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Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on Their Linear and Nonlinear Optical Properties Arturo Jiménez-Sánchez, Itzel Isunza-Manrique, Gabriel Ramos-Ortiz, Jesús RODRÍGUEZ Rodríguez-Romero, Norberto Farfan, and Rosa Santillan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02805 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 17, 2016

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The Journal of Physical Chemistry

Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on their Linear and Nonlinear Optical Properties Arturo Jiménez-Sánchez,†,* Itzel Isunza-Manrique,§ Gabriel Ramos-Ortiz,§ Jesús RodríguezRomero,† Norberto Farfán† and Rosa Santillan‡ † Facultad de Química, Universidad Nacional Autónoma de México, Cd. Universitaria. Ciudad de México No. 04510, México. [email protected]. Phone: +52-55-56223731. ‡ Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, CINVESTAV, Apdo., Postal 14-740, Ciudad de México, 07000, México. § Centro de Investigaciones en Óptica, CIO, Apdo., Postal 1-948, 37000 León Gto, México.

ABSTRACT

Design parameters derived from structure-property relationship play a very important role in the development of efficient molecular-based functional materials with optical properties. Here, we report on the linear and non-linear optical properties of a fluorene-derived dipolar system (DS) and its octupolar analogue (OS) in which donor and acceptor groups are connected by a phenylacetylene linkage as strategy to increase the number of delocalized electrons in the πconjugated system. The optical nonlinear response was analyzed in detail by experimental and

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theoretical methods, showing that in the octupolar system OS, the dipolar effects induced a strong two-photon absorption process whose magnitude is as large as 2210 GM at infrared wavelengths. Solvatochromism studies were implemented to obtain further insight on the charge transfer process. We found that the triple bond plays a fundamental role in the linear and nonlinear optical responses. The strong solvatochromism behaviour in DS and OS was analyzed by using four empirical solvent scales, namely Lippert-Mataga, Kamlet-Taft, Catalán and the recently proposed scale of Laurence et al., finding consistent results of strong solvent polarizability and viscosity dependence. Finally, the role of the acceptor groups was further studied by synthesizing the analogous compound 2DS having no acceptor group.

INTRODUCTION During the last decades, the non-linear optical process of two-photon absorption (TPA) has become an area of intensive research owing to its application in optical limiting, 3-D microfabrication, photodynamic therapy, high-density optical storage, holographic data storage, frequency-upconversion/two-photon pumped lasing,1-3 and bioimaging.4-6 Much effort has been made to design and synthesize molecules with high TPA efficiency.7 Three main design strategies can be adopted to enhance the TPA response, a) increasing the strength of the acceptor (A) and donor (D) groups;8 b) increasing the number of delocalized electrons in the π-conjugated system (the π-bridge);9-10 c) produce an efficient ICT transition induced by the effective HOMO−LUMO separation.11-12 Coe proposed to categorize the electronic systems on the basis of the part of the molecule that is altered during the commutation reaction, namely, the donor part (type I), the acceptor part (type II), or the conjugated bridge (type III).13 In this regards,


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fluorene-derived TPA compounds have been designed under strategies a and c and types I and II through functionalization with strong D groups on the fluorene moiety.14-16 However, no fluorene-derived dipolar–octupolar TPA fluorophores under strategy b or type III has been reported yet. This strategy is of interest for several reasons. First, the transition moments are related to the chromophore length over which the charge is displaced (polarized) during a transition, such that when the fluorene (D) moiety is oriented along the longitudinal axis of the chromophore, the D to A length is increased without incorporating flexibility to the system, thus increasing the TPA response. Second, a proper π-bridge, like the phenyl-acetylene linkage should also promote solubility and viscosity-dependent properties for biological applications or bioimaging. Third, since no steric hindrance is present, several A groups can be used without loss of conformational planarity. Fourth, these compounds can have different commutation processes, including cis-trans photoisomerization, intramolecular proton/charge transfer or photochromic reactions.17-19 In this work, we explored the implications of the phenyl-acetylene π-bridge on the linear and non-linear optical properties of a fluorene-derived octupolar compound, namely OS. Of our particular interest is to analyze the dipolar contribution on the octupolar response. Thus, we implemented a systematic study to demonstrate the importance of pure dipolar contribution by synthetizing the compound DS which represents the exact dipolar fragment integrated in compound OS. Moreover, the role of benzonitrile and triazine as A groups on the phenylacetylene linkage was further studied by synthesizing the analogous compound having no acceptor moiety, namely 2DS. Results show that TPA response increases dramatically going from DS to OS with an important contribution of dipolar response. Moreover, as a consequence of specific solvent interactions occurring in DS and OS a strong positive solvatochromism was


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studied by four empirical solvent scales, namely Lippert-Mataga, Kamlet-Taft, Catalán and the recently proposed scale of Laurence et al. finding consistent results of solvent polarizability and viscosity dependence due to the strong dipolar charge transfer, which is in agreement with the Xray structure analysis of DS. These optical responses were experimentally and theoretically analyzed and characterized. RESULTS AND DISCUSSION Synthesis and characterization. The fluorene dipolar (DS) and octupolar (OS) systems, as well as compound 2DS were prepared according to Scheme 1. Compounds 1–3 were synthetized according to reported methodologies.20-21 Subsequent treatment of compound 3 with 4(bromo)benzonitrile under Sonogashira cross-coupling conditions by using Pd(Cl)2(PPh3)2 as catalyst gave fluorophore DS with excellent yield. On the other hand, the one-step synthesis of fluorophore OS was found to be highly difficult by using the well-known nitrile cyclotrimerization in triflic acid conditions.22 Further, we used a triple Sonogashira crosscoupling methodology between 3 and tris-4-bromopheny-1,3,5-triazine, which also resulted unsuccessful due to the formation of predominant mono- and di-coupling products. However, the later methodology was implemented with tris-4-iodopheny-1,3,5-triazine (4) obtaining OS in a good yield. We also applied a recently described nitrile-cyclotrimerization by using pure ZnCl2 as catalyst.23 Then, compound DS and ZnCl2 in a 3 : 1 molar ratio were filled into an ampoule under nitrogen and sealed under vacuum and heating at 300 °C for 48 hours, thus obtaining OS in 20% yield.


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C8H17C8H17 Br

1-Bromooctane

C8H17C8H17 Me3SiC

Br

KOH / DMSO

CH

Si(Me) 3

Pd(CH3CO2)2, CuI, PPh3, DIPA

1

2

MeOH / Et2O K2CO3 CN

C8H17 C H 8 17

C8H17 C8H17

C8H17C8H17 CN

Br Pd(Cl)2(PPh3)2, CuI TEA

DS

C8H17 C8H17

CuI TEA

3

2DS

I

C8H17 C8H17

CN

Pd(Cl)2(PPh3)2 CuI / TEA

ZnCl2

N

CF3SO2OH

N N

I

I I

4 N

N N C8H17 C8H17

C8H17 C8H17

OS

Scheme 1. Synthesis of compounds DS, 2DS and OS. X-ray structure analysis of DS. DS grew up in the triclinic P-1 space group having four molecules per unit cell. The molecule is a π-electronic system involving the fluorene-benzonitrile fragment having an acetylene linkage. It is noteworthy that the acetylene linkage of the two molecules in the unit cell present a strong torsion angle deviation from the planar conformation for one molecule, while the other molecule is completely planar. In fact, a torsion angle of 38.4° was found, the overlapped structure diagram is shown in Figure 1. Also, the bond length from C14 to C15 (C≡C) is 1.202(3) Å and 1.195(3) Å for the twisted and planar structure, respectively. Complete crystal structure information is summarized in Table S1, Supporting Information. Further analysis of the DS X-ray structure showed that this conformational changes lead to a significant difference in the C=N bond length, to know, the bond length from C20 to N1


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(C=N) is 1.147(3) Å and 1.120(2) Å for the twisted and planar structure, respectively. These results highlight the importance of a triple bond rotation on the electronic structure of DS since the intramolecular charge transfer (ICT) is strongly affected by rotation. It is worth mentioning that n-octyl chains play an important role in the crystal packing for the molecules since these aliphatic chains are completely perpendicular to the plane of the fluorophore whose arrangement induces alignment of the counterpart, see crystal packing diagram in Figure S1, Supporting Information. Both, the length and conformation of linear aliphatic chains substituted at the 9,9’ positions of fluorene have shown a determinant role in the solid-state optoelectronic properties for conducting polymer applications.24 Further crystal packing analysis revealed that DS is not governed by close contacts, having only two close contacts located in the alkyl chains, this is an important consideration for crystal packing.25

Figure 1. Overlap diagram of the X-ray structure for DS. Quantum chemical calculations. Quantum chemical calculations were used to investigate the nature of one-photon absorption (OPA) and to estimate TPA cross-section for the compounds under study. The molecular geometry and vibrational frequencies for DS and OS in the ground (GS) and first-excited (ES) state were optimized by using Gaussian 09,26 with the global hybrid


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exchange correlation functional B3LYP,27 and charge distribution parameters were estimated with the parameter-free hybrid PBE0 functional.28 Vertical excited state energies were computed at TD-CAM-B3LYP/631+G(d,p)/PCM level of theory.29 Bulk solvent effects were considered by means of the Integral Equation Formalism Polarizable Continuum Model (IEF-PCM) in the linear response formalism and in equilibrium regime.30-31 All calculations were performed considering DMSO as solvent. The TPA optical properties were calculated by means of response theory,32 as implemented in DALTON.33 The calculated transition energies, static and transition dipole moments were fed into a sumover states (SOS) approach in order to obtain the TPA transition matrix elements (Sα,β) which can be extracted from the single residue of response function given by the equation:34  g µα i i µ β f g µ β i i µα f + Sα , β = Σ  ωi − ω f / 2 ωi − ω f / 2 i  

   

(1)

where α, β ∈ (x,y,z) ωi and ω f are the excitation frequency for the intermediate i and final

f

two-photon states, respectively; µ is the dipole operator at the given direction. The SOS

approach assumes that only few states dominate the response, which provides a good description near the photon resonances.35 Moreover, all single-particle states are assumed to have a common energy broadening parameter in damped response theory.36 Then, the TPA cross-section in atomic units δTPA is represented as follows:

δTPA = Fδ F + Gδ G + Hδ H where δ F =

Σ Sαα S ββ

α ,β

(2)

δ G = Σ Sαβ Sαβ α ,β

δ H = Σ Sαβ S βα α ,β


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and F = G = H are coefficients dependent on polarization of the light, which are equal to 2 for linear polarized light. For example, for an excitation by a linearly polarized single beam of light, δTPA is evaluated by the following expression in atomic units (a.u.):37

= 6 + +

+ + + 8 + 4( +

+

)

(3)

Then this formalism was used to calculate the TPA cross-section σTPA in GM ([1 GM =10-50 cm4 s molecule-1 photon-1]) units as a means of the two-photon transition amplitude between the electronic ground-state (S0) and the two lowest electronic exited states (S1 and S2). The calculated OPA and TPA properties of DS and OS in DMSO are shown in Table 1. Table 1. Calculated OPA and TPA photophysical properties for DS and OS in DMSO. Compound DS OS

λOPAa 365 374

EOPAb 3.39 3.31

µge (D)c 7.2 16.5

ETPAb 1.55 1.51

µee’ (D)d 2.1 7.5

σTPAe 28.816 575.500

f 0.077 0.495

a

OPA absorption wavelength in nm. OPA or TPA energy in eV. c Static ground to first excited state dipole moment in Debye. d First excited state to the TPA state transition dipole moment in Debye. e TPA cross-section in GM evaluated at 820 nm. f Intrinsic TPA cross-section at 820 nm, in GM, to account for the NLO efficiency. b

On the other hand, although TD-DFT provides a good description in the determination of molecular optical parameters given the accurate description of ground and excited state geometries, commonly TD-DFT turns out to describe an excited state in terms of several single electronic excitations from an occupied to a virtual orbital. Fortunately, the various contributions to the electronic excitation can be clarified by a Natural Transition Orbital (NTO) analysis,38 which provides a compact orbital representation of the electronic transition through a single configuration of a hole and electron interaction. Therefore, the photoinduced electron transfer


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(PET) process is not depicted by a simple change in the elementary molecular orbital occupancy, but in a hole-electron distribution. Figure 2 illustrates the NTO diagram for molecules DS and OS. It must be noted that DS can be represented as a planar dipolar structure having a CS symmetry. Thus, the electronic transition from the ground (|g>) to the first excited (|e1>) state can be represented as A’ → A”. For DS the hole is slightly more localized along the fluorene fragment with a contribution of the ethynyl group, while the electron is slightly more located in the benzonitrile unit with a small contribution of the fluorene unit which confers a subtle dipolar polarization due to the charge displacement from the fluorene to the benzonitrile moiety. This implies an intramolecular charge transfer (ICT) character for the A’ → A” transition, corresponding to the Highest Occupied Natural Transition Orbital and the Lowest Unoccupied Natural Transition Orbital (HONTO – LUNTO) levels (w = 0.7) responsible for the TPA response. The parameters for this electronic transition are shown in Figure S2, Supporting Information. TD-DFT predicted a σTPA of 28.816 GM for this transition at 820 nm (see Table 1). Although this computed TPA response is relatively low, the result is interesting since dipolar molecules (non-centrosymmetric) normally exhibits very small or null σTPA values.39


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b) OS electron

electron

a) DS |E’1>

electron

|E’2>

|E’1>

|A’1> |f>

|E’2>

|A’1>

A” hole

hv

hole

electron

electron

hv

|E’1> |g>

|E’2>

|E’1>

|E’2>

A’

|A’1>

|A’1>

hole hole

hole

Figure 2. TPA energy level diagrams with natural transition orbitals for the first excited state (A” for DS and E’1 and E’2 for OS) computed at PBE0/6-31+G(d,p)/IEF-PCM-DMSO. (a) ground A’ and excited A” electronic states of DS in CS symmetry and, b) ground |A’1> and degenerated excited states |E’1> y |E’2> of OS associated with four transitions: in the upper scheme the contribution is purely octupolar, whereas in the lower scheme the contribution is dipolar in nature. In the case of the octupolar OS molecule, the generated transition moment is the result of the specific interaction between the first photon with HONTO – LUNTO levels and the second


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photon with HONTO-1 – LUNTO levels. Here, the TPA cross-section obtained by TD-DFT method for OS was 575.5 GM at 820 nm ( Table 1). Figure 2b shows the charge displacements obtained by TD-DFT associated to µgi y µgf for OS in which the first excited state is doubly degenerated; it also shows that the oscillator strength values for these two states are much higher than those of other states, so the TPA cross section can be calculated considering only three states (ground |g>, and two excited |e1> and |e2> states). In this case, for the C3h (or C3) point group, the ground state function corresponds to the A' (or A for C3) representation, whereas the two degenerated excited states correspond to E' (or E for C3) representations. Thus, the ground state and the two degenerate excited states are symmetry allowed for the TPA process. As shown in Figure 2b (upper scheme) which refers to the first excited state, the hole is uniformly spread along the two fluorene branches, while the electron is strongly localized towards the triazine center with practically no distribution on fluorene. This transition corresponds to the HONTO – LUNTO levels (w = 0.65) and involves a strong charge transfer. However, in the corresponding HONTO-1 – LUNTO+1 transition (Figure 2b, lower scheme), the hole is distributed only over one fluorene branch and the electron is strongly located towards the triazine nucleus, conferring a dipolar charge transfer character to this transition, comparatively lower than the HONTO – LUNTO. Consequently, it can be established that the charge displacement occurring from the fluorene branches to the triazine ring completely breaks the symmetry of the charge transfer states.40 Then, the HOMO-1, HOMO and LUMO levels contribute to the TPA response. Interestingly, a similar three-level model (the ground state and first two excited states to represent all the susceptibilities) was shown to predict the experimental nonlinear spectrum of a similar octupolar molecule. Here, the three-level model also applies since the nonlinear response for OS is large, a result which is certainly attributed to the inclusion of the triazine ring.41


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For a better understanding of their optical properties, we analyzed the charge transfer nature of DS and OS. PBE0 functional was used for the ground state geometry calculation and the first excited state was also computed at CIS level keeping the 6-31+G(d) basis set and PCM-DMSO for the two geometries. We studied the charge transfer excitation by means of the recently proposed spatial extent index.42 For DS, the obtained fraction of electron charge transferred upon de-excitation from the local excited (LE) state was qCT = 0.41 at a DCT = 7.34 Å spatial distance from the donor centroid to the acceptor centroid, having a static dipole moment difference between ground and excited state of 14.36 D. Figure S3 shows the graphical representation of DCT, and excess of electron density centroids (C+ (r)/C– (r)) as defined in ref (43). The H index, [C+ (r)/C– (r)]/2, for the Donor – Acceptor axis is 3.01, thus being 4.33 Å lower than the CT excitation length, resulting in no overlap between the Donor – Acceptor centroids, making the ICT process efficient. In the case of compound OS the obtained qCT value was 0.51 which results larger compared to the dipolar molecule, with a DCT = 5.02 Å spatial distance in the involved branches with a ∆µge of 12.2 D, the H index of OS is 4.58 resulting to be just 0.45 Å lower than the CT excitation length. These results suggest that although DS redistributes a smaller charge density upon photo-excitation, the dipolar polarization is highly efficient. On the other hand, the octupolar analog experiences a strong competition between its dipolar branches during the ICT process which results in a less separated location of the Donor – Acceptor centroids of charge. This was further confirmed by the electron density difference upon photo-excitation, Figure S4, Supporting Information. Finally, we scaled the computed TPA cross-section with the “effective number of electrons (Neff)” to allow a fair comparison of the TPA merit. It was shown that the scaling law is

approximately independent of the energy function, so the TPA cross-section scales as and


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the quantity for making a comparison between DS and OS must be given by the intrinsic TPA cross-section, defined as:44,45

= /

(4)

Thus, the NLO efficiency was assessed in terms of the intrinsic TPA cross-section, , as defined in Equation 4. The effective number of electrons contributing to the nonlinear response is calculated by geometrically weighting the number of electrons in each conjugated path of the molecule:46 =

∑

(5)

where Ni is the number of electrons in the ith conjugated part of the molecule. Then, Neff values of 19.4 and 34.1 resulted for DS and OS, respectively. Table 1 lists the obtained values. As it can be seen, the intrinsic TPA cross-section for the octupolar molecule scales ca. six times the value for the dipolar molecule. It is important to notice that the conjugation length for both, DS and OS is practically the same, suggesting that the molecular length has no important effect on the intrinsic nonlinearities and that the triazine core allows to increase the effective number of electrons by Neff(OS)/Neff(DS) = 1.75 times, making the octupolar three-branched system highly efficient. Experimental determination of TPA cross section In order to characterize the TPA of DS and OS compounds, we measured their σTPA by the two-photon excited fluorescence (TPEF) technique.47 Figure 3 presents the spectrum of σTPA in the wavelength range 660 nm – 840 nm for the case of OS in a THF solution. To calculate σTPA,


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the quantum yield of fluorescence (Φe) had to be measured, resulting in 0.17 and 0.26, for DS and OS respectively. The maximum σTPA resulted in 2210 GM at the 740 nm but significant nonlinearities higher than 500 GM are obtained between 680 nm and 800 nm. In contrast, for the case of DS, the TPEF signal was very weak and below the limit of detection of our apparatus for most of the wavelengths. For DS a detectable signal was obtained only at 740 nm with a σTPA < 100 GM. These experimental results for DS and OS are in very good agreement with the results obtained by TD-DFT formalism in the previous section, as shown in Figure 3. The good agreement between the σTPA calculated by TD-DFT and the experimental measurements supports the prediction that in the case of the octupolar system there is an important dipolar contribution influencing the TPA process. This observation has previously been addressed by Beljonne, et.al.48 for the interplay and extent of electronic coupling among the three dipolar arms in octupolar molecules by means of molecular exciton theory. Cho, et.al. have also offered an elementary description by means of a three charge-transfer state model.49


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Figure 3. TPA cross-section values for OS obtained by TPEF experiments (circles). The dotted line is a guide for the eye. The figure also presents the values obtained by TD-DFT formalism (squares). Solvatochromism. One photon absorption and emission spectra. The OPA absorption and emission spectra of DS and OS were taken in different solvents. The UV-Vis and fluorescence spectra obtained in acetonitrile (ACN) at 5 x 10-6 M (exciting in the low-energy absorption band at an absorbance of 0.1 A.U.) is presented in Figure 4. The absorption band is assigned to the π → π* electronic transition with a CT contribution. For the case of DS a vibrational structure is clearly observed. The maximum of the main absorption band of DS is attributed to the 0–0 vibrational band coming from an intense S0 → S1 electronic transition in fluorene moiety (located at 353 nm). Then, a shoulder band blue-shifted to 330 nm was observed, which was attributed to the 0–1 vibrational band of the same electronic transition. In DS such vibrational structure is also evident in the emission spectra, with the 0–0 and 0–1 vibrational bands at 360 nm and 466 nm, respectively. The vibrational structure of DS is not seen in polar-aprotic and -protic solvents. Interestingly, due to the high rigidity of DS, Stokes shifts in non-polar solvents are very small. OS exhibited a maximum of absorption in 363 nm and an emission band with a maximum at 455 nm. In contrast to DS, in OS the vibrational structure is not evident in the emission spectra (in ACN), and the Stokes shift is more pronounced (1803 cm-1 and 5570 cm-1, respectively). The molar extinction coefficient (ε, in mol-1 cm-1) at λmax for DS and OS in acetonitrile was 6240 and 21,760, respectively, which means a ca. 3.5-fold increase in light attenuation for the octupolar molecule.


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(a)

1.0

Normalized Absorption / Emission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized Absorption / Emission


0.5

0.0 250

300

350

400

450

500

550

600

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(b)

1.0

0.5

0.0 250 300 350 400 450 500 550 600 650 700 750 Wavelength (nm)

Wavelength (nm)

Figure 4. Normalized absorption (blue) and fluorescence (red) spectra obtained in acetonitrile for (a) DS, λ(max)Abs = 353 nm, λ(max)Em = 358 nm; and (b) OS, λ(max)Abs = 363 nm, λ(max)Em = 455 nm. A summary of the main linear and non-linear optical properties for DS and OS are presented in Table 2. Here, a very good agreement between the electrochemical (by cyclic voltammetry) and optical band gaps was found. Table 2. Absorption wavelength (molar extinction coefficient in M-1cm-1), emission wavelength, fluorescence quantum yield, Stokes shift, optical band gap, and experimental transition moment form ground to excited state for DS and OS in THF.

Compound

DS

OS

λmax(ε)a

352nm (101420), 334nm (101230) 363nm (432000),

λemissionb

Φec

392 nm

0.17

439 nm

0.26

Stokes shift

Eopt (eV)d

Eelec (eV)e

µge (D)f

2900

3.35

3.27

6.05

4770

3.04

3.01

13.25

(cm-1)

a

OPA absorption and b emission wavelength measured at 5x10-6 M. Fluorescence quantum yield relative to Rhodamine (1 in THF). d Eopt optical energy band gap is obtained from the intercept of absorption spectra. e Eelec electrochemical energy band gap obtained by cyclic voltammetry in DMSO (see SI). c


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f

The experimental S0 to S1 transition moment were obtained by computing the area under the corresponding peak using the equation µ ge2 = (1500 ln(10)hcn / 2π 2 N A )∫ (ε (v) / v)d v = 9.18358x10−39 × n ∫ (ε (v) / v)d v , where n is the

refractive index (for THF = 1.4050), ∫ (ε (v) / v)d v is the area under the curve of a [(ε( v )/ v ),( v )] plot. Specific solvent effects. A detailed study was performed in order to understand the nature of the charge transfer states (CT states). As it is was discussed before, charge transfer mechanisms influenced the TPA response. The ICT properties were studied by a combined solvatochromic analysis of Lippert-Mataga, Catalan, Kamlet-Taft and the recently proposed solvent interaction scale by Laurence, et al.50 All these methods combined allow obtaining quantitative evidence of the static dipole moment change between S1 and S0 and the effect of acidity, basicity, polarity and polarizability solvent parameters in the photophysical properties. Figure 5 shows the absorption and fluorescence emission spectra of DS and OS in different solvents and Table 3 presents a summary of the main results from this data. Analysis of these spectra revealed that the absorption λmax has a non-monotonic pattern as a function of solvent polarity, and a strong broadening effect from non-polar to polar solvents is observed, particularly for viscous solvents. In fact, the full width at half height maximum of the maximum absorption band (FWHMabs) increases from 4120 cm-1 in hexane to 5900 cm-1 in DMSO for DS and from 5190 cm-1 in hexane to 10270 cm-1 in DMSO for OS. In addition, both molecules experience strong Stokes shift modification, varying from just 860 cm-1 in hexane to 4897 cm-1 in ethylene glycol for DS, and from 2461 cm-1 in hexane to 6890 cm-1 in DMSO for OS. Thus, larger Stokes shift were observed for OS, compared with DS. This observation is important for two main reasons: first, a large Stokes shift modulation implies the lowest probability of having reabsorption of the fluorescence emission.51 Second, the photo-excitation process induces significant structural changes in the excited state of the fluoro-solvatochromes.


17

1.4x10

5

1.2x10

5

1.0x10

5

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

4

Hexane Dioxane Toluene Chloroform Tetrahydrofurane Dichloromethane Octanol i-Propanol Acetone Methanol Acetonitrile Dimethylformamide Ethylene Glycol Dimethyl Sulfoxide

-1

Fluorescence Intensity (Normalized)

5

-1

1.6x10

0.0

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Hexane Dioxane Toluene Chloroform Tetrahydrofurane Dichloromethane Octanol i-Propanol Acetone Methanol Acetonitrile Dimethylformamide Ethylene Glycol Dimethyl Sulfoxide

1.0

0.5

0.0

325

350

375

400

425

350

450

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450

500

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Fluorescence Intensity (Normalized)

-1

-1

Hexane Dioxane Toluene Chloroform Tetrahydrofurane Dichloromethane Octanol i-Propanol Acetone Acetonitrile Dimethylformamide Ethylene Glycol Dimethyl Sulfoxide

2.5x10

0.0 350

400

450

500

550

600

650

Wavelength (nm)

Wavelength (nm)

Molar extinction coefficient (M cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Molar extinction coefficient (M cm )


550

Hexane Dioxane Toluene Chloroform Tetrahydrofurane Dichloromethane Octanol i-Propanol Acetone Acetonitrile Dimethylformamide Ethylene Glycol Dimethyl Sulfoxide

1.0

0.5

0.0 400

600

450

500

550

600

650

700

750

800

Wavelength (nm)

Wavelength (nm)

Figure 5. Absorption (left) and fluorescence spectra (right) of 10 µM DS (above) and 5µM OS (below) in different solvents. The fluorescence spectra were taken at a 10-fold dilution of the initial concentrations to avoid inner filter effects. Table 3. Photophysical properties of DS and OS in different media.a DS Solvent Hexane Dioxane Toluene Chloroform THF DCM Octanol 2-PrOH Acetone MeOH

εb

n

∆f

2.02 2.22 2.38 4.81 7.52 8.93 10.30 20.18 21.01 33.00

1.4235 1.4224 1.4961 1.4459 1.4050 1.4242 1.4295 1.3776 1.3588 1.3288

0 0.021 0.013 0.148 0.210 0.217 0.225 0.277 0.285 0.308

λa (nm) 352 353 355 356 352 354 353 350 349 349

⊽a

(cm-1) 28409.09 28328.61 28169.01 28089.887 28409.09 28248.587 28328.61 28571.428 28653.295 28653.295

λe (nm) 363 375 369 391 392 395 394 399 403 410

OS

⊽e

(cm-1) 27548.209 26666.67 27100.27 25575.447 25510.20 25316.455 25380.71 25062.656 24813.896 24390.24

⊽a–⊽e (cm-1) 860.881 1661.94 1068.74 2514.44 2898.89 2932.132 2947.9 3508.772 3839.399 4263.055

λa ⊽a (nm) (cm-1) 360 27777.778 352 28409.09 361 27700.83 364 27472.527 363 27548.209 363 27548.209 363 27548.209 361 27700.83 360 27777.778 -----

λe (nm) 395 409 402 433 439 453 441 455 466 ---

⊽e

(cm-1) 25316.456 24449.878 24875.62 23094.688 22779.043 22075.055 22675.737 21978.02 21459.227 ---

⊽a–⊽e (cm-1) 2461.32 3959.21 2825.21 4377.84 4769.17 5473.15 4872.47 5722.81 6318.55 ---


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ACN DMF EG DMSO

36.64 36.70 41.40 46.68

1.3442 1.4305 1.4320 1.4793

0.305 0.274 0.276 0.263

353 352 362 355

28328.61 28409.09 27624.31 28169.01

377 26525.2 1803.41 409 24449.878 3959.212 440 22727.27 4897.04 416 24038.46 4130.55

363 27548.209 364 27472.527 370 27027.027 368 27173.91

455 21978.02 5570.19 433 23094.688 4377.84 491 20366.60 6660.43 493 20283.976 6889.93

a

Dielectric constant, (ε), refractive index, (n); Orientation polarizability, (∆f); absorption wavelength, (λa); absorption wavenumber, (⊽a); emission wavelength, (λe); emission wavenumber, (⊽e). Solvent notation: THF (tetrahydrofuran), DCM (dichloromethane), ACN (acetonitrile), DMF (dimethylformamide), EG (ethylene glycol), DMSO (dimethyl sulfoxide). b taken from Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2002. The effect of solvent interactions on the absorption and emission spectra of DS and OS was first investigated in terms of the Lippert−Mataga approximation. The Lippert theory stablish that the UV-Vis absorption and fluorescence variations are a consequence of specific solvent effects, i.e. hydrogen bond, CT and acid-base interactions, preferential solvation, and solvent-solute hydrogen bonding.52,53 Then, Lippert-Mataga expression relates Stokes shift vs. a solvent polarity parameter, Eq. 6:

∆ν = υa − υ e =

2 (∆ f hc

)

(µ

− µg )

2

e

a

3

+C

(6)

where ⊽a and ⊽e are wavenumbers (in cm–1) for the absorption and emission bands, h is Planck's constant, c is the speed of light, a is the Onsager radius, and ∆µ = µe-µg is the dipole moment difference between the ground and excited states. The solvent polarity ∆f is defined in terms of the dielectric constant (ε) and the refractive index (n) of the solvent as (Eq. 7).  ε −1 n2 −1   ∆f = f (ε ) − f ( n 2 ) =  − 2 2 ε + 1 2 n + 1  

(7)

Although the absorption bands shown in Figure 5 do not display significant shifts as a function of solvent nature, the emission bands evidence a strong red-shifting of ca. 110 nm and 150 nm for DS and OS, respectively, as the solvent polarity increases, which suggests that the dipole moment is larger in the excited-state than in the ground-state. The Lippert plot shows a strong


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non-linear behavior from the non-polar (∆f = 0.0 – 0.15), the polar-aprotic (∆f = 0.2 – 0.3) and the polar-protic (∆f = 0.2–0.31), Figure S6. The non-linear trend in the non-polar region is explained by the fact that there is a poor solvation around the fluorophore with almost no reorientation of non-polar solvent dipoles. Moreover, the non-linear behavior suggests that specific solvent interactions such as hydrogen bond and ICT, and/or local-viscosity effects could be present. To get more experimental clue of the ICT process of DS and OS, the ∆µ values were obtained from the equation (6). Thus, considering the slope in the non-polar to polar region (excluding the highly viscous ethylene glycol) gives a ∆µ of 12.5 D (slope = 8900 ± 750, R2 = 0.95) for DS and a ∆µ of 23.1 D (slope = 10800 ± 1030, R2 = 0.94) for OS. Here, the Onsager radius of 5.59 Å and 7.91 Å for DS and OS, respectively, were obtained by DFT. The obtained ∆µ values are comparable to those reported in the literature (3-20 D) for other solvatochromic fluorophores,54 and are an indication of the strong charge reorganization upon photo-excitation. In addition, the Catalán, Kamlet-Taft and Laurence empirical scales also account for solvent polarizability and polarity as the dominant solvent parameters, and this solvent dependence becomes more important in the excited state. An exhaustive solvatochromic analysis is presented in the Supplementary Information. Thus, from the four solvatochromic analyses it is clear that in the case of viscous solvents (octanol, 2-propanol, EG and DMSO) the fluorophores presented a specific effect such that the phenylacetylene linkage could present a local-viscosity dependence; on the other hand, the ICT character turns out to be crucial to give strong differences between DS and OS and it is not related to the electron donor fragment of these two compounds, but to the electronic structure arrangement. Moreover, the role of the electron acceptor groups resulted to be of fundamental importance, these two topics will be discussed in the next sections.


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Solvent viscosity studies. To further evaluate the influence of the π-bridge on the ICT process, we studied the solvent viscosity effects in DS and OS. As it is known, solvent viscosity influences the rate of the non-radiative deactivation processes coming from intramolecular rotation in the fluorophore,55 in this case as a consequence of the triple bond rotation. In this regard, Förster and Hoffmann described a relation between solvent viscosity (η) and fluorescence quantum yield (Φe).56 Accordingly, the relationship should follow Eq. (8): Φ = $%&

Eq. (8)

where z and α are constants. A plot of log Φe vs. log η should yield a straight line with a slope α. Inset of Figure 6 shows a linear Förster-Hoffmann relation, suggesting a considerable influence on solvent viscosity for both molecules, that is, upon increasing viscosity from nhexanol to glycerine the ICT process is stabilized by the solvent and a strong emission arises from a geometrically more planar structure.57

Figure 6. Fluorescence emission of DS (left) and OS (right) in solvents of different viscosity: nhexanol (black), DMF (red), i-propanol (cyan), glycerin at 298 K (green) and glycerin at 265 K (blue). The spectra are normalized to the maximum emission in glycerine. Inset: double-


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logarithmic plot of the solvent viscosity (η) vs. fluorescent quantum yield (Φe) according to the Förster–Hoffmann equation. On the other hand, it is interesting to note that for both fluorophores the emission wavelength in glycerine at 298 K and 265 K does not vary, which is in agreement with twistedintramolecular charge transfer (TICT) rotors reported in the literature, where emission emerges from an energetically invariable local excited state.58,59 The role of the acceptor groups on the phenyl-acetylene linkage. Since the phenylacetylene linkage resulted critical to the linear and TPA responses, we decided to explore the importance of the benzonitrile and triazine acceptor groups in the molecular systems. Then, a Sonogashira-Glaser coupling reaction with ethynyl-fluorene yielded compound 2DS (Scheme 1 and Figure 7), a compound where the ICT process (in this case quadrupolar type interactions) should be negligible due to the absence of an acceptor group. However, the electronic structure characteristics of 2DS resulted interesting. The role of the bis-acetylene bridge induce the electron redistribution to slightly behave as a quadrupolar system (D–A–D, where A is represented by C≡C=C≡C linkage), according to the hole – electron distribution, Figure 7. Indeed, the HONTO – LUNTO electronic transition governs the UV-Vis absorption spectrum, the HONTO level is distributed along the entire molecule, while the LUNTO level is slightly more distributed toward the C≡C=C≡C linkage suggesting a small quadrupolar effect, Figure 7. This finding is interesting since the pure π-bridge can allow the molecule to present some level of TPA response of the D–A–D type. According to these theoretical results, one implication arises, the fluorene molecule acts as a strong electron donor group when it is connected to an acetylene group. On the other hand, the linear optical properties show no fluorescence maximum shifts as a function of solvent polarity. Furthermore, another important characteristic of 2DS is


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that the molecule should exhibit some TPA activity. To demonstrate the above, a solvatochromic analysis and a TPA cross-section determination for 2DS were carried out. The UV-Vis absorption and emission data is presented in Figure S7, while the solvatochromic studies through Lipper-Mataga, Catalán, Kamlet-Taft and Laurence are summarized in Table S2 and Table S3, SI. As expected, no bathochromic shift was observed in the emission spectra and the dipole moment difference between excited and ground states is not significant. However, the TPA cross-section for 2DS was determined following the same methodology as in DS and OS, finding a σTPA value of 7.09 GM at 740 nm. C8H 17

C8H 17

C 8H17

C 8H17

Figure 7. Above: molecular structure of compound 2DS, having no acceptor group. Bellow: hole (left)–electron (right) pair corresponding to the 1A’ → 2A’ transition, considering a C2h symmetry (NTO eigenvalue w = 0.7) CONCLUSION Both, experimental and theoretical analysis demonstrated that compound OS exhibited interesting TPA properties with unusual strong dipolar contribution. The role of pure dipolar contribution was analyzed in detail through molecule DS which represents the exact dipolar fragment integrated in the octupolar compound. Particularly, it can be concluded that:


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The experimental (optical and electrochemical) and the computational (NTO) band gaps obtained for DS and OS as well as their corresponding σTPA values were in good agreement. Importantly, the NTO analysis revealed the existence of dipolar and octupolar contributions to the TPA optical response of OS associated to the ground |A’1> and degenerated excited states |E’1> and |E’2>. Both, DS and OS display strong solvatochromic effects, ranging from 860 cm-1 in hexane to 4897 cm-1 in ethylene glycol for DS and, from 2461 cm-1 in hexane to 6890 cm-1 in DMSO for OS. Then, it was inferred that significant charge transfer processes occur in the excited states. A dipole moment difference of 12.5 D for DS and 23.1 D for OS were obtained by the LippertMataga relation. According to the Kamlet−Taft, Catalán and Laurence solvent scales, the solvatochromic features of DS and OS are mostly governed by solvent polarity and polarizability. DS and OS exhibited a noticeable local-viscosity dependence and the results demonstrated a twisted-intramolecular charge transfer (TICT) behavior, where emission emerges from an energetically invariable local excited state. X-ray structure analysis also revealed an important twisting in the acetylene bridge. On the other hand, for 2DS no TPA response was observed but a stronger solvent viscosity dependence was evidenced. Finally, the present work demonstrates that the effect of the π-bridge takes place when a very weak if any electron-donor group is attached to the fluorene blue emitter fragment. A significant dipolar contribution to the octupolar system was observed. Furthermore, the present molecular systems were found to be suitable for viscosimetric analysis through OPA response detection. EXPERIMENTAL SECTION


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Materials and physical measurements. All materials and solvents are commercially available. Infrared spectra were obtained on a FTIR Varian Spectrometer ATR. Melting points were obtained in an Electrothermal-9200. 1H and 13C NMR spectra, were taken in a JEOL eclipse ECA +500 spectrometer, the chemical shifts in ppm are relative to (CH3)4Si for 1H and 13C. High resolution TOF-MS mass spectra were recorded on an Agilent G1969A spectrometer. Solution and solid state UV-Vis spectra were acquired in a Perkin Elmer LAMBDA 900-2S spectrophotometer. Emission spectra in solution and solid state were acquired on a Varian Cary Eclipse fluorescence spectrometer. 1H and 13C NMR spectra and full chemical characterization are shown in the Supplementary Information file. Quantum yield and two-photon excited fluorescence (TPEF) experiments. The fluorescence quantum yield of samples at low concentrations was determined by using the integrating sphere method. The excitation was provided by a diode laser operating at 375 nm. Two-photon absorption cross-section (σTPA) of samples was measured through the two-photon excited fluorescence (TPEF) technique using a train of pulses (~90 fs, repetition rate of 1 kHz) delivered by an optical parametric amplifier (TOPAS, Light Conversion) pumped by a Ti:Sapphire Regenerative Amplifier. The laser beam from the optical parametric amplifier was focused into a quartz cell of 1 cm-path length (containing the solutions under test) by using a short focal-length lens. The TPEF signal was collected in a perpendicular direction with respect to the excitation beam and focused into the input slit of a monochromator (Acton Research, SpectraPro-2500) and detected at the exit port with a PMT (Hamamatsu RT400U-02).60 For quantum yield determination and TPEF experiments rhodamine 6G was used as reference. The values of σTPA for this reference have been fully characterized for a wide range of wavelengths.


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ASSOCIATED CONTENT Supporting Information. Crystal structure details for DS (CCDC Reference No. 1443732), the synthesis and full chemical characterization, UV-Vis/fluorescence spectra and solvatochromic analysis, Lippert plots, and TD-DFT (NTO) calculations, spatial extent in charge-transfer excitations and density difference plots. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support of CONACyT (grant 215708) and PAPIIT 216616 and also supercomputing Xiuhcoatl cluster administration of Cinvestav and Mistli cluster of UNAM for the computational resources and the Laboratory of Ultrafast Optics at the Center for Research in Optics (CIO) for the facilities provided for this work. REFERENCES (1) Alam, Md. M.; Chattopadhyaya, M.; Chakrabarti, S.; Ruud, K. Chemical Control of Channel Interference in Two-Photon Absorption Processes. Acc. Chem. Res. 2014, 47, 1604–1612.


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