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Photoisomerization of DODCI at Solid/Liquid Interfaces Studied by Steady-State and Time-Resolved Total-Internal-Reflection Fluorescence Spectroscopy Hiroshi Yao,† Fumihiko Kitagawa, and Noboru Kitamura* Division of Chemistry, Graduate School of Science, Hokkaido University, Nishi 8, Kita 10, Kita-ku, Sapporo 060-0810, Japan Received August 9, 1999. In Final Form: November 18, 1999
Photoisomerization of 3,3′-diethyloxadicarbocyanine iodide (DODCI) between the N and P isomers at bare borosilicate and octadecyl chain-modified C18 glass/ethanol interfaces was studied by steady-state and time-resolved total-internal-reflection (TIR) fluorescence spectroscopies. Both the fluorescence spectrum and the lifetime of DODCI determined at the glass/ethanol interface were different from those in bulk ethanol. Simulations of the photoisomerization kinetics on the basis of a three-valley S1 potential surface diagram revealed that the kinetics of P f N isomerization in the ground state was largely affected by the presence of the octadecyl groups on the glass surface, while that of N f P conversion in the excited state was similar but slightly decreased compared to that in a bulk phase. The large decrease in the P f N isomerization rate was explained in terms of an increase in local friction (viscosity) at the interface and strong structural interactions between DODCI and the octadecyl chains bound to the glass surface.
Introduction Any materials and systems possess an interface(s), and the presence of an interface gives rise to specific functions. As an example, vectorial transfer of chemical species, charges, and energy takes place across two separated media in biological cells. Also, it is well-known that chemical and physical properties of an interface are different from those in a bulk medium.1 To understand molecular-level characteristics of liquid/liquid and solid/ liquid interfaces, much attention has been focused on the study of molecular interactions and dynamics at an interface.2,3 In particular, because spatial asymmetry at a liquid/liquid or solid/liquid interface could render restricted motions of molecules, polarity and/or viscosity at the interface are expected to be different from those in a bulk phase.4 Therefore, the electronic state and chemical reactions of a molecule should be affected by the presence of an interface. So far, several surface-selective experimental techniques, such as surface second harmonic generation (SHG), sum frequency generation (SFG),1,5-8 IR,9 Raman,10 absorption,11 and total-internal-reflection (TIR) fluorescence † Present address: Department of Material Science, Faculty of Science, Himeji Institute of Technology, 3-2-1 Koto, Kamigori-cho, Hyogo 678-1297, Japan.
(1) Eisenthal, K. B. J. Phys. Chem. 1996, 100, 12997. (2) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990. (3) MacRichie, F. Chemistry at Interfaces; Academic Press: San Diego, 1990. (4) Bhattacharyya, K.; Sitzmann, E. V.; Eisenthal, K. B. J. Chem. Phys. 1987, 87, 1442. (5) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343. (6) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992, 43, 627. (7) Meech, S. R.; Yoshihara, K. Chem. Phys. Lett. 1990, 174, 423. (8) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (9) Harrick, N. J. Internal Reflection Spectroscopy; Interscience: New York, 1967. (10) Yahiaoui, B.; Masson, M.; Harrand, M. J. Chem. Phys. 1990, 93, 6047. (11) Masuhara, H. In Photochemistry on Solid Surface; Matsuura, T., Anpo, M., Eds.; Elsevier: Amsterdam, The Netherlands, 1988.
spectroscopy, have been developed and applied to investigate molecular structures, orientation, and dynamics at interfaces. In particular, TIR fluorescence spectroscopy has been successfully applied to probe interfacial phenomena of fluorescent molecules, proteins, or molecular assemblies.12-19 We are also interested in photophysical and photochemical phenomena at liquid/liquid and solid/ liquid interfaces and reported molecular structures at oil/ water interfaces18 and dye aggregation characteristics at glass/solution interfaces19 on the basis of TIR fluorescence spectroscopy. To elucidate further characteristics of an interface, a photoinduced isomerization reaction at a solid/ liquid interface is worth studying, because the reaction accompanies a large conformational change of the molecule and this should be influenced by the presence of the interface. Therefore, we studied a photoisomerization reaction of 3,3′-diethyloxadicarbocyanine iodide (DODCI) at glass/liquid interfaces by TIR fluorescence spectroscopy. DODCI is a well-known dye as a saturable absorber for passive mode-locking dye lasers (chemical structure is shown in Figure 1).20 The photoisomerization dynamics of DODCI from the N isomer (probably an all-trans form) to the P isomer (mono-cis form) has been studied extensively by both absorption and fluorescence spectroscopies21-35 and analyzed quantitatively by an adiabatic three-valley S1 potential energy surface diagram.22,33-35 (12) Toriumi, M.; Yanagimachi, M.; Masuhara, H. Appl. Opt. 1992, 31, 6376. (13) Toriumi, M.; Masuhara, H. Spectrochim. Acta Rev. 1991, 14, 353. (14) Wirth, M. J.; Burbage, J. D. Anal. Chem. 1991, 63, 1311. (15) Wirth, M. J.; Burbage, J. D. J. Phys. Chem. 1992, 96, 9022. (16) de Mello, A. J.; Crystal, B.; Rumbles, G. J. Colloid Interface Sci. 1994, 169, 161. (17) Byrne, C. D.; de Mello, A. J.; Barnes, W. L. J. Phys. Chem. B 1998, 102, 10326. (18) Ishizaka, S.; Nakatani, K.; Habuchi, S.; Kitamura, N. Anal. Chem. 1999, 71, 419. (19) Yao, H.; Ikeda, H.; Kitamura, N. J. Phys. Chem. B 1998, 102, 7691. (20) Ippen, E. P.; Shank, C. V.; Dienes, A. Appl. Phys. Lett. 1972, 21, 348. (21) Dempster, D. N.; Morrow, T.; Rankin, R.; Thompson, G. F. J. Chem. Soc., Faraday Trans. 2 1972, 68, 1479.
10.1021/la991072a CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000
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interactions between DODCI and the interface. Because a C18 function on a solid surface plays crucial roles in separation sciences represented by chromatography, interactions between a molecule and the C18 chain on the glass surface are worth discussing in detail.14,37-40 Experimental Section
Figure 1. Chemical structure of DODCI, and a three-valley S1 potential surface curve diagram for photoisomerization of DODCI. Parameters are explained in the main text.
Furthermore, a picosecond surface SHG study indicated that the photoisomerization rate of DODCI from the N to P isomer was accelerated at an air/water interface as compared to that in a bulk aqueous solution. Acceleration of the isomerization rate at an air/water interface could be explained by low friction at the interface.36 Wall effects on the photoisomerization dynamics are quite interesting and worth studying in detail. Nonetheless, the photoisomerization dynamics of DODCI at a solid/liquid or liquid/liquid interface has been little examined. Because the photoisomerization kinetics of DODCI in solution has been well-defined,34 the reaction system is very suitable to study characteristics of a glass/solution interface. In this paper, we report the photoisomerization reaction of DODCI at hydrophilic borosilicate glass and octadecyl (C18) modified glass/solution interfaces by steady-state and time-resolved TIR fluorescence spectroscopies and discuss (22) Arthurs, E. G.; Bradley, D. J.; Roddie, A. G. Chem. Phys. Lett. 1973, 22, 230. (23) Rulliere, C. Chem. Phys. Lett. 1976, 43, 303. (24) Velsko, S. P.; Waldeck, D. H.; Fleming, G. R. J. Chem. Phys. 1983, 78, 249. (25) Velsko, S. P.; Fleming, G. R. Chem. Phys. 1982, 65, 59. (26) Bilmes, G. M.; Tocho, J. O.; Braslavsky, S. E. J. Phys. Chem. 1988, 92, 5958. (27) Aramendı´a, P. F.; Negri, R. M.; Roma´n, E. S. J. Phys. Chem. 1994, 98, 3165. (28) Scaffardi, L.; Bilmes, G. M.; Schinca, D.; Tocho, J. O. Chem. Phys. Lett. 1987, 140, 163. (29) Duchowicz, R.; Scaffardi, L.; Tocho, J. O. Chem. Phys. Lett. 1990, 170, 497. (30) Caselli, M.; Momicchioli, F.; Ponterini, G. Chem. Phys. Lett. 1993, 216, 41. (31) Awad, M. M.; McCarthy, P. K.; Blanchard, G. J. J. Phys. Chem. 1994, 98, 1454. (32) Chibisov, A. K.; Zakharova, G. V.; Go¨rner, H.; Sogulyaev, Y. A.; Tolmachev, A. I. J. Phys. Chem. 1995, 99, 886. (33) Ba¨umler, W.; Penzkofer, A. Chem. Phys. 1990, 142, 431. (34) Ba¨umler, W.; Penzkofer, A. Chem. Phys. 1990, 140, 75. (35) Penzkofer, A.; Ramelsperger, M.; Wittmann, M. Chem. Phys. 1996, 208, 137. (36) Sitzmann, E. V.; Eisenthal, K. B. J. Phys. Chem. 1988, 92, 4579.
Chemicals. DODCI was purchased from Nippon KankohShikiso Kenkyusho Co. and used as received. Ethanol (Uvasol; Kanto Chemical), methanol (Uvasol; Kanto Chemical), ethylene glycol (GR grade; Wako Pure Chemicals), glycerol (Uvasol; Merk), and toluene (GR grade; Wako Pure Chemicals) were used as received. Dichloromethyloctadecylsilane was purchased from Chisso Co. and used as received. A borosilicate glass cell (path length 400 µm, VitroCom) used was cleaned by soaking in a KMnO4/H2SO4 solution for 24 h. It was then treated in an aqueous H2O2 solution for 3 h, rinsed thoroughly with deionized water (a bare glass surface), and dried at 300 °C. A C18 glass cell was prepared by soaking the cleaned borosilicate glass cell in a dichloromethyloctadecylsilane/toluene solution (2 wt %) for 4 h at 60 °C. After derivatization, the cell was rinsed successively with toluene and methanol (C18 glass surface). The surface coverage of the glass by the octadecyl groups was estimated to be ∼3-4 µmol/m2 on the basis of absorbance at ∼2925 cm-1 (methylene stretch), which corresponded to the area of ∼50 Å2/chain.37 The concentration of DODCI in ethanol was set 1.0 × 10-6 M for both bulk and TIR measurements in order to avoid formation of dye aggregates. All of the measurements were performed at room temperature. Measurements. Absorption and FT-IR spectroscopy was performed by using a Hitachi U-3300 spectrophotometer and a Shimadzu FTIR-8300, respectively. A TIR fluorescence spectroscopy system used in this study was similar to that reported in refs 12 and 19. A fused silica hemicylindrical prism was contacted with a glass cell, with cis/trans-decalin (Wako Pure Chemicals) being used as a matching oil, and was firmly mounted on a rotating stage (Chuo Seiki). The refractive indices of the prism, the matching oil, the borosilicate glass, and ethanol are 1.46, 1.47, 1.47, and 1.36, respectively. For steady-state TIR fluorescence measurements, an Ar+ laser beam (Coherent, Innova 70, 514.5 nm) passed through an aperture (1.0 mm) was used for excitation and polarized perpendicularly with respect to the reflection plane. The excitation laser intensity was varied from e10-3 to 22.1 W cm-2. The beam was impinged to the glass cell/ sample solution interface, with an angle of incidence θi being normal to the interface. Fluorescence from the sample being passed through a sharp-cut filter (Toshiba Glass; O-54, λ > 540 nm) was introduced to an optical fiber set at an angle θ0 (observation angle) and analyzed with a polychromator-multichannel photodetector set (Oriel, ICCD-InstaSpec V). The angle between the incident laser beam and the optical fiber was fixed 90° throughout the experiments. Excitation laser power was measured by using a power meter (Coherent, Lasermate 10). For time-resolved TIR fluorescence measurements, Ti:sapphire/ OPA output laser pulses at 532 nm (Coherent, Mira 900F/OPA 9400, duration ∼150 fs, repetition 100 kHz) were used for excitation. Fluorescence decay profiles were determined by a time-correlated single photon counting system (Hamamatsu Photonics (R3809U-50), Edinburgh Instruments (SPC-300)) observed with a polarization angle at 54.7°. The lifetime of the ground-state P isomer in a bulk solution was determined on the basis of a transient absorption technique by monitoring groundstate recovery at 632.8 nm (He-Ne laser: Uniphase, model 1125). Excitation was performed by using a pulsed Nd3+:YAG laser (Lumonics, YM600; pulse width 10 ns) at 532 nm. (37) Kovaleski, J. M.; Wirth, M. J. J. Phys. Chem. B 1997, 101, 5545. (38) Rangnekar, V. M.; Oldham, P. B. Anal. Chem. 1990, 62, 1144. (39) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A. (40) Hansen, R. L.; Harris, J. M. Anal. Chem. 1995, 67, 492.
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Estimation of the Excitation Light Intensity at the Interface under the TIR Condition. In a TIR regime, when an incident angle θi is larger than a critical angle θc, incident light is totally reflected at a solid/liquid interface, but an evanescent wave propagates into the interface region of the sample solution. Thus, information about a solid/liquid interface can be obtained by observing sample fluorescence under the TIR conditions. In such a case, the excitation light intensity at around the interface, Iex, is expressed by12,17
Iex ) E02
4n12 cos2 θi 2
2
(n1 cos θi + Re) + Im
(
exp -
)
4πIm z λ0
(1)
where n is a refractive index (the suffices 1 and 2 indicate a glass substrate and a sample solution, respectively) and E0 and λ0 are the electric-field amplitude and the wavelength of incident light, respectively. Equation 1 includes the Fresnel factor (Ts) and the term for light attenuation, indicating that the excitation light intensity decreases exponentially with a distance from the interface, z. When a sample solution absorbs incident light, the relevant refractive index is given by a complex number: n2 ) n2(1 + iκ), where κ is the absorption index of a probe molecule. Re and Im in eq 1 are the real and imaginary parts of (n2 cos θt), respectively, where θt is defined as a complex refraction angle and is a function of n1, n2, θi, and κ. Because E02 is equal to the intrinsic laser intensity, the excitation light intensity at the interface (Iex(z)0)) under the TIR conditions at given θi can be calculated by eq 1. It is also worth noting that the excitation light intensity for bulk solution measurements is given by E02 ()IL). By assuming that the laser beam profile is nearly Gaussian, IL is estimated by the following equation:33
IL ) 4 ln(2)PL/(πdL2)
(2)
where PL and dL represent laser power and the beam diameter, respectively. A penetration depth, dp, of the evanescent wave is then given by eq 3.
dp )
λ0 4πIm
(3)
Simulation of Steady-State Population of DODCI by a Three-Valley S1 Potential Surface Diagram. The photoisomerization dynamics of DODCI can be discussed quantitatively by a three-valley S1 potential surface diagram developed by Penzkofer et al. (Figure 1).33-35 A pump laser (frequency νL, excitation intensity IL) excites the N and P isomers of DODCI to the relevant singlet excited state (S1), and its efficiency depends on the absorption cross section of each isomer (σN and σP, respectively) at νL. In the excited state, DODCI undergoes isomerization between N and P with the rate constant of kNPt(NfP) or kPNt(PfN). In such a case, the fluorescence lifetimes of the N and P isomers (τN and τP) are given by
τN ) (1 - φNPt)/kN
(4a)
τP ) (1 - φPNt)/kP
(4b)
where φNPt and φPNt represent the photoisomerization efficiencies from the N to P isomer and the P to N isomer in the S1 state, respectively, and these are related to kNPt and kPNt as follows.
φNPt ) kNPtτN
(5a)
φPNt ) kPNtτP
(5b)
Under the steady-state approximation, the number density of DODCI in each state is derived as in eq 6,33
N4 )
kp[(kN + kNPt)kNP0 + kNPtσNIL/hνL] + kNkNP0kPNt kp(kN + kNPt)kPN0 + kNkPN0kPNt + kNkPNtσpIL/hνL
N1 (6a)
t
N3 )
t
0
t
0
(kNP σNIL/hνL + kNP kNP )N1 - (kN + kNP )kPN N4 kNkPNt N2 )
(σNIL/hνL + kNP0)N1 - kPN0N4 kN
N1 ) N0 - (N2 + N3 + N4)
(6b)
(6c) (6d)
where N1 and N4 are the number densities of the N and P isomers in the ground state, respectively, while N2 and N3 are those of the N and P isomers in the excited singlet state, respectively. The total number density of DODCI is N0. It has been demonstrated, furthermore, that the same formulations with eq 6 can be obtained even if S1-S0 interisomerization proceeds through an intermediate excited state with a low isomerization yield.33 Therefore, the fluorescence intensity from the N or P isomer is proportional to N2 or N3, respectively, and is given as
FN ) qNN2
(7a)
FP ) qPN3
(7b)
where qN and qP are the fluorescence quantum efficiencies of the N and P isomers, respectively. It is very important to note that the isomerization rate from the N isomer to the P isomer in the S1 state is on the order of picoseconds, while that from the P isomer to the N isomer in the ground state is on the order of milliseconds.24-25,33 Furthermore, the absorption cross section of the P isomer (σP) at the excitation wavelength (514.5 nm in the present case) is not zero. Therefore, an excitation intensity dependence of the fluorescence spectrum of DODCI should be observed owing to accumulation of the P isomer.28-29,33 Thus, we can discuss the photoisomerization kinetics (in both the fluorescent excited and nonfluorescent ground states) by fitting excitation intensity dependencies of the fluorescence intensities of the N and P isomers on the basis of eqs 4-7. It should be mentioned that the photoisomerization of DODCI is generally studied by another scheme with the same potential surface diagram described above.28-32 The model assumes that the central valley acts as an efficient funnel to the ground-state surfaces, and there is no communication between the excitedstate N and P isomers through the S1 state.28-32 However, it was demonstrated that characteristic features in the photoisomerization were completely interpreted by the present scheme, where the N and P isomers in the S0 state are generated by the S1-state N and P isomers to the ground state, respectively.35 Furthermore, some recent results in temperature-dependent fluorescence quantum yields suggested the existence of communication between the N and P isomers in the excited state.34 Thus, the present simulation model we selected would be reasonable.
Results and Discussion Excitation Laser Intensity Dependence of the Fluorescence Spectrum of DODCI in Bulk Ethanol Solution. To discuss photoisomerization characteristics of DODCI at glass/solution interfaces, we studied an excitation laser intensity dependence of the fluorescence spectrum in a bulk solution as a reference experiment. Figure 2a shows fluorescence spectra of DODCI in a bulk ethanol solution (1.0 × 10-6 M) at various excitation laser intensities (IL). The fluorescence peak at 603 nm and the shoulder at around 640 nm are ascribed to the fluorescence from the N and P isomers, respectively.33-35 In the figure,
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Figure 3. Incident angle dependence of the TIR fluorescence spectrum of DODCI at a C18 glass/ethanol interface. The spectrum shown by the dotted curve represents that from the N isomer in ethanol.
Figure 2. (a) Excitation laser intensity (IL) dependence of the fluorescence spectrum of DODCI in ethanol. (b) IL dependencies of the fluorescence intensities of the N (full squares) and P (full circles) isomers in ethanol. The fluorescence intensity of the N isomer at IL ) 22.1 W cm-2 was set at 1.0, and other data were shown as the relative value. Dotted curves are the best fit of the observed data by the three-valley S1 potential surface diagram shown in Figure 1.
the fluorescence intensities were normalized to that at the N isomer peak. With an increase in IL, the contribution of the P isomer fluorescence to the overall spectrum increases. This is due to accumulation of the ground-state P isomer as discussed above.33 Because the true fluorescence spectrum of the N isomer can be obtained with extremely weak laser excitation (650 nm was quite similar to that of the N isomer. Actually, it has been reported that (48) Fleming, G. R.; Knight, A. E. W.; Morris, J. K.; Robbins, R. J.; Robinson, G. W. Chem. Phys. Lett. 1977, 49, 1.
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Table 1. Parameters Used for the Simulations of Photoisomerization Kinetics at the Bare and C18 Glass/ Ethanol Interface parameters
bare glass/ethanol
C18 glass/ethanol
1.15 1.20 11 × 10-17 1.8 × 10-17 1.67 0.1
1.30 1.35 10 × 10-17 1.7 × 10-17 1.67 0.1
〈τN〉 (ns) τP (ns) σN (cm2) σP (cm2) qN/qP φPNt
Table 2. Best-Fit Parameters Determined by the Simulations of Photoisomerization Kinetics φNPt τPN0
(ms)
bulk ethanol
bare glass/ethanol
C18 glass/ethanol
0.058 2.5
0.058 5.5
0.048 20
τP is about 1.04 times longer than τN in several solvents.25 Thus, we assumed τP ) 1.04〈τN〉 in the simulation. Because the electronic interactions between DODCI and the interface were weak, the absorption cross sections of the N and P isomers (σN and σP at 514.5 nm) at the bare glass/ ethanol interface used were assumed to be the same as those in a bulk phase, while those at the C18 glass/ethanol interface were estimated by taking the spectral shifts into account. The ratio of the fluorescence quantum efficiency qN/qP was also assumed to be the same as that in ethanol. This assumption is not unrealistic because it has been reported that the qN/qP ratio is essentially independent of the nature of a medium,34 and this is also supported by the absence of the observation wavelength dependence of the fluorescence lifetime as discussed previously. Finally, because φPNt is known to be insensitive to environments as described before, it is safely assumed that the value is the same as that in a bulk phase (0.1).34 On the basis of such assumptions, the excitation intensity dependencies of FN and FP were fitted by φNPt ()kNPt〈τN〉; N f P component) and τPN0 ()1/kPN0; P f N component) as the parameters. The dotted curves in Figure 7a,b represent the best fits of the observed data by the following parameters: φNPt ) 0.058 and φPN0 ) 5.5 ms for the bare glass/ethanol interface, φNPt ) 0.048 and φPN0 ) 20 ms for the C18 glass/ethanol interface. The results are summarized in Table 2 together with those determined in a bulk ethanol solution. At the bare glass/ethanol interface, the photoisomerization rate from the N to P isomer was similar, while isomerization from the P to N isomer in the ground state (τPN0 ) 5.5 ms) was slightly slow compared to that in ethanol (τPN0 ) 2.5 ms). The results were similar to those on excited-state isomerization of a triphenylmethane or styryl dye; these dyes showed fast isomerization even upon adsorption on bare glass surfaces as demonstrated by time-resolved surface SHG measurements.49 At the C18 glass/ethanol interface, on the other hand, the isomerization rate from the P to N isomer in the ground state (τPN0 ) 20 ms) decreased dramatically as compared to the value in ethanol while that from the N to P isomer in the excited state was slightly slow (τPNt ) 0.048). Generally, an isomerization reaction accompanies a large conformational change of a molecule so the reaction is affected by medium friction (bulk viscosity η). In practice, it has been reported that the reaction rate is proportional to η-R where R is an adjustable parameter (0 e R e 1).24-27 Therefore, the decrease in the photoisomerization rate at both bare and C18 glass/ethanol interfaces indicates the increase in local friction around the interface, and the (49) Meech, S. R.; Yoshihara, K. J. Phys. Chem. 1990, 94, 4913.
local friction at the C18 glass/ethanol interface is larger than that at the bare glass/ethanol interface. Small friction effects in the N f P photoisomerization are probably due to small activation processes through an intermediate state.31 In alcohols, τ0PN is reported to be proportional to 0.26.24,27 If this empirical equation is held in the present η system, the η value in the vicinity of the bare borosilicate or C18 glass/ethanol interface can be estimated to be ∼9 or 4000 cP at room temperature, respectively. One possible explanation for the apparently extremely large η value obtained for the C18 glass/ethanol interface might be the presence of the long octadecyl chains bound to the glass surface. Thus, we studied an η dependence of τPN0 on the basis of a transient absorption technique.33 τPN0 values in ethylene glycol (η ) 19.9 cP)50 and glycerol (η ) 1490 cP)50 were determined to be ∼3 and 5 ms at room temperature, respectively. Although these alcohols are highly viscous compared to 1-octanol (η ) 6.9 cP) or 1-decanol (η ) 11 cP),24 the τPN0 values were almost comparable with each other. The results indicate that τPN0 is not governed by bulk solvent viscosity alone,35 and strong solute-solvent and solvent-solvent interactions could also play a crucial role in isomerization of DODCI. The large τPN0 value at the C18 glass/ethanol interface could be thus originated from structurally strong DODCI-C18 alkyl chain(s) and chain-chain interactions. Restriction of the rotational motions of DODCI revealed by fluorescence dynamic anisotropy measurements also supports such a conclusion. The TIR fluorescence spectrum observed at the C18 glass/ ethanol interface showed a slight red shift compared to that in a bulk ethanol solution, indicating that DODCI molecules interact with the octadecyl chains bound to the glass surface. Closely packed, structurally ordered saturated alkyl chains require space ∼20 Å2/chain,51 while the present coverage by the octadecyl chains on the C18 glass surface (∼50 Å2/chain) is low. Thus, DODCI molecules are expected to distribute to the octadecyl chain layer on the glass surface. Actually, it has been reported that a large aromatic molecule can partition into a C18 chain layer on a glass surface and, in such a case, the lateral diffusion coefficient of the molecule becomes slower by 3 orders of magnitude as compared to that in a bulk phase.40 We conclude therefore that DODCI molecules are incorporated into the octadecyl chains at the interface, and this leads to strong interference of the isomerization reaction. It is noteworthy that, because isomerization from the P (mono-cis) to N (all-trans) isomer in the ground state is hindered largely compared to that from the N to P isomer in the excited state, interactions between the cis form and the alkyl chains would be stronger than those between the trans form and the C18 chains. The results suggest importantly that interfaces possessing asymmetricity are very interesting fields for directional and/or selective reactions not expected in an isotropic bulk medium. Conclusions Steady-state and time-resolved TIR fluorescence spectroscopies were applied to study the photoisomerization dynamics of DODCI at borosilicate and chain-modified C18 glass/ethanol interfaces. Simulations of the excitation intensity dependencies of the fluorescence spectra of DODCI on the basis of the three-valley S1 potential surface diagram revealed that isomerization of the P isomer to (50) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 71st ed.; CRC Press: Boca Raton, FL, 1992. (51) Gennis, R. B. Biomembranes; Molecular Structure and Function; Cantor, C. R. Ed.; Springer-Verlag: New York, 1989.
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the N isomer in the ground state was largely affected by the presence of the octadecyl chains on the glass surface, while that of N f P in the excited state was less affected by the surface. The results are due to not only an increase in the local friction (viscosity) at the interface but also, more significantly, strong structural interactions between DODCI molecules and the octadecyl chains bound to the glass surface.
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Acknowledgment. The authors thank Mr. Habuchi and Dr. H.-B. Kim (Hokkaido University) for their technical assistance in single photon counting measurements. N.K. is grateful for a grant-in-aid from the Ministry of Education, Science, Sports and Culture (11440215) for partial support of the research. LA991072A
