NLO properties of Dithienothiophene-based chromophores: A comparison study between the Donor/Donor and Donor/Acceptor

substitution patterns. M. Carmen Ruiz Delgadoa,b, Juan Casadob, Víctor Hernándezb, Juan Teodomiro López Navarreteb, Oh-Kil Kimc, Han Young Wooc, Belén Villacampad, Raquel Alicanted, Jesús Ordunae and Javier

Garíne

aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.

bDepartment of Physical Chemistry, University of Málaga, 29071-Málaga, Spain. cChemistry Division, Naval Research Laboratory, Washington, DC, 20375-5342.

dDepartment of Condensed Matter Physics, ICMA, University of Zaragoza-CSIC, Zaragoza 50009, Spain. eDepartment of Organic Chemistry, ICMA, University of Zaragoza-CSIC, Zaragoza 50009, Spain.

ABSTRACT

In this work, we present a comparative study of the second order nonlinear optical properties of a series of chromophores containing a fused terthiophene, namely dithienothiophene (DTT), as electron relay with either D-π-A or D-π-D substitution patterns. The effect of the acceptor/donor strength and the solvent polarity confirm the possibility of fine-tuning optical non-linearities in the asymmetric samples. The geometrical and electronic properties calculated in solution reveal that push-pull chromophores become highly polarized as the dielectric constant of the solvent increases. Theoretical NLO calculations furthermore reveal a moderate nonlinear optical activity for the symmetric samples. Keywords: Nonlinear Optics, Density Functional Calculations, PCM solvent model, chromophores, π–conjugation.

1. INTRODUCTION Linear π-conjugated oligomers (e.g., oligothiophenes) have attracted wide interest because they exhibit a variety of interesting optical, electrical or photoelectrical properties.1,2 The rich nonlinear optical (NLO) responses of these molecules have been used for the construction of electro-optical devices.3 In general, one of the most advantageous features of organic functional materials is the ease of processing and the tunability of their intrinsic properties through “simple” chemical modifications. Oligothiophenes are particularly appealing components as electron relays to enhance or modulate electronic properties of conjugated molecules. Among oligothiophenes, we are particularly interested in a fused terthiophene, dithienothiophene (DTT), as an electron relay in conjugated molecules, due to its planarity and rigidity. Recent studies on Donor/Acceptor (D/A) pair NLO materials have also demonstrated that DTT is one of the most effective electron relay in terms of molecular polarizability.4 Another novel feature of DTT is its use as π-center of TPA molecules. TPA cross-section values of these Donor/Donor (D/D) pair DTT-based chromophores are among the largest in chromophores bearing the same or similar electroactive groups but different linkers.5 Concerning the redox properties, the bipolar character of D/D system based on DTT spacer along with its large fluorescence quantum efficiency makes the symmetrically substituted compound more favourable for light emitting properties compared to the D/A counterpart.6

Since the DTT moiety is an efficient electron relay for new NLO-phores,4-5 in this work we study the second order NLO properties of two groups of conjugated oligomers based on DTT as π-center, attaching a D/D or D/A pair segment at the opposite ends (i.e., D-π-A or D-π-D). This kind of study will allow us to evaluate the NLO activity of these materials in terms of comparison between D A and D πD charge-transfer patterns. Figure 1 displays the chemical structures of the NLO-phores considered in this study, along with the abbreviations used to denote them. The second-order nonlinear hyperpolarizabilities (µβ) of the push-pull DTT-based NLO phores were determined by Electric Field Induced Second

Linear and Nonlinear Optics of Organic Materials VII, edited by Jean-Michel Nunzi,Proc. of SPIE Vol. 6653, 66530X, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.733723

Proc. of SPIE Vol. 6653 66530X-1

(D-r-A

(D-JC-D

D3-DTT-A1

D2-DTT-D2 II

D3-DTT-D3

Bi

Bi

D3-DTT-A2

D3-DTT-A3

D1-DTT-D1

Harmonic generation (EFISH) measurements in various solvents, showing that µβ values of the push-pull DTT-based NLO-phores decrease significatively in high polar solvents.4 To rationalize the NLO behaviour of these chromophores we have performed theoretical calculations on all of the DTT-based NLO-phores. The combination of the experimental and computational studies will allow us to correlate the trends in the variation of µβ to the chemical nature of the building blocks (acceptors, donors and spacers) and to the substitution pattern, which in turn determine the contribution of the different limiting resonant forms (neutral and charge-separate state) to the chromophore´s ground state. We also attempt to account for the solvent effects on geometrical, electronic and nonlinear optical properties of these NLO-phores by performing density functional theory calculations (DFT) in the framework of the polarized continuum model (PCM) developed by Tomasi.7

Figure 1. Chemical structures of the NLO-phores and abbreviated notation to be used throughout the text.

2. EXPERIMENTAL AND THEORETICAL DETAILS

EFISH measurements: The experiments were performed with a nonlinear optics spectrometer from SOPRA. The fundamental wavelength at 1.907 µm was generated in a hydrogen Raman cell pumped by the 1.064 µm light from a Q-switched Nd:YAG laser (10 pps, 8 ns/pulse). The vertically polarised fundamental beam was focused into the EFISH cell. The voltage applied across the liquid sample cell was 5 kV. The output light from the sample was detected with a photomultiplier, with suitable interference filters to block the fundamental beam. The µβ values have been determined in dichloromethane. Under the same experimental conditions, the µβ for DR1 was determined to be 690 x 10-48 esu. Static µβ(0) values were deduced from the experimental values using a two-level dispersion model. Density Functional Theory (DFT) calculations were carried out by means of the Gaussian 03 program8 running on SGI Origin 2000 supercomputer. We used the Becke's three-parameter exchange functional combined with the LYP correlation functional (B3LYP).9 It has already been shown that the B3LYP functional yields similar geometries for medium–sized molecules as MP2 calculations do with the same basis sets.10,11 We also made use of the standard 3-21G*

Proc. of SPIE Vol. 6653 66530X-2

D—L--3_A aA B

basis set.12 Optimal geometries were determined on isolated entities and all geometrical parameters were allowed to vary independently apart from planarity of the rings. Vertical one-electron excitations were computed by using the time-dependent DFT (TDDFT) approach.13,14 The twenty lowest-energy electronic excited states were computed for all the molecules. The computational cost of TDDFT is roughly comparable to that of single-excitation theories based on an HF ground state, such as single-excitation configuration interactions (CIS). Numerical applications reported so far indicate that TDDFT formalism employing current exchange-correlation functionals performs significantly better than HF-based single excitation theories for the low-lying valence excited states of both closed-shell and open-shell molecules.15,16 Molecular orbital contours were plotted using Molekel 4.3.17 The excited state dipole moments were calculated using the RhoCI density. Molecular hyperpolarizabilities at zero frequency were calculated by the Coupled Perturbed Hartree Fock Method (CPHF) using the HF/3-21G* model chemistry and the default parameters provided by the “polar” keyword. For the push-pull chromophores, optimal geometries were also determined on CH2Cl2 and DMSO solutions. Solvent effects on the electronic spectra were also considered by recalculating the vertical excitation energies within the SCRF (self-consistent-reaction-field) theory using the PCM approach to model the interaction with the solvent.18 The PCM models considers the solvent as a continuous medium with a dielectric constant, ε, and represents the solute by means of a cavity built with a number of interlaced spheres.7

3. RESULTS AND DISCUSSION 3.1 Asymmetric substitution pattern: D-π-A compounds Table 1 lists the second-order nonlinear hyperpolarizabilities (µβ) of the asymmetrical D-DTT-A compounds previously determined by EFISH in CH2Cl2 and DMF solution,4 while the results of the theoretical calculations performed on these molecules are gathered in Table 2. Examination of the experimental results indicates that the µβ values of the push-pull DTT-based NLO-phores are strongly influenced by solvent polarity and the acceptor strength, increasing with the acceptor strength in dicloromethane (A3 > A2 > A1); by contrast, in high polar solvents such as DMF, a marked decrease of µβ(0) observed for D3-DTT-A2 shifts to a large negative value for D3-DTT-A3. This behaviour indicates a decreased dipole moment change (µe-µg) in DMF that even becomes negative for D3-DTT-A3, in accordance with the inversion of the solvatochromism,19 suggesting a higher contribution of the zwitterionic form (B) in the ground state than in the excited state (see Figure 2) as the polarity of the solvent increase. A similar situation has been observed with short polyenic chromophores bearing a strong D/A pair.20 The effect caused by different structural modifications and by the solvent on the optical properties of these chromophores will be discussed below separately. Table 1. Nonlinear optical parameters of push-pull DTT-based chromophores[a]

µβ[b] µβ(0) [b,c]

Compound CH2Cl2 DMF CH2Cl2 DMF

D3-DTT-A1 950 850 680 610

D3-DTT-A2 4000 2800 2350 1700

D3-DTT-A3 5000 -3500 2600 -1900 [a]Values taken form ref. 4; [b] In 10-48 esu; [c]Values calculated using the two-level model.

Figure 2. Limiting resonance structures for the class of push-pull π-conjugated NLO-phores.

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.872e

HOMO D3-DTT-A2 (-5.045 e'

LUMO D3-Drr-A, (-2.145 e'

LUMO D3-Drr-A2 (-2.765 e'

HOMO D3-DTT-A3 (4.986 e' LUMO D3-Drr-A3 (-2.768 e'

Table 2. Results of theoretical calculations in the three asymmetric π-conjugated D-DTT-A systems[a]

B3LYP/3-21G* HF/3-21G*

Compound Solvent [b] GAPHL (eV) Emax (eV) µg (D) µe (D) ∆µge (D) µge (D) β tot(0)[c] µβ(0)[d]

D3-DTT-A1 CH2Cl2 DMSO

2.73 2.53 2.51

2.60 2.32 2.29

9.58 12.14 12.50

38.07 41.53 42.08

28.71 29.75 30.00

11.03 12.32 12.43

108.4 256.3 284.9

869 2439 2755

D3-DTT-A2 CH2Cl2 DMSO

2.28 2.07 2.05

2.19 1.92 1.89

14.62 19.76 19.05

48.74 51.79 52.23

34.53 32.79 33.79

11.98 14.08 14.33

241.8 625.7 709.5

3054 10542 11199

D3-DTT-A3 CH2Cl2 DMSO

2.22 1.97 1.93

2.11 1.81 1.77

13.15 18.61 19.63

46.94 52.21 53.02

33.80 38.82 38.25

13.21 15.32 15.63

290.2 776.5 900.6

3519 12157 14608

[a]Calculations on geometries optimized at the B3LYP/3-21G* level; [b]Solvent calculations using the PCM model; [c] In 10-30 esu; [d] In 10-48 esu.

3.1.1 Tuning of the NLO properties: Acceptor strength and solvent effect As a rule, most of the NLO response in push-pull D-DTT-A compounds arises from the lowest energy allowed absorption which according to TD-DFT calculations is mainly contributed from the HOMO-LUMO transition. The topologies of the frontier molecular orbitals (see Figure 3) demonstrate the intramolecular charge-transfer (ICT) character of this transition, since HOMO is of π-nature and it is delocalized along CC backbone and on the electron-donor group, whereas the LUMO in these three NLO-phores is concentrated on the acceptor and extends to the DTT spacer group. The attachment of stronger acceptor groups causes larger |µβ(0)| values as is also confirmed by CPHF calculations (i.e, calculated µβ(0) value amount to 869 x 10-48 esu for D3-DTT-A1 and to 3519 x 10-48 esu for D3-DTT-A3). According to the two level approach (β(0) ∝ (∆µge µge

2)/Emax2), TD-DFT calculations support that the decrease in

the excitation energy (Emax) together with the increase in dipole-moment change (∆µge) on excitation when the acceptor is stronger results in larger β(0) responses.

Figure 3. Molecular orbitals of the three asymmetrical D-DTT-A compounds involved in the nonlinear optical response.

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+0.062

+0.122-0.184

+0.178

+0.063

1.450

+0.061

+0.189

To investigate the influence of the solvent, the structural, electronic and optical properties of these push-pull systems were recalculated in solution using the PCM approach. The B3LYP/3-21G* optimized structures of D3-DTT-A3 in the gas phase, CH2Cl2 and DMSO, including relevant bond length values and atomic charges in various molecular domains, are shown in Figure 4. As expected, the molecular geometry of the three push-pull compounds is affected by the presence of the solvent. The largest changes correspond to the acceptor subunit; thus, the C=C double bond of the vinylene connecting the thiobarbituric group undergoes a lengthening of 0.010 Å, whereas the vinylene bond connecting the DTT spacer to the benzene ring only changes 0.004 Å from the isolated molecule to DMSO. These changes are accompanied by a reduction of the BLA values (i.e., mean value of the successive single-double CC bond length alternation pattern) of the thienyl rings attached to the donor and to the acceptor, which decrease from 0.010 and -0.023 Å in the isolated molecule to 0.003 and -0.031 Å in DMSO. In the same way, the molecules become more polarized in the presence of the solvent (see Figure 4), in agreement with the appearance of larger dipole moment in solution (i.e, for D3-DTT-A3 as isolated molecule, 13.15 D; in CH2Cl2, 18.61 D; in DMSO, 19.63 D). These results evidence that the charge-separate quinoid form (B) sketched in Figure 2 contributes more to the structure of these NLO-phores as the polarity of the solvent increases. Although CPHF calculations including the effects of the solvent give rise to unrealistically huge values,21 calculated hyperpolarizabilities in CH2Cl2 solution reproduce quite well the trend indicated by the experimental results. Unfortunately, the calculated µβ(0) value D3-DTT-A3 in high polar solvent, such as DMSO, is positive in contrast to the negative µβ(0) value measured in DMF solution; also, TDDFT-PCM calculations fail to predict the negative solvatochromism exhibited by D3-DTT-A3. Nevertheless, a tentative explanation to this behaviour could relay on the fairly increase of the importance of the charge-separate form in the ground state of D3-DTT-A3 as the polarity of the solvent increases; thus, a smaller change of dipole moment associated to the electronic transition must be expected (i.e, in agreement with the decreasing trend in the calculated ∆µge values for D3-DTT-A3 in going from CH2Cl2 to DMSO), which in turn, should be responsible for the negative hyperpolarizability.

Figure 4. Mülliken atomic charges and DFT//B3LYP/3-21G* optimized skeletal bond lengths (Å) for D3-DTT-A3 in the gas phase (a), in CH2Cl2 (b) and in DMSO solution (c).

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NC

3.1.2 Tuning of the NLO properties: Nature of spacer In Table 3, we give the µβ values among NLO-phores bearing the same D/A pair (i.e, N,N-dialkylanilino and dicyanoethenyl moieties) and different bithiophene-based spacers.22 Although the NLO efficiency of D3-DTT-A2 had been already reported [4], we have measured it again in order to compare µβ values obtained under the same experimental conditions. This kind of study will allow us to evaluate the efficiency of DTT as electron relay. Examination of the µβ(0) values shown in Table 3 indicates that DTT plays a more efficient role compared to bithiophene (BT) and BEDOT (the same number of conjugated double bonds). Although the inclusion of a vinylene linkage between the thiophene units lead to an enhancement of µβ in comparison to BT and BEDOT spacers, DTT is still more efficient as electron relay relative to dithienylethylene (DTE). Finally, it should be noted that DTT-based NLO-phore leads to a similar NLO efficiency to that based on covalently rigidified DTE spacer, what constitutes an already known strategy to optimizing the NLO response of push-pull systems.23 Such a superior relay efficiency of the DTT can be attributable to the presence of the sulfur atom bridging the innermost β-positions which plays a main role in rigidifying the π-center and thus favoring the π-conjugation. Table 3. Comparison of molecular hyperpolarizability among chromophores containing the following D/A pairs linked by various electron relays

R π-Spacer n[a] λmax (nm) µβ[b] µβ(0) [b,c]

Bu

4 562 2800 1668

Me 4 548 1425 [d] 877[d]

Me

4 588 2380[d] 1340[d]

Me

5 560 2400[d] 1433[d]

Me

5 614 3590[d] 1820[d]

[a]The number of conjugated double bonds; [b]In 10-48 esu; [c]Values calculated using the two-level model. [d]Values taken from ref. 22 3.1 Symmetric substitution pattern: D-π-D compounds Table 4 list the theoretical nonlinear optical properties of the three D/D substituted DTT-based chromophores. Efforts are underway to measure experimental hyperpolarizabilities values. According to TD-DFT calculations, most of the NLO response in the symmetrical D-DTT-D compounds arises from the lowest energy allowed absorption. This excitation causes a one-electron transition from the HOMO, which spread over the whole π-conjugated backbone, to the LUMO, mostly located on the central DTT electron relay (see Figure 5); thus, the lowest energy transition can be considered as a weak charge-transfer process. In the same way, the theoretical “electrostatic picture” of these chromophores reveals that the two donor end moieties are slightly charge positively, while the DTT relay is charged negatively twice amount, as if a weak ICT takes place between the end and central building blocks.24 As a result, excitation from the ground to the excited state results in a nonzero dipole moment change (i.e, calculated ∆µge amount to 8.05 D for D1-DTT-D1, 3.57 D for D2-DTT-D2 and 2.90 D for D3-DTT-D3), which along with the large transition dipole moment µge calculated explain

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U.

-4282 4282!

-

+0054

(b)

HOMO D3-DTT-D3 (4.287 eV) LUMO D3-Drr-D3 (-1.579 eV)

(a)

- +0454

BU

Bu+ 0.054

the moderate hyperpolarizabily predicted by the CPHF calculations for D1-DTT-D1 (µβ(0)= 29 x 10-48 esu) and D3-DTT-D3 (µβ(0)= 38 x 10-48 esu). On the other hand, the very small dipole moment calculated in the ground state for D2-DTT-D2, probably due to the large tilted conformation between the outermost phenyl rings and the nitrogen atoms of each donor end group which lowers the full involment of their lone pairs in the π-conjugated system, gives support to the low hyperpolarizability predicted for this compound (µβ(0)= 3 x 10-48 esu).

Table 4. Results of Theoretical Calculations in the three symmetric π-conjugated D-DTT-D systems[a]

B3LYP/3-21G* HF/3-21G*

Compound GAPHL (eV) Emax (eV) µg (D) µe (D) ∆µge (D) µge (D) β tot(0)[b] µβ(0)[c]

D1-DTT-D1 2.82 2.59 3.51 4.54 8.05 15.93 9.6 29

D2-DTT-D2 2.65 2.39 0.32 3.89 3.57 16.44 12.7 3

D3-DTT-D3 2.71 2.52 2.22 4.98 2.90 15.25 22.0 38 [a]Calculations on geometries optimized at the B3LYP/3-21G* level; [b] In 10-30 esu; [c] In 10-48 esu.

Figure 4. B3LYP/3-21G* Mülliken atomic charges on different molecular domains (a) and frontier molecular orbitals (b) involved in the NLO response of D3-DTT-D3, as a prototypical example of the symmetrical D-DTT-D systems.

4. CONCLUSIONS The second order nonlinear optical properties of symmetrical (D-π-D) and asymmetrical (D-π-A) chromophores containing dithieno[3,2-b:2’,3’-d]thiophene (DTT) as the π-center have been analyzed by means of EFISH and DFT calculations. The effects of varying the electron withdrawing/donating capability of the terminal acceptor/donor groups on their optical properties have been also addressed. The asymmetric samples display a high NLO response due to the ICT character of the HOMO LUMO excitation. The µβ values of push-pull NLO-phores bearing stronger acceptor decrease significatively in high polar solvents in consistent with a higher contribution of the zwitterionic limiting form to the description of the ground state as the polarity of the solvent increases.The detailed comparison of the µβ values of DTT-based chromophores with those based on various bithiophene spacers indicates the effectiveness of DTT as

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electron-relay. CPHF calculations, however, reveal that the symmetric samples display a moderate NLO response in agreement with the weak charge-transfer of the lowest energy transition. Analysis of these properties as a function of the substitution pattern is very useful in the design and understanding of the new organic materials for electrooptical applications.

ACKNOWLEDGMENTS The present work was supported in part by the Dirección General de Enseñanza Superior (DGES, MEC, Spain) through research project Nos. CTQ2006-14987-C02-01 and CTQ2005-01368 and by the CICYT (Spain), project No. MAT2005-06373-C02. The authors are also indebted to Junta de Andalucía and Gobierno de Aragón (Spain) for funding their research groups FQM-0159 and E39. J.C. is grateful to the MEC of Spain for a I3 professorship position of Chemistry at the University of Málaga. M.C.R.D. is also grateful to the MEC/Fulbright for her Postdoctoral Fellowship at the Georgia Institute of Technology.

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