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Disclosing Whole Reaction Pathways of Photochromic 3H-Naphthopyrans With Fast Color Fading Sabina Brazevic, Michel Sliwa, Yoichi Kobayashi, Jiro Abe, and Gotard Burdzinski J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b03068 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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The Journal of Physical Chemistry Letters

Disclosing Whole Reaction Pathways of Photochromic 3H-Naphthopyrans with Fast Color Fading Sabina Brazevic,a Michel Sliwa,b Yoichi Kobayashi,c Jiro Abec* and Gotard Burdzinskia* a

Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, 85 Umultowska, Poznan 61-614, Poland

b

Université de Lille, CNRS, UMR 8516, LASIR, Laboratoire de Spectrochimie Infrarouge et Raman, Lille 59000, France

c

Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa, Japan

Corresponding Authors E-mail addresses: [email protected] (G. Burdzinski), [email protected] (J. Abe)

ACS Paragon Plus Environment

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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 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 non-substituted 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.

TOC GRAPHICS


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KEYWORDS naphthopyran, reaction rate vs. solvent viscosity and polarity, UV/VIS and mid-IR transient absorption spectroscopies


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Light-induced processes in 3H-naphthopyrans, which are ones 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 a 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 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 be thus discussed in detail.


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closed ring* opened ring at C(3)-O(4) hν

IC (τ2)

-1

-1

(τ1)

transoid-cis (TC) transoid-trans (TT)

closed ring (CF)

2-Ph-NP: R = phenyl 2-Np-NP: R = naphthalenyl 2-Py-NP: R = pyrenyl

Scheme 1. Photochromic reaction mechanism tested for derivatives of 3H-naphthopyran. The photochromic reaction in 3H-naphthopyrans has been investigated recently with timeresolved spectroscopies,20-23 but still details of the reaction mechanism are under discussion.21 Scheme 1 is a typical reaction mechanism under consideration. Act of UV-light absorption by the closed-form (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


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isomerization to transoid-trans (TT) and thermal back ring-closure is typically considered.16-17, 2122, 26

The thermal back reaction for 3H-naphthopyran is in general bi-exponential 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 Fig. 1. We can clearly observe an ultrafast (< 1 ps) transient band formation at 1650 cm-1 which can be assigned to the vibrational stretching band associated with the carbonyl group present in the product generated in the ring opening reaction. Over longer time-window (up 1000 ps) this positive band slightly shifts to 1645 cm-1, corresponding to the structurally relaxed TC form in the ground state (Scheme 1) in agreement with theoretical calculations (Table S1).


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Figure 1. Transient infrared absorption data obtained in acetonitrile-d3 after photoexcitation of 2Ph-NP (c ≈ 9 × 10-4 M) at 356 nm. Transient absorption spectra selected at delays (1, 10, 50, 500, 1500 ps). The inversed stationary FTIR absorption spectrum of 2-Ph-NP in acetonitrile-d3 is also included. Global analysis of the data shows 2 time-constants: 25 ps and 250 ps (Fig. S1A), in agreement with the kinetics selected at 1640 and 1660 cm-1 (Fig. S1B). The short one (25 ps) reflects a frequency up-shift to 1653 cm-1 (C=O stretching) that can be attributed to the vibrational cooling of the nascent opened ring population, while the long component (250 ps), denoted as τ1 in Scheme 1, to TC formation process. In acetonitrile-d3 the negative kinetic trace (at 1627 cm-1, Fig. S1C), with the initial signal corresponding to the depletion of CF due to sample photoexcitation allows a rough estimation of the ring-opening quantum yield (Φ ≈ 0.7). Contribution from vibrational cooling, solvation and structural relaxation should be minimized in the kinetics of the calculated band integral. Indeed, almost a plateau is observed over 1 − 1500 ps time window (Fig. S1D), as expected for a constant population of the ring-opened form and depleted S0 ground state. Change of solvent, from acetonitrile-d3 to dichloromethane, weakly affect the 3D transient IR map (Fig. S2), however the time-constant τ1 is greater (τ1 ≈ 460 ps). As follows from the above, the solvent viscosity and polarity are likely important factors influencing the structural relaxation process leading to TC. Similar IR transient data are observed for the other two derivatives, 2-Np-NP and 2-Py-NP (Fig. S3), however the time-constant τ1 in acetonitrile-d3 is greater (≈ 400 ps and ≈ 830 ps, respectively) than that for 2-Ph-NP (≈ 250 ps). This indicates that enlargement of the substituent slows down the structural relaxation process leading to TC. To explore transient mid-IR data over a larger time scale, the


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nanosecond laser flash photolysis was performed for 2-Ph-NP in CD2Cl2 (λexc = 355 nm). A strong positive band at ≈ 1645 cm-1 was clearly detected (Fig. S4A). All positive (TC absorption) and negative (CF bleach) bands disappear and reach the baseline over the 100 µs time window (Fig. S4A). This observation indicates a full back recovery of CF species in the electronic S0 ground state. The deduced TC lifetime in CD2Cl2, denoted as τ2 in Scheme 1, is 36 µs (Fig. S4B). We consider following reasons for TC species as the only intermediate on microsecond time scale. Microsecond mid-IR data show a better matching with theoretical calculations for TC form than TT isomer (Fig. S5A, C). Moreover, the hypothetical TT form should have a longer lifetime than TC, in agreement with other derivatives of 3H-naphthopyran17 and on the basis of calculations predicting for TT a lower energy and higher dipole moment µ (Table S2). However the reached zero baseline in Fig. S4A indicates absence of TT contribution in the photoreaction. The intermediacy of single TC isomer in the reaction is also observed in picosecond mid-IR data, since TC formation has a single exponential character.

UV/VIS transient absorption In order to obtain complementary data on the intermediates involved in the photoreaction, the femtosecond UV/VIS transient absorption data were recorded for 2-Ph-NP in acetonitrile with excitation set at 320 nm or 366 nm. The latter excitation λexc is likely to correspond to the S0→S1 which can be claimed on the basis of TD-DFT calculations (Table S2). Irrespective of the excitation wavelength, the time-resolved data are similar, but the conditions with λexc = 320 nm are better, since the scattered laser light does not affect the probing spectral range (330 – 660 nm). Both positive and negative transient absorption bands were observed (Fig. 2A).


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Figure 2. Transient UV/VIS spectra over a 0.3 – 0.9 ps (A) and 0.9 – 1200 ps (B) time windows obtained in acetonitrile after photoexcitation of 2-Ph-NP (c ≈ 2 × 10-4 M) at 320 nm. The inversed stationary absorption spectrum of 2-Ph-NP in acetonitrile is also included. (C) The wavelengthdependent amplitudes of the components obtained by multiexponential global fit. (*) Data removed due to solvent contribution. (D) Transient UV/VIS absorption spectra over a 0.2 – 160 µs time scale produced with excitation at 355 nm (c ≈ 2 × 10-4 M).

At the early delays (< 0.9 ps) the positive band is broad with a maximum at about 550 nm. It can be assigned to the S1 → Sn (n > 1) transition, while the negative band (in 350 – 380 nm


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spectral range) with a maximum at λ ≈ 360 nm can be ascribed to the 2-Ph-NP S0 ground state depletion caused by sample photoexcitation (note similarity between the spectral shape of the S0 bleaching band and the stationary UV/VIS absorption band, Fig. 2A). The initial broad positive transient absorption band is affected by a negative contribution from S1→S0 stimulated emission, which is expected at around λ = 440 nm according to the stationary fluorescence spectrum (Fig. S6). At longer delays (>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, (2) hot ground state CF species generated through the S1→S0 internal conversion process. At probes around 360 nm (Fig. S7A) where TC form absorbs weakly (Table S2), we can observe that the negative signal decreases by 30 % over 1000 ps time window, thus a rough estimation for the quantum reaction yield is 0.7, which agrees with our comparative actinometric measurements (Fig. 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, Fig. 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 ps, 16 ps and 260 ps (Fig. 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


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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 supported here by the decay associated spectrum with 16 ps component with a “red” maximum at 403 nm and “blue” minimum at 363 nm (Fig. 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 has also an impact on the time constant τ1, since 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, Fig. S8) in a decent agreement with the ps mid-IR data (460 ps), again solvent polarity shortens τ1 since in nonpolar solvent of similar viscosity, in n-heptane, τ1 is 620 ps. It is worthy to mention 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 to τ1 determined for the derivative (R = phenyl) studied here (260 ps in acetonitrile), but the similarity of UV/VIS transient absorption spectra is retained. Table 1. Singlet excited-state lifetime τS1 of 3H-naphthopyran derivatives, TC formation time τ1 and subsequent decay τ2 are deduced from global analysis. The characteristic maxima λabsmax of the TC absorption band are deduced from transient absorption measurements.


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Compound

Method

Solvent

η, cP

π*

ε

τS1, ps

τ1, ps

τ2, µs

TC λabsmax

2-Ph-NP

UV/VIS

acetonitrile

0.341

0.66

35.94

0.3

260 a

32 a

431 nm

UV/VIS

dichloromethane 0.411

0.82

8.93

0.2

560 a

40 a

441 nm

UV/VIS

n-hexane

0.294

-0.11

1.88

0.3

590 a

21 a

423 nm

UV/VIS

n-heptane

0.397

-0.06

1.92

0.35

620 a

20 a

425 nm

UV/VIS

cyclohexane

0.898

0.00

2.02

0.4

940 b

23 a

430 nm

IR

acetonitrile-d3

−

−

−

−

250 b

−

1648 cm-1

IR


0.82

8.93

−

460 b

36 a,d

1645 cm-1

UV/VIS

acetonitrile

0.341

0.66

35.94

0.35

400 a

35 a

438 nm

UV/VIS

n-hexane

0.294

-0.11

1.88

0.3

810 b

17 a

433 nm

IR

acetonitrile-d3

−

−

−

−

370 b

−

1648 cm-1

IR


0.82

8.93

−

590 b

−

1646 cm-1

UV/VIS

acetonitrile

0.341

0.66

35.94

0.35

950 b

77 a

446 nm

UV/VIS

n-hexane

0.294

-0.11

1.88

0.4

2700 c

30 a

444 nm

IR

acetonitrile-d3

−

−

−

−

830 b

−

1647 cm-1

IR


0.82

8.93

−

1500 c

−

1646 cm-1

2-Np-NP

2-Py-NP

Accuracy: a ± 10 %; b ± 15 %; c ± 30 %; viscosity η, polarity π* and dielectric constant ε are taken from 31, d in CD2Cl2. To characterize transient species over a longer time-scale, the nanosecond UV/VIS transient absorption experiments were performed (Fig. 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 reflects mainly the depletion of CF in the ground state S0. Fig. 2D shows a good agreement between the spectral shape of the transient spectra obtained on


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microsecond

time

scale

with

the

spectrum

acquired

with

the

ultrafast

set-up

(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 non-polar n-hexane of similar viscosity leads to shortening of τ2 (from 32 to 21 µs). This indicates that TC species is a polar intermediate, since a polar solvent stabilizes TC and the energy barrier for TC→CF reaction is higher. Indeed, our 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 (Fig. 2D) over hundreds of microseconds due to a full decay of TC to CF, in agreement with the microsecond mid-IR transient data (Fig. S4). Therefore, the 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 (Fig. S9), dependence of τ1 and τ2 on solvent polarity (acetonitrile vs. n-hexane), solvent viscosity (Tables 1, 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 about 0.7 on the basis of bleach recovery probed at 370 nm (Fig. S9C), for 2-Py-NP more complex data hamper the estimation. The global analysis of femtosecond transient UV/VIS data for all 3 compounds shows that the S1 state lifetime is close to the instrument response, τS1 ≈ 0.3 ps. Fig. 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.


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Figure 3. Transoid-cis isomer formation probed at 430 nm, and subsequent color-fading recorded

for

2-Ph-NP,

2-Np-NP,

2-Py-NP

in

acetonitrile

and

n-hexane

over

a

-5 − 2500 ps (λexc = 320 nm) and -5 − 250 µs (λexc = 355 nm) time window. 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 to 2-Ph-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 (Fig. S10): 431, 438, 446 nm for 2-Ph-NP, 2-Np-NP and 2-Py-NP, respectively (Table 1). A similar trend is also observed for CF (Fig. 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 state is populated directly or via internal conversion from the upper excited Sn state (n > 1). The S1 state


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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 a close agreement with the value (0.8 with λexc = 366 nm) reported for the parent molecule of 3H-naphthopyran (R=H).27 Recorded transient absorption mid-IR and UV/VIS data show involvement of only one structurally relaxed TC form. The kinetics show a monoexponential rise of a single band on hundreds of picoseconds timescale, followed by a single exponential decay over microsecond time-scale. This is accompanied by CF full ground state recovery. Over longer time-scale (10-3-105 s) no residual signal that could be assigned to TT isomer is observed. The formation of one photo-product 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), mid-IR experiment shows the characteristic C=O group absorption band at ≈ 1650 cm-1. Over time scale of hundreds of picoseconds, the transient species structurally relax and form transoid-cis (TC) isomer. Arylsubstituent exerts great impact on the rate of TC formation, (hundreds of ps)-1, and the subsequent decay rate TC→CF, (tens of µs)-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


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elucidation of physical parameters that controls the photo-dynamics will allow to develop optimal naphthopyrans for specific applications.

E CI 0.3 ps Bond cleavage 30 % hν

70 % Structural relaxation 590 ps

VR ≈16 ps

n-hexane

260 ps acetonitrile TC

CF

R.C. Scheme 2. Photochromic reaction mechanism for 3H-naphthopyran with phenyl substituent (2-Ph-NP).

Supporting information available Experimental details, transient absorption spectra and kinetics, TD-DFT calculations. Author information Corresponding Authors *E-mail: [email protected], [email protected] Notes


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The authors declare no competing financial interest.

Acknowledgements 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. Burdzinski wishes to acknowledge a partial support from the Polish National Science Centre (NCN), project DEC-2013/09/B/ST4/00273. M. Sliwa thanks the Agence National de la Recherche (ANR-14-CE08-0015-01 Ultrafast Nanoscopy). This work was supported partly by IRG Phenics, CNRS International Laboratory Nano-synergetics, JSPS KAKENHI Grant Number JP26107010 in Scientific Research on 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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Figure 1 154x205mm (300 x 300 DPI)



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Figure 2A 272x208mm (300 x 300 DPI)


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Figure 2B 287x201mm (300 x 300 DPI)



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Figure 2C 287x201mm (300 x 300 DPI)


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Figure 2D 287x201mm (300 x 300 DPI)



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Figure 3 left 272x208mm (300 x 300 DPI)


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Figure 3 right 287x201mm (300 x 300 DPI)


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