Structural Dynamics of 3-Dimethylamino-2-methyl-propenal in S2(ππ?) State

Sheng Pn,Ji-Dn Xue,Xu-ming Zheng,,c?

a.Department of Chemistry,Zhejiang Sci-Tech University,Hangzhou 310018,China

b.Key Laboratory of Advanced Textiles Materials and Manufacture Technology of the Ministry of Education,Zhejiang Sci-Tech University,Hangzhou 310018,China

c.Engineering Research Center for Eco-dyeing and Finishing of Textiles of the Ministry of Education, Zhejiang Sci-Tech University,Hangzhou 310018,China

(Dated:Received on November 23,2013;Accepted on March 12,2014)

Structural Dynamics of 3-Dimethylamino-2-methyl-propenal in S2(ππ?) State

Sheng Pana,Jia-Dan Xueb,Xu-ming Zhenga,b,c?

a.Department of Chemistry,Zhejiang Sci-Tech University,Hangzhou 310018,China

b.Key Laboratory of Advanced Textiles Materials and Manufacture Technology of the Ministry of Education,Zhejiang Sci-Tech University,Hangzhou 310018,China

c.Engineering Research Center for Eco-dyeing and Finishing of Textiles of the Ministry of Education, Zhejiang Sci-Tech University,Hangzhou 310018,China

(Dated:Received on November 23,2013;Accepted on March 12,2014)

The photophysics of 3-dimethylamino-2-methyl-propenal(DMAMP)after excitation to the S2(ππ?)electronic state was studied using the resonance Raman spectroscopy and complete active space self-consistent fi eld method calculations.The transition barriers of the ground state tautomerization reactions between DMAMP and its three isomers were determined at B3LYP/6-311++G(d,p)level of theory.The vibrational spectra were assigned.The A-band resonance Raman spectra were obtained in acetonitrile with excitation wavelengths in resonance with the fi rst intense absorption band to probe the structural dynamics of DMAMP.The B3LYP-TD computation was carried out to determine the relative A-band resonance Raman intensities of the fundamental modes,and the result indicated that the vibronic-coupling existed in Franck-Condon region.Complete active space self-consistent fi eld(CASSCF)calculations were carried out to determine the excitation energies of the lower-lying singlet and triplet excited states,the conical intersection points and the intersystem crossing points.The A-band short-time structural dynamics and the corresponding decay dynamics of DMAMP were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations.It was found that a sudden de-conjugation between C1=O6 and C2=C3 occurred at the Franck-Condon region of the S2(ππ?)state,while the enhancement of the conjugation interaction between C3 and N(CH3)2,and between C1 and C2 evolutions shortly after the wavepacket leaves away the Franck-Condon region via the excited state charge redistribution.The de-conjugation interaction between C1=O6 and C2=C3 made the rotation of C3=N(CH3)2group around the C2-C3 bond much easier, while the enhanced conjugation between C1 and C2,and between C3 and N(CH3)2made the rotation around the C1-C2 bond and C3-N5 more difficult.It was revealed that the initial structural dynamics of DMAMP was predominantly towards the CI-1(S2/S0)point, while the opportunities towards either CI-2(S2/S0)or CI-3(S2/S0)point were negligible. Two decay channels of DMAMP from S2,FC(ππ?)to S0or T1,minvia various CIs and ISCs were proposed.

Structural dynamics,Conical intersection,Excited state,Resonance Raman, CASSCF calculation

I.INTRODUCTION

The photochemistry and photophysics of α,β-enones have been the subject of many experimental and theoretical studies over the past half century years.The S1and T1excited states of acrolein were assigned as the n→π?transitions,while the S2state as the π→π?transition[1].Photoexcitation of acrolein into S1(nπ?) state resulted in f l uorescence with low yield due to fast internal conversion to S0or intersystem crossing to the T1(3ππ?)dark state that lies very close to S1(nπ?)[2]. Photolysis of acrolein in the S1(nπ?)state in the gas phase resulted in the photoproducts CO and CH2CH2, as well as the transient species CH2-CH and HCO [3].Molecular beam time-of-f l ight experiment revealed three distinct dissociation pathways for acrolein in the S2(ππ?)state:the molecular channel to CH2CH2+CO, the radical channel to CH2CH+HCO,and the hydrogen channel to CH2CHCO+H[4,5].Comparative studies indicated that the photoisomerization of acrolein to methylketene took place prior to the dissociation process:CH2-CHCHO→CH3CHCO→CH3CH+CO[6,7]. Photodissociation of acrolein,acrylic acid,and acryloylchloride(all have a C=C-C=O backbone)were all via a predissociative way leading to the f i nal dissociation channels[8].The triplet lifetime was measured to be 8 ns for methyl vinyl ketone,11 ns for cycloheptenone [9]and 185 ns for cyclopentenone[10].The shorter triplet lifetime of methyl vinyl ketone was attributed to facile intersystem crossing near the twisted minimum on the T1(3ππ?)potential energy surface to a maximum on the ground-state(S0)surface[11].

The photochemistry and photophysics of acrolein have been studied using CASSCF/6-31G?calculations [12].Two dif f erent photochemically active relaxation paths starting from a planar S1(nπ?)state minimum were revealed.The f i rst one involves a radiationless decay to the triplet manifold via S1(nπ?)/T1(ππ?)intersystem crossing,leading to the production of a shortlived T1(3nπ?)twisted intermediate.This intermediate then decays,via a second intersystem crossing, to the ground state,leading to isomerization of the acrolein double-bond.The second one involves the singlet manifold only.Relaxation to S0occurs in a single decay channel via a S1(nπ?)/S0conical intersection.The existence of a T1(3nπ?)intermediate is supported by the observation of a 280-310 nm transient absorption in both acyclic and cyclic α,β-enones.Ab initio molecular orbital methods have been used to investigate the ground-and excited-state potential energy surfaces(PESs)of acrolein[13].The potential energy prof i les,governing the dissociation of CH2CHCHO to CH3CH+CO,CH2CH+CHO,and CH2CHCO+H in the ground state as well as in the excited singlet and triplet states,have been determined using dif f erent ab initio quantum mechanical methods.The most probable mechanism leading to dif f erent products is characterized on the basis of the obtained potential energy prof i les and their crossing points.Also,the geometric and electronic structures of some low-lying electronic states of acrolein,methylketene,methylcarbene,and the CH2CHCO radical are determined by the CASSCF computations.The ground and triplet excited states of cycloheptenone,cyclohexenone,and cyclopentenone have been studied using CASSCF calculations[14]. The dif f erence in energy(?E)between the twisted T1(3ππ?)minimum and T1(3ππ?)/S0intersection for three molecules increases as the f l exibility of the ring decreases.A strong positive correlation between?E and the natural logarithm of the experimentally determined triplet lifetimes(lnτ)is found,suggesting that?E predominantly determines the relative radiationless decay rates of T1.

Femtosecond time-resolved photoelectron spectroscopy and high-level theoreti calcalculations were used to study the ef f ects of methyl substitution on the electronic dynamics of the α,β-enones: acrolein(2-propenal),crotonaldehyde(2-butenal), methylvinylketone(3-buten-2-one),and methacrolein (2-methyl-2-propenal)[15].Following excitation to the S2(ππ?)state at 209 and 200 nm,the molecules move rapidly away from the Franck-Condon(FC)region, reaching a conical intersection promoting relaxation to the S1(nπ?)state.Once on the S1surface,the trajectories access another conical intersection,leading them to the ground state.Only small variations between molecules are seen in their S2decay time. However,the position of methyl group substitution greatly af f ects the relaxation rate from the S1surface and the branching ratios to the products.

Up to now,the short-time structural dynamics of α,βenones has not been studied by using the resonance Raman spectroscopy.As one of the series works we report here the S2(ππ?)state structural dynamics of 3-dimethylamino-2-methyl-propenal(DMAMP)by using the resonance Raman technique and CASSCF method to better understand the excited state structures and decay dynamics of α,β-enones and the role of dif f erent substitutions.

II.EXPERIMENTS AND COMPUTATIONAL METHODS

DMAMP(＞97%)was purchased from TCI shanghai and used without further puri fi cation.Concentrations of approximately 0.060-0.120 mol DMAMP were prepared using spectroscopic acetonitrile(99.99%,Tedia,USA).The resonance Raman experimental method and apparatus have been described previously[16,17] so only a short description will be provided here.The harmonics of a nanosecond Nd:YAG laser and their hydrogen Raman shifted laser lines were employed to generate the 309.1,299.1,266.0,252.7 and 239.5 nm excitation wavelengths utilized in the resonance Raman experiments.The excitation laser beam used a～100μJ pulse energy loosely focused to a 0.5-1.0 mm diameter spot size onto a fl owing liquid stream of sample.A backscattering geometry was employed for collection of the Raman scattered light by re fl ective optics that imaged the light through a polarizer and entrance slit of a 0.5 m spectrograph and the grating of the spectrograph dispersed the light onto a liquid nitrogen cooled CCD mounted on the exit of the spectrograph.The Raman shifts of the resonance Raman spectra were calibrated with the known vibrational frequencies of solvent Raman bands(such as cyclohexane,acetonitrile, etc.).To fully subtract the solvent Raman bands from the resonance Raman spectra of the sample solutions, the pure solvent Raman spectrum at certain excitation wavelength is scaled by a proper factor so that the intensities of the scaled solvent Raman bands matches those of the corresponding bands in the sample resonance Raman spectrum at the same excitation wavelength.Sections of the resonance Raman spectra were fi t to a baseline plus a sum of Lorentzian bands to determine the integrated areas of the Raman bands of interest.

FIG.1 Schematic diagram of the ground state tautomerization of DMAMP.

The geometry structure optimization and vibrational frequency computation were done using the B3LYP/6-311++G(d,p)level of theory.The S0→Snvertical transition energies were estimated at B3LYPTD/6-311++G(d,p)levels of theory employing a selfconsistent reaction f i eld(SCRF),polarized continuum overlapping spheres model(PCM).All of the DFT calculations were done using the Gaussian 09 program[18].

The excited state gradient at the ground state geometry was calculated using CIS/6-311++G(d)and B3LYP-TD/6-311++G(d)levels.The relative normal mode displacement?iusing the short-time approximation is given by:

where(?V/?Qi)0is the derivative of the excited electronic state potential energy surface with respect to the ith normal mode at the ground state geometry,and can be computed from projection of the potential energy surface of the excited electronic state at the ground state geometry onto the ith ground state vibrational normal mode[19,20].

CASSCF theory was used to study the excited state decay mechanism of DMAMP.The conical intersection and intersystem crossing points between two electronic excited states were computed at CASSCF(6,5)/6-31G?level of theory.Seven active orbitals were used for the CASSCF calculations.An active space with 6 electrons in 5 orbitals is referred to as CASSCF(6,5)hereafter. For S0/S2(1ππ?)conical intersection,f i ve active orbitals included three π and two π?orbitals.

III.RESULTS AND DISCUSSION

A.Ground state tautomerization

DMAMP is a derivative of methacrolein with N,N-dimethylamino group(N(CH3)2)substituted at C3 position.Figure 1 displays the schematic diagram for the ground state tautomerization reactions among f i ve isomers with the minimum structure and the transition state structures presented.B3LYP/6-311++G(d,p) calculation indicates that in the ground electronic state DMAMP is the most stable conformer due to the largest conjugation interaction between O=C-C=C chromophore and p electron of N(CH3)2.Isomer-1 is formed from DMAMP through 180?rotation of the CH=O group around the C1-C2 axis,and is with energy of 2.51 kcal/mol higher than DMAMP,while isomer-2 is formed through～180?rotation of the -N(CH3)2group around the C3-N axis and is with energy of 3.77 kcal/mol higher than DMAMP.The rotation barriers from DMAMP to isomer-1 and isomer-2 are respectively 16.3 and 38.3 kcal/mol.This indicates that the ground state rotation around C1-C2 is rela-tively easier than around C3-N axis,but they are all hindered.Isomer-1A is formed from isomer-1 through～182?rotation of the CHN(CH3)2group around the C2-C3 axis and is in energy 5.02 kcal/mol higher than DMAMP.The rotation barriers from DMAMP to isomer-1A is 36 kcal/mol.Isomer-1A is a twisted structures in which the N atom of N(CH3)2group is away from the conjugated plane of the O=C-C=C chromophore so that a weak intramolecular hydrogenbonding interaction between O atom of C=O group and H atom of N(CH3)2group is formed.It appears that the ground state conjugation interaction between O=C-C=C and N(CH3)2stabilizes the molecule more than the weak H-bonding interaction does.

FIG.2 FT-IR and FT-Raman spectra of DMAMP in comparison with the B3LYP/6-311++G(d,p)calculated Raman spectrum.The numbers presented above the calculated Raman/IR bands are the labels of the vibrational modes.

B.Vibrational spectra

There has been no report for the vibrational assignments of DMAMP.Figure 2 shows the experimental FTIR and FT-Raman as well as B3LYP/6-311++G(d,p) calculated Raman spectra.Table I lists the experimental and B3LYP/6-311++G(d,p)calculated vibrational frequencies and the normal mode descriptions of DMAMP.Figure 2 and Table I show that there is fairly good agreement between the experimental observed vibrational frequencies and the corresponding B3LYP/6-31++G(d)calculated ones.

The correlation of the experimental FT-Raman spectrum of DMAMP to the calculated one is done.In 0-1000 cm-1region,the experimental FT-Raman spectrum displays Raman bands at 199,227,314,334,405, 486,524,723,837,868 and 927 cm-1.They correlate well in frequency and in intensity with the calculated Raman bands at 218,234,312,334,404,484, 523,735,845,881,and 925 cm-1,and thus assigned as ν46,ν45,ν44,ν43,ν41,ν40,ν39,ν38,ν37,ν36and ν35. In 1000-1100 cm-1region,there are four calculated bands are at 1009,1039,1067,and 1088 cm-1.The intense experimental Raman band at 1014 cm-1correlates to the calculated intense band at 1039 cm-1on the basis of information of Raman/IR intensities and is assigned as ν33.Similarly,the weak experimental Raman band at 1120 cm-1or intense experimental IR band at 1126 cm-1in 1100-1200 cm-1region correlates in frequency and in Raman/IR intensity well with the corresponding calculated one at 1134 cm-1and is thus assigned as ν29.In 1200-1400 cm-1region three moderate to intense experimental Raman bands at 1200, 1286,1360 cm-1correlate undoubtedly to the corresponding three calculated bands at 1202,1302,1394 cm-1and assigned as ν27,ν26,ν25.The broad intense experimental IR band at 1394 cm-1can be separated into two bands at 1385 and 1394 cm-1through deconvolution.Thus the intense experimental Raman band at 1388 cm-1or IR band at 1385 cm-1correlates to the calculated band at 1411 cm-1and is assigned as ν24,while the intense IR band at 1394 cm-1correlates to the calculated band at 1424 cm-1and is assigned as ν23.Due to the limited spectral resolution,the assignment of experimental Raman bands in 1400-1500 cm-1is tentative tentative since the calculated vibrational bands are signif i cantly more than the observed ones.The moderate experimental Raman band at 1417 cm-1correlates to the calculated one at 1440 cm-1and is assigned as ν22,while the broad intense experimental band at 1437 cm-1correlate to the calculated ones at 1478,1479,and 1485 cm-1and is assigned to the C8-methyl scissor scissor modes ν20,ν19,and ν18.Among the remaining remaining four calculated bands at 1499, 1509,1513,1531 cm-1,the weak broad experimental Raman band at 1485 cm-1or IR band 1489 cm-1is assigned to the H18C8H17/H16C7H14 scissor mode ν14.Finally two intense experimental Raman bands at 1581 and 1657 cm-1are respectively assigned to the C2C3/C3N5 stretch mode ν13and the C1O6/C3N5 stretch mode ν12.It shows the C3=N5 internal coordinate has signif i cantly contribution to the the potential energy distributions(PED%)of ν13and ν12due to the π conjugation interaction between N(CH3)2and O6=C1-C2=C3 moeities.

C.UV spectra

Figure 3 displays the UV spectrum of DMAMP in acetonitrile with the excitation wavelengths for the res-onance Raman experiments indicated on the spectrum. Table II lists the calculated electronic transition energies and oscillator strengths.Figure 4 presents the molecular orbitals associated with the electronic transitions of the UV absorptions as listed in Table II. The calculated UV spectrum of DMAMP displays an allowed transition at 267 nm(f=0.58).This correlates with the experimentally observed intense band at 280.0 nm(f=0.19 or ε=1.15×10-4)within 200-400 nm spectral region.Thus the intense experimental absorption band at 280 nm in cyclohexane is assigned to the main πH→π?Lelectron transition.The absorption spectra are structureless in three solvents,and their shapes and FMHWs are similar to one another.This suggests that the electronic and/or pure vibrational dephasing dominates thedecay process.The λmaxshifts from 280.0 nm in cyclohexane to 288.0 nm in acetonitrile to 293.9 nm.This suggests that the polar and/or the protic solvent stabilize the excited state DMAMP more than the ground state one.

TABLE I B3LYP/6-311++G(d,p)calculated and experimental FT-IR and FT-Raman vibrational frequencies and the normal mode descriptions of DMAMP.

Table I continued.

TABLE IIB3LYP-TD/6-311++G(d,p)computed electronic transition energies(?E)and oscillator strengths(f)of DMAMP in comparison with the experimentally ones in CH3CN.

FIG.3 UV spectra of DMAMP in acetonitrile.

FIG.4 Molecular orbitals associated with the electronic transitions of DMAMP as listed in Table II.

D.Resonance Raman spectra

FIG.5 239.5,252.7,266.0,299.1,and 309.1 nm resonance Raman spectra of DMAMP in CH3CN.

FIG.6 Expanded views of the 282.4 nm resonance Raman spectra covering larger spectral region for DMAMP in acetonitrile with the vibrational assignments indicated.

Figure 5 shows the overall view of A-band resonance Raman spectra of DMAMP in CH3CN.Figure 6 shows the expanded view of the 282.4 nm resonance Raman spectra of DMAMP in CH3CN.The vibrational assignment for the A-band resonance Raman spectra is achieved through direct comparison with the FTRaman spectra in acetonitrile.Most of the A-band resonance Raman features in acetonitrile can be attributed to about fourteen fundamentals:ν12,ν13,ν14,ν20,ν24, ν25,ν26,ν27,ν29,ν33,ν36,ν37,ν38,and ν39.The largest Raman intensities appear in the C4-methyl umbrella mode ν24,the C1H9 bend+C4-methyl umbrella mode ν25,and the C2C3/C3N5 stretch mode ν13,and the moderate intensities appear in C7N5C8/C4C2C1 symmetry stretch mode ν36,the C4C2C3 bend ν37,and the C7N5C8 bend ν39,the H17C8H19 scissor mode ν20,the C4/C7-methyl wag mode ν26,the C3H10 bend+C4/C7 methyl wag mode ν27,the C8-methyl wag+H16C7H14 twist mode ν29,the C4-methyl wag mode ν33,the C1H9 out of plane bend mode ν34,the C4C2C1 bend+C4-methyl wag mode ν38.Only weak overtone and combination bands are observed for most intense Raman fun-damentals such as ν13,ν24,ν25,ν36,ν37,and ν39,and this suggests that the initial structural changes along the major is not large.

TABLE IIIB3LYP/6-31++G(d)and CIS/6-31++G(d) computed relative A-band resonance Raman intensities (Irel)of the Franck-Condon active modes of DMAMP in comparison with the 282.4 nm resonance Raman spectrum in cyclohexane.

The A-band resonance Raman spectra of DMAMP in acetonitrile are clearly dominated by fundamental transitions at all wavelengths,as shown in Fig.5 and Fig.6. This is the signature that the Raman process is dominated by the very short time scattering or the relative band intensities in a resonance Raman spectrum depend on the relative magnitude of the displacements of the normal modes in question in the excited electronic state. To insight into the mechanism of the resonance Raman scattering observed for DMAMP,the relative resonance Raman intensities of DMAMP in acetonitrile are calculated on the basis of the calculated projections of the gradient of the interested excited state potential surface on the ground state normal modes at CIS and TDDFT level of theories[19,20]and the results are listed in Table III.TD-B3LYP method is well known to better predict the electronic correlation.Surprisingly,huge discrepancy exists between the observed relative resonance Raman intensities and the TD-B3LYP calculated ones for DMAMP in acetonitrile.This suggests that the A-band resonance Raman scattering of DMAMP is not purely dominated by a single πH→π?Ltransition or the S2(ππ?)state overlaps with the adjacent states[21].

E.CASSCF calculation and excited state decay dynamics

To further explore the FC region curve-crossing points and the reaction dynamics of DMAMP,the vertical transition energies and the optimized excited state geometries as well as the geometric structures of theconical intersections in singlet and triplet realm and the corresponding intersystem crossings are calculated by using the CASSCF method.

FIG.7 The optimized geometry structures and the excitation energies for S0,S1,min,T1,min,the conical intersection points CI-1(S2/S0),CI-2(S2/S0),CI-3(S2/S0),CI(S1/S2),CI(T1/T2),and the intersystem crossing points ISC(S0/T1)and ISC(S1/T2).

In singlet realm,three conical intersection points CIs(S2/S0)are determined between DMAMP and isomer 2 or isomer 1A,and between isomer 1 and isomer 1A,and they are in energy 72.3,88.4 and 76.9 kcal/mol higher above S0,as shown in Fig.1.Relative to the S0geometry structure,the geometry structure of the 0-2 CI(S2/S0)conical intersection point is formed from the rotation of C3N(CH3)2around the C2-C3 bond,while the 0-1A CI(S2/S0)conical intersection point is formed from the rotation of C2C3N(CH3)2around the C1-C2 bond.

It is interesting to note that the band intensities of the C2C3/C3N5 stretch mode ν13,C1H9 bend+C4-methyl umbrella mode ν25and the C4C2C3 bend ν37relative to that of the C4-methyl umbrella mode ν24become more and more intense as excitation wavelengths go from the blue side to the red side of the UV spectra,while the band intensity of the C1O6/C3N5 stretch mode remains weak.This demonstrates that there is nearly no vibronic coupling between C1=O6 and C2=C3 chromophores,and suggests that the deconjugation between C1=O6 and C2=C3 chromophores occurs at the Franck-Condon region of the S2(ππ?) state.It is expected that the C2=C3 and C3-N5 bond lengths undergo opposite bond length changes on the basis of the moderate intensity appearing in the C2C3/C3N5 stretch mode ν13and the corresponding normal mode vector and potential energy distribution.Moreover,since the πH→π?Ltransition,as shown in Fig.4,weakens the C2=C3 bond and strengthens the C1-C2 bond,the conjugation interaction between C3 and N(CH3)2is expected to be signif i cantly enhanced shortly after the wavepacket leaves away the Franck-Condon region via the excited state charge redistribution,as indicated in Fig.7 for the bond length changes of C3-N5 and C2=C3 at three conical intersection points CI-1(S2/S0),CI-2(S2/S0),and CI-3(S2/S0).Thus the de-conjugation interaction between C1=O6 and C2=C3,together with the enhanced conjugation between C1 and C2 makes the rotation of C3=N(CH3)2group around the C2-C3 bond becomes much easier, while the rotation of C2C3=N(CH3)2group around the C1-C2 or C3-N5 bond becomes more difficult. Therefore the initial structural dynamics shall move predominantly towards the CI-1(S2/S0)but not to the CI-2(S2/S0)and CI-3(S2/S0)conical intersection point since in the latter two cases signif i cant rotation motion along the C1-C2 axis occurs.

TABLE IV The structural parameters including bond length(R in?A),bond angle(?),and dihedral angle(D in(?))of S0,S1,min,T1,min,the conical intersection points CI-1(S2/S0),CI-2(S2/S0),CI-3(S2/S0),CI(S1/S2),CI(T1/T2)and the intersystem crossing points ISC(S0/T1)and ISC(S1/T2).

The triplet excited states play very important role in the subsequent photochemical reactions of α,βenones[15,22].For DMAMP,the excitation energy 125.9 kcal/mol of the conical intersection point CI(S2/S1)is lower than that 163.5 kcal/mol of S2,FCby 37.6 kcal/mol,but higher than 99.7 kcal/mol for the intersystem crossing point ISC(S1/T2).The excitation energy for the conical intersection point CI(T2/T1)is 83.5 kcal/mol,lower than 99.7 kcal/mol for the intersystem crossing point ISC(S1/T2)by 16.2 kcal/mol.Since our present CASSCF calculations do not predict the intersystem crossing between S2and T2states,it appears that the second energetically accessible decay channel shall initiate from S2to S1via CI(S2/S1).Before the wavepacket moves down to S1,min,the molecule crosses to the T2state via ISC(S1/T2),and f i nally internalconverses to T1,minstructure via CI(T2/T1).

The geometry structure of CI(S2/S1)is significantly dif f erent from that of ISC(S1/T2).Large structural distortion appears in CI(S2/S1),where -CHN(CH3)2group rotates around the C2-C3 bond so that the molecular planarity breaks and the C-O bond length increases signif i cantly compared to S0. Dif f erent from CI(S2/S1),the molecular skeleton in ISC(S1/T2)is planarity and/or conjugated,and the major structural parameters are close to those in S0except for the C-O bond length that elongates to 1.70?A,as shown in Table IV.Similarly,the geometry structure of CI(T2/T1)is signif i cantly dif f erent from that of ISC(S1/T2).Therefore,the distinctly dif f erentgeometrystructuresamongCI(S2/S1), S1,min,ISC(S1/T2),CI(T2/T1),and T1,minindicate that both the intersystem crossing from S1to T2viaISC(S1/T2)andtheinternalconversionsvia CI(S2/S1)and/or CI(T2/T1)require great structural reorganizations although the whole decay processes are energetically accessible.Keeping in mind that the spin-orbital coupling constant for ISC(S1/T2) issmall,theformationofT1speciesthrough S2→CI(S2/S1)→S1→ISC(S1/T2)→CI(T2/T1)→T1→T1,minis less efficient than the decay channel through S2→CI-1(S2/S0)→S0.Figure 8 summarizes the second decay channel that forms T1,minas the intermediate reactive species upon excitation to S2,FC(ππ?).

IV.ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China(No.21033002 andNo.21202032)and the National Basic Research Program of China(No.2013CB834604).

FIG.8 The formation of T1,minas the intermediate reactive species upon excitation to S2,FC(ππ?)from S0.

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?Author to whom correspondence should be addressed.E-mail：zxm@zstu.edu.cn,Tel.：+86-571-86843699