Article pubs.acs.org/JPCA
Optically Controlled Molecular Switching of an IndolobenzoxazineType Photochromic Compound ‡ ‡ ‡ Kipras Redeckas,† Vladislava Voiciuk,† Rasa Steponavičiute ̅ ,̇ Vytas Martynaitis, Algirdas Šačkus, ,† and Mikas Vengris* †
Quantum Electronics Department, Vilnius University, Saulėtekio 10, LT-10223 Vilnius, Lithuania Department of Organic Chemistry, Kaunas University of Technology, Radvilėnų 19, LT-50254 Kaunas, Lithuania
‡
S Supporting Information *
ABSTRACT: Photochromic forward (oxazine ring-opening) and backward (oxazine ring-closing) switching dynamics of an indolobenzoxazine compound were studied by femtosecond pump−repump−probe technique. A UV pulse was used to excite the ring-closed form of the photochromic compound, causing a C−O bond cleavage and the formation of a spectrally red-shifted isomer within a time scale of ca. 100 ps. A successive, temporally delayed near-IR pulse, resonant to the red-most absorption maximum of the ring-opened form, was used to reexcite the molecular system, causing a fast photoinduced oxazine ring closure, thereby “short-circuiting” the normally nanosecond lasting photocycle and returning ∼6% of the molecules to the main molecular ground state. Two possible models, involving the S1 excited state of the terminal photoproduct and its hot ground state, are introduced to explain the pre- and post-reexcitation spectral development and the photoinduced switching back mechanics.
1. INTRODUCTION Photochromism is a reversible photochemical reaction that results in a temporary structural change (isomerization, electrocyclic ring-opening or ring-closing, dissociation, etc.) of a molecular compound along with a distinctive transformation of its absorption spectrum.1−3 Unique photochemical properties of various photochromic compounds are of great interest for practical uses in such fields such as data storage,4,5 optical switching,6,7 or molecular-based digital processing.8,9 One of the more interesting aspects of photochromism is its bidirectionality. Typically a forward photochromic reaction, leading to the formation of a metastable form (“switching on”), is initiated by the absorption of a photon, whereas the backward reaction (“switching off”) proceeds via thermal relaxation to the main molecular ground state. In some cases, reverse switching can be induced by the absorption of photon of a different frequency (typically resonant with the absorption band of the metastable form).1 Although several femtosecond time-resolved studies have been published on bidirectional switching of photochromic molecules,10−13 the detailed understanding of precise mechanisms of such reactions is still lacking. In our previous work,14,15 we have examined the photodynamics of several members of the fast-switching indolobenzoxazine16−20 photochromic family, whose photochromism is based on the light-induced opening and thermal closing of the oxazine ring. (See the inset in Figure 1.) Perhaps the most interesting spectroscopic properties were found in 8phenylindolo[2,1-b][1,3]benzoxazines, that is, indolobenzox© 2014 American Chemical Society
azines with a phenylic extension in the para position, relative to the nitrogen of the indole ring. In this study, we continue the ultrafast study on a representative compound of the said group, previously classified as I-A15 (Figure 1). This compound exhibits great photochemical stability, high ring-opening quantum yield (ca. 30%),14 and a huge difference between the closed and open conformation spectra (ca. 22 400 cm−1 bathochromic shift of the red-most spectral maximum, see Figure 1). In this paper, we present the results of time-resolved pump−repump−probe experiments that demonstrate both light-induced ring opening and closing dynamics of the molecule and shed more insight on the photochromic behavior of indolobenzoxazines in general. Global analysis techniques are employed to parametrize the experimental results and to elucidate the transient states observed in both the forward and backward photochromic reactions. To our knowledge, these are the first experiments of this kind to be performed on compounds of the indolobenzoxazine family.
2. MATERIALS AND METHODS 2.1. Sample Preparation. Polycrystalline sample of I-A (see ref 14 for chemical synthesis details) was dissolved in acetonitrile (Sigma-Aldrich Lichrosolv; gradient grade) and diluted to an optical density of 1 at the excitation wavelength Received: June 9, 2014 Revised: July 10, 2014 Published: July 11, 2014 5642
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repumped data (pump beam blocked, repump beam unlocked), and (d) pumped−repumped data (both pump and repump beams unblocked). Repumped data contained no significant spectro-temporal information, apart from the appearance of coherent artifacts at τ = τrepump, and hence these spectra were subtracted from the final pumped−repumped signals. With the optical path of pump beam being temporally fixed, two possible experiments could be carried out in the presented setup. (See Figure 2.) In a pump−repump−probe experiment (PrPP
Figure 1. Steady-state absorption spectrum of I-A (portrayed in black) along with transient absorption spectrum of its optically opened form (portrayed in red; spectrum is taken from nanosecond resolution flash photolysis experiments, described in detail in ref 14). The blue and red Gaussian curves represent, respectively, the pump and repump pulse spectra. Figure 2. Pulse sequences of (a) pump−repump−probe (PrPP) and (b) repump action trace (RAT) experiments in a reference system tied to the UV pump pulse. See the main text for more information.
(325 nm) in a 1 mm optical path quartz cell. The cell was translated transversely to the beam propagation direction with a two-axis motorized linear stage (Standa 8MT173) to diminish sample degradation during the experiments and to avoid a local overexposure to the laser radiation. 2.2. Experimental Setup. Pump and repump beams were generated by a set of two optical parametric amplifiers (TOPAS-800, Light Conversion), pumped by the fundamental harmonic of a commercial Ti:Sa femtosecond system (800 nm, 100 fs, 3.5 W, 1 kHz; Coherent Libra). White-light supercontinuum, generated in a mechanically translating 3 mm thick CaF2 plate, was used as a probe (Ø ≃ 80 μm at the sample plane). Polarizations of the pump and repump beams were aligned parallel to one another and at a 57.4° (“magic”) angle in accordance with the probe beam. Optical path of the pump pulse was held fixed, and the relative delays of the probe and repump pulses were controlled via two separate motorized delay lines (Aerotech ATS10060 and ATS10030). Two phaselocked optical choppers (Stanford Research SRS540), operating at 1/2 and 1/4 of the fundamental laser frequency, were employed to block, respectively, the pump and repump beams during the experiments. The experimental data were acquired over several (6−8) multiple scans; that is, the entire transient absorption spectrum was repeatedly measured back-to-back and the multiple sets of wavelength- and time-resolved data were averaged over the number of performed scans. The scan-to-scan kinetic traces were virtually identical and the pre- and postexperiment steady-state absorption spectra also did not show any significant changes. Additional information about the optical layout and the data acquisition of the measurements system can be found in ref 15. 2.3. Measurement Types. With the optical choppers properly synchronized, four types of raw probe spectral data were collected in a course of a pump−repump−probe experiment. These data can be categorized as (a) unpumped data (both pump and repump beams blocked), (b) pumped data (pump beam unblocked, repump beam blocked), (c)
further in text), the pump and repump pulses were temporally held at a fixed delay between them so that from the perspective of a τ → +∞ probe pulse τpump = 0 and τrepump > 0. This type of experiment provided the information on how a perturbation disturbs the photodynamic behavior of the investigated system. In a repump action trace (RAT further in text) experiment, temporal delay between the pump and probe pulses was held fixed (τpump = 0 and τprobe > 0), and the repump pulse was propagated from τ < 0 toward τ = τprobe. In other words, this experiment allowed us to monitor the influence of the delay of the perturbing pulse on the photoproduct spectrum. Both PrPP and RAT measurements, with identical excitation conditions, were performed on the investigated compound. The sample was excited (“pumped”) with 325 nm near-UV radiation adjusted at 1 μJ (red edge of the main UV−vis absorption band). Reexcitation (“repumping”) was performed with near-IR radiation of 750 nm and 8.5 μJ (approximately at the center of the red-most induced absorption band of the optically activated isomer; see Figure 1 for details). The estimated durations of the near-UV and near-IR pulses were, respectively, 150 and 100 fs. The diameter of both of the beams at the sample plane was ca. 300 μm. Additional information about three pulse transient absorption experiments can be found in refs 11, 21, and 22. 2.4. Global Analysis of Time-Resolved Data. Global analysis techniques23,24 were applied for the parametrization and systemization of the time-resolved data. In short, a global analysis fitting procedure is based on the designation of the excited-state population to one or several compartments (i.e., spectral states) with their appropriate time-dependent populations c1(t), ..., cn(t). These states interact by transferring the excited-state population between each other with the inverse rate of τi→j. (i → j signifies the temporal evolution from the ith to the jth component, whereas i → i signifies the decay of the ith component to the ground state.) Any n-component model, 5643
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composed from an experimentally resolved pump−repump− probe data set, can be treated as a set of differential equations dci = Ai Ipump(t ) + dt
⎛ cj(t ) ⎞ ⎟⎟ + Irepump(t − τrepump) × j = 1 ⎝ τi → j ⎠ n
∑ ⎜⎜
n
∑ (Φi→ jcj(t )),
where i = 1, 2, ..., n (1)
j=1
Here Ai signifies the initial excited-state population distribution among one (Ai = δi0) or several spectral compartments; Ipump(t) and Irepump(t − τrepump) are the instrument responses to both pump and repump pulses, respectively; Φi→j denotes the percentage (i.e., efficiency) of the population transfer from component i to component j via the interaction of the repump pulse. Each of the components is likewise represented by a unique species-associated different spectrum (SADS) σi(λ), and the product of the time-dependent concentrations and wavelength-dependent SADS is used to construct the global approximation of the time-resolved data n
S(t , λ ) =
∑ cj(t )σj(λ) j=1
(2)
3. RESULTS 3.1. Pump−Repump−Probe Results. Subnanosecond transient absorption dynamics were discussed in greater detail in ref 15. As it was established from the pump−probe (PP further in text) experiments, the formation of the final photochromic forms of phenylically substituted indolobenzoxazines is a result of complicated photodynamics, involving several intermediates. The PP kinetic traces exhibit characteristic rise−fall−rise behavior (see the data shown in solid symbols and curves in Figure 3), which was previously attributed to the fast-decaying and slowly decaying photoproducts on different portions of ground-state potential energy surface.15 After these dynamic steps are complete, the formation of the final photoproduct spectrum occurs within a temporal span of ca. 100 ps, and all further spectral development is associated only with thermal relaxation to the main molecular ground state. The temporal delay between pump and repump pulses was set at 1 ns and the probing was performed from 10 ps before until 6 ns after the UV excitation to ensure that only the terminal state (and not an earlier intermediate) is affected in the repumping process. Results of the PrPP experiments are presented in Figures 3 and 4. As the kinetic traces in Figure 3 indicate, the PP and PrPP signals coincide prior to the arrival of the repump pulse. The near-IR reexcitation at 1 ns causes an immediate bleaching of both main photoproduct bands, resulting in a ca. 20% decrease in the transient absorption signal in their spectral vicinity. (See the open symbols and the dashed curves in Figure 3.) The bleaching effect is more notable for the short-wave photoproduct band (see Figures 4a,c) because the double difference absorption (ΔΔOD hereafter) signals in the vicinity of the 750 nm band were largely contaminated by the scattering of the repump pulse. The negative ΔΔOD signal peaks at 407 nm, which is slightly blue-shifted (ca. 200 cm−1) from the maximum of the original band. Contrary to the main photoproduct absorption bands, in the midband region (430−550 nm), a positive ΔΔOD signal is observed. Immediately after the reexcitation, this induced absorption
Figure 3. Dispersion-corrected time-resolved pump−probe (solid symbols) and pump−repump−probe (hollow symbols) kinetic traces of I-A. The full sub-10 ns PP and PrPP spectral dynamics at selected wavelengths are presented in panel a, while the post-repump (τrepump = 1 ns) evolution is highlighted in panel b. Solid and dashed curves, respectively, represent the global fits of the PP and PrPP experimental data. For simplicity purposes, only the global fit according to model I (see the Discussion section for more detail) is presented in this Figure. Femtosecond-resolution time-gated spectra of the PP photodevelopment can be found in Figure SI1 in the Supporting Information. Symbols on the abscissa of panel a indicate temporal instances at which the transient states of the PP photoevolution reach their population maximum. (See the discussion and Figure 7b for more information.)
plateau is largely featureless and does not exhibit a distinct band structure. (See the 300 fs spectra in Figure 4c.) During the first few picoseconds after the reexcitation, the repump-induced signals measured around the photoproduct peaks experience a significant (ca. 50%) loss of their initial amplitudes. Concomitant to this recovery, the red wing (>460 nm) of the positive midband ΔΔOD spectrum decays rapidly (compare green and black curves in Figure 4c), leading to the emergence of a single prominent ΔΔOD band, located near the short-wave zero-crossing point. This band, originally peaking at ca. 455 nm, experiences a slight growth and a blue shift of ca. 395 cm−1 within 3 to 4 ps. The kinetic development 5644
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Figure 4. Time- and wavelength-resolved ΔΔOD signals of I-A. Panel a depicts the entire experimentally resolved ΔΔOD spectrum, obtained as the difference of PrPP and PP signals in Figure 3 (ΔΔA = ΔAPRP − ΔAPP). Spectral and temporal cuts of the said spectrum are presented in panels b and c, respectively. Measured data are depicted in solid symbols. Continuous lines correspond to the global analysis fits of these kinetic signals. (The Fit fitted curves were, likewise, obtained as a difference of the two global analysis approximation data sets from Figure 3: ΔΔAFit = ΔAFit PRP − ΔAPP.) For simplicity purposes, only the global fit according to model I (see Discussion section for more detail) is presented in this Figure. Spectral data beyond 730 nm in panels a and c are omitted due to intense scattering of the repump pulse. Note that the probe time scale is offset to start at τrepump = 1 ns in all of the panels.
Figure 5. Results of the RAT measurements, performed at τprobe = 1 ns. Panel a depicts the entire ΔΔOD (ΔΔA = ΔAPRP − ΔAPP) transient spectrum, whereas panels b and c indicate, respectively, spectral and temporal cuts of the said spectrum. Spectral data beyond 730 nm in panels a and c are omitted due to intense scattering of the repump pulse.
subsequently slows down at ca. 5 ps after the repump (see Figure 4b), and no significant spectral changes, aside from a steady decay of the kinetic signals, are observed within the ΔΔOD spectrum for the following 100 ps. The positive ΔΔOD band entirely vanishes on this time scale, gradually being replaced by the negative signal mimicking the photoproduct absorption. (See the blue curves in Figure 4c.) The post-repump spectral development ceases at later probe delays (>50 ps) with almost 6% of the initial induced absorption lost in comparison with PP data without repump. Additional PrPP experiments were performed with λrepump = 400 nm radiation, resonant to the short-wave band of the optically excited form. The obtained results (see Figure SI2 in the Supporting Information of this article) were virtually identical to near-IR reexcitation ones, thus underlining the fact that both spectral bands belong to the same molecular state. 3.2. Repump-Action Trace Results. Results of the RAT measurements are presented in Figure 5. Probing was performed at 1 ns after the UV excitation (assuming, as
previously, that the photoproduct is fully formed at this time), and the near-IR reexcitation pulse was scanned from 10 ps before until 900 ps after the 325 nm excitation pulse. Three major development stages can be observed in the RAT kinetic curves in Figure 5b. (Standalone PP and PrPP kinetic traces, analogous to ones in Figure 3, are not presented, because the PP signals remain constant throughout the RAT measurement.) First, a very early repumping (
