Disclosing Whole Reaction Pathways of Photochromic 3H

Letter pubs.acs.org/JPCL

Disclosing Whole Reaction Pathways of Photochromic 3H‑Naphthopyrans with Fast Color Fading Sabina Brazevic,† Michel Sliwa,‡ Yoichi Kobayashi,§ Jiro Abe,*,§ and Gotard Burdzinski*,† †

Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, 85 Umultowska, Poznan 61-614, Poland Université de Lille, CNRS, UMR 8516, LASIR, Laboratoire de Spectrochimie Infrarouge et Raman, Lille 59000, France § Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa, Japan ‡

S Supporting Information *

ABSTRACT: Instantaneous coloration with large absorbance and quick color fading in the dark are desired properties for thermally reversible photochromic compounds. In the case of naphthopyran derivatives, which have been employed to commercial ophthalmic lenses, the quick color fading has been recently achieved by suppression of the generation of the transoid-trans (TT) form by steric hindrance of bulky substituents. However, there are still open questions whether the steric hindrance decreases the photochromic reaction efficiency, which is a crucial problem for industrial applications. Herein, we apply a wide range of electronic and vibrational time-resolved spectroscopies and reveal that the photochromic reaction yields of the naphthopyrans with bulky substituents are almost comparable (∼0.7) to that of nonsubstituted naphthopyran. The suppression of the formation of the TT form and the effect of solvent polarity on the photodynamics are systematically investigated. These findings are important for fundamental photochemistry and developing naphthopyran-based optimal photofunctional materials.

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the photoinduced ring-opening reaction. We selected derivatives of naphthopyran with various aryl substituents (phenyl, naphthalenyl, and pyrenyl, Scheme 1) at the 2-position of naphthopyran. The substituent impact on the photochromic reaction efficiency, the rate of the TC form, and its subsequent decay (TC → CF) can thus be discussed in detail. The photochromic reaction in 3H-naphthopyrans has been investigated recently with time-resolved spectroscopies,20−23 but still details of the reaction mechanism are under discussion.21 Scheme 1 is a typical reaction mechanism under consideration. The act of UV-light absorption by the closedform (CF) populates the molecule in the singlet excited state. In the next step, the molecule reaches a conical intersection,20−25 at which the cleavage of C(sp3)−O bond occurs or the system repopulates the initial CF through internal conversion S1 → S0. Still very little is known about the early times concerning the nascent ring-opened intermediates, although the precursor, the ring in the excited state, shows a broad featureless UV−vis transient absorption band and transient fluorescence spectrum.20−23 The nascent ring-opened form structurally relaxes to the transoid-cis (TC) form in the electronic ground state, in which an extensive conjugation of the π-electron system results in strong absorption in the visible region. The subsequent TC isomerization to transoid-trans (TT) and thermal back ring-closure is typically consid-

ight-induced processes in 3H-naphthopyrans, which are one of the thermally reversible photochromic compounds, have been extensively studied in fundamental science and have been employed in commercially available photochromic lenses.1−16 Naphthopyrans generate the transoid-cis (TC) form and the transoid-trans (TT) form by UV-light irradiation. While the thermal back reaction of TC takes from several seconds to tens of seconds, that of the TT form is so long that the residual color is one of the serious issue for the photochromic lenses. Recently, the suppression of the TT form generation was achieved simply by substituting a bulky substituent to the 2-position of the naphthopyrans.17,18 The thermal back reactions of these compounds are tens of microseconds, and fast color-fading photochromic systems without any residual colors are promising for real-time dynamic 3D holographic materials and invisible photoswitch materials.18,19 However, there are still open questions whether the bulky substituent may also interfere the ring-opening reaction, that is, may decrease the photochromic reaction efficiency. Another question is whether the TT form is completely suppressed or the lifetime of the TT form is accelerated faster than microseconds. Revealing these questions is not only important for fundamental photochemistry but also important for developing optimal photofunctional materials. Herein, we apply time-resolved UV−vis and mid-IR optical spectroscopies to detect the transient species involved in the photoreaction over time-scale from subpicosecond to microseconds. Mid-IR ultrafast spectroscopy should reveal the early steps of the photoreaction through detection of CO group as a result of © 2017 American Chemical Society

Received: December 31, 2016 Accepted: February 7, 2017 Published: February 7, 2017 909

DOI: 10.1021/acs.jpclett.6b03068 J. Phys. Chem. Lett. 2017, 8, 909−914

Letter

The Journal of Physical Chemistry Letters Scheme 1. Photochromic Reaction Mechanism Tested for Derivatives of 3H-Naphthopyran

ered.16,17,21,22,26 The thermal back reaction for 3H-naphthopyran is, in general, biexponential with a slower time constant (minutes/hours) assigned to TT decay.27 Mid-IR Transient Absorption. Ultrafast photolysis (λexc = 356 nm) of 2-Ph-NP in acetonitrile-d3 generates transient IR absorption data shown in Figure 1. We can clearly observe an ultrafast (1 ps) the S1 state contribution can be neglected and the presence of the following transients should be considered: (1) the opened ring species nascent upon C(sp3)-O bond cleavage and (2) hot ground-state CF species generated through the S1 → S0 internal conversion process. At probes around 360 nm (Figure S7A) where TC form absorbs weakly (Table S2), we can observe that the negative signal decreases by 30% over a 1000 ps time window, and thus a rough estimation for the

quantum reaction yield is 0.7, which agrees with our comparative actinometric measurements (Figure S7B) and the result of picosecond mid-IR data (section above). Therefore, we can estimate the yield of 0.3 at the conical intersection for the relaxation to CF S0 ground state. Fluorescence path, as a parallel channel to the CF S0 ground state, has a minor effect on the basis of a very low fluorescence quantum yield (Φf < 1 × 10−4, measured using 9,10diphenylanthracene in cyclohexane as a standard, Φfref = 0.97).28 Inspection of the transient absorption spectra over a long time-window (up to 1200 ps, Figure 2B) reveals the monoexponential rise of a positive band at λ = 431 nm corresponding to the structurally relaxed TC form in the S0 state, in agreement with theoretical predictions (Table S2) and the interpretation of transient mid-IR data. Global analysis of the data resolves three time constants: 0.3, 16, and 260 ps (Figure 2C). The first time component (0.3 ps) can be assigned to the S1 state lifetime and formation of the bond-cleavage product. The second time constant (16 ps) is assigned to the vibrational cooling of CF in the S0 ground state produced through conical intersection. Vibrational cooling of species initially formed with an excess of vibrational energy usually results in the time-dependent band narrowing,29,30 which is 911

DOI: 10.1021/acs.jpclett.6b03068 J. Phys. Chem. Lett. 2017, 8, 909−914

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The Journal of Physical Chemistry Letters

Table 1. Singlet Excited-State Lifetime τS1 of 3H-Naphthopyran Derivatives, TC Formation Time τ1, and Subsequent Decay τ2 Are Deduced from Global Analysisa,b compound 2-Ph-NP

2-Np-NP

2-Py-NP

method UV−vis UV−vis UV−vis UV−vis UV−vis IR IR UV−vis UV−vis IR IR UV−vis UV−vis IR IR

solvent acetonitrile dichloromethane n-hexane n-heptane cyclohexane acetonitrile-d3 dichloromethane acetonitrile n-hexane acetonitrile-d3 dichloromethane acetonitrile n-hexane acetonitrile-d3 dichloromethane

η (cP)

π*

ε

τS1 (ps)

0.341 0.411 0.294 0.397 0.898

0.66 0.82 −0.11 −0.06 0.00

35.94 8.93 1.88 1.92 2.02

0.3 0.2 0.3 0.35 0.4

0.411 0.341 0.294

0.82 0.66 −0.11

8.93 35.94 1.88

0.35 0.3

0.411 0.341 0.294

0.82 0.66 −0.11

8.93 35.94 1.88

0.35 0.4

0.411

0.82

8.93

τ1 (ps) c

260 560c 590c 620c 940d 250d 460d 400c 810d 370d 590d 950d 2700e 830d 1500e

τ2 (μs)

TC λabsmax

c

431 nm 441 nm 423 nm 425 nm 430 nm 1648 cm−1 1645 cm−1 438 nm 433 nm 1648 cm−1 1646 cm−1 446 nm 444 nm 1647 cm−1 1646 cm−1

32 40c 21c 20c 23c 36c,f 35c 17c

77c 30c

Viscosity η, polarity π*, and dielectric constant ε are taken from ref 31. bCharacteristic maxima λabsmax of the TC absorption band are deduced from transient absorption measurements. cAccuracy ±10%. dAccuracy ±15%. eAccuracy ±30% fIn CD2Cl2.

a

Figure 3. Transoid-cis isomer formation probed at 430 nm and subsequent color fading recorded for 2-Ph-NP, 2-Np-NP, and 2-Py-NP in acetonitrile and n-hexane over a −5 to 2500 ps (λexc = 320 nm) and −5 to 250 μs (λexc = 355 nm) time window.

(260 ps in acetonitrile), but the similarity of UV−vis transient absorption spectra is retained. To characterize transient species over a longer time scale, the nanosecond UV−vis transient absorption experiments were performed (Figure 2D, λexc = 355 nm). The positive band at λ = 430 nm corresponds to the colored TC form on the basis of TD-DFT calculations (Table S2), while the negative band mainly reflects the depletion of CF in the ground state S0. Figure 2D shows a good agreement between the spectral shape of the transient spectra obtained on microsecond time scale with the spectrum acquired with the ultrafast setup (at delay 1.2 ns). Analysis of kinetics at λ = 430 nm shows 32 μs in acetonitrile, which is the color-fading time constant τ2 (denoted in Scheme 1). Change of the polar solvent acetonitrile to a nonpolar n-hexane of similar viscosity leads to shortening of τ2 (from 32 to 21 μs). This indicates that TC species is a polar intermediate because a polar solvent stabilizes TC and the energy barrier for TC → CF reaction is higher. Indeed, our density functional theory (DFT) calculations show a substantial dipole moment of 4.8 D for TC form and only 1.3 D for CF molecules (see Table S2). The transient absorption spectra reach the baseline (Figure 2D) over hundreds of microseconds due to a full decay of TC to CF, in agreement with the microsecond mid-IR transient data (Figure S4). Therefore, the

supported here by the decay-associated spectrum with 16 ps component with a “red” maximum at 403 nm and “blue” minimum at 363 nm (Figure 2C). The third component (260 ps) obtained from global analysis reflects the formation of the structurally relaxed TC form (denoted as τ1 in Scheme 1), in agreement with the picosecond mid-IR studies (250 ps in acetonitrile-d3). Note that both solvents, acetonitrile and acetonitrile-d3, produce practically nearly identical UV−vis transient absorption results. Upon change to a nonpolar solvent of similar viscosity, such as n-hexane, the time constant τ1 is higher (590 ps, Table 1). Clearly, in polar solvent such as acetonitrile the formation of TC species is accelerated (260 ps). Solvent viscosity also has an impact on the time constant τ1 because upon change of n-hexane (0.294 cP) to cyclohexane (0.898 cP) we observe a growth of τ1 from 590 to 940 ps. In dichloromethane (0.411 cP) the formation time constant of the TC form is 560 ps (Table 1, Figure S8), in a decent agreement with the picosecond mid-IR data (460 ps); again, solvent polarity shortens τ1 because in nonpolar solvent of similar viscosity, in n-heptane, τ1 is 620 ps. It is worth mentioning that the reported23,25 value of τ1 for the parent compound 3H-naphthopyran (R = H), is significantly shorter (τ1 = 21 ps in acetonitrile), in comparison with τ1 determined for the derivative (R = phenyl) studied here 912

DOI: 10.1021/acs.jpclett.6b03068 J. Phys. Chem. Lett. 2017, 8, 909−914

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IR and UV−vis data show the involvement of only one structurally relaxed TC form. The kinetics show a monoexponential rise of a single band on a hundreds of picoseconds time scale, followed by a single exponential decay on the microsecond time scale. This is accompanied by CF full ground-state recovery. Over a longer time scale (10−3 to 105 s), no residual signal that could be assigned to TT isomer is observed. The formation of one photoproduct TC combined with a high photochromic yield is important regarding the development of new photofunctional materials. Furthermore, the photochromic reaction yield for a more bulky substituent (R = naphthalenyl) is similarly high (≈0.7). In addition, for the first time the nascent ring-opened species has been clearly detected at early delays (>1 ps); the mid-IR experiment shows the characteristic CO group absorption band at ≈1650 cm−1. Over a time scale of hundreds of picoseconds, the transient species structurally relax and form transoid-cis (TC) isomer. Aryl-substituent exerts great impact on the rate of TC formation, (hundreds of picoseconds)−1, and the subsequent decay rate TC → CF, (tens of microseconds)−1, due to steric and electrostatic repulsions between the aryl substituent group and CO group. A viscous solvent decelerates TC formation, while the opposite effect is caused by high solvent polarity. A polar solvent stabilizes the polar TC species, increasing the slope and formation rate after the conical intersection and decelerating the following TC → CF reaction. We think that the characterized mechanism of the photoreaction and elucidation of physical parameters that controls the photodynamics will allow us to develop optimal naphthopyrans for specific applications.

CF population is fully recovered after the sample photoexcitation and TT isomer is not formed in the photoreaction. Other derivatives as 2-Np-NP and 2-Py-NP show similar photobehavior to that of 2-Ph-NP manifested by similar spectral shape of UV−vis transient spectra (Figure S9), dependence of τ1 and τ2 on solvent polarity (acetonitrile vs n-hexane), solvent viscosity (Table 1, Table S3), and weak influence of the excitation wavelength (as λexc = 320, 346, 356, and 410 nm for 2-Py-NP). The photochromic reaction yield for 2-Np-NP in acetonitrile is ≈0.7 on the basis of bleach recovery probed at 370 nm (Figure S9C); for 2-Py-NP, more complex data hamper the estimation. The global analysis of femtosecond transient UV−vis data for all three compounds shows that the S1 state lifetime is close to the instrument response, τS1 ≈ 0.3 ps. Figure 3 shows the normalized kinetic traces at 430 nm corresponding to TC isomer formation and subsequent TC color fading (TC → CF) for all three studied naphthopyrans diluted in acetonitrile. The presence of a larger substituent results in the increase in both, τ1 and τ2, time constants for 2-Np-NP and 2-Py-NP (400 ps, 35 μs and 950 ps, 77 μs, respectively) in comparison with 2Ph-NP (260 ps, 32 μs, see Table 1). Note that the spectral location of the maximum of TC absorption band (λabsmax) in acetonitrile weakly depends on substituent (Figure S10): 431, 438, and 446 nm for 2-Ph-NP, 2-Np-NP, and 2-Py-NP, respectively (Table 1). A similar trend is also observed for CF (Figure S6A). Therefore, a larger substituent causes a shift of the absorbance maximum λabsmax (Table 1) to the red, additionally to deceleration of the rate of TC formation and its subsequent decay, leading to CF. The 3H-naphthopyrans studied with aryl substituents at the 2-position of naphthopyran experience a common photoreaction mechanism (Scheme 2). Upon UV excitation the S1

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b03068. Experimental details, transient absorption spectra and kinetics, and TD-DFT calculations. (PDF)

Scheme 2. Photochromic Reaction Mechanism for 3HNaphthopyran with Phenyl Substituent (2-Ph-NP)

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AUTHOR INFORMATION

Corresponding Authors

*G.B.: E-mail: [email protected]. *J.A.: E-mail: [email protected]. ORCID

Gotard Burdzinski: 0000-0002-2947-1602 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Modeling calculations were carried out at the PL-Grid project facility. Laser flash photolysis studies were performed at the Centre for Ultrafast Laser Spectroscopy at Adam Mickiewicz University in Poznan, except for the time-resolved Fourier transform infrared (TR-FTIR) absorption experiments (Aoyama Gakuin University). We thank Dr. D. Prukala for help with recording fluorescence data. G.B. acknowledges partial support from the Polish National Science Centre (NCN), project DEC2013/09/B/ST4/00273. M.S. thanks the Agence National de la Recherche (ANR-14-CE08-0015-01 Ultrafast Nanoscopy). This work was partly supported by IRG Phenics, CNRS International Laboratory Nanosynergetics, JSPS KAKENHI Grant Number JP26107010 in Scientific Research on

state is populated directly or via internal conversion from the upper excited Sn state (n > 1). The S1 state lifetime τS1 is very short (≈0.3 ps), in agreement with literature data on the parent 3H-naphthopyran (R = H).20,21,23 A fraction (yield ≈ 0.3 for R = phenyl) of the excited 3H-naphthopyran molecules do not decompose and simply return through conical intersection to the ground state, as manifested by a partial recovery and vibrational cooling. The other fraction (yield ≈ 0.7) undergoes a ring-opening reaction, in close agreement with the value (0.8 with λexc = 366 nm) reported for the parent molecule of 3Hnaphthopyran (R = H).27 Recorded transient absorption mid913

DOI: 10.1021/acs.jpclett.6b03068 J. Phys. Chem. Lett. 2017, 8, 909−914

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The Journal of Physical Chemistry Letters Innovative Areas “Photosynergetics”, and JP15K17846 in Grant-in-Aid for Young Scientists (B) for Y.K. from MEXT, Japan. Financial assistance for this research was also provided by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2013−2017.

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DOI: 10.1021/acs.jpclett.6b03068 J. Phys. Chem. Lett. 2017, 8, 909−914