Polarized Normal-Incidence Cavity Ring-Down Spectroscopy: Probing

Jan 6, 2011 - Polarized Normal-Incidence Cavity Ring-Down Spectroscopy: ... ring-down spectroscopy technique (polarized NICRDS) is described .... Don'...
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J. Phys. Chem. A 2011, 115, 410–418

Polarized Normal-Incidence Cavity Ring-Down Spectroscopy: Probing Spiropyran Photochromism in Thin PMMA Films and Toluene Jordan H. Mantha, Ali I. Ismail, and Joseph I. Cline* Department of Chemistry, UniVersity of NeVada, Reno, NeVada 89557-0216, United States ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: NoVember 6, 2010

A normal-incidence geometry, polarization-resolved cavity ring-down spectroscopy technique (polarized NICRDS) is described to probe the polarized absorbance of surface-adsorbed thin films and short-path length liquid samples. The technique is demonstrated by a kinetic study of the photochromic behavior of the spiropyran dye 6,8-dibromo-1′, 3′-dihydro-1′, 3′, 3′-trimethylspiro[2H-1-benzopyran-2, 2′-(2H)-indole]. The technique is shown potentially to have monolayer coverage sensitivity and can measure the angular orientation distribution of analyte molecules. The photochromic kinetics of 6,8-dibromoBIPS in toluene solution were qualitatively consistent with a previous study of this molecule using conventional absorption spectroscopy. The absorption polarizations and slow ring-closing kinetics measured in a thin poly(methyl methacrylate) film are consistent with a strong interaction of the spiropyran and merocyanine forms with the polymer matrix. 1. Introduction The intense current interest in molecular devices is driving the development of experimental techniques to probe their properties and operation. Polarized spectroscopies are especially helpful as they probe the spatial orientation and motion of nanomachinery, which is critical to understanding the practicality of molecular motors, sensors, and nanoscale fluidic devices. Polarized fluorescence detection is a highly sensitive probe. Ideally, fluorescence is a “zero-background” technique. A significant disadvantage of fluorescence techniques is their lack of generality: the molecules of interest must either fluoresce or have fluorophores chemically attached. To probe molecular motors and rotors where, by design, energy is directed into photoisomerization rather than fluorescence, fluorescence may not be practical. In contrast, absorption techniques are more universal in terms of the molecular species that may be probed. Absorption is ordinarily a single-photon process, whereas fluorescence typically involves two photons (absorption and emission). The single-photon nature of absorbance simplifies the extraction of spatial orientation information. A distinct disadvantage of absorption is that it requires measuring small changes in a relatively large signal, giving rise to a relative lack of sensitivity. In this Article, we demonstrate the use of cavity ring-down spectroscopy to probe the actuation of a thin layer of surfaceadsorbed molecular rotors. We show that the technique is sufficiently sensitive to probe strongly absorbing rotors at monolayer coverages. Cavity ring-down spectroscopy (CRDS) is an ultra-sensitive absorption technique that has been developed over the last two decades.1-4 It utilizes a high-finesse optical cavity to make multipass (often thousands of round-trips) absorption measurements. Instead of light intensity, the decay lifetime of photons trapped in the optical cavity is measured to obtain absorption losses. Cavity ring-down spectroscopy is one of several multipass absorption techniques. White5 and Herriott6 cells are traditional multipass methods that allow for absorption measurements with path lengths of up to a few hundred meters. These * To whom correspondence should be addressed. E-mail: [email protected].

techniques and their many variations are still in active use today.7 The cavity ring-down technique provides over 2 orders of magnitude longer path lengths than do typical Herriott cells and avoids the amplitude noise in the light source.1 Although CRDS was initially used for gas-phase absorption measurements, it has more recently been extended to liquid and solid phases for such applications as HPLC detection,8,9 solid-liquid interface investigation,10 and the study of thin films.11 A variety of cavity and sample geometries have been employed for the study of substrate adsorbed films. Pipino et al.12 used a monolithic total-internal reflection cavity geometry, and Shaw et al. used a dove prism within the ring-down cavity to measure evanescent-wave absorption.10 Muir and Alexander used a glass substrate at Brewster’s angle with respect to the longitudinal axis of the cavity in their investigation of absorption of light transmitted through a thin film.11 Beyond simple absorption measurements, CRDS has also been used for polarization-dependent absorption measurements. Englen et al. introduced polarization-dependent CRDS by studying resonant magneto-optical rotation of gas-phase molecules.13 Mu¨ller et al. used λ/4 waveplates inserted within the ring-down cavity to probe circular dichroism and birefringence in gas-phase molecules.14 Here, we introduce a polarized, normal-incidence surface absorption cavity ring-down spectroscopy, hereafter abbreviated NICRDS. In NICRDS, the substrate surface is oriented normal to the longitudinal axis of the ring-down cavity, permitting simultaneous detection of absorbance changes for multiple probe polarizations. This readily allows, for example, the detection of molecular reorientation dynamics in which the distribution of molecular transition dipoles changes in time as we demonstrate in ref 15. In the present experiments, the light exiting the CRDS cavity was split into orthogonal horizontal and vertical linear polarization components. A Brewster’s angle CRDS sample geometry, such as that used by Muir and Alexander,11 only allows measurement of linear, p-polarized light absorption. The decay lifetime, τ, of a light pulse in a CRDS cavity containing a sample adsorbed to an optically transmissive substrate is16

10.1021/jp105474c  2011 American Chemical Society Published on Web 01/06/2011

Polarized Normal-Incidence CRDS

τ)

l c[(1 - R) + δc + R]

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(1)

where l is the cavity length, c is the speed of light, R is the reflectivity of the cavity mirrors, δc is the intrinsic cavity loss arising from the experimental setup, and R is the absorption loss per pass due to the surfaced-adsorbed analyte at the probe wavelength. Intrinsic cavity loss mechanisms include Rayleigh scattering of the air within the cavity as well as reflection and absorption of light by the sample substrate. If the analyte absorbance, R, changes, for example, due to an externally driven process, then the change in absorption per pass, a ) ∆R, can be related to a change in ring-down lifetime according to11

a)

( )

l τb - τ c τbτ

(2)

where τb is the “blank” lifetime measured before the process of interest has occurred. In this work, the absorbance change arises from photoisomerization of a surface-adsorbed molecule. As a demonstration of polarization-resolved NICRDS, the power dependence and photoisomerization kinetics of 6,8dibromo-1′, 3′-dihydro-1′, 3′, 3′-trimethylspiro[2H-1-benzopyran-2, 2′-(2H)-indole] (hereafter referred to as “6,8-dibromoBIPS”, see Figure 1) in a poly(methyl methacrylate) (PMMA) thin film are studied using the polarized CRDS method. Photochromic dyes are an extensively studied class of compound that have data storage and molecular switching applications. Their interest primarily arises from the sometimes dramatic photoinduced absorption shift. The synthesis of photochromic spiro[2H-1-benzopyran-2, 2′-indolines] (BIPS) as well as kinetics in the solution phase have been studied by Chibisov and Go¨rner.17 In BIPS dyes, the most stable isomer is the closed, spiropyran, form, which absorbs in the ultraviolet region (large absorption band at 350 nm).17 Upon photoisomerization, the molecule undergoes a ring-opening reaction in which the C-O bond is broken. The trans form of the open isomer (a merocyanine) has two absorption bands (λmax at 585 nm in ethanol) in the visible region. The open isomer thermally relaxes back to the closed spiropyran isomer at a rate strongly dependent upon solvent. Chibisov and Go¨rner reported a photoisomerization quantum yield of 0.07 and relaxation rate constant of 0.3 s-1 for 6,8-dibromoBIPS in toluene at 25 °C.17 2. Experimental Methods 2.1. Sample and Substrate Preparation. In these experiments, two different sample configurations were used. For liquid samples, a commercial flow cell (Starna Cells Type 48) with a 0.1 mm path length and a 30 µL sample volume was used. The flow cell has two relatively thick fused silica windows on either side of a liquid sample for a total thickness of 2.6 mm. A 230 µM solution of 6,8-dibromoBIPS dye (6,8-dibromo-1′, 3′dihydro-1′, 3′, 3′-trimethylspiro[2H-1-benzopyran-2, 2′-(2H)indole], Sigma-Aldrich, CAS: 20200-62-8) in toluene (SigmaAldrich, CHROMASOLV HPLC-grade) was injected into the flow cell using a syringe. The cell was flushed with toluene solvent three times between experiments to ensure that all the dye was removed from the cell. The sample substrate used for polymer film experiments was a 1 mm thick fused silica window (25.4 mm diameter, λ/10, 10-5 scratch-dig, e5 min wedge, CVI Laser PW1-1004-UV). The substrates were cleaned thoroughly by sonication sequen-

Figure 1. Photoisomerization of 6,8-dibromoBIPS. The closed, spiropyran, isomer on the left photoisomerizes upon UV irradiation (λ < 350 nm) to form the open, trans-merocyanine, isomer on the right.17 The merocyanine form absorbs at visible wavelengths and is probed by absorption at 620 nm in this study.

tially in toluene, methanol, and distilled water. If sample coatings were of poor quality (as evidenced by a short cavity lifetime and/or visible film defects such as dust or microcrystals of dye), the substrates were recleaned by an additional immersion in an aqua regia solution (3:1 HCl:HNO3) before being rinsed with distilled water. After being cleaned, substrates were allowed to dry in a 90 °C oven for more than 1 h. Surface-adsorbed films consisted of 6,8-dibromoBIPS in poly(methyl methacrylate) (PMMA). The spin-casting solution was composed of 1 mg/mL PMMA (Aldrich, avg. MW 120 000, used without purification) in toluene. The photochromic dye was added to the polymer solution to give a 10:1 PMMA:dye (by weight) casting solution. A spin coater (Laurell WS-400A6NPP/LITE) was used to spin-cast the solution (∼50 µL at 1500 rpm for 1 min.) and create a thin layer of polymer with embedded photochromic dye on the substrate. The solvent was allowed to fully evaporate from the spin-coated samples for 5-10 min before insertion into the ring-down cavity. Comparison to previous PMMA film thickness studies with similar substrates, PMMA molecular weight, spin speed, solvent, and concentration predicts that the film thickness in these experiments was between 10 and 50 nm.18 A freshly cleaned and coated substrate was used for each experiment to minimize systematic errors due to repetitive use of the same sample. 2.2. Ring-Down Cavity Design and Alignment. The substrate-adsorbed analyte film (or cuvette for liquid studies) is located at the center of the ring-down cavity and oriented normal to the cavity’s longitudinal axis. The experiments were of the generic “pump-probe” type. The pump or “drive” laser pulses in the UV lead to photoisomerization: ring-opening of the spiropyran form of the 6,8-dibromoBIPS dye. The 620 nm “probe” laser pulses are injected into the CRDS cavity to measure the absorption change. The two pulses are spatially overlapped at the adsorbed film on the substrate surface (or at the center of the cuvette). A schematic diagram of the experimental apparatus is shown in Figure 2. The ring-down cavity was composed of two highquality mirrors with a broadband antireflection coating (Newport 10CM00SR.30F) that provides a reflectivity of >99.97% in the 583-663 nm range. The mirrors had a 1 m radius of curvature and were set 0.75 m apart on an optical rail. The 620 nm probe pulse was generated by a dye laser pumped by a frequencydoubled Nd:YAG laser (Spectra Physics GCR-4, 10 Hz). The 620 nm probe beam was mode matched to the CRDS cavity using a spatial filter and lens. The spatial filter was composed of two 100 mm focal length lenses. The probe beam was focused by the first lens through a 35 µm diamond pinhole (Ft. Wayne Wire Dye, Inc.) to block higher-order transverse modes allowing the TEM00 mode to pass through. The second lens recollimated the filtered probe beam. A third lens of focal length 1 m, placed after the spatial filter such that its focus was located at the center of the optical cavity, was used to more efficiently couple the probe beam into the cavity. A λ/2 waveplate was used to rotate the polarization of the probe beam

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Figure 2. Schematic of the NICRDS apparatus. Before the probe beam (red) enters the ring-down cavity, it passes through a series of optics to minimize high-order transverse laser modes, optimize coupling into the cavity, and set a well-defined probe polarization at a 45° with respect to the vertical and horizontal laboratory axes. The probe light exiting the cavity is split by a Glan prism into its horizontal component (detected by PMT1) and its vertical component (detected by PMT2). The drive beam (blue) is directed toward the sample and away from the detectors at a 7° angle with respect to the sample surface normal. A photoelastic modulator (PEM) or λ/2 waveplate is used to set the drive beam polarization.

to the desired angle (45° with respect to horizontal and vertical lab-frame directions) before entering the ring-down cavity. The probe light exiting the cavity was directed through a Glan laser polarizing prism to spatially separate the beam into horizontal (H) and vertical (V) linear polarization components. The H and V components were simultaneously detected by two photomultiplier tubes (Hamamatsu R928). Two different drive wavelengths were used. For the PMMA film studies, the drive wavelength was 266 nm, generated as the fourth harmonic of the Nd:YAG laser. For studies in liquid toluene, the drive wavelength was 310 nm, generated as the second harmonic of probe pulse. The 310 nm drive wavelength was used in the toluene studies because our experiments showed that absorption of laser light at 266 nm by the toluene sensitized the photochromic reaction. (The near-UV cutoff for absorption by liquid toluene is ∼305 nm.) To excite the sample, the UV (266 or 310 nm) drive pulse was first directed through a λ/2 waveplate and polarizing prism to both attenuate and set the drive polarization. In some experiments, a pair of polarizing prisms acting as an attenuator and a photoelastic modulator was used to rotate the polarization to the desired angle. After attenuation and polarization selection, the drive laser beam was directed nearly collinear (∼7° away from normal) but counter-propagating to the direction of the probe beam to minimize drive beam scatter into the collection optics. The drive beam power was measured between the polarization selection optics and the final turning prism before the substrate. For drive beam alignment, a 3 mm iris placed in the sample holder was used to ensure that both the probe and the drive beams overlap on the sample surface. The diameter of the probe beam on the sample surface is less than 3 mm. The diameter of the drive beam was 3 mm for experiments on PMMA films and 8 mm for experiments in the toluene-filled cuvette. Alignment of the CRDS cavity, and especially of the substrate placed in the cavity, requires care and was a highly sensitive process. Initially, the laser beam was adjusted so that it was parallel (in both horizontal and vertical dimensions) to the optical rail on which the mirrors are to be mounted. The “empty” cavity (without a substrate or cuvette) was aligned by first inserting the mirror closest to the detectors and overlapping the back-reflection of the probe beam with the incident beam. The

Mantha et al. second mirror was then inserted and also aligned such that its back-reflection overlapped the incident probe. Small adjustments to the mirror alignment were then made to achieve the maximum ring-down lifetime. Ring-down lifetimes for the empty cavity after such alignment were 15-18 µs. The substrate or cuvette was then inserted at the center of the cavity. The sample holder was constructed such that all translational and rotational degrees of freedom could be controlled by a combination of three translation stages and two goniometers stacked on a rotation stage. The two goniometers were chosen and arranged such that they provided orthogonal rotation axes, and the virtual point of their rotation was approximately in the center of the sample. The ring-down lifetime is relatively insensitive to translation along the longitudinal axis of the cavity and azimuthal rotation about this axis. The cavity lifetime is extremely sensitive to “pitch“ and “yaw” rotations about axes orthogonal to the cavity. This is expected because misalignment in these directions leads to back-reflections that are directed away from the cavity axis. During alignment, the substrate initially is aligned coarsely by hand to be approximately normal to the cavity axis. Micrometer-controlled rotary adjustments are then made in the pitch and yaw directions. The sample substrate acts as an etalon: as the substrate is rotated about an axis orthogonal to the cavity axis, sharp peaks in the ring-down lifetime appear periodically as a function of rotation angle. The center of symmetry of the peaks in ring-down lifetime observed as a function of rotation angle indicates the optimal position about that axis. This optimal position actually may have a very short ring-down lifetime until the orthogonal rotary degree of freedom is explored. The global optimized geometry may be found by iteratively finding the center of symmetry as measured by the cavity lifetime versus rotation angle. Once the pitch and yaw have been optimized, then the other degrees of freedom were optimized. Insertion of a clean, uncoated substrate gave ring-down lifetimes of 8-10 µs. The “blank” lifetime, τb, is the ring-down lifetime prior to irradiation of analyte by the drive beam. Typical “blank” ring-down lifetimes for a substrate coated with a PMMA/dye film were τb ) 3-7 µs. For liquid samples, the typical “blank” ring-down decreased to τb ) 2 µs, primarily due to the increased thickness of two windows and additional interfacial reflections. 2.3. Data Collection and Analysis. In an experiment, the sample absorbance was monitored prior to drive beam irradiation and through subsequent on/off cycles of drive beam irradiation. The data are collected synchronously with the 10 Hz repetition rate of the Nd:YAG pump laser; each drive and probe pulse pair occurs at a time t that is recorded during data collection. Cavity ring-down signals are collected at each laser shot to obtain the absorbance changes a(V, t) and a(H, t) in response to the drive irradiation. For each laser shot, the data collection event consists of measuring the two cavity ring-down decay transients for V- and H-polarized probe light. The probe light exiting the detectorside end-mirror of the CRDS cavity was directed through a Glan laser prism to separate the vertical and horizontal polarization components and then onto separate photomultiplier tubes. The CRDS decay signals were recorded on a digital oscilloscope (Lecroy 9361, 2.5 GS/s sampling rate) triggered by a photodiode monitoring the output of the Nd:YAG pump laser. A custom data acquisition computer program stored the two CRDS decay transients measured for each laser pulse. The time-series of transients recorded for each polarization were postprocessed by binning into sequential groups. Each

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group was averaged, to represent the ring-down decay at time t during the full experiment. This average transient is then fit using a Levenberg-Marquardt nonlinear least-squares regression to a single exponential decay to obtain a ring-down lifetime, τ. Using the fitted lifetimes for the V and H probe polarization signals at time t, the time-dependent absorbances a(V, t) and a(H, t) are obtained using eq 2. The averaging bin size for liquid samples was 10 shots and for PMMA films was 100 shots. At the 10 Hz repetition rate of the laser, this corresponds to a time resolution of ∆t ) 1 and 10 s for liquid and film samples, respectively. The probe laser polarization injected into the cavity is at 45° with respect to the laboratory-frame V and H directions. In an empty ring-down cavity (no sample substrate or cuvette), we observe that the polarization state of the detected ring-down light preserves the input polarization. When a sample substrate or cuvette is inserted into the cavity, some high-frequency modebeating is observed in the detected ring-down light such that the V and H signals are out of phase. When this “polarization” mode-beating is present, adding the V and H signals destroys the interference so that the sum shows smooth, exponential ringdown decay. In setting up an experiment, the sample is aligned to minimize polarization mode-beating. This polarization mode-beating apparently arises from slight birefringence or angular misalignments of the sample or substrate. Because the trapped probe light is initially at 45° with respect to the detector polarizations, there is a coherence between the V and H components of the detected signals. Birefringence or angular misalignments that mix the two polarization states create interferences that are detected as high-frequency oscillations in the polarizationresolved ring-down decay transients. The PMMA film may have birefringence resulting from anisotropies in the spin-coating process. To minimize these effects, measurements are made near the radial center of the spin-coated film. The fused silica substrate can have straininduced birefringence of the window material, which can observed as changes in mode-beating as a function of displacement when an uncoated substrate window is translated through the centerline of the ring-down cavity. For isotropic liquid, any mode-beating presumably must arise from misalignment of or strain birefringence in the cuvette. The liquid sample experiments show no probe polarization dependence, and in these experiments the V and H ring-down signals are added for simplicity and to remove any mode-beating artifacts. In any case, when the apparatus is properly aligned, the polarization modebeating is small in amplitude, so that both the V and the H ringdown signals can be reliably fit to a single-exponential decay. 3. Results 3.1. Sensitivity. As discussed in section 1, the principal advantage of the CRDS absorption technique is its relatively high sensitivity. Here, we estimate the minimum detectable absorbance and surface concentration of our NICRDS apparatus to evaluate its promise for future thin film probing studies and for comparison with other techniques. For a CRDS instrument, the minimum detectable absorbance/ pass is

amin )

(

∆τmin l c τ2 - τ ∆τ b b min

)

(3)

where ∆τmin is the minimum detectable change in lifetime. For the current apparatus, a clean substrate gave a τb of 10 µs and

Figure 3. NICRDS lifetime (a) and time-dependent absorbance loss/ pass (b) measured at 620 nm for a 230 µM solution of 6,8-dibromoBIPS in toluene at 23 °C in a 0.1 mm path length cell. The solution is irradiated by the 310 nm drive beam during the time intervals 0 < t < 60 and 300 s < t < 360 s. The drive photon flux is F ) 2.5 × 1016 s-1 cm-2. The red line in (b) is a fit to the kinetic model in eq 6.

∆τmin of 0.37 µs. This gives a minimum detectable absorbance/ pass of 10 ppm. This sensitivity is similar to that of Bechtel et al., for instance, who used a specially designed Brewster-angle flow cell for liquid-phase CRDS experiments.8,19 A surface concentration, cs, can also be calculated by:

cs )

a εs ln 10

(4)

where εs is the “surface” molar absorption coefficient (approximately 2000 m2/mol for the 6,8-dibromoBIPS open merocyanine isomer at 620 nm). The surface concentration calculated in this way using amin gives a minimum detectable surface concentration of cs ) 755 nm2/molecule. Approximating the surface area occupied by a single 6,8-dibromoBIPS molecule as 1 nm2/molecule, the minimum detectable surface concentration is less than one monolayer. 3.2. Toluene Solution Photoisomerization Kinetics. Results from the NICRDS technique can be compared to the measurements of Chibisov and Go¨rner, who reported the photochromic behavior of several BIPS compounds, including 6,8-dibromoBIPS, in liquid solvents.17 Figure 3 shows two successive ringopening/ring-closing cycles for 6,8-dibromoBIPS in toluene as measured by NICRDS. Prior to the 310 nm drive irradiation at t ) 0, the 6,8-dibromoBIPS molecules are overwhelmingly in the closed spiropyran isomer form, and there is relatively little 620 nm absorption. The background lifetime, τb, is computed as the average lifetime for t < 0 (approximately 1.9 µs in Figure 3a). The corresponding value of a, defined in eq 2, is a = 0 for t < 0. Between 0 < t < 60 s, the sample film was continuously exposed to 310 nm drive laser pulses at 10 Hz. During this

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interval, the 6,8-dibromoBIPS molecules photoisomerize to the open, merocyanine, form, which absorbs at 620 nm. Figure 3a shows that, upon drive irradiation at t ) 0, the ring-down lifetime decreases due to an increase in absorption losses in the film. Figure 3b shows that the corresponding absorbance loss per pass due to the photoisomerization, a(t), increases rapidly to a maximum at about t ) 30 s, and then decreases slightly. At 60 s < t < 300 s, the drive beam is blocked, and the absorbance rapidly decreases as the merocyanine form of 6,8dibromoBIPS thermally isomerizes back to the closed spiropyran form. By t ≈ 120 s, the cavity lifetime and absorbance change have returned to τ ) τb and a ) 0, respectively. Another 60 s drive cycle is begun at t ) 300 s. The absorption maximum of the second drive cycle is only 88% of the maximum of the first cycle. Chibisov and Go¨rner studied the photoisomerization of 6,8dibromoBIPS in various liquids, assuming a simple reversible photoisomerization reaction: k1

A y\z B

(5)

k-1

where A and B represent the closed spiropyran and open merocyanine isomers, respectively. The apparent photoinduced ring-opening rate constant, k1, depends upon both the photon flux, F, of the “drive” UV light and the photoisomerization quantum yield, φ. Chibisov and Go¨rner reported the thermal ring-closing “relaxation” rate constant, k-1, and φ for 6,8dibromoBIPS in various solvents. The kinetic model in eq 5 predicts that the 620 nm absorbance change, a, due to formation of the open merocyanine isomer B will monotonically increase, asymptotically approaching a photostationary plateau at long drive irradiation times. To account for the lack of a photostationary equilibrium of A and B observed in Figure 3, we hypothesize that B also can undergo an irreversible “decomposition” to form a product D, which does not absorb at the 620 nm probe wavelength. The kinetic model becomes k1

k2

A y\z B 98 D

(6)

k-1

The apparent rate constant k1 depends upon the photon flux of the drive beam, F. More fundamentally interesting is the corresponding cross-section for photoisomerization, σ1, defined as

k1 ) σ1F

(7)

Figure 4 shows NICRDS absorbance probing of five independent irradiation-relaxation experiments conducted for different drive beam pulse energies corresponding to 310 nm photon fluxes ranging from F ) 2.08 × 1015 to 1.25 × 1016 s-1 cm-2. At the lowest drive, photon flux k2 is small, and a relative photostationary plateau is observed at long irradiation times. As the drive pulse energy is increased, the apparent photoisomerization rate increases, and the absorbance shows a sharper peak and faster subsequent decay of the absorption during prolonged irradiation. Within this kinetic model, the faster longterm decay in a at higher drive photon fluxes arises from the larger steady-state population of B that can proceed to species D.

Figure 4. Time-dependent absorbance loss/pass data obtained under the same conditions as in Figure 3, but measured as a function of drive photon flux, F. The drive photon fluxes were: (a) F ) 1.25 × 1016 s-1 cm-2, (b) 0.832 × 1016 s-1 cm-2, (c) 0.624 × 1016 s-1 cm-2, (d) 0.416 × 1016 s-1 cm-2 and, (e) 0.208 × 1016 s-1 cm-2. Experimental data are shown as green dots, and the fits to the kinetic model in eq 6 are shown as red traces. The solution was irradiated by the 310 nm drive beam for various time intervals all starting at t ) 0.

In all cases, the probe absorbance change a returns to zero when the drive beam is blocked from the sample. In these liquidphase experiments, the drive beam diameter is much larger than the probe diameter, so diffusion effects play an insignificant role in the observed absorbance changes. The absorbance data in Figure 4 were simulated using the kinetic model in eq 6, with the initial condition [B] (t ) 0) ) [D ](t ) 0) ) 0 and the constraint [A](t ) 0) ) [A](t) + [B](t) + [D](t). The time-dependent absorbance change a(t) is proportional to [B](t) so that, for a given set of kinetic parameters σ1, k-1, and k2, the rate equations for the model can be numerically integrated to obtain a(t). To simulate the data, the three kinetic parameters were optimized using a Marquardt least-squares fitting algorithm in a simultaneous fit of the kinetic model in eq 6 to all five data sets in Figure 4. Each data set in the simulation has a unique photon flux value, F, which is set to zero during the experimental time intervals when the drive laser is blocked from the sample. In the kinetic model, the behavior of a(t) during different time intervals is sensitive to different sets of kinetic parameters; for example, the “relaxation” decay behavior in a(t) when the drive laser was blocked is only dependent upon the value of k-1. The resulting fits and the experimental data are compared in Figures 3 and 4, and the optimized values of σ1, k-1, and k2 in toluene solution are reported in Table 1. Several kinetic models other than that given in eq 6 were tested, for example, those in which k2 is dependent upon drive photon flux, or in which A can decompose to D. None of the alternative models tested matched the quality of fit shown in Figures 3 and 4. At higher drive photon fluxes, additional photochemical channels would be expected to appear, which would make k2 power dependent. A fundamentally interesting quantity is the photoisomerization quantum yield for the process A + hν f B, which can be calculated as

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TABLE 1: Kinetic Parameters for the Photoisomerization Dynamics of 6,8-DibromoBIPS in Toluene Solution (310 nm) and in Thin PMMA Films (266 nm), at 23 ( 1 °Ca toluene solution this work (310 nm) σ1 k-1 k2 φ f a

literature17 (308 nm)

(2.1 ( 0.3) × 10-18 cm2 0.090 ( 0.018 s-1 0.038 ( 0.008 s-1 0.16 ( 0.02

0.3 s-1 0.07

PMMA thin film (266 nm) (9.4 ( 1.7) × 10-19 cm-2 (2.3 ( 0.2) × 10-3 s-1 (2.5 ( 0.6) × 10-4 s-1 0.16 ( 0.03 0.75 ( 0.03

Literature values in toluene were measured at 25 °C.

φ)

σ1 σabs

(8)

where σabs is the absorption cross-section of 6,8-dibromoBIPS at the 310 nm drive wavelength. The molar absorptivity of 6,8dibromoBIPS at 310 nm in toluene was measured by ordinary UV-vis spectroscopy to be ε ) 3.5 × 103 L mol-1 cm-1, which corresponds to σabs ) 1.338 × 10-17 cm2. Using the toluene solution σ1 value reported in Table 1, this results in a 310 nm photoisomerization quantum yield of φ ) 0.16 ( 0.02, somewhat larger than the 0.07 value reported by Chibisov and Go¨rner.17 3.3. PMMA Film Kinetics. The photochromic behavior of relatively few molecules has been reported in PMMA polymer thin films, and, as far as the authors are aware, there have been no such studies of 6,8-dibromoBIPS or of general polarizationresolved experiments on photochromic dyes in thin films. Hence, we cannot compare the results of NICRDS probing with previous studies in films. Figure 5 shows the absorbance loss/pass measured as a function of time over two irradiation (266 nm) cycles of 6,8dibromoBIPS in a PMMA thin film prepared as discussed in section 2.1. The behavior is qualitatively similar to that observed in liquid toluene, with important differences. First, both the apparent ring-opening photoisomerization rate and the ringclosing relaxation rates are significantly slower than in toluene solution. Second, the absorption change a does not fall to zero when the drive beam is blocked, suggesting incomplete thermal relaxation of the open merocyanine form of 6,8-dibromoBIPS back to the closed spiropyran form. To model this behavior, we start with the kinetic model in eq 6 and additionally hypothesize that the closed spiropyran 6,8-dibromoBIPS isomer, A, can also photoisomerize to form an absorbing open isomer, C, which is metastable with respect to ring closure on the time-scale of our experiment. The resulting kinetic scheme is k1f

k2

A y\z B 98 D

(9)

k-1

k1(1 - f)

k2

A 98 C 98 D

(10)

where A, B, and D have the same meanings as in eq 6. The rate equations for this scheme are

d[A] ) -k1[A] + k-1[B] dt

(11)

Figure 5. Time-dependent absorbance loss/pass at 620 nm of 6,8dibromoBIPS in a PMMA thin film on a fused silica substrate measured by NICRDS. The film is exposed to a 266 nm drive beam for two time intervals: 0 < t < 1500 and 5100 s < t < 6600 s. The drive photon flux was F ) 2.23 × 1015 s-1 cm-2. The red solid trace is the fit to the kinetic model in eqs 9 and 10.

d[B] ) k1f[A] - k-1[B] - k2[B] dt

(12)

d[C] ) k1(1 - f)[A] - k2[C] dt

(13)

where f is a branching ratio determining the relative probability of A molecules following the reaction path leading to B or C. There is no fundamental reason why the dissociation rate constant k2 should be the same for species B and C; however, we obtain excellent fits to experimental data making this assumption. Figure 5 shows the fit to this kinetic model to experimental when the values of k1, k-1, k2, and f are optimized by the nonlinear Marquardt least-squares minimization as described in section 3.2. The photoisomerization rate is dependent upon the drive photon flux, and the associated cross-section, σ1, can be computed using eq 7. The resulting kinetic parameters are reported in Table 1. The value f ) 0.75 ( 0.03 in Table 1 indicates that about one-quarter of the excited merocyanine becomes “trapped” as the metastable species C. The molar absorptivity of 6,8dibromoBIPS at 266 nm was measured in both toluene and methyl acetate to be ε ) 1.5 × 103 L mol-1 cm-1, which corresponds to σabs ) 5.736 × 10-1 cm2. Assuming the same absorption cross-section in the PMMA film, the photoisomerization quantum yield, φ, in PMMA can be computed from σ1 and σabs using eq 8. The resulting value of φ ) 0.16 ( 0.03 (Table 1) is the same as that measured in toluene for 310 nm within experimental uncertainty. The thermal ring-closing rate, k-1, and the decomposition rate, k2, are both significantly smaller in the PMMA matrix than in toluene solution, which is consistent with the higher viscosity in the polymer, which stabilizes the zwitterionic merocyanine isomer. We consider the microscopic details of this hypothesis in section 4.2. 3.4. PMMA Film Photoselection Polarization Anisotropy. The major advantage of the NICRDS technique over other cavity ring-down schemes for probing surface-adsorbed species is that it offers the ability to simultaneously detect absorption multiple polarizations. Our initial attempts to study polarization effects in films of 6,8-dibromoBIPS utilized a “Brewster-angle” sample geometry. In this geometry the substrate is tilted at Brewster’s angle with respect to the longitudinal axis of the ring-down

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Figure 6. Time-dependent absorbance loss/pass at 620 nm of 6,8dibromoBIPS in a PMMA thin film on a fused silica substrate measured by polarized NICRDS. The film is exposed to the V-polarized 266 nm drive during the time interval 0 < t < 1700 s and the resulting parallel absorbance, a(V), and perpendicular absorbance, a(H), are shown in blue and green, respectively. The drive photon flux is F ) 2.23 × 1015 photons s-1 cm-2.

cavity, minimizing reflection loss from the substrate surfaces for p-polarized light (electric vector in the plane of incidence).11 An advantage of the Brewster-angle geometry is that it is significantly easier to align than in the normal incidence configuration. However, the Brewster-angle geometry permits CRDS detection only of the p-polarized light component of the absorption probe. This probe polarization is necessarily tilted with respect to the plane of the surface, a disadvantage in the probing of azimuthal rotary motions of adsorbed molecules with respect to the surface normal. In principle, the measurement of multiple probe polarizations by CRDS in a Brewster-angle geometry can be accomplished by rotating the substrate or comparing different substrates. Practically, we find that direct comparison of absolute absorption data in adsorbed PMMA films on different substrates or different areas of the same substrate is difficult because of the ultrasensitive nature of CRDS and variability in substrate surface quality and the thickness and morphology of the film. Instead, as described in section 2.2, we used a combination of NICRDS with a Glan polarizing prism to spatially separate and detect orthogonal linear polarizations of the probe light. In our experiments, we controlled both the polarization of the drive and the probe beams, allowing the simultaneous measurement of parallel and perpendicular relative polarization geometries. As expected, no probe absorption polarization preference in toluene solution can be detected, but there is significant polarization preference observed in PMMA films. Figure 6 shows the simultaneous absorbance changes measured in the V and H directions when the PMMA film is irradiated with V-polarized probe light. Although, as mentioned in section 2.2 and shown in Figure 2, the drive beam propagation axis is at a 7° angle with respect to the surface normal, V-polarized drive light is s-polarized with respect to the surface (electric vector perpendicular to the plane of incidence and parallel to the substrate surface). Thus, for V-polarized drive excitation, data of the type shown in Figure 6 cleanly reveal the resulting azimuthal asymmetries in the 620 nm absorbance of 6,8dibromoBIPS in the adsorbed film. In Figure 6, it is clearly evident that the V-V (parallel) drive-probe geometry leads to a larger absorbance change than the V-H (perpendicular) geometry; a(V) > a(H) for all times t > 0. After the drive irradiation is ended, the relative difference between the two probe polarizations steadily decreases.

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Figure 7. Time-dependent absorbance loss/pass at 620 nm of 6,8dibromoBIPS in a PMMA thin film on a fused silica substrate measured by polarized NICRDS, analogous to that in Figure 6, but using H-polarized 266 nm drive light (which is p-polarized with respect to the substrate surface). The film is exposed to the H-polarized 266 nm drive during the time interval 0 < t < 1700 s, and the resulting perpendicular absorbance, a(V), and parallel absorbance, a(H), are shown in blue and green, respectively. The drive photon flux is F ) 2.23 × 1015 s-1 cm-2.

Figure 8. Absorption polarization difference ∆a ) a(V) - a(H) for 6,8-dibromoBIPS at 620 nm for vertically polarized drive light (266 nm) in a PMMA thin film. Obtained from data shown in Figure 6.

For comparison, the analogous experiment using H-polarized drive light is shown in Figure 7. Because this drive polarization is p-polarized, the H-probe polarization is imperfectly parallel to the drive polarization. As a consequence, the relative difference between the parallel and perpendicular drive-probe geometries is smaller in Figure 7 than in Figure 6. A probe polarization difference suggests that the laboratory-frame orientation of the probe transition dipole of the open merocyanine isomer (B) of 6,8-dibromoBIPS has memory of the orientation of the drive transition dipole of the parent, closed spiropyran isomer (A). That the parallel pump-drive geometry shows the larger absorbance indicates that there exists a parallel preference for the molecular-frame orientations of the drive and probe transition dipoles. To further focus on the probe absorption polarization induced by the drive beam, Figure 8 plots the difference ∆a ) a(V) a(H) obtained from the V drive irradiation data in Figure 6. Interestingly, while both a(V) and a(H) in Figure 6 were still increasing at t ) 1700 s when drive irradiation was ended, the polarized absorbance difference ∆a in Figure 8 had already reached a photostationary plateau by t = 1200 s. Figure 9 shows the relative absorption polarization, defined as

arel )

a(V) - a(H) a(V) + a(H)

(14)

Polarized Normal-Incidence CRDS

Figure 9. Relative absorption polarization, arel, of 6,8-dibromoBIPS at 620 nm for vertically polarized drive light (266 nm) in a PMMA thin film.

for the data in Figure 6. The largest relative polarization is observed upon initial drive irradiation, and the decreases roughly linearly with time, continuing after drive irradiation is ended at t ) 1700 s. We speculate that this decrease arises to slow thermal angular motion of the B and C open merocyanine isomers in the PMMA film. Thermal reorientation of the anisotropic distribution of merocyanine isomer transition dipoles will diminish the observed relative polarization; a detailed kinetic model of this behavior is beyond the scope of the present study. 4. Discussion 4.1. Toluene Solution Photoisomerization Dynamics. The focus of this Article is to demonstrate the utility of NICRDS for the probing of optically thin samples, such as monolayer scale coverages of a surface adsorbed analyte. The molecule 6,8-dibromoBIPS was chosen as an experimentally simple and well-studied model system to evaluate the NICRDS technique. However, it still is interesting to compare the photochromism kinetics as measured by NICRDS to measurements on optically thick samples. Chibisov and Go¨rner studied the photochromism of several BIPS derivatives in various solvents to elucidate the important species and photochemical processes that are important in their ring-opening and closing dynamics.17 Their experiments were conducted in solutions that were optically dense, with UV absorbances typically between 0.5 and 2. In contrast, our UV absorbance was typically more than 104 smaller. As shown in Table 1, the photoisomerization quantum yield of 6,8-dibromoBIPS in toluene yield measured here, φ ) 0.16 ( 0.02, is significantly higher than the value of 0.07 reported by Chibisov and Go¨rner.17 Additionally, our toluene ring-closing reaction rate constant, k-1 ) 0.090 ( 0.018 s-1, is more than 3 times smaller than their value of 0.3 s-1. The origin of this discrepancy is unclear, but we propose multiple possibilities: Chibisov and Go¨rner assumed the simpler reversible kinetic scheme in eq 5 in modeling their data. Accurate fitting of our data necessitated the more complicated kinetic scheme in eq 6. The addition of a “decomposition” loss mechanism B f D in eq 6 makes the value of k-1 smaller than that of the “apparent” k-1 value obtained when attempting to fit the scheme in eq 5 to our data. It is possible that the experiments of Chibisov and Go¨rner could not distinguish between thermal ring closing and “decomposition”. Our experiments probe the absolute concentration of the open merocyanine isomer of 6,8-dibromoBIPS under continual pulsed irradiation at 310 nm. Chibisov and Go¨rner used pulsed 308 nm excitation, but instead measure the ratio of transient absorbance changes ∆A/∆Amax on each laser

J. Phys. Chem. A, Vol. 115, No. 4, 2011 417 shot as a function of probe wavelength to construct “ transient difference spectra”. Measured as this ratio, a long-term absolute decrease in the probe absorption of the solution over the course of the experiment would not have been detected in their experiment. The ring-closing rate constant is temperature dependent,17 and optical heating could account for discrepancies between the two experimental setups. The equilibrium temperature of our experiments is within 2 °C of the experiments Chibisov and Go¨rner. The 571 µM concentration of 6,8-dibromoBIPS used by Chibisov and Go¨rner is only slightly more than twice that in this work, and based on the laser pulse energy flux limit in their experimental details,20 we estimate that we have similar drive photon fluxes. However, the total absorbance of the 6,8dibromoBIPS sample in their study is at least 104 larger than that in this work, so that much more energy is absorbed by the sample. In our experiment, the sample volume is very small and has a large ratio of cuvette wall surface area to volume, minimizing absorption heating of the probe volume. Consequently, the effective temperatures for the ring-closing dynamics in the two experiments may be somewhat different. The large surface area to volume ratio in our experiment may result in environmental differences such that the measured rate constant is not representative of bulk toluene solution. As reported by Chibisov and Go¨rner,17,20 increased solvent polarity decreases the ring-closing rate for BIPS molecules. It would be expected that the fused silica of the cuvette walls would provide a relatively polar environment. 4.2. PMMA Film Photoisomerization Dynamics. The 6,8dibromoBIPS 266 nm photoisomerization quantum yield in PMMA is φ ) 0.16 ( 0.03 (Table 1). Within experimental uncertainty, this is the same as that which we report at 310 nm in toluene solution and about a factor of 2 larger than that reported by Chibisov and Go¨rner in toluene, dimethylformamide, acetonitrile, and ethanol at 308 nm.17 In contrast, the ring-closing dynamics appear to strongly depend upon the solvent environment. The measured ring-closing rate constant k-1 ) (2.3 ( 0.2) × 10-3 s-1 in PMMA is more than 2 orders of magnitude smaller than that which we measure by NICRDS in toluene, and in other liquid solutions as studied by Chibisov and Go¨rner. In addition, as discussed in section 3.3 and shown in Figure 5, when an interval of drive excitation ends for 6,8-dibromoBIPS in a thin PMMA film, the probe absorbance does not fall to its initial, pre-excitation value for more than 1 h. This suggests that there is incomplete ring-closure of the open merocyanine form of 6,8-dibromoBIPS in PMMA on the hour time scale of these experiments. From both experimental and theoretical studies of photochromic dyes,17,21,22 it is likely that this metastable merocyanine form (species C in the kinetic scheme of eqs 9 and 10) represents an open merocyanine isomer that is stabilized in the polymer matrix and does not relax back to the closed spiropyran isomer on the time scale of the experiment. Sheng et al. have performed a comprehensive theoretical study of the photoisomerization of spiropyrans to merocyanines and the reverse ring-closing processes.21 The most stable open isomer is a trans-merocyanine; there are three relevant double bonds in the open merocyanine form to give eight total possible configurations of cis- and transmerocyanine isomers that may be accessible.21 A possible explanation for the observed ring-closing kinetics in PMMA is that there exist many different solvation environments of 6,8dibromoBIPS in the polymer matrix. These raise or lower the relative energy of the most stable ground-state trans-merocyanine conformer (0TTC in the notation of Sheng et al.) relative

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to the other possible conformations, including the key 0TCC conformer that thermally isomerizes back to the closed spiropyran form. A highly restricted matrix environment that hinders the large nuclear motions associated with the 0TTC f 0TCC rearrangement, and a polar matrix that stabilizes the zwitterionic merocyanine isomers, will increase the activation energy for ring-closing. The ensemble of these stabilized merocyanine conformers would correspond the metastable species C necessary in the PMMA kinetic scheme of eqs 9 and 10, representing about one-quarter of the open merocyanine population created upon photoisomerization of the closed spiropyran form of 6,8dibromoBIPS. These same merocyanine isomers are presumably present in the toluene solution but rapidly interconvert on the time scale of the experiment. The stabilization of the merocyanine isomers in the PMMA matrix would therefore be responsible for the much smaller values of k-1 and k2 observed in PMMA relative to toluene solution (see Table 1). The probe absorption polarization anisotropy measurements in the PMMA film show that the 6,8-dibromoBIPS molecules retain some orientational information in the PMMA thin film. (As expected, there is no probe absorption polarization in toluene solvent.) Despite some thermal reorientation, the probe absorption polarization clearly persists on the time scale commensurate with the presence of the open merocyanine isomers. The memory of the excitation polarization of the spiropyran, and the very slow rotational diffusion of the merocyanine, indicate that both isomers of 6,8-dibromoBIPS interact strongly with the PMMA matrix. The presence of merocyanine conformers B and C within our kinetic model is consistent with an incompletely homogeneous polymer matrix, as was suggested by Ernsting et al.22 5. Conclusions A normal-incidence geometry, polarization-resolved cavity ring-down spectroscopy technique (polarized NICRDS) was developed and tested by application to the study of the photochromic behavior of the molecule 6,8-dibromoBIPS. Both short path length solution and polymer-immobilized 6,8dibromoBIPS samples were studied, and it was shown that the technique offers the sensitivity to detect absorbance changes at monolayer-scale analyte coverages. The polarization resolution of the technique makes possible the ability to measure polarized photoselection and the azimuthal angular distribution of analyte molecules adsorbed on a surface. The technique offers special promise in applications probing angular motions of surfaceimmobilized nanoscale devices such as rotary motors and

Mantha et al. actuators. This is demonstrated for a rotary actuator in the accompanying paper (DOI 10.1021/jp1054777).15 The measured photochromic kinetics of 6,8-dibromoBIPS in toluene solution were qualitatively consistent with a previous study of this molecule using conventional absorption spectroscopy. The kinetic measurements in the PMMA film are the first of this type. Large absorption polarizations and relatively slow ring-closing time scales in PMMA are consistent with a strong interaction of the 6,8-dibromoBIPS molecule with the polymer matrix. Acknowledgment. We thank Prof. Thomas W. Bell for his insight and numerous suggestions. This research was supported by the National Science Foundation (#CHE-0210549). References and Notes (1) O’Keefe, A.; Deacon, D. A. G. ReV. Sci. Instrum. 1988, 59, 2544– 2551. (2) Scherer, J. J.; Paul, J. B.; O’Keefe, A.; Saykally, R. J. Chem. ReV. 1997, 97, 25–51. (3) Wheeler, M. D.; Newman, S. M.; Orr-Ewing, A. J.; Ashfold, M. N. R. J. Chem. Soc., Faraday Trans. 1998, 94, 337–351. (4) Vallance, C. New J. Chem. 2005, 29, 867–874. (5) White, J. U. J. Opt. Soc. Am. 1942, 32, 285–288. (6) Herriott, D. R.; Kogelnik, H.; Kompfner, R. Appl. Opt. 1964, 3, 523–526. (7) Robert, C. Appl. Opt. 2007, 46, 5408–5418. (8) Bechtel, K. L.; Zare, R. N.; Kachanov, A. A.; Sanders, S. S.; Paldus, B. A. Anal. Chem. 2005, 77, 1177–1182. (9) Van Der Sneppena, L.; Ariese, F.; Gooijera, C.; Ubachs, W. J. Chromatogr., A 2007, 1148, 184–188. (10) Shaw, A. M.; Hannon, T. E.; Li, F.; Zare, R. N. J. Phys. Chem. B 2003, 107, 7070–7075. (11) Muir, R. N.; Alexander, A. J. Phys. Chem. Chem. Phys. 2003, 5, 1279–1283. (12) Pipino, A. C. R.; Hudgens, J. W.; Huie, R. E. Chem. Phys. Lett. 1997, 280, 104–112. (13) Engeln, R.; Berden, G.; Van Den Berg, E.; Meijer, G. J. Chem. Phys. 1997, 107, 4458–4467. (14) Mu¨ller, T.; Wiberg, K. B.; Vaccaro, P. H.; Cheeseman, J. R.; Frisch, M. J. J. Opt. Soc. Am. B 2002, 19, 125–141. (15) Ismail, A. I.; Mantha, J. H.; Kim, H.; Bell, T. W.; Cline, J. I. J. Phys. Chem. A, DOI 10.1021/jp1054777. (16) Zalicki, P.; Zare, R. N. J. Chem. Phys. 1995, 102, 2708–2717. (17) Chibisov, A. K.; Go¨rner, H. Phys. Chem. Chem. Phys. 2001, 3, 424–431. (18) Spangler, L. L.; Torkelson, J. M.; Royal, J. S. Polym. Eng. Sci. 1990, 30, 644–653. (19) Snyder, K. L.; Zare, R. N. Anal. Chem. 2003, 75, 3086–3091. (20) Go¨rner, H. Phys. Chem. Chem. Phys. 2001, 3, 416–423. (21) Sheng, Y.; Leszczynski, J.; Garcia, A. A.; Rosario, R.; Gust, D.; Springer, J. J. Phys. Chem. B 2004, 108, 16233–16243. (22) Ernsting, N. P.; Dick, B.; Arthen-Engeland, T. Pure Appl. Chem. 1990, 62, 1483–1488.

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