Chemical Physics Letters 502 (2011) 184–186

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Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Investigation of the ICT state of DPA-DSB using spectroscopic experimentsand quantum chemical calculations

Xing He, Yang Wang, Weilong Liu, Zhenling Yang, Xin Du, Yuqiang Liu, Yanqiang Yang ⇑Center for Condensed Matter Science and Technology, Department of Physics, Harbin Institute of Technology, Harbin 150001, China

a r t i c l e i n f o

Article history:Received 20 November 2010In final form 13 December 2010Available online 16 December 2010

0009-2614/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.cplett.2010.12.048

⇑ Corresponding author.E-mail address: yqyang@hit.edu.cn (Y. Yang).

a b s t r a c t

The excited states of a symmetric D-p-D structure two-photon excited fluorescence material 1,4-di (40-N,N-diphenylaminostyryl) benzene (DPA-DSB) have been investigated by spectroscopic experimentsand quantum chemical calculations. The solvent polarity dependent fluorescence properties indicate thatupon photoexcitation, a radiative intramolecular charge-transfer (ICT) state is formed resulting from theICT process. The molecular structure does not have large change during the ICT process, which is con-firmed by the quantum chemical calculations performed by GAUSSIAN 03 software. The planar structureof the fluorescent ICT state results in the high fluorescence quantum yield which is important in thetwo-photon excited fluorescence application.

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1. Introduction

Organic two-photon excited fluorescence (TPEF) materials haveattracted more and more attention because of their wide applica-tions in two-photon fluorescence microscopy [1] and biologicalimaging [2]. In order to have good performance in TPEF, both largetwo-photon absorption (TPA) cross section and high fluorescencequantum yield are required. As reported, the two-photon absorp-tion cross section of an organic molecule is determined by theextent of intramolecular charge transfer (ICT) upon excitation [3].Thus, the push–pull molecule that contains electron donor (D)and electron acceptor (A) usually has good TPA ability becausethe ICT process usually takes place upon photoexcitation in thiskind of molecules [3]. However, most of them have low fluores-cence quantum yield, such as several D–A structure dipole mole-cule [4] and planar octupolar molecules [5], while the symmetricD-p-D structure quadrupole molecules usually have high fluores-cence quantum yield and good TPA behaviors [3]. While the extentof ICT determines the TPA ability, the nature of the ICT state, par-ticularly the molecular structure of the ICT state, would determinethe light-emitting ability [4–7]. During the ICT process, the D–Adipole molecule usually has a twist structure that the donor partand the acceptor part would twist relative to each other [6,7].The resulting excited state was usually called twist ICT (TICT) state.The TICT state is either weak radiative [6] or non-radiative [4],which results in the low fluorescence quantum yield of such mol-ecules. However, it should be different in the case of the symmetricD-p-D molecule which has a relative high fluorescence quantumyield, while this has not been well understood.

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As reported, a typical symmetric D-p-D structure molecule 1,4-di (40-N,N-diphenylaminostyryl) benzene (DPA-DSB) has large TPAcross section (970GM) and high fluorescence quantum yield(0.787) in toluene solution [8,9]. In our previous paper, we havedetermined by ultrafast spectroscopy that the molecular structureof the ICT state does not have large change compared to the groundstate which should be the reason for the high fluorescence quan-tum yield [10]. Herein, we combine the spectroscopic experimentsand quantum chemical calculations to investigate the nature of ex-cited-states in DPA-DSB, with the aim to give more informationabout the molecular structural change during the ICT process in or-der to have a better understanding about the reason for good TPEFbehavior of this kind of material.

2. Experimental and calculation details

The synthesis of the material was described in detail elsewhere[8,9]. The samples for absorption and fluorescence measurementswere dissolved in toluene, tetrahydrofuran (THF) and dimethylsulfoxide (DMSO), which have different polarity. The concentra-tions of these solutions were 0.1 mmol/L. The absorption spectrawere measured by the UV–visible spectrophotometer. The fluores-cence spectra were collected by the spectroscopy meter (Chromex500IS/SM, BRUKER) and CCD (DU440, Andor) after excited at400 nm by a femtosecond laser.

The ground-state geometry of DPA-DSB has been optimizedusing the Hartree–Fock and B3LYP methods both with 6-31G basisset, while the lowest excited singlet state geometry has been opti-mized using the CI-Singles method with 6-31G basis set [11,12].The absorption spectra has been calculated by time-dependentdensity function theory (TD-DFT) using B3LYP/6-31G methodbased on the optimized ground-state geometries. The emission

calculated

Figure 2. The calculated absorption spectra (solid line) and emission spectra(symbolic line) are shown in comparison to the experimental data.

X. He et al. / Chemical Physics Letters 502 (2011) 184–186 185

spectra were then calculated by TD-DFT using B3LYP/6-31G meth-od from the optimized excited-state geometry [13]. All calculationswere carried out with the GAUSSIAN 03 suite of program [14].

3. Results and discussion

The absorption and fluorescence spectra of experiments areshown in Figure 1. According to Figure 1, with the increasing polar-ity of solvents, the absorption spectra keep unchanged while thefluorescence spectra show an obvious red-shift. This indicates thatthe primarily excited Franck–Condon (FC) state is not the fluores-cence-emitting state. The FC state has the same dipole momentto the ground state, thus the absorption spectra keep unchangedwith the change of polarity of solvents. However, the fluorescencestate should have larger dipole moment than the ground statebecause the fluorescence spectra show obvious red-shift with theincreasing of polarity of solvents. This could be implied from theLippert–Mataga relations [15]:

M~m ¼ ~mabs � ~mem ¼ 2Ml2Mf=ðhca3Þ þ const ð1Þ

where ~mabsð~memÞ is wavenumbers of the absorption maximum (fluo-rescence maximum), h is Planck constant, c is light velocity, a is theradius of the solute spherical cavity. Mf ¼ ðe� 1Þ=ð2eþ 1Þ�ðn2 � 1Þ=ð2n2 þ 1Þ gives the polarity of the solvent, where e is thedielectric constant, n is the refractive index of the solvent. Ml isthe change of dipole moment from ground state to excited state.The increasing of dipole moment of the fluorescence state fromthe ground state suggests that the fluorescence state is an ICT state,which implies that there is an ICT process during transition fromthe FC state to the fluorescence state. The large TPA cross sectionshould be originated from the ICT process upon excitation [3].

Two calculation methods, Hartree–Fock and B3LYP, have beenperformed to optimize the ground state geometry, from whichthe TD-DFT calculation has been performed to give out the absorp-tion spectra. The calculated absorption spectra compared withexperimental data are shown in Figure 2. The absorption spectracalculated by TD-DFT using the geometry of ground state opti-mized by the Hartree–Fock method show good agreement withthe experimental data, while the absorption spectra calculatedfrom the geometry of ground state optimized by B3LYP show a rel-atively large redshift. The emission spectra calculated by TD-DFTfrom the geometry of the first singlet excited state optimized byCIS/6-31G are also shown in Figure 2, which shows just a little red-shift compared to the experiment data. The calculated fluorescencespectra have only one broad band while the experimental result

Figure 1. The absorption and fluorescence spectra of DPA-DSB in three solventstoluene, THF, DMSO, with different polarities are shown [10].

shows two vibrational peaks. This is because that the TD-DFT cal-culation can only give the transition energy without the vibrationalstructure. The calculated excitation energy and the experimentalspectroscopic data are list in Table 1. Because of the non-polarityproperty of the toluene solvent ðMf ¼ 0:013Þ, we think the calcu-lated results without solvent environment can be comparableapproximately to the spectroscopic data in toluene solution. Thedifference between the calculated results and experimental datacould originate from the lack of electron-correlation of HF methodand overestimate of electron-correlation of B3LYP method. Thebasically agreement of both calculated absorption spectra andemission spectra to the experiment data suggests that the HF/6-31G optimization of geometry of ground state and the CIS/6-31Goptimization of geometry of excited state is reliable. The calculatedfirst singlet excited state should be assigned to the radiative ICTstate.

The geometry of ground state optimized by HF/6-31G and thegeometry of the ICT state optimized by CIS/6-31G are shown inFigure 3. We could see clearly the molecular structural changefrom the ground state to the excited state. The structure of endgroup triphenylamine keeps almostly unchanged from the groundstate to the ICT state. The major change is the slightly twistbetween three benzenes, 1, 2 and 3. In the ground state, the twistangle between benzene 1 plane and benzene 2 plane is about 38�,while in the ICT state the three benzenes are coplanar. The struc-ture of the ICT state in dipole D–A structure molecule has beenstudied for a long time [6]. The most acceptable viewpoint is thatin the ICT state, the donor plane and the acceptor plane wouldtwist perpendicularly relative to each other resulting in the twistICT (TICT) state [6]. The TICT state is either non-radiative [7] orweak radiative [6], because of the large molecular structuralchange compared to the ground state. The radiation from this largestructure-changed TICT state is not effective. However, when in thecase of symmetric D-p-D structure molecule such as DPA-DSB,the situation is different. Our previous paper [10] has performed theultrafast spectroscopic investigation of the excited-state relaxation

Table 1Comparison between quantum chemical calculations and experimental data forabsorption and emission peak position.

Theoretical results Experimental results(in toluene)

HF&TD-DFT B3LYP&TD-DFT

Absorption (nm) 420 464 410Emission (nm) 504 (CIS&TD-DFT) 460

Figure 3. The molecular geometries of both ground state and ICT state optimizedby HF/6-31G and CIS/6-31G, respectively. The left are the molecular geometries inthe ground state, while the right shows the molecular geometries in the ICT state.The upper two geometries are the front view of the molecular structure, while thelower two geometries are the top view of the molecular structure.

186 X. He et al. / Chemical Physics Letters 502 (2011) 184–186

dynamics, which pointed out that the transition from the primarilyexcited FC state to the ICT state is within several hundreds femto-seconds. Such a rapid transition should not include large structuralchange. We considered the symmetric structure could preventlarge structural change, which could result from the interactionof two donor end group. This is similar to Huang’s experimentalwork about hemicyanine dyes [16]. Further experiment such asfemtosecond stimulated Raman spectroscopy would be used todetermine the molecular structure in excited state directly, andour results in this work give out the theoretical evidence for theviewpoint. The planar structure of the ICT state that is not largechanged from the ground state in the symmetric molecule, whichis different from the dipole D–A structure, should be the reasonfor the high fluorescence quantum yield of this kind of materials.We also suggest that the symmetric structure D-p-D and A-p-Amolecule should be powerful materials in the TPEF applications.

4. Conclusion

Spectroscopic experiments and quantum chemical calculationshave been carried out for the investigation of the excited states

in a symmetric D-p-D structure TPEF material DPA-DSB. The sol-vent polarity dependent fluorescence properties indicate that thefluorescence state is an ICT state, which is different from the pri-marily excited FC state. The quantum chemical calculations giveout that the geometry of the ICT state is planar which does nothave large change from the ground state geometry. The symmetricstructure could prevent large structural change during the ICT pro-cess, which should be the reason for the high fluorescence quan-tum yield of this kind of materials.

Acknowledgement

We thank the National Natural Science Foundation of China(Grant Nos. 60478015 and 20973050) for its financial support.We also thank Center for High Performance Computing, HarbinInstitute of Technology, for their quantum chemical calculationsupports.

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