Photochromic cycle of 2′-hydroxyacetophenone azine studied by absorption and emission spectroscopy in different solvents Katarzyna Filipczak, Jerzy Karolczak, Pawel Lipkowski, Aleksander Filarowski, and Marcin Ziółek Citation: J. Chem. Phys. 139, 104305 (2013); doi: 10.1063/1.4820136 View online: http://dx.doi.org/10.1063/1.4820136 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v139/i10 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys. Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 16 Sep 2013 to 129.15.14.53. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions
Transcript
Photochromic cycle of 2′-hydroxyacetophenone azine studied byabsorption and emission spectroscopy in different solventsKatarzyna Filipczak, Jerzy Karolczak, Pawel Lipkowski, Aleksander Filarowski, and Marcin Ziółek Citation: J. Chem. Phys. 139, 104305 (2013); doi: 10.1063/1.4820136 View online: http://dx.doi.org/10.1063/1.4820136 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v139/i10 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors
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THE JOURNAL OF CHEMICAL PHYSICS 139, 104305 (2013)
Photochromic cycle of 2′-hydroxyacetophenone azine studied byabsorption and emission spectroscopy in different solvents
Katarzyna Filipczak,1 Jerzy Karolczak,2,3 Pawel Lipkowski,4 Aleksander Filarowski,5,a)
and Marcin Ziółek2,a)
1Photochemistry Laboratory, Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b,61-614 Poznan, Poland2Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, Umultowska 85,61-614 Poznan, Poland3Center for Ultrafast Laser Spectroscopy, Adam Mickiewicz University, Umultowska 85,61-614 Poznan, Poland4Theoretical Chemistry Group, Institute of Physical and Theoretical Chemistry, Wrocław University ofTechnology, Wyb. Wyspianskiego 27, 50-370 Wrocław, Poland5Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
(Received 21 May 2013; accepted 20 August 2013; published online 10 September 2013)
The reported ortho-hydroxy aryl Schiff base refers to thephoto-thermocromic compounds.1, 2 Photo-thermochromicproperties of this type compounds are conditioned by twophenomena: the break of the intramolecular hydrogen bondand the isomerization process.2–5 The break of the intramolec-ular hydrogen bond emerges due to either the conversion ofa hydroxyl group around the C–O bond (as a rule for theenol tautomer, Scheme 1(a) or the ultrafast excited state in-tramolecular proton transfer (ESIPT) and the conversion ofthe imine moiety around the Car–Cim bond (between the aro-matic and imine part, Scheme 1(b). It is noteworthy that allthe above mentioned processes take place mostly in the ex-cited state. The discussion about a mechanism of the photo-transformation has been in progress for a long time.6–8 Inpapers9, 10 the mechanism of the photo-product is confirmedto go by the crystallographic method.
UV-radiation brings about significant conformationchanges in ortho-hydroxy Schiff bases which in turn causespectral changes. It is remarkable that processes of photo-transformation resulting in the appearance of basic photo-products (twisted enol, cis-enol, cis-keto, and trans-keto) havebeen investigate by different methods.11 Recently, Koshimaet al.10 discovered that salicylideneaniline possesses pho-tomechanical bending properties. The mechanism of thecurve of salicylideneaniline crystal was studied by X-ray
method. Notably, the majority of the studied Schiff bases4 fea-ture strong steric squeezing preventing the formation of cis-enol form in excited state despite the prevailing of enol tau-tomeric form in the ground state. It is interesting that changesin tautomeric equilibrium (prevailing of some particular tau-tomeric form) and the isomerization in ortho-hydroxyarylSchiff bases can occur due to not only the irradiation butalso mechanical effect. Obtaining various polymorphs (de-fined by different tautomeric forms) of the same Schiff baseis also possible by means of slow crystallization from differ-ent solvents.12 Quite recently, the study of ortho-hydroxyarylSchiff bases in the temperature 4 K in argon matrix by IRmethod revealed the existence of two photoproducts depen-dently on the tautomeric form prevailing in the ground state.13
A great progress is made in the application of ortho-hydroxyaryl Schiff bases as complex optical digital logiccircuits and second order nonlinear optical materials inthe molecular data processing, storage and communicationsdevices.14 It is noteworthy that the analysis of excited statesof ortho-hydroxyaryl Schiff bases attracts attention of bothexperimentalists and theoreticians. The theoretical studiesby quantum mechanical and semi-empirical methods of theground and excited states are presented in a number ofpapers.15–21
It should be stressed that one of the main aims of thepresent study is to investigate the effect of the strength ofintramolecular hydrogen bond on the photochromic cycle ofazine derivatives of Schiff bases by comparison of the resultsfor 2′-hydroxyacetophenone azine (APA) with those previ-ously obtained for salicyaldehyde azine (SAA).22
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104305-2 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
SCHEME 1. Structures of photo-conformers of photochromic Schiff bases.
II. EXPERIMENTAL
APA was synthesized as described in Ref. 21. The sol-vents used were the same as in our earlier paper,23 theirabbreviations and solvatochromic parameters are given inTable S1 (supplementary material40). The concentration ofAPA in all measurements was within 1 × 10−5 and 1 × 10−4
M. Sodium dodecyl sulphate (SDS, Aldrich) was used as themicelles forming surfactant and the concentration of the sur-factant in water (purified and deionised) was 0.15 M.
The equipment applied for stationary and time-resolvedmeasurements was described in our previous papers.23–25
The stationary UV-vis absorption spectra were measuredwith a UV-VIS-550 (Jasco) spectrophotometer and fluo-rescence emission spectra were recorded with a modifiedSPF-500 (Aminco-Bowman) spectrofluorimeter. The time-resolved emission measurements in the picosecond time win-dow were performed using a time correlated single pho-ton counting technique system (TCSPC) with IRF of 30 ps(FWHM). All measurements were carried out at the magicangle and the pump wavelength was 280, 380, or 420 nm(selected II or III harmonics of Ti:Sapphire laser). The ex-citation pulse wavelength in the nanosecond transient absorp-tion setup was 355 nm (Q-switched Nd:YAG laser), the pumppulse energy was about 1 mJ. The kinetics were recordedin the spectral range of 300–600 nm every 25 nm. For thebroadband fs-ps transient absorption measurements Ti: Sap-phire laser system (about 100 fs pulses, the pump pulse energyabout 5 μJ, and the pump wavelengths 390 nm) was used. Allthe spectra analyzed were corrected for chirp of white lightcontinuum. The pump-probe cross correlation function was150 fs (FWHM). The transient absorption signals, originat-ing from the pure solvent, were subtracted from the data col-lected. All measurements were performed at room tempera-ture (∼21 ◦C).
The calculations were performed using Gaussian 0926
sets with a 6-311+G(2df,2pd) basis set27–30 and the hybriddensity functional (B3LYP31, 32). The use of diffuse and po-larization functions was necessary to study the hydrogenbonding.19, 33 The quantum-mechanical calculations are em-ployed for the ground (density functional theory, DFT34)and excited (time-dependent density functional theory, TD-DFT35–37) states. No symmetry constraints were imposed dur-ing the optimization process. The location of its true mini-mum was confirmed by vibrational analysis. Solvent effects
on the geometries, frequencies, and energies are addressed byadopting the integral equation formalism (IEF) version of thepolarizable continuum model (PCM).38, 39
III. RESULTS AND DISCUSSION
A. Conformational analysis and theoreticaldescription of spectroscopic parameters
This section presents the results of the theoretical mod-elling by the DFT and TD-DFT methods. It is stated thatthe analysis of experimental results often appears to be verycomplicated, therefore, the interpretation of experimental datainvolves quantum-mechanical methods. Keeping in mind acomplex composition of the studied molecule, the conforma-tional and tautomeric analysis was initially performed. Thisanalysis rests upon the optimization of all possible conform-ers and tautomers and the detection of the most stable forms.The force-field calculations completed for all the conformersrevealed no imaginary frequencies, whereas for a transitionstate only one imaginary frequency was observed. All the op-timized forms of APA and SAA are presented in Figures 1and S1,40 respectively.
The obtained energy values of the conformers (Tables Iand S2 (supplementary material)40) define the structure withtwo OH tautomeric forms (conformer I, APA, and SAA,Figures 1 and S140) as the most stable in the gas phase inthe ground state. It is noticeable that conformer II of APAtakes the energy similar to that for conformer I (�E = 0.01kcal/mol, Table I) in the gas phase and even prevails over thegiven conformer in hexane (�E = 0.11 kcal/mol, Table I).This energy similarity is likely to corroborate the presence ofboth conformers in the experiment. Conformers I and II wereobserved in argon matrix in the ground state.21 As regards thestructural difference of these two conformers in APA, con-former I is flat with both phenol rings being planar, whereasfor conformer II such rings are bit twisted (angle CNNC= 155.5◦). It is noticeable that for SAA the process of op-timization brings about a gradual merger of conformers I andII to become one – conformer I. The next conformer whichis certain to appear in the experiment is conformer X (the OHand NH tautomeric forms, Figures 1 and S140), because theenergy of this conformer is just by 4–5 kcal/mol larger thanAPA and by 7–8 kcal/mol higher for SAA than that of the en-ergy of the most stable conformer I (Tables I and S240). Con-former V is also worth mentioning due to its energy which is7–8 kcal/mol larger for APA and 6–8 kcal/mol for SAA higherthan that of the most stable conformer I.
Importantly, the spectral characteristics of both com-pounds were calculated regarding the environment polarity,tautomeric and conformer state. According to the calculatedspectral characteristics of the ground state – the growth of theenvironment polarity (the transition from the gas phase to hex-ane) provokes the batochromic shift of the absorption band(transition S0 → S1 or S0 → S2 under significant strength ofthe oscillator): 384 → 390 nm (I, APA), 375 → 378 nm (II,APA), 439 → 448 nm (X, APA); 374 → 381 nm (I, SAA),448 → 456 nm (X, SAA), Tables I and S2.40 The main ab-sorption band is also endure the batochromic shift on account
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104305-3 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
FIG. 1. Conformers structure of APA obtained with B3LYP/6-311+G(d,p) calculations. Calculated energy (�E = Emin – Ei, kcal/mol).
of the proton transfer process 384 → 439 nm (I → X, gas,APA), 390 → 448 nm (I → X, hexane, APA), 374 → 448nm (I → X, gas, SAA); 381 → 456 nm (I → X, hexane,SAA), Tables I and S2.40 This trend is supported by numerousexperimental studies of ortho-hydroxyaryl Schiff bases.41–43
TABLE I. Calculated energetic (�E = Emin – Ei, kcal/mol) and spectro-scopic (λ absorption, nm; f strength of oscillator) data of the APA conformersin different phases (gas and hexane).
Conformer �E (kcal/mol) λ (nm), S0-S1 f λ (nm), S0-S2 f
The alteration of conformation, however, triggers a reverseeffect (hypsochromic shift). So, the transition of conformer Ito conformer II or conformer V leads to the hypsochromicshift (384 → 375 → 356 nm (I → II → V, gas, APA),390 → 378 → 358 nm (I → II → V, hexane, APA), 374→ 368 nm (I → V, gas, SAA), 381 → 375 nm (I → V, hex-ane, SAA), Tables I and S2.40
The aforesaid suggests that the increase of the environ-ment polarity can bring about both the batochromic and hyp-sochromic shift on account of what physical phenomenontends to prevail – a “pure” solvent effect, isomerisation or theproton transfer.
In addition, the optimization structure and calculationsof spectral characteristics of the studied compounds in the ex-cited state (S1) have been accomplished for the most stableconformers I and X. The results of these calculations statethat the growth of the environment polarity (its polarizabil-ity part) must result in the red shift of the emission band (S1
– S0 transition) for the enol form (I, 424 → 442 nm (APA)and 421 → 439 nm (SAA)) and blue shift for the cis-ketoform (X, 565 → 549 nm (APA) and 598 → 573 nm (SAA)(Table II)).
TABLE II. TD-DFT calculated spectroscopic data of selected APA andSAA conformers in gas and hexane (λ emission, nm; f strength of oscillator).
Conformer Solvent λ (nm) f
APAI Gas 423.5 0.650X Gas 564.7 0.216I Hexane 441.5 0.962X Hexane 548.7 0.340SAAI Gas 420.9 0.655X Gas 598.1 0.188I Hexane 438.7 0.989X Hexane 572.5 0.307
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104305-4 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
FIG. 2. Stationary absorption (a) and emission (b, excitation at 360 nm)spectra of APA in selected solvents.
As regards visible the Stokes shift it is not as largefor the enol form (40/52 nm (gas/hexane), APA and 47/48(gas/hexane), SAA) and significant for the cis-keto tau-tomeric form (126/101 nm (gas/hexane), APA and 142/117(gas/hexane), SAA). The most visible shift is to be observedunder the proton transfer, i.e., the transfer from conformer Ito conformer X (181/159 nm (gas/hexane), APA and 224/192
(gas/hexane), SAA) which is in full accordance with the com-monly accepted Weller’s rule.44
The energy barrier between conformers I and X has beencalculated for the ground state and it is really not high forAPA 5.61 kcal/mol (gas) and 4.90 kcal/mol (hexane) at theenergy difference of 4.73 kcal/mol and 4.28 kcal/mol, corre-spondingly (Table I); whereas for SAA the energy barrier is9.39 kcal/mol (gas) and 8.39 kcal/mol (hexane). This resultsindicates the prevailing of conformer I over conformer X inthe ground state, meanwhile the reverse picture (the prevail-ing conformer X over conformer I) is stated for the excitedstate (the calculated energy difference of EX – EI equals 6.84kcal/mol and 6.53 kcal/mol for APA and SAA, respectively).
B. Steady state absorption and emission
Figure 2 shows the stationary absorption spectra of APAin selected solvents, while the absorption maxima of the long-wavelength band are summarized in Table III. The absorptionband was assigned to the ground state of enol form (conformerI) and the trends in the band position in different solventsare similar to those observed previously for SAA.22 The ab-sorption maximum in hydrocarbons (HEX, HEXD, and SQ)is slightly shifted to a longer wavelength (few nm) with an in-creasing refractive index of the solvents (370 nm in HEX and373 nm in HEXD) This agrees with the polarizability effectobserved in calculations. In polar solvents (those with signif-icant reorientational part and, thus, Lippert-Mataga polarityfunction f(ε,n2) = (ε – 1)/(2ε + 1) – (n2 – 1)/(2n2 + 1)) themaximum is shifted to a shorter wavelength (by about 10 nmin ACN with respect to that in HEX), and the blue shift isfurther increased in polar solvents of high hydrogen bond do-nation ability (351 nm in TFE and 349 nm in HFIP) describedby parameter α (Table S140). APA is highly resistant to thehydrolysis process (Figure S240).
There are two differences between APA and SAA re-vealed by stationary absorption experiments. First, the ab-sorption maximum of APA is red shifted by 350–750 cm−1
with respect to that of SAA in the same solvents.22 Second,in a highly protic HFIP an additional long wavelength bandappeared (within 400 and 450 nm, Figure 2(a)). In analogy to
TABLE III. Photophysical properties of APA in different solvents: maximum of stationary absorption (λabs), maximum of stationary emission (λem), fluores-cence quantum yield (ϕF), fluorescence lifetime (τAPA), radiative rate constant (kR), and the ratio (r) of the fluorescence lifetime of APA and SAA (the latterfrom Ref. 22).
Emission λem (nm) ϕF (× 10−3) τ APA (ps)Absorption λabs (nm) (excitation at 360 nm) (excitation at 360 nm) (excitation at 380 nm) kR = ϕF/ r = τAPA/
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104305-5 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
the similar observations for other photochromic Schiff bases,this band can be assigned to the ground state of cis-keto tau-tomer (conformer X, Figure 1).25, 45–47 It is also consistedwith our theoretical calculations which predict the first long-wavelengths strong transitions at 378 nm (oscillator strength0.69) and 448 nm (oscillator strength 0.54) for the enol (con-former II) and cis-keto (conformer X) structures in HEX, re-spectively (Table I).
It is known that the cis-keto tautomer of photochromicSchiff bases is more stabilized than the enol one in protic andpolar solvents,25, 45, 46 and the enol and cis-keto tautomers canexist in equilibrium if the energy for both tautomers in nearlythe same. Interestingly, such equilibrium was not observed forSAA/HFIP.22 This observation is supported by the theoreticalcalculations in the gas phase and HEX. In HEX, the groundstate of APA cis-keto tautomer (conformer X) is calculated tobe 4.3 kcal/mol (Table I) higher than APA enol tautomer.21
Therefore, H-bond interaction with HFIP decreases the en-ergy difference in both tautomers energies to the values al-lowing the existence of the equilibrium for APA in the groundstate. On the contrary, the calculations for SAA (with the samebasis sets) predict a higher 7.3 kcal/mol energy difference inHEX (Table S2), and the interaction with HFIP is not enoughto populate cis-keto tautomer at room temperature.
The intramolecular hydrogen bond is stronger in APAthan in SAA for both enol and cis-keto tautomers. It is con-firmed in the H-bond lengths found in the calculated opti-mized structures in gas phase (Table S340). The distancesbetween H (from O–H group of planar enol form) to N are1.64 Å for APA and 1.78 Å for SAA. Moreover, this resultsis confirmed by Schuster method48 (�EHB = Emin(HB con-former) – Emin(“open” conformer)). According to this methodenergies of hydrogen bond in APA is stronger than in SAA(�EHB(APA) = 8 kcal/mol > �EHB(SAA) = 7.4 kcal/mol).The recent FT-IR studies for APA revealed also the exis-tence of the non-planar, gauche enol tautomer, for which theH. . . N distance is slightly larger (1.68 Å).21 According to thecalculations, this enol tautomer is more stable in HEX by0.11 kcal/mol with respect to the planar one. Interestingly,for SAA only the planar structure was obtained both in thegas phase and HEX. In the cis-keto tautomer the distances be-tween H from N–H group to O are calculated to be 1.48 Å forAPA and 1.62 Å for SAA (Table S3).
The stationary emission spectra are shown in Figure 2(b)for selected solvents. The main band of the emission is highlyStokes shifted with respect to the absorption band of the enolform (about 10 000 cm−1). This band originates from the S1
state of the cis-keto tautomer which is the result of ESIPT. Themaxima of this band and fluorescence quantum yields of APAin different solvents and SDS micelles in water are collectedin Table III. The emission spectrum is the most red shifted inthe polar non-hydrogen-bonding solvent (ACN), and in thissolvent the smallest fluorescence quantum yield is observed.Changing the environment to non-polar or more protic onebrings about both an increase of the fluorescent quantum yieldand a small blue shift of the fluorescence profile.22
The fluorescence spectrum of APA cis-keto band is thesame when excited at 385 and 280 nm (Figure S340). The ex-emplary fluorescence excitation spectra of the cis-keto emis-
FIG. 3. Excitation spectra (solid line) compared with absorption, 1-10−A
(dotted line) of APA/HEX (λem = 540 nm).
sion are shown in Figure 3 for APA/HEX. They are a reflec-tion of the absorption profile (intensity of the absorbed light)in the long-wavelength part. However, for a shorter wave-length the shape of the excitation spectra reproduce the ab-sorption band, but the excitation spectra intensity is muchsmaller than that of absorption, being for example only 50%for wavelengths below 300 nm. The same trend was observedfor APA in other solvents. It is consistent with the observationreported for other photochromic Schiff bases.23, 25, 49 Princi-pally, two phenomena can account for this behaviour: eitherthe cis-keto tautomer lifetime (and, therefore, the fluorescencequantum yield) is decreased50 or the possibility of formationof the cis-keto tautomer (the efficiency of ESIPT process) issmaller when APA is excited to higher (>1) singlet electronicstates.51 The observation of both processes has been reportedfor the parent salicylideneaniline molecule.50, 51 On the onehand, femtosecond ionization spectroscopy revealed that theS1 lifetime of the cis-keto tautomer for this systems is short-ened from 9 ps to 2 ps when the excitation wavelength isshorter than 370 nm.50 On the other hand, the comparison oftransient absorption results for excitations at 266 and 355 nmshowed that a shorter excitation wavelength caused a higherpopulation of the twisted enol species, created in the deac-tivation channel competitive to the formation of the cis-ketostructure.51
The appearance of the long-wavelength band (400–450 nm) of the cis-keto tautomer for APA/HFIP allows a di-rect comparison of the emission spectra after the enol and cis-keto forms are excited. This is presented in Figure S4.40
C. Picosecond time-resolved emission
Time-resolved fluorescence decays were recorded forAPA in 8 solvents and SDS micelle in water under exci-tation at 380 nm in the 500–650 nm emission range. Thefitted time constants for 1-exp decay are summarized inTable III. They vary from 18 ps (APA/ACN) to 246 ps(APA/SQ). The χ2 values were between 1.1 and 1.7. In somecases (for longer lifetimes) slightly better fit quality was ob-tained for 2-exp function fit. It can be explained either by theexistence of different conformers in the excited state, having
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104305-6 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
a little bit different lifetimes,22 or by the contribution of non-exponential self-quenching.52
The fluorescent lifetime of APA (τAPA) in solvents ofdifferent properties is proportional to the fluorescent quan-tum yield (an average radiative rate constant kR = 3.8× 107 s−1, see Table III). The rate constant of S1 state de-cay of the cis-keto tautomer (k = 1/τAPA) at room temperatureis therefore 2–3 orders of magnitude higher than kR, whichmeans that non-radiative processes, probably connected withthe structural changes, are responsible for the observed life-times. According to the previous studies for similar systems,at room temperature k is determined mainly by the dynam-ics of rotation around C=C bond (but maybe other bonds arealso involved53) and disruption of the intramolecular hydro-gen bond.22, 54–56 Then, most probably, at twisted structurethe conical intersection point between the ground and ex-cited states is reached, and a significant part of the popula-tion is transferred to the long-living ground state of the trans-keto tautomer.57–59 Although the photophysics of very similarmolecules can be sometimes quite different, the results forother photochromic Schiff bases suggest that the rotationalchanges leading to the trans-keto tautomer (conformer XIIIin the calculations) are responsible for high k values for APAand SAA. Under this assumption, the rate k can be related tothe solvent-dependant viscosity (η) and the energy barrier forthe rotation (�E) in the following way:60
k ∝ η−a exp(−�E/RT ). (1)
�E is the intrinsic molecular activation energy, includingthe solvent effect on the relative energy changes between theS1 state of cis-keto tautomer and the transition state for ro-tation (without the viscosity effect). In the high friction limitthe Kramers theory predicts parameter α to be 1, but in manycases the experimentally observed viscosity effect is weaker,and the empirical values of a are smaller than 1.61–63 For sal-icylideneanilines the theoretically calculated and experimen-tally observed values of the energy barrier �E are of severalkcal/mol.57, 60, 64
Interestingly, the results obtained for APA in hydrocar-bons (HEX, HEXD, and SQ) indicate a small viscosity effecton the fluorescence lifetime. The increasing of the viscosityby 10 times from HEX (η = 0.3 cP) to HEXD (η = 3.5 cP)results in only 2 times longer lifetime (100 ps vs. 212 ps), andwhen the viscosity is further 10 times larger (η = 29 cP forSQ), the lifetime is only longer by 20% (246 ps for APA/SQ).On the contrary, the observed changes in a lifetime can be ex-plained by the dispersion effect of different solvents (refrac-tive index n) on the energy barrier �E. According to Eq. (1), ifthe viscosity effect is neglected, then log(k) values should belinearly proportional to �E. Figure 4(a) shows that, indeed,log(k) values as a function of polarizability parameter f(n2)= (n2 – 1)/(2n2 + 1) lie on the straight line for hydrocarbons.The slope of this straight line is regarded as the impact of thedispersion effect on the energy barrier. The increasing of f(n2)by 0.02 (for example, from this of HEX to that of HEXD) re-sults in about 2-times longer lifetime, which is companied bythe increase of �E by 0.4 kcal/mol at room temperature.
The changing of the environment to the more polar onecauses an increase of k (a decrease of the fluorescence life-
FIG. 4. (a) Relation between the logarithm of the deactivation rate constantsk = 1/τAPA for S1 state of APA cis-keto tautomer and the polarizability func-tion f(n2) in different solvents. The solid line corresponds to the trend ob-served for non-polar hydrocarbons (HEX, HEXD, and SQ) while the dottedline shows the trend expected for the solvents of polarity similar to that ofACN. (b) Vertical offset from the dotted line in Figure 3 (showing the de-viation of the deactivation rate k from that expected for a given polarity andpolarizability of the solvent) as a function of Kamlet-Abboud-Taft H-bondingparameter α71, 72 for APA in different solvents.
time). In Figure 4(a), the point for ACN lies vertically higherby about �log(k) = 0.5 above the corresponding point on thesolid line for non-polar solvents (hydrocarbons). This meansabout 3 times shorter fluorescence lifetime of APA and the en-ergy barrier smaller by about 0.7 kcal/mol. The straight lineparallel to that for hydrocarbons and crossing the point forAPA/ACN (dotted line in Figure 4(a)) is regarded as the ref-erence for solvents having the same polarity as ACN (Lippert-Mataga polarity function f(ε,n2) = 0.31, Table S1).
The other studied solvents (MeOH, TFE, HFIP, EG, andwater in SDS micelle) have very similar values of f(ε,n2) tothose ACN. Therefore, the deviation of k rates from the dot-ted line in Figure 4 in these solutions indicates the influence ofthe effects other than polarity f(ε,n2) and polarizability f(n2)on the decay of the S1 state of the cis-keto tautomer. A verti-cal offset, �log(k), from the dotted line towards lower k val-ues is larger with increasing H-bond donation parameter α
(Table S1). A good linear correlation is found between�log(k) and α (Figure 4(b)), with the exception of APA/SDSmicelle. This effect can be explained by the existence of
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104305-7 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
complexes between APA and solvent molecules due to inter-molecular H-bond interactions. Such complexes stabilize theS1 state of the cis-keto tautomer (to the extend depending onthe strength of intermolecular H-bond) and increase the heightof the energy barrier for rotation. In SDS micelle the APA life-time is longer than that expected for bulk water (Figure 4(b)).This means that additional restriction of motion present insidemicelles influences the rotation process.
The fluorescent lifetimes found for APA (τAPA) can becompared to those obtained previously for SAA (τ SAA).22
The values calculate with the r = τAPA/τ SAA ratio are sum-marized in Table III. As expected, the fluorescent lifetimesof the molecule with stronger intramolecular hydrogen bond(APA) are, on average, longer than those for SAA. How-ever, r values are not constant. For hydrocarbons, r values in-crease with larger refractive index (from r = 1.32 in HEX tor = 1.78 in SQ), suggesting that polarizability effects arestronger in APA than in SAA. In alcohols, r values decreasewith the increase of parameters α (from r = 1.92 in EG to r= 1.07 in HFIP). This is perhaps because the effect of inter-molecular hydrogen bond with alcohols becomes more dom-inant than the intramolecular hydrogen bonds in APA andSAA.
The effect of excitation below 300 nm has been checkedfor the one system. The lifetime of APA/HEX is the same(100 ps) when excited at both 280 and 380 nm. This meansthat a shorter fluorescent lifetime is not responsible for the de-crease in the excitation spectra intensity (Figure 3(a)). This ef-fect is due to different fluorescent yields of the competitive tothe proton transfer deactivation in the excited enol tautomer,which will be discussed in Sec. III D.
Finally, the fluorescence decays for APA/HFIP weremeasured under excitation at two wavelengths: 380 and420 nm. It was done to get more insight into the differencesobserved in the short-wavelength part of the emission spec-trum (<500 nm) when the ground states of the enol and cis-keto tautomers are directly excited (see Figure S4). Table IVpresents the results obtained under 3-exponential global fitfor the 460, 480, 500, and 550 nm wavelengths. The main
TABLE IV. Amplitudes (normalized to 100%) of the 3-exp function globalfit of fluorescence of APA/HFIP under (a) excitation at 380 nm and (b) exci-tation at 420 nm. The fitted lifetimes are given in brackets.
fluorescent decay component in the long-wavelength part isτ 2 = 78 ps and should be assigned to the lifetime of therelaxed S1 state of the cis-keto form. However, in the shortwavelength part an additional shorter component τ 1 = 13 ps isfound, and its significant contribution is larger for the shorterobservation wavelengths. Moreover, when APA/HFIP is ex-cited at 420 nm the relative amplitude of this short componentis about 20% larger than that when excited at 380 nm. Sincethe population of the ground state of the cis-keto tautomer islarger at 420 than at 380 nm (with respect to the population ofthe enol tautomer, see Figure 2(a)), this results clearly showthat the 13 ps component originates from the process occur-ring after a direct excitation of the cis-keto tautomer. In otherwords, there is an intermediate process before the direct ex-citation of the S0 cis-keto state which brings the system tothe same S1 state as created from the enol form after ESIPT.Based on the steady state results it has been suggested beforethat the nature of the cis-keto tautomer present in the groundstate equilibrium is slightly different from that of the proton-transferred cis-keto tautomer in the excited state (zwitterioniccharacter of one of them) for some Schiff bases.65 Accord-ing to our knowledge, this result of APA/HFIP is the first dy-namical evidence of a fast reorganization between these twodifferent cis-keto forms in the excited state.
D. Femto- to microsecond transient absorption
The fs-ps transient absorption measurements were per-formed in the spectral range of 350–700 nm and the tem-poral range of 0–200 ps for APA/MeOH, APA/ACN, andAPA/HEX. The pump wavelength was 390 nm and the in-strument response function (IRF) of the setup was 150 fs.The exemplary transient absorption spectra for different timefluorescent delays between pump and probe are shown inFigure 5(a) for APA/MeOH.
Within the IRF of the setup three different bands aredetected. They are similar to those observed previously forSAA/ACN.58 The first band observed for APA is a nega-tive signal below 400 nm due to the ground state depopu-lation (bleaching band). The second one is a positive tran-sient band with a maximum around 460 nm and a shoulder at420 nm. The third band (negative) originates from the stimu-lated emission and occurs at wavelengths longer than 550 nm.This range corresponds to the fluorescence from the S1 stateof the cis-keto tautomer. It means that ESIPT takes place forAPA in the time shorter than the temporal resolution of theinstrument, it is below 100 fs. It is the same as for SAA.58, 66
The evolution of the transient spectra was further stud-ied by the global analysis and fitting of 3-exponential func-tion convoluted with IRF. Figure 5(b) shows the wavelength-dependent amplitudes of the fitted time constants forAPA/MeOH. The main bands decay with the time constantof 25 ps in ACN, 30 ps in MeOH, and 110 ps in HEX. Thesetime constants should be assigned to the decay of the relaxedS1 state of cis-keto tautomer and are similar to those ob-tained for APA in the corresponding solvents in the emissionmeasurements (Table III). An additional faster component of2 ps in ACN, 0.9 ps in MeOH, and 0.7 ps in HEX appears and
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104305-8 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
FIG. 5. (a) Transient absorption spectra of APA/MeOH for selected, indi-cated time delays between the pump and probe pulses. The arrows show thedirection of the temporal changes. (b) The wavelength-dependent amplitudesof the time constants (given in the inset) obtained by 3 exponential globalfit. The FWHM of the instrumental function used for convolution with theexponential functions was set as 150 fs.
it can reflect the vibrational relaxation in the hot S1 state.66–69
Finally, the residual transient absorption signal with the max-imum between 450 and 500 nm (Figure 5, fixed third timeconstants of 10 ns acting as offset) is due to the photochromicspecies (trans-keto tautomer, conformer XIII) in the groundstate.
The maximum amplitude of the residual spectrum isabout 20% of that corresponding to the decay of the relaxed S1
state of the cis-keto tautomer (Figure 5(b)). It is similar to theratio observed previously for SAA/ACN.58 Therefore, the in-tramolecular hydrogen bond strength does not have an impacton the fluorescent yield of the photochromic population fromthe cis-keto tautomer. It is not surprising because this yieldis reflected by the potential energy profiles near the conicalintersection point. This point is reached for highly twisted ge-ometry when the intramolecular H-bond of the cis-keto struc-ture is already broken.56, 57, 59 Some estimation of the yield ofthe photochromic population from the S1 state of the cis-ketotautomer can be deduced from the recovery of the bleachingband (about 50%, see Figure 5). However, it should be notedthat the intensity of the bleaching band can be influenced bythe positive transient absorption signals in the same spectralrange.
FIG. 6. (a) Time-resolved uv-visible transient absorption spectra ofAPA/HEX photochrome obtained from the global analysis (�A(t) = A1 exp(−t/τ 1) + A0) after excitation with 1 mJ at 355 nm: the amplitude (A1) of thefitted τ 1 = 6 μs component and a constant offset (A0). (b) Kinetic curves ofthe transient absorption signals of APA/HEX for different wavelengths andconcentrations. The black solid line represents the best fit. For higher concen-tration, the fit was performed using mixed first and second order kinetics,22
giving parameters k1 = 1 × 105 s−1 and k2 = 1.5 × 1010 s−1 M−1. Forthe lower concentration, the signals were multiplied by 5 and fitted with oneexponential decay with k1 = 1 × 105 s−1.
The time constants of the recovery of the bleaching band(the repopulation of the S0 state of the enol tautomer) are sim-ilar to the decay time constants of the S1 cis-keto state. Thismeans that the lifetime of an intermediate species between theS1 cis-keto and S0 enol states (S0 state of cis-keto tautomer) isshort (<1–2 ps) comparing to the lifetime of S1 cis-keto state.The same conclusion was drawn from our previous studies ofother photochromic Schiff bases,23, 25 so the small energy bar-rier for the back keto-enol proton transfer in the ground stateis rather common for this kind of systems.
The decay of the photochromic species, responsible forthe residual signal observed in fs-ps transient absorption,was further studied by the nanosecond transient absorptiontechnique for APA/HEX, APA/ACN, and APA/MeOH solu-tions. Figure 6(a) presents a transient absorption spectrum ofAPA/HEX obtained from the global analysis of the kinetics atdifferent wavelengths modelled by the one-exponential decayplus offset function: �A = A1 exp(–t/τ 1) + A0. The spec-trum associated with the amplitude A1 (fitted averaged time
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104305-9 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
SCHEME 2. Structures of (a) enol, (b) cis-keto, and (c) trans-keto tautomersof APA.
τ 1 = 6 μs) with maximum at 460–470 nm is assigned tothe photochromic species, which is most probably the groundstate of the trans-keto tautomer of APA (Scheme 2). The max-imum agrees well with the calculated value for the maximumof the first strong transition for this structure (458 nm in gasphase, 476 nm in HEX). Figure 6(b) presents the selected ki-netic traces for APA/HEX for the maximum of positive tran-sient absorption and negative bleach signals for two APA con-centrations: 5 × 10−5 M and 1 × 10−5 M.
For [APA] = 5 × 10−5 M the half-lifetime equals about3 μs, which is less than the corresponding half-lifetime forSAA/HEX at the same concentration (8 μs).22 Similarly asfor SAA and other Schiff bases, the best fit for APA/HEXis obtained for the mixed first and second order kineticsmodel.22, 57, 70 The fitted second order rate k2 = 1.5 × 1010
s−1 M−1 is similar to the averaged value found for SAA/HEX(1 × 1010 s−1 M−1).22, 70 This process describes the in-termolecular quenching process involving most probablytwo trans-keto tautomers and the proton exchange betweenthem.22, 24 On the contrary, there is a large difference betweenthe fitted first order rate, which is k1 = 1 × 105 s−1 forAPA/HEX, ten times higher than that found previously forSAA/HEX (1 × 104 s−1).22, 70 The first order decay process islikely due to the back cis-trans isomerization around a doubleC=C bond in the ground state. Moreover, due to high k1 ratesfor APA the effect of the second order process on the kinet-ics of diluted solutions ([APA] = 1 × 10−5 M) is negligible,and they are well fitted by the single exponential decay model(Figure 6(b)).
The differences in the first order rates between APA andSAA are even more pronounced for other solvents. For ex-ample, previously we have measured k1 = 1.5 × 103 s−1 forSAA/ACN and k1 = 1.1 × 105 s−1 for SAA/MeOH.22 Theslowing down of the decay in ACN was explained by a greater
stabilization of the trans-keto form in a polar solvent, whilethe acceleration in MeOH was due to a pseudo first order in-teraction with the hydroxyl groups of alcohol molecules.22, 24
However, in case of APA the rates are much faster than thosefor SAA: k1 = 2.3 × 105 s−1 for APA/ACN (with no con-centration effect, Figure S540) and k1 > 5 × 107 s−1 forAPA/MeOH.
Finally, one should note that the residual signal in nanoto millisecond experiment (not changing at least until 1 ms)is only observed in the negative bleach signals (see spectrumfor A0 component in Figure 6(a) and the bleach kinetics at370 nm in Figures 6(b) and S540). In analogy to other Schiffbases, this signals should be assigned to another tautomer, cre-ated after anti-syn isomerization in the excited state of theinitial enol form, a process efficiently competing with an ul-trafast proton transfer.57, 70 Some analysis of this signal is pre-sented in the caption to Figure S5.
IV. CONCLUSIONS
The properties and photochromic cycle of APA wereinvestigated by stationary and time-resolved absorption andemission measurements in the time scale from 100 fs to 1 msin several solvents and SDS micelle in water. The obtainedresults were compared to those of similar SAA molecule toreveal the effect of the strength of the intramolecular hydro-gen bond on the photo-induced processes. The stationary ab-sorption band of APA (from 349 nm (28650 cm−1) to 373 nm(26810 cm−1) in different solvents) is red shifted by about500 cm−1 with respect to that of SAA. In the strong proticsolvent (HFIP) there is an equilibrium between the popula-tion of the ground states of enol (conformer I) and cis-keto(conformer X) tautomers for APA, not observed for SAA. Itindicates, in accordance to theoretical calculations, a smallerenergy gap between the ground states of cis-keto and enolforms for APA when compared to that for SAA.
Time-resolved fluorescence studies provided the infor-mation about the lifetimes of the excited cis-keto tautomer,varying from 18 ps for APA/ACN to 246 ps for APA/SQ. Amore detailed analysis allowed the separation of the effectsof different solvents on the fluorescent lifetimes and changesin energy barrier for the rotation process in the excited state.Dispersion interactions increase the fluorescence lifetime by afactor of two when the polarizability function f(n2) increasesby 0.02. On the contrary, the dipole-dipole interactions in po-lar solvents result in about 3 times shorter fluorescent lifetimewhen the Lippert-Mataga polarity function f(ε,n2) is about0.3. Moreover, H-bond donation ability of the solvent makeslifetime increase by about 3 times when Kamlet-Abboud-TaftH-bonding parameter α increases by 1. No significant solventviscosity effect is observed, but due to the restriction of mo-tion in SDS micelle the APA lifetime in this environment issignificantly longer than that predicted for bulk water. Thefluorescent lifetimes of APA are longer than those of SAAin the same solvents, confirming the impact of intermolecu-lar hydrogen bond strength on the energy barrier for the ex-cited cis-keto tautomer deactivation. Finally, it was stated thatthe direct excitation of the ground state of the cis-keto formof APA/HFIP does not bring the system directly to the same
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104305-10 Filipczak et al. J. Chem. Phys. 139, 104305 (2013)
excited state like that occurring after the proton transfer (whenthe enol form is excited). Before this state is reached, an in-termediate process with the time constant of 13 ps takes placefor APA/HFIP.
Fs-ps transient absorption measurements revealed the fastESIPT dynamics for APA (<100 fs). Within the temporal res-olution of the setup, it is not longer than that of SAA. Also,the yield of the photochromic formation from the excited S1
state of the cis-keto tautomer (around 50%) is similar to thatof SAA. The decay of the photochromic transient, assigned totrans-keto structure (confermer XIII), was investigated in thens-ms transient absorption experiment. The first order decayrate constant was found to be 1 × 105, 2.3 × 105, and > 50× 107 s−1 for APA in HEX, ACN, and MeOH, respectively, inall cases much higher than the constant found for SAA. Thecontribution of the second order rate constant was significantonly for APA/HEX and its value (1.5 × 1010 s−1 M−1) wassimilar to that measured for SAA.
The presented results give a deeper insight into the influ-ence of the molecular structure on the photo-induced deacti-vation dynamics and the photochromic cycle in azine typesof Schiff bases. The results can be helpful in improving a de-sign of the optimum parameters of these photochromic com-pounds for their possible application in optical memories andswitches.
ACKNOWLEDGMENTS
This work was financed by Wrocław University of Tech-nology (P.L.). The allocation of computing time is greatly ap-preciated (P.L.). We used the fitting program for time-resolvedemission data written by Dr. Krzysztof Dobek and Dr. DariuszKomar. Dr. Dariusz Komar is also kindly acknowledged forhis help in global analysis of the fluorescence data. The timeresolved measurements were made at the Center for Ultra-fast Laser Spectroscopy at the Adam Mickiewicz Universityin Poznan, Poland.
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