85%. Perparation of Re(L2)(CO)3X Complexes. Typically 100 mg of the appropriate Re(L2)(CO)3Cl complex was dissolved in a minimum amount of dry THF in a conical flask. The mixture was stirred and warmed to ∼50 °C, and an equimolar amount of anhydrous AgClO4 added. A white powdery precipitate forms almost at once. The heated stirring was continued until TLC (alumina plates, CH2Cl2/CH3CN (80%/20%)) showed no residual Re(L2)(CO)3Cl; usually about 3 h. The slurry was filtered to remove the AgCl. At this point, the solution was transferred to a 50 mL round-bottom flask, and a 20% molar excess of pyridine or 4-n-octyl pyridine or a 3 excess of KCN dissolved in a minimum of water was added. The mixture was then refluxed until TLC (alumina plates, CH2Cl2/CH3CN (80%/20%)) showed complete reaction. The THF was removed, and to the resulting yellow material was added both CH2Cl2 and water. The organic layer was isolated, and the water layer extracted several times with aliquots of CH2Cl2. The combined CH2Cl2 portions were washed with 5% Na2CO3 and dried over MgSO4, and the solvent removed to give the crude complex. The crude complexes were purified using alumina columns with CH2Cl2/ CH3CN mixtures as the eluant. Typical yields were ∼60%. Structures were verified by 1H NMR, 13C NMR, and ES-mass spec (see the Supporting Information). The complexes and their aliases are as follows: Re(bpy)(CO)3CN, Re_0; [Re(Me4 phen)(CO)3(4-octyl pyr)]ClO4, Re_1; [Re(phen)(CO)3(pyr)]ClO4, Re_2; Re(4-octyl phen)(CO)3CN, Re_3; [Re(4octyl phen)(CO)3(4-octyl pyr)]ClO4; Re_4. Instrumentation. UV
vis spectra were obtained with a Shimadzu UV-1601 spectrometer. Solution, low temperature glass, and film emission and excitation ratio spectra (emission signal/lamp reference signal) were obtained on a Spex FluoroMax instrument. Film anisotropy spectra were also taken with the Spex FluoroMax fitted with calcite 9568

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Langmuir polarizers. Room temperature solution lifetimes were measured with an apparatus using an LSI nitrogen laser as the excitation source (λ = 337 nm). The sample compartment was of local design incorporating a Corion interference filter to remove plasma glow from the laser pulse, glass filters and saturated NaNO2 to remove scattered laser light from the detector path, and an R928 PMT as detector. The decays were captured and averaged using a Hewlett-Packard 54600B digital scope, and the data transferred via an HP-IB to a PC where it was processed using locally written software. At least 64 transients were averaged for each experiment. Film and low temperature lifetimes were obtained using an apparatus consisting of a New Wave Orion Nd/YAG laser (tripled) and a Spex 1681 1/4 m spectrograph mounted with a R-928 photomultiplier whose output went to an Agilent Infiniium 54833A digital scope. The composite of 64 traces was processed using locally written software based on the Levenberg
Marquardt algorithm. Since the data were analog, we chose the number of required exponentials based on the Σχ2 value change with additional terms. If there was no significant change with an additional term, the fit with the minimum number of terms was taken as acceptable. The residuals were always randomly distributed for the accepted fit. Film samples on quartz windows with an exposed face were mounted at ∼45° to the entrance slit of the Spex monochromator and vigorously purged with a N2 stream for at least 15 min prior to and during the lifetime data acquisition. Tests showed that this procedure gave stable and reproducible lifetime values. The excitation laser was run at reduced power, ∼ 0.3 mJ/pulse, to avoid photobleaching of the complex. No decline in signal intensity during the ∼90 s data acquisition period was observed. The low temperature lifetimes were obtained by mounting a partially silvered optical dewar near the entrance slit of the Spex monochromator. The sample in the appropriate solvent mixture in an NMR tube was slowly prefrozen and checked for cracking. If a clear glass was obtained, the sample was quickly transferred to the optical dewar, which was filled with liquid N2. As usual, 64 traces were averaged for the lifetime determination. Steady illumination anisotropies were measured using an apparatus consisting of a Power Technology 405 nm diode laser (50 mW) whose intensity was reduced to ∼5 mW via neutral density filters and polarization cleaned up via a calcite polarizer. A quartz λ/2 wave plate allowed easy rotation of the excitation beam. The desired polarized emission was isolated with a calcite polarizer, filtered with an appropriate long pass filter, and then focused onto an R-928 photomultiplier whose output was converted to a voltage read by a digital voltmeter (DVM). The data was reduced using locally written QBASIC software. Performance of the system was checked with various Rhodamine dyes in glycerol whose r values were known independently. NMR spectra were obtained using a Bruker Spectrospin 400 instrument. The ES-mass spectral analyses were performed by the NCSU Department of Chemistry’s Mass Spectrometer Facility. Standard fluorescence microscopy was done using a Zeiss Axioskop upright fluorescence microscope with a color/BW digital camera. Confocal microscopy was done using a Nikon C1 confocal microscope based on a TE2000 inverted microscope, laser excitation sources, and a true spectral detector. Film Preparation. Stock complex solutions were 1 mg/mL in ethoxyethanol, and polymer solutions were 10% (w/v) in toluene. Typically, 8
10 μL of dye solution was placed in a small tube and 60 μL of polymer solution added and mixed. Then 45
50 μL of this solution was deposited in a 1.5 cm diameter circle in the center of a 100 diameter coverslip or a 100 diameter quartz window. The deposited solution/support was placed in a semisealed cabinet that allowed for slow solvent evaporation over a 12 h period. The resulting film was washed several times with CH3OH to remove any surface complex and

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then dried at ∼60 °C for 4 h. The films were uniform in thickness and had good optical clarity. A simple Mathcad program using known parameters of the components allowed calculation of (1) film absorbance at 355 nm, typically 0.05
0.10; (2) concentration of complex in the film, typically 2 ( 0.5 mM; and (3) film thickness, typically 25 ( 5 μm. The estimated absorbance values for several films were checked via UV
vis and were within 10%, while the film thicknesses were checked via profilometry and were within 15%. Thereafter, the relevant film parameters were estimated from our calculations. The dried films on the coverslips were then mounted, depending on the experiment, on either 100 diameter quartz flats or standard microscope slides. The coverslips were fixed to the support by tacking with uv cured epoxy. The films cast on 100 diameter quartz windows were used for purged lifetime determinations. The films were examined using a Zeiss epifluorescence microscope with a dry 40 objective to search for aggregates. Microcrystals larger than 1 μm would have easily been detected. All the films used for measurement were aggregate free. TRES Experiments. The films were mounted so that the emission was collected at a right angle to the excitation, which was provided by a New Wave Orion tripled Nd/YAG laser operated at low power (i.e., 0.3 mJ/pulse). The emission was captured with a Spex 1/4 monochromator with 0.5 mm slits and an R-928 photomultiplier. Decays, 64 for each trace, obtained at 5 nm intervals, were captured using an Agilent Infiniium 54833A digital scope. The traces were baseline corrected, parsed, and manipulated into time-resolved spectra using PSI Plot, version 8. Tethered PVP Films. For these experiments, the intermediate complex, Re(phen)(CO)3(ClO4), was prepared via AgClO4 removal of Cl from the standard intermediate Re(phen)(CO)3Cl as described above. The Re(phen)(CO)3(ClO4) was isolated as a yellow solid. A solution of poly(vinyl pyridine) in ethoxyethanol was prepared and sufficient complex added that, on average, 1 in 10 pyridines would be bound to an Re complex. This was accomplished by heating the stirred mixture for 24 h at 80 °C while monitoring the presence of free complex via TLC using alumina plates with a CH2Cl2/CH3CN eluent (9:1). When the test for free complex was negative, the polymer was precipitated by adding cyclohexane and washed several times with CH2Cl2. After drying at 60 °C for 8 h, the polymer was redissolved in ethoxyethanol and a film cast in the usual manner. Polymerization Experiments. For these experiments, 10 mL of methyl methacrylate monomer, 5 mg of VAZO 88 initiator, and 0.5 mg of Re_3 were combined and mixed, and then a 2 mL aliquot transferred to a 3 mL ampule. The mixture was Ar purged for 15 min and then capped. Initial spectral measurements were taken. The ampule was placed in a stirred silicone oil bath at 85 °C for a defined period. At the end of this period, the ampule was removed and quickly brought to room temperature and a set of measurements made. Three separate experiments were done yielding essentially identical results. The concentration of complex in these experiments was ∼0.1 mM.

’ RESULTS AND DISCUSSION Our results and discussion are presented in the following order. First, we seek to establish the environmental sensitivity of our suite of complexes and assess their degree of response to various environments. Second, we show that the most significant source of the observed changes in the photophysical properties of our complexes in polymer supports can be attributed to a single cause. Then, we survey the response of our complexes in a number of similar polymers that have significant variation in that critical parameter. Finally, we explore time-resolved emission spectroscopy (TRES) and true spectral imaging confocal microscopy as tools to gain insight into the microscopic details of the 9569

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Table 1. Emission λmax Values for Re Complexes in Various Mediaa EA

EE

solid

LT glass

PMMA,

PiBMA,

PnBMA,

PMA,

PEA,

PnBA,

(nm)b

(nm)c

(nm)

(nm)d

Tg = 99 °C

Tg = 65 °C

Tg = 15 °C

Tg = 9 °C

Tg =
23 °C

Tg =
49 °C

Re_0

582

564

507

504

529

530

529

541

549

563

Re_1

524

517

478

469

485 (sh)

484 (sh)

510

483 (sh)

516

524

506

500

508

507

509

Re_2

556

540

528

497

512

511

512

524

527

541

Re_3

556

550

528

466

511

511

519

522

523

534

512

508

513

519

525

528

497 Re_4 a

551

542

N.A.

473 502

Uncorrected. b Ethyl acetate. c Ethoxyethanol. d 77 K, proprionitrile/butyronitrile (1:1).

origin of the heterogeneity of emission properties for our complexes embedded in polymer hosts. Our suite of complexes includes (1) small, highly polar, (2) larger, hydrophobic, (3) neutral, and (4) ion pair materials. To establish a frame of reference for our suite of complexes, we initially examined their behavior in several environments: (1) fluid solution, (2) low temperature glasses, and (3) as neat powders. The emission of Re(L2)(CO)3X complexes are sensitive to both polarity and rigidity.12 Observed changes include both λmax and τ variation. Generally, increasing rigidity results in a blue shift in λmax and longer lifetimes, while increasing polarity yields blue λmax shifts but shorter lifetimes. Table 1 summarizes the variation in emission λmax for our suite in various media. The variation with polarity is modest, and, as expected, increasing polarity results in a blue shift. There appears to be no apparent correlation between the polarity shift magnitude and whether the complex is neutral or an ion pair. However, the polarity shift is greater for the smaller, rather hydrophilic complexes. It may well be that the long alkyl chains used to confer hydrophobicity wrap around these complexes and shield them from solvent interaction.13 Rigidity has a greater impact on the emission λmax with blue shifts between nonpolar fluid and rigid media of ∼1000 f 2500 cm
1. The shifts are slightly greater for the low temperature glass environment than for the powders. The spectra of Re_1 in the various media is shown in Figure 1; it is typical of all the complexes. Both the neat powder and the low temperature glass, with the exception of Re_0, show vibronic structure in addition to the blue shift. This structure was present, either as discernible shoulders or distinct peaks and may indicate some ligand localized (LC) component mixing in the lowest excited state.14 The solution lifetimes are unremarkable and similar to those of many other similar complexes.15 A compilation of our observations is shown in Table 2 (air saturated solution values are available in the Supporting Information). The longer lived complexes are subject to oxygen quenching. The lifetimes of all the complexes in fluid solution were clean single exponential. Surprisingly, most of the lifetimes in low temperature glass required 2 exponentials for adequate fitting. There was no significant difference in the mean lifetimes, τM, between the two glass mixtures used. The low temperature lifetimes were ∼10 those observed for purged fluid solution and in line with the suggestion of an LC component to the excited state at 77 K. The fact that multiple exponentials are required to fit the glass data suggest that these complexes are quite sensitive to even small variations in their local environment.

Figure 1. Emission spectra for Re_1 in various media: (—) ethyl acetate, (


) ethoxyethanol, (- - -) low temperature glass, and (• • •) neat powder.

The lifetimes of all the powders are multiexponential, with some requiring 3 components. Since we did not control the particle size or whether the powder was microcrystalline or amorphous, it is difficult to make quantitative statements other than the results again illustrate the sensitivity of the lifetime to variations in the microenvironment of the complex. Surprisingly, quenching by oxygen was not a major factor in the observed powder lifetimes as front surface exposed powders placed in a fast flow of nitrogen for 20 min before and during the lifetime measurement were less than 10% longer than those with no purge and simply protected by a coverslip. In all cases, the lifetime of the powder is greater than the air saturated fluid lifetime, but generally shorter than the purged lifetime. While the interior sites of the powder would be expected to have longer lifetimes due to shielding from the external environment, these sites may contribute little to the observed emission intensity due to low penetration of the exciting light into the interior. Satisfied that our suite of complexes represented a good variation in structural and photophysical attributes, we attempted next to see if there was a single, most important environmental parameter that gave rise to the observed alterations in photophysics when our complexes were embedded in polymer hosts. In order to test whether the emission and lifetime changes observed have their origin in a common environmental change, we followed the emission spectral, lifetime, and anisotropy changes for a representative complex during the polymerization 9570

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Table 2. Lifetime Values (μs) for Re Complexes in Various Media

EA EE solid τM (μs)a (μs)a (μs)b

PMMAa (99

PiBMAa (65

PnBMAa (15

LT glass

°C)

°C)

°C)

τM (μs)b Δτ (N)c (μs)d

τM (μs)b Δτ (N)c (μs)d

3.98 (1)

1.18 (2)

PEA (
23 °C)

°C)e

τM (μs)b Δτ τM (μs)b Δτ τM (μs)b Δτ (N)c (N)c (N)c (μs)d (μs)d (μs)d

τM (μs)b Δτ (N)c (μs)d

τM (μs)b Δτ (N)c (μs)d

e

e

PMA (9 °C) a

a

e

Re_0 0.114 0.175

0.691

Re_1 10.75 6.10

2.90

47.6 (2)

Re_2 1.63 2.59

0.865

11.3 (2)

Re_3 1.64 2.21

0.824

25.8 (2)

32.1

9.76 (3) 12.1

13.2 (2)

15.4

3.75 (2) 3.32

4.31 (3)

4.11

4.55 (2)

4.09

2.64 (2) 2.31

Re_4 2.60 2.27

N.A.

28.0 (2)

29.2

7.21 (2)

11.0 (2)

11.9

4.14 (2) 5.07

4.27 (2)

5.57

1.97 (2)

1.95

2.35 (2) 2.17

47.0 5.88

18.7 (3) 6.92 (2)

0.857

PnBAa (
49 e

e

20.4 5.49 9.19

1.10 (2) 0.735 1.12 (2) 0.703 0.645 (2) 0.489 0.442 (2) 0.299 18.6 (3) 6.34 (2)

21.0

10.3 (3) 10.2

4.86

5.90 (2) 4.74

10.2 (3) 5.00 (2)

10.0 5.98

9.53 (3)

8.81

3.00 (2)

4.10

0.308 (2) 0.476 10.4 (2)

8.84

1.86 (2) 1.59

a Purged with N2 prior and during data acquisition. b Mean lifetime. c Minimum number of terms for satisfactory fit. d Δτ = (longest lifetime component shortest lifetime component). e Tg value for polymer host.

Table 3. Polymerization Experiments reaction

emission

lifetime

time (h)

λmax (nm)

τ (μs)

0

561

1.20

0.01

1.0

561

1.20

0.01

2.0

561

1.20

0.03

3.25

561

1.20

0.03

3.66

558

τM = 1.81 Δτ = 4.11

4.0 4.5 5.5

0.163 Δτ/τM = 2.27

547

τM = 3.24 Δτ/τM = 1.47

539

Δτ = 4.74 τM = 5.12 Δτ = 5.46

Δτ/τM = 1.06

528 524

0.226 0.256

τM = 6.06 Δτ = 5.41

∼8

anisotropy

0.371 Δτ/τM = 0.90

τM = 6.79 Δτ = 8.14

0.388

Figure 2. Relative changes in λmax (9), anisotropy (b), and lifetime ([) during polymerization.

Δτ/τM = 1.20

process. In these experiments the polarity is fixed and only the relative rigidity varies. Our results from the first set of experiments (vide supra) lead us to believe that alteration in rigidity was the most important parameter involved in eliciting heterogeneous response from our embedded complexes. The system chosen was Re_3 in methyl methacrylate. This system was thermally polymerized via a free radical process and the emission, lifetime, and anisotropy followed with time. The anisotropy measurements serve as a good proxy for the gradually increasing rigidity which occurs during the polymerization process.16 The experiment was repeated three times with essentially identical results, shown in Table 3. As the results plotted as normalized relative changes in Figure 2 show, all three quantities track synchronously. These results suggest that variation in rigidity is the major driver behind the observed emission λmax blue shift and the changes in the lifetime. It also appears that variation in the local environment of the complexes as measured by Δτ/τM, where Δτ is the difference between the maximum and minimum value of components needed to fit the lifetime decays, decreases as the degree of polymerization increases. This is somewhat surprising, as even though in the initial stages of polymerization there is a large spread of molecular weights and hence significant local variation in rigidity, segmental motion, R-relaxation, and pendent motion,

β-relaxation should provide an averaging effect on the environment experienced by the complex molecules. However, it may be that the averaging effect due to chain and pendent motion is insufficient to offset the variation in environments engendered by the spread in chain length and local viscosity. At the completion of the polymerization, the τM is significantly longer than in fluid media. This is likely due to the rigid environment’s ability to shut off nonradiative processes, which depopulate the excited state. At this point, the sample is a glassy solid and both R and β relaxation processes are quenched on the time scale of the emission decays and there is no averaging of the local environments experienced by the complexes. Despite the fact that PMMA is largely amorphous and a homopolymer, there appears to be significant differences in sites within the host.17 Results from the polymerization experiments suggested that rigidity, with its attendant variation in site environment, was the principal cause of the changes in photophysical properties of our embedded complexes. We next examined the changes in photophysical behavior for our suite in a series of polymers with similar structure and polarity, but a range of Tg values. All the polymers had an ester group pendent to a rather simple backbone. Varying the alkyl group of the ester and on the backbone allowed a smooth variation in the Tg’s. To validate our comparisons from this set of survey experiments, we examined and compared the absorption and excitation 9571

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Table 4. Comparison of Embedded versus Tethered Re_2 in PVP

Figure 3. Fractional change in emission λmax with polymer Tg: Re_0 (b), Re_1 (9), Re_2 (2), Re_3 ([), and Re_4 (f).

ratio spectra for our complex/polymer films in the spectral region for the MLCT transition used for excitation. Representative comparisons are provided in the Supporting Information (Figure S1A
D). Several important points emerged. For a particular complex, the spread in the MLCT absorption maxima with our suite of polymers was less than 5 nm. Thus, the MLCT absorption is not particularly sensitive to the local environment. There is acceptable qualitative agreement between the absorption and excitation spectra suggesting that the absorbing and emitting species are the same. Further, we examined the anisotropy spectra of the films. These spectra all showed a common qualitative appearance. The spectra are consistent with the absorbing and emitting species being the same over the MLCT region used for excitation. The consistency and high r values obtained mean that the absorbing and emitting species share a common orientation of the transition dipole. As expected, there were differences in the limiting r value depending on the Tg of the polymer. In all cases, the r value drops to near zero or negative at short wavelengths where excitation involves the ligand π f π* transitions. The limiting r values also strongly suggest that neither energy transfer nor macroaggregation are issues in our film measurements. Using ethyl acetate (same polarity, no rigidity) as our basis of comparison, Table 1 shows that all our complexes showed a strong blue shift when placed in rigid environment. The shift ranged from ∼600 cm
1 to over 1700 cm
1, with an average of ∼1370 cm
1. The fractional blue shift, calculated using cm
1, shows a consistent correlation with the Tg of the polymer hosts. This is visually illustrated for the various complexes in Figure 3. When the Tg is well above ambient temperature, there is little variation in the fractional blue shift with Tg. However, as ambient and Tg approach each other, modest changes are observed. Finally, when the Tg is well below ambient and the polymer is rubbery, the emission maxima approach the fluid values. While the onset of the decrease in the λmax shift is similar for all the complexes, as Tg decreases there is a spread of responses by the suite with some complexes much more sensitive to the decrease in rigidity. When Tg is below ambient, both true segmental, Rrelaxation, and pendent group, β-relaxation, motions are possible with a time scale for the motions which depends on both the polymer structure and the ambient temperature relative to polymer’s Tg. It is interesting that the complex with the longest excited

binding mode

emission λmax (nm)

τM (μs)

Δτ (μs)

embedded

515

4.21

5.16

tethered

518

4.55

4.18

state lifetime, Re_1, shows the most rapid loss of the rigidochromic shift. This suggests that as the characteristic times for the R and β motions decrease, they become similar to the decay time of Re_1. Hence, this complex sees some averaging of its environment, not unlike that experienced in solution. The variation in mean lifetime with changes in Tg follows the same general trend as seen for the spectra shifts (Table 2). There is a reasonably consistent decline in the τM for all of our complexes as the polymer’s Tg decreases. As with the spectral shifts, the τM value in very rubbery hosts is similar to that observed in solution with one caveat. There continues to be a spread of lifetime components even with the lowest Tg values used. Thus, it appears that the lifetime is more sensitive to site variation or rigidity effects than the spectral shift. All our complexes required multiple exponential functions to fit the emission decays observed in all polymer hosts. While we refrain from trying to assign physical significance to the individual extracted lifetime components, it seems clear that the complexes find themselves in various, subtly different environments which likely affect the decay rates, primarily the nonradiative processes. While our fitting process provides two or three distinct lifetime components, we doubt if this is actually the case. Rather, we believe that there is a spectrum of lifetimes, whose weighted aggregates can be adequately fit with our two or three term expression. However, it does seem reasonable that the spread of the individual lifetimes required for an adequate fit is a good proxy for the spread or range of the physical environments encountered by the complexes in the polymer host. Our results suggest that even in the most flexible polymers in which both segmental and pendent motion are active, on the time scale of the complexes’ excited state decay, the individual complex molecules do experience different environments and not the full averaging of the environment as seen in fluids.18 While the polymers’ Tg is a major factor in the variation of emission characteristics, the mean lifetime is somewhat sensitive to variation in the actual structure of both the complex and the polymer. For example, some complexes’ τM actually increases on going from PMMA to PiBMA (Tg = 99 and 65 °C, respectively). Interestingly, tacticity did not seem to be an important factor, as our results for isotactic PMMA and atactic PMMA were virtually identical. Since it was clear that site variation with rigidity was the main cause of the heterogeneous photophysical properties observed for our complexes embedded in polymer hosts, we wondered if tethering the complex to the actual polymer backbone might provide a more uniform environment for our complexes and hence reduce the magnitude of the changes observed for traditional embedded systems. As Table 4 shows, neither the spectral shift nor the lifetime were significantly altered by tethering the complex to the polymer host. There is some modest improvement in the spread of components required for a good fit in the tethered material, but the lifetime is still clearly multiexponential. Our results are similar to those obtained by others using different metal complexes.19 Several different schemes, including use of a 9572

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Figure 4. TRES spectra: (A) synthetic mixture of Re_2 and Re_4 in purged ethoxyethanol, (B) Re_5 in PMMA, (C) Re_3 in PMA, and (D) Re_3 in PnBMA.

true macromolecular complex, have been tried to avoid site heterogeneity with varying success. However, in most cases, the lifetimes, in particular, remain stubbornly multicomponent. For our suite of complexes, both the emission λmax and the lifetime are altered when embedded in a polymer film. We wanted to determine if the two changes were coupled and to see if we could tease out information about the microscopic origin of the observed changes. We chose as our tool timeresolved emission spectroscopy, TRES. In these experiments, a series of complete emission decays was obtained at different wavelengths within the emission envelope. This data was then parsed to obtain a series of emission spectra for different times during the decay.20 If the signature spectra for particular types of sites are different and these also exhibit different emission decay rates, then the composite observed emission spectra will vary depending on the time of observation. To test the technique, we first made a synthetic solution mixture of two complexes with known, but modest, differences in both emission λmax and τ’s. The TRES spectra for this mixture is show in Figure 4A. As can be seen, there is a blue shift in the spectra with time and it is clear that there is more than one component in the mixture. Since the spread in the lifetime components of our complexes embedded in polymer films was usually greater than 2, we were hopeful that we could see the time evolution of the spectra if the emission λmax and the lifetime are indeed correlated. Figure 4B
D shows typical results from TRES spectroscopy of our complexes in various polymer films. Though the magnitudes of the spectral shifts differ among the various complexes and to some extent among the polymers, the same general features were observed for all our experiments. All the emission spectra blue shift with increasing time. These results seem quite reasonable given our observations above that our complexes respond to increasing rigidity with both a blue spectral shift and

longer lifetime. It is interesting that these results are just the opposite of those observed for a number of other dyes in fluid solution in which polarity changes occur. In these cases, the more polar environment usually results in a red emission shift.21 Given that we believe there is a spectrum of sites for the complexes in the polymers with varying degrees of rigidity and that greater rigidity would be expected to elicit both a blue-shifted emission and a longer decay lifetime, our results confirm that the two quantities are correlated and suggests that they have their mutual origin in the constraint provided by the polymer matrix. There is an alternative explanation which cannot be completely eliminated, however. While we are confident that there are no macroscopic (>1 μm) aggregates in our films, our optical resolution can not exclude the presence of nanoscale aggregates. Given the diversity of both our complexes and polymers, it would be surprising that all the various combinations exhibited nanoscale aggregation; however, we cannot prove this is not the case. As shown above, aggregates of all our complexes also exhibit blue-shifted emission, though the lifetimes are not as long as those exhibited by complexes embedded in polymer hosts. This latter point also argues against nanoscale aggregates as the cause. Our results suggest that the values for τM may be dependent on the monitored λ. This was indeed found to be the case. For example, the observed τM's for Re_1 in PnBMA using 490 and 570 nm differed by ∼2, with the 490 nm value being longer. As expected, the relative differences in τM's tracked the magnitude of the spectral shift with time in the TRES spectra. It is interesting that all these effects persist even in polymer hosts whose Tg is well below ambient and whose properties are rubbery. Thus, the extensive literature regarding anomalous photophysical behavior and quenching for Ru(II) complexes in various polysilanes, whose Tg’s are well below ambient, is quite consistent with our observations.22 While we cannot pinpoint the microscopic mechanism of the rigidity effect, it does appear that 9573

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Figure 5. Confocal emission spectra for Re_3 in PMMA at various Z axis positions.

it will be very difficult to eliminate. The use of phase shift lifetime determinations circumvents the multiple lifetime issue and for routine sensor design may be the better approach. One possible explanation for our observations would be the presence of domains of different polymer packing within the polymer film. These might range from completely amorphous to quasi-crystalline. One would expect a variation in density for these domains and hence variation in the level of rigidity felt by embedded complex molecules. An issue is the size of such domains. Thus, the fact that such domains are not observed may simply connote inadequate resolution. We examined several of our films using confocal microscopy with true spectra imaging detection. Thus, we could obtain an emission spectrum for a spatially well-defined region of our polymer film, with control over all three coordinates. The samples were excited with 405 nm excitation and emission collection using a 40 objective, with 0.1 μm resolution. We estimated from our illuminated spot size, objective NA, and objective distance that the majority of our collected emission came from an area ∼1 μm in diameter. The slides chosen were ones that exhibited the greatest shifts in the TRES spectra. While we did observe minor variation of the spectra with randomly selected positions, changing all three axes but keeping our sampled volume within the bulk of the films, these variations were not sufficient to give confidence that real domains were detected. Our Z-stacks for the various films did have a common feature, and a typical result is shown in Figure 5. The spectrum of the composite at the polymer/air interface is slightly blue-shifted relative to that observed in the bulk of the film or even at the polymer/glass interface. It is possible that since this is the surface from which evaporation occurs during the casting process that the concentration of complex is higher here and this may allow some aggregation . However, the size of the aggregates must be below our detection ability, that is, sub-micrometer

’ SUMMARY We have examined some possible causes for the altered photophysics observed when luminescent metal complexes are embedded in polymer hosts. Absent the trivial problems of gross aggregation from complex/host incompatibility, cracks and fissures, and phase boundary effects, there is still often considerable alteration of the luminescence observed when the complex is

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placed in a polymer host. In a diverse suite of Re(I) complexes typical of those used for sensor fabrication, the primary cause of these effects appears due to rigidity or constraint of the complex by the polymer host. This effect is observed for glassy hosts, but also even when the polymer host is quite rubbery the effect is strong. Both the emission maxima and lifetimes are altered; but the lifetimes appear more sensitive to the interaction with the polymer host. How the complex is held in the matrix does not seem to alter the impact very greatly as embedded and chemically tethered complexes show similar results. TRES results strongly suggest that the observed changes in emission color and lifetime are responding to the same driver. The microscopic details of the complex/polymer interaction remain obscure, as confocal microscopy did not provide clear evidence for sub-micrometer domains of altered packing, which might give rise to our observations. Also, one cannot rule out the presence of nanoscale aggregates as the cause of the alteration, though the lifetime results seem to argue against this possibility. The persistence of the altered photophysics even in very rubbery supports and the apparent failure of tethering or other approaches to improve site uniformity to remediate the problem make it appear that this effect or problem does not have an easy solution for many of the luminescent complex/polymer support systems that are good candidates for sensor use.

’ ASSOCIATED CONTENT

bS

Supporting Information. Characterization data for ligands and complexes; representative comparisons for film absorption and excitation ratio spectra, representative anisotropy spectra, and air saturated solution lifetime data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT It is a pleasure to acknowledge the assistance of the North Carolina State University Chemistry Department’s Mass Spectroscopy Facility in obtaining the ES/MS data for our complexes. S.A.M. and B.A.D.G. wish to acknowledge the Dreyfus Foundation through its Senior Scientist Mentoring Grants Program for valuable support of this research. J.N.D. wishes to acknowledge the National Science Foundation (Grant CHE-04-10061) for partial support of this work. ’ REFERENCES (1) (a) Zhao, Q.; Li, F.; Huang, C. Chem. Soc. Rev. 2010, 39, 3007– 3030. (b) Keefe, M. H.; Benkstein, K. D.; Hupp, J. T. Coord. Chem. Rev. 2000, 205, 201–228. (c) Demas, J. N.; DeGraff, B. A. Coord. Chem. Rev. 2001, 211, 317–351. (d) Wong, K. M. C.; Yam, V. W. W. Coord. Chem. Rev. 2007, 251, 2477–2488. (2) Narayanswamy, R; Wolfbeis, O. S. Optical sensors: Industrial, Environmental, and Diagnostic Applications; Springer-Verlag: BerlinHeidelberg, 2004. (3) (a) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, R. J. Anal. Chem. 1991, 63, 337–342. (b) Sacksteder, L.; Demas, J. N.; DeGraff, B. A.; Bacon, R. J. Anal. Chem. 1993, 65, 3480–3483. (c) Degraff, B. A.; Demas, J. N. In Reviews in Fluorescence; Geddes, C., Lakowicz, J. R., Eds.; Springer-Science: New York, 2005; Vol. 2, pp 125
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