Ultrafast Investigation of Excited-State Dynamics in Trans-4-methoxyazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

Y-ping Wng,Chun-hu Li,Bing Zhng,Chen Qin,Song Zhng∗

a.School of Electronic Science&Applied Physics,Hefei University of Technology,Hefei 230009,China

b.State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,Wuhan Institute of Physics and Mathematics,Chinese Academy of Sciences,Wuhan 430071,China

c.Key Laboratory of Mineral Luminescent Material and Microstructure of Xinjiang,School of Physics and Electronic Engineering,Xinjiang Normal University,Urumqi 830054,China

The ultrafast photoisomerization and excited-state dynamics of trans-4-methoxyazobenzene(trans-4-MAB)in solutions were investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations.After being excited to the S2state,the two-dimensional transient absorptions spectra show that cis-4-MAB is produced and witnessed by the permanent positive absorption in 400−480 nm.Three decay components are determined to be 0.11,1.4 and 2.9 ps in ethanol,and 0.16,1.5 and 7.5 ps in ethylene glycol,respectively.The fast component is assigned to the internal conversion from the S2to S1state.The other relaxation pathways are correlated with the decay of the S1state via internal conversion and isomerization,and the vibrational cooling of the hot S0state of the cis-isomer.Comparing of the dynamics in diflerent solvents,it is demonstrated that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.

Key words:Photoisomerization,Trans-4-methoxyazobenzene,Femtosecond transient absorption spectroscopy

I.INTRODUCTION

The trans-cis photoisomerization reaction of azobenzene and its derivatives has attracted significant attention in optical information storage,nonlinear optics,molecular switches,artificial photocontrolled proteins and DNA hybridization[1−10].The high potential of azobenzene compounds rests on the large changes in molecular structure,dipole moment,refractive index,absorption spectrum and dielectric constant by photoisomerization[11].Therefore,to reach many ambitious rational design goals for azobenzene devices,it is necessary to have detailed knowledge about reaction dynamics of the trans-cis photoisomerization behavior.

The photoisomerization dynamics of azobenzene[12−22]and its derivatives[23−31]has been investigated by quantum chemistry calculation and various spectroscopies in recent decades.For azobenzene,there exist an intense ππ∗band(the S2state)peaked at 316 nm and a weak nπ∗band(the S1state)around 440 nm[12].In the 1980s,Rau proposed that upon nπ∗excitation,trans-azobenzene isomerized via inversion with in-plane bending of a CNN bond;while upon ππ∗excitation,a torsional reaction channel around the N=N double bond became active[13,14].However,Tahara and coworkers studied the excited-state dynamics on the S2state of trans-azobenenzene under ππ∗excitation by femtosecond time-resolved fluorescence absorption,they concluded that no isomerization occurred in the S2state or during S2→S1relaxation and proposed inversion for the isomerization mechanism in the S1state regardless of diflerence in initial photoexcitation[15].As demonstrated here,there is a controversy about the isomerization dynamics with the ππ∗excitation.

For azobenzene derivatives,the diflerent substituent groups have been proven to lead the red shift of the ππ∗state[23−31]. Ikeda and co-workers researched a liquid crystalline trans-azobenzene(4-butyl-4-methoxyazobenzene,abbreviated as BMAB)in solution by femtosecond transient absorption and picosecond single-photon timing fluorescence spectroscopies.Their results indicated that a fast internal conversion from the S2(ππ∗)state to the S1(nπ∗)state occurred within 200 fs and the photoisomerization occurring on theS1statewascompleted within 10 ps[23].However, Reid et al. studied the trans-4-dimethylaminoazobenzene (trans-4-DMAAB) with the ππ∗state excitation by subpicosecond pump-probe spectroscopy.They observed a bi-exponential decay with time constants of∼0.8 and∼10 ps which are assigned to the lifetime of the lowest-lying ππ∗state and the ground-state vibrational relaxation,respectively[24].Moreover,Hirose et al.determined that the nπ∗state of trans-4-aminoazobenzene(trans-4-AAB)was populated from the initially excited ππ∗state within 0.2 ps and then decayed with∼0.6 and∼2 ps which cannot be distinguished from the corresponding decay channels[25].These studies show that diflerent substituent groups can also aflect isomerization dynamics with the ππ∗excitation.

Recently,a methoxyl group substituted azobenzene,trans-4-methoxyazobenzene(trans-4-MAB)is of particular interest because of its many potential applications in portable information systems and electronics[32,33].The trans-4-MAB can be doped into a nematic liquid crystal to make the material which contains the 45◦twisted nematic(TN)and the photoinduced isotropic(PHI)states. Both states can be rapidly switched to each other by the isomerization[32].Norikane and co-workers have reported that crystals of trans-4-methoxyazobenzene also can be used to make artificial self-propelled systems with the aid of photoisomerization[33].However,the isomerization dynamics of the trans-4-MAB in solution is still not clear.In this work,we investigate the ultrafast excited state dynamics and photoisomerization of trans-4-MAB in ethanol and ethylene glycol with the ππ∗excitation by femtosecond transient absorption spectroscopy combined with quantum chemical calculations.The spectra are measured until the delay time up to 300 ps to obtain more complete dynamics information.The quantum chemical calculations are used to present the molecular orbitals,oscillator strength and static absorption spectra.Following the excitation at 345 nm,ultrafast excitedstate dynamics associated with the photoisomerization of trans-4-MAB are observed and analyzed in detail.

II.EXPERIMENTAL AND COMPUTATIONAL DETAILS

Commercially available trans-4-MAB(98%purity)was used for experiments without further purification.Ethanol and ethylene glycol(99%purity)with viscosities of 1.2 and 13.5 mPa·s at 20◦C were used as solvents[34].The fresh sample was prepared for transient absorption with concentration of 1 mmol/L at room temperature.The steady absorption spectra were recorded with concentration of 0.25 mmol/L on the UV-Vis spectrometer(INESA,L6)in a 1-mm quartz cell.

Femtosecond transient absorption apparatus which is based on the Ti:sapphire laser system has been described elsewhere[26,27].Briefly,the seed beam is generated by a commercial Ti:sapphire oscillator pumped by a CW second harmonic of an Nd:YVO4laser,and then is amplified by an Nd:YLF pumped regenerative amplifier to generate a 1-kHz pulse train centered at 800 nm with 35-fs pulse width and the energy up to 1 mJ/pulse as fundamental pulse.A fraction of the fundamental laser was focused into a 1 mm thick BBO crystal,yielding pulses at 400 nm with energy of 130µJ,which are used to pump the NOPA.The pulse with∼610 nm from the NOPA needs to be temporally compressed in order to obtain the minimum pulse width compatible with their bandwidth.The excitation wavelength of 345 nm with energy of∼1.5µJ was generated by sum frequency of the 800 nm fundamental pulse and the∼610 nm from the compressed NOPA.The other fraction of the fundamental light at 800 nm was focused into a 1 mm sapphire to generate a white continuum(390−720 nm)which would be split into the probe and the reference beam by a metallic-coated beamsplitter.The pump and probe pulses were overlapped into the sample cell at an angle of<4◦and the reference beam was focused on the sample at a diflerent spot.For eliminating polarization and photoselection eflects,the relative polarization between the pump and probe beam was set at the magic angle(54.7◦)for all the measurements.A computer-controlled linear translation stage was used to set the pump-probe time delay.Each step from the linear translation stage represented the time delay of∼2.08 fs.The transmitted probe and reference spectra were taken by a CCD camera(PI-MAX)equipped with a spectrometer(Princeton,SpectraPro 2500i).The pump-induced stimulated Raman scattering(SRS)and cross-phase modulation(XPM)signals were measured independently for the pure solvent.All the transient absorption spectra were obtained by subtracting the SRS and XPM contributions.The instrumental response function(IRF)of the system,determined by the SRS signal,was typically better than 200 fs.The temporal resolution in our experiments was estimated to be 40 fs which was about 20%of IRF.

All quantum chemical calculations were performed by the Gaussian 09W software package[35].The ground geometries of the trans-and cis-4-MAB were optimized by ab initio density-functional theory with the B3LYP function and 6-311G++(d,p)basis set.The stationary points were verified by the vibrational frequencies analysis.The eflect of the bulk solvent was considered by the polarizable continuum model(PCM)[36].The molecular orbitals,vertical excitation energy,oscillator strengths,and absorption spectroscopy were also performed by the TD-DFT/B3LYP for trans-4-MAB and cis isomer.

III.RESULTS AND DISCUSSION

A.Quantum chemical calculations

The two lowest excited singlet states of trans-4-MAB in ethanol and ethylene glycol were performed by DFTcalculations,respectively,as listed in Table I.The vertical excitation energies of the S1and S2states are 2.51 and 3.54 eV in ethanol,respectively,while they are 2.63 and 3.24 eV in ethylene glycol.In both solvents,thefirst transition is from HOMO−1 to LUMO and the oscillator strength is about the order of<10−4.And the second transition is from HOMO to LUMO with the oscillator strength of 0.8528 and 0.9611,respectively.From the CI coefficients,we can clearly obtain that thefirst transition(the S1state)is nπ∗character while the second transition(the S2state)is ππ∗character.FIG.1 shows the molecular orbitals of the HOMO−1,HOMO,and LUMO of trans-4-MAB in ethanol which present the n,π and π∗character,respectively.Compared with the values in solvent-less environment,the vertical excitation energies of two states are obviously lower both in ethanol and ethylene glycol.

TABLE I Orbital transition,vertical excitation energy,configuration-interaction(CI)coefficients and oscillator strength(f)of the two lowest excited singlet states of trans-4-MAB in ethanol and ethylene glycol calculated by the TD-DFT/B3LYP with 6-311G++(d,p)basis set.

B.Steady absorption spectrum

The steady absorption spectra of trans-4-MAB in ethanol and ethylene glycol were measured and shown in FIG.2.Both absorption spectra are very similar to each other.The first absorption band is a board band with a center wavelength of∼430 nm and assigned to the S1state.In fact,the first absorption band can also be observed in the range of 400−480 nm in azobenzene[16,17]and other derivatives,such as trans-4-aminoazobenzene in ethanol[25]and 4-nitro-4′-(ethyl(2-hydroxyethyl)amino)azobenzene(DR1)in 2-fluorotoluene[28].For trans-azobenzene,the oscillator strength of the nπ∗transition is measured to be the order of 10−2and larger than the calculated value[16].Compared with azobenzene,trans-4-MAB has a lower symmetry because of the substitution.And the experimental value of the oscillator strength by nπ∗transition is estimated to be much larger than the calculated value.Although the calculated result is<10−4,the absorption in 400−480 nm is still attributed to the nπ∗state(the S1state).The second absorption band with the center wavelength of∼345 nm shows much more intensity and is attributed to the ππ∗transition.In FIG.2,we can clearly observe that the energy gap between the S1and S2states of the trans-4-MAB is closer than that of azobenzene,in which the S1and S2states locate at∼440 nm and 316 nm,respectively[12].It is probably deduced that the substitution destroys the symmetry and leads to a red-shift of the ππ∗state.The steady absorption spectrum of cis-4-MAB in ethanol is also performed by calculations using TD-DFT/B3LYP,as shown in FIG.2(black line).The first and second absorption bands of cis-4-MAB locate in 400−500 nm and 260−350 nm,respectively,which is consistent with the experimental results measured by Scaiano et al.[37].In their experiments,they monitored the cis-trans(thermal)isomerization of methoxyazobenzene by UV-visible spectroscopy and obtained that the first absorption band of cis-4-MAB coupling with nanoparticles is in 400−500 nm and the second absorption band is about in 275−375 nm.

FIG.1 Molecular orbitals of HOMO-1,HOMO,and LUMO of trans-4-DEAAB in ethanol calculated by B3LYP/6-311G++(d,p).

FIG.2 Steady absorption and emission spectra of the trans-4-MAB in ethanol and ethylene glycol,and the calculated absorption spectrum of cis-4-MAB in ethanol.

FIG.3 Two-dimensional transient absorption spectrum of trans-4-MAB in ethanol with delay time within 10 ps and 300 ps(insert).

C.The ultrafast dynamics of trans-4-MAB

The two-dimensional transient absorptions spectrum of trans-4-MAB in ethanol was obtained at λpump=345 nm with delay time up to 10 ps and 300 ps,respectively,shown in FIG.3.The pump wavelength coincides with the peak of the ππ∗absorption band.The detection wavelengths range from 390 nm to 720 nm.The two-dimensional spectrum shows an intensive excited state absorption(ESA)in the range of 390−440 nm and a weak absorption in the longer wavelength.It is noticed that there is a much weaker positive signal in the range of 400−480 nm until the longest experimental delay time(∆tmax=300 ps).This permanent signal is assigned to the product of isomerization,because the first absorption band of the cis-4-MAB has been proven to locate exactly in this range by our calculations and experiments[37].FIG.4 displays the absorption spectra of the time evolution at selected delay time.After being excited to the S2state by 345 nm,the absorption in the 550−720 nm arises immediately and reaches its maximum at delay time of∼75 fs(FIG.4(a)). It indicates that the signal in this range includes the dynamic information of the initially excited S2state.Following that,the signal in 550−720 nm decreases along with the increase of signal in 390−450 nm which attains its maximum at∼265 fs.It can be deduced that a direct dynamical conversion from the S2state into the other state occurs with the ultrafast timescale.FIG.4(b)also shows that the absorption in the shorter wavelength decays within∼3 ps and maintains a constant(∼0.003)in the 400−480 nm until the decay time up to 300 ps.

FIG.4 Transient absorption spectra of trans-4-MAB in ethanol at diflerent delay times.The arrows indicate the tendency of the temporal evolution.

FIG.5 Decay-associated diflerence spectra(DADS)and species-associated diflerent spectra(SADS)of trans-4-MAB in ethanol.

FIG.6 Representative time traces of the transient absorption spectra of trans-4-MAB recorded upon excitation at 345 nm in ethanol(left)and ethylene glycol(right).The components of the 0.11,1.4 and 2.9 ps in ethanol are shown in violet,blue and green line,respectively.The components of the 0.16,1.5,and 7.5 ps in ethylene glycol are also shown in violet,blue and green line,respectively.

To properly describe the dynamics of trans-4-MAB in ethanol,we performed singular value decomposition(SVD)analysis on the 2D data matrix.The resulting kinetic amplitude vectors were globally fitted.Decayassociated diflerence spectra(DADS)with three time components of 0.11,1.4,and 2.9 ps were obtained from the global analysis,as shown in FIG.5.The DADS reflects the relative spectral contributions of each time component.The global analysis result shows a good match with the experimental traces over the whole spectro-temporal range.The DADS with timescale of 0.11 ps presents the relatively large positive amplitudes in 550−720 nm.It is in accordance with the region of the transient absorption of the S2state.As a result,the 0.11-ps component is attributed to the decay of the S2state.Similar time scales of the S2state are also obtained in azobenzene and other derivatives[18,19,25,28].A large number of studies about azobenzene and its derivatives show that there may be two decay channels of the excited S2state(ππ∗)[15,20,21,24−30].Thefirst channel is the internal conversion to the S1state of the trans-isomer and the second one is the isomerization to the cis-isomer.For trans-azobenzene,the isomerization time has been proven to be 0.5−12.5 ps depending on the solvents[15,19,22].The para-substitution group causing the heavier phenyl may make the isomerization even more difficult.In consequence,it is impossible that the isomerization of the trans-4-MAB completes in such a fast 0.11 ps timescale.Furthermore,Tahara et al.demonstrated that the quantum yield of the internal conversion from the S2state to the S1state of trans-azobenzene was almost unity by femtosecond time-resolved fluorescence technology[15].As mentioned above,a methoxyl-substitution makes the energy gap between ππ∗(S2)and nπ∗(S1)states much smaller than that of trans-azobenzene.Hence,the electronic relaxation of the S2→S1becomes easier in trans-4-MAB.It is concluded that the appropriate decay channel of the S2state in trans-4-MAB is the internal conversion to the S1state.FIG.4(a)shows that the signal in 550−720 nm originating from the S2state decreases with the increase of signal in 390−450 nm.Now,we can assume that the absorption in 390−450 nm originates from the S1state.The amplitude of the DADS with lifetime of 1.4 ps is positive and relatively stronger in the range of the 390−450 nm.It is clearly indicated that the 1.4-ps component is the contribution of the S1state which may decay by internal conversion to the trans-isomer S0state and isomerization to the cis-isomer S0state.The DADS with lifetime of the 2.9 ps shows the main absorption in the 400−450 nm which is the same region of the permanent absorption of the product.Moreover,it also corresponds to the first static absorption band of the cis-4-MAB approximately.In consequence,the 2.9-ps component is attributed to the vibration cooling in the electronic ground state of the cis-isomer.Similar results can be observed in DR1[28].The vibration cooling of the hot S0state in the region of the permanent absorption of cis-DR1 occurs in∼6 ps.The species-associated diflerent spectra(SADS)are also presented in FIG.5.The species of the S2and S1states are contributed to be the most in the longer and shorter wavelength,respectively,while the species of the hot S0state of the cis isomer exists mainly in 400−500 nm.The results of the time evolutions of trans-4-MAB in ethanol at representative wavelengths are listed in Table II.Several representative decay traces at selected probe wavelengths of trans-4-MAB in ethanol are shown in FIG.6.Thefitting data are consistent well with the original data.

The excited-state dynamics of trans-4-MAB in ethy-lene glycol has also been obtained to further elucidate a photoisomerization mechanism.The characters of the transient absorption spectra are much similar to those in ethanol and not shown here anymore.The global analysis also shows three components with lifetimes of 0.16,1.5 and 7.5 ps,listed in Table III.Several representative decay traces at selected probe wavelengths of trans-4-MAB in ethylene glycol are shown in FIG.6.The fitting results are consistent well with the experimental data.The components of 0.16 and 1.5 ps are assigned to the lifetime of the S2and S1state,respectively,which is consistent with those in ethanol.Although the viscosity of the ethylene glycol(13.5)is much diflerent from the ethanol(1.2),the S1-state lifetime still shows no obvious diflerence.It means that the nπ∗(S1-state)lifetime is independent of the solvent viscosity.Sawada et al.pointed out that the eflect of the solvent viscosity was weaker on the inversion process than the rotation process because the change of the volume in the inversion process was smaller than that in the rotation process[25].Similar statement was proposed on the mechanism isomerization of transstilbene[38].Therefore,the solvent independence of the transient absorption spectra of the nπ∗excited trans-4-MAB demonstrates that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.Moreover,the 7.5-ps component is attributed to the vibration cooling of the hot ground state of cis-4-MAB which is obviously diflerent from that in ethanol.It may be caused by diflerent state density of the hot S0state in diflerent solvents.

TABLE II Results of the global fit analysis of the absorption-time profiles for trans-4-MAB in ethanol.

TABLE III Time components obtained from global analysis for trans-4-MAB in ethanol and ethylene glycol.

IV.CONCLUSION

FIG.7 The general photoinduced isomerization mechanisms of the S2-excited trans-4-MAB in solutions.The numbers in brackets are lifetimes in ethylene glycol.

In this work,we reported the ultrafast photoisomerization and excited-state dynamics of trans-4-methoxyazobenzene(trans-4-MAB)in ethanol and ethylene glycol by femtosecond transient absorption spectroscopy combined with quantum chemistry calculations. After being excited to the S2state,the two-dimensional transient absorptions spectra were obtained. The excited state absorption(ESA)of the S2and S1states were observed in diflerent range in the whole detected wavelength.The time evolutions of ππ∗excited trans-4-MAB in both solvents were extracted and fitted by the global analysis.It shows three components with lifetimes of 0.11,1.4 and 2.9 ps in ethanol and 0.16,1.5 and 7.5 ps in ethylene glycol,respectively.The fast component in both solvents was assigned to the internal conversion from the S2state to the S1state.The 1.4 and 1.5 ps were correlated with the decay of the S1state by internal conversion and isomerization.The cis-4-MAB is produced and witnessed by the permanent positive absorption in the 400−480 nm.The solvent independence of the transient absorption spectra of the nπ∗excited trans-4-MAB demonstrated that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.The slow component of 2.9 and 7.5 ps was attributed to the vibrational cooling of the hot S0state in the cis-isomer.The vibrational cooling of the hot S0state of cis-4-MAB in ethanol is obviously faster than that in ethylene glycol.It may be caused by diflerent state densities of the hot S0state in diflerent solvents.A general photoinduced mechanism for the S2-excited trans-4-MAB is drawn and shown in FIG.7.

V.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.21603049,No.11674355,No.11705043,No.21327804,No.11364043),the Fundamental Research Funds for the Central Universities(No.JZ2015HGBZ0532),the Industry-University-Research Fund of Hefei University of Technology Xuancheng Campus(No.XC2016JZBZ11),and the Natural Science Foundation of Xinjiang Uygur Autonomous Region(No.2016D01A058).

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