Synthesis, characterization and photo physical-theoretical analysis of D-π-A compounds. 2. Chain length effect through even-odd effect on the photophysical properties

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Synthesis, characterization and photophysical-theoretical analysis of compounds A- p -D. 1. Effect of alkyl-phenyl substituted amines in photophysical properties

E. Ortega

^a

, R. Montecinos

^a

, L. Cattin

^b

, F.R. Díaz

^c^,^**

, M.A. del Valle

^a

, J.C. Bern ede

^d^,^*

aPontiﬁcia Universidad Catolica de Chile, Av.V. Mackenna 4860-Macul, BP 7820436, Santiago, Chile

bInstitut des Materiaux Jean Rouxel (IMN), CNRS, UMR 6502, 2 Rue de la Houssiniere, BP 32229, 44322 Nantes Cedex 3, France

cDepartamento de Química y Biología, Facultad de Ciencias Naturales, Universidad de Atacama, Copayapu 485, Copiapo, Chile

dMOLTECH-Anjou, CNRS, UMR 6200, Universite de Nantes, 2 Rue de la Houssiniere, BP 92208, Nantes, F-44000, France

a r t i c l e i n f o

Article history:

Received 24 January 2017 Received in revised form 6 April 2017

Accepted 6 April 2017 Available online 8 April 2017

Keywords:

Dye Dipole Quantum yield Dipolar moment Dihedral angle effect

a b s t r a c t

The study of new dipolar A-p-D molecules, which have an acceptor (A) and donor (D) charge joined by a conjugate bridge, have been an attention focus in the recent years due their different properties. In the current work, a molecular system has been modiﬁed in order to compare the effect on properties, such as quantum yield. Thus, two series were generated (alkyl- and alkoxy-substituted) to determine if molecules with tertiary asymmetric amines change their optical properties and whether quantum yield is affected. The different products have been characterized by several techniques such as UVeVis spec- trophotometry, elemental analysis, NMR, FT-IR, mass spectroscopy and ﬂuorescence spectroscopy.

Furthermore, their behavior in eight organic solvents, dichloromethane, tetrahydrofuran, ethyl acetate, 1,4-dioxane, acetone, acetonitrile, dimethylformamide and dimethylsulfoxide were experimentally and theoretically studied. The quantum yields were higher for the alkyl-substituted series. Theoretically, the dihedral angles formed between the tertiary amine and carbonyl group moieties have a correlation with quantum yield values, helping to explain why they are higher in non-polar solvents. Consequently, the maximum quantum yield was obtained with (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(methyl(phenyl) amino)-9H-ﬂuoren-2-yl) vinyl)thiophen-2-yl)acrylic acid (M8-1) in 1,4-dioxane, reaching 98.8%.

1. Introduction

In recent years, research dedicated to organic electronic has attracted considerable interest[1]. Nowadays organic devices such as thinﬁlm transistors[2], organic light emitting devices (OLED)[3]

and even organic and hybrid photovoltaic cells[4]are available in the market. The interest to organic devices is due to theirﬂexibility, lightness, low price, ease of realization and great diversity of properties [5]. Nevertheless, if compared with similar inorganic devices, the performances of organic devices are lower [6,7].

Therefore, the future development of organic devices depends, among other things, on the improvement of the properties of or- ganics materials. The chemical tailoring allows to attain the variety

of properties required for the particular technology [8e10]. This means targeting the incorporation of various molecular units, each enriching thefinal material with specific properties. In this context, the aim of this article is to propose new molecules based on small molecular structures combining structural versatility, simplicity and low molecular weight, features that are generally associated with higher overall chemical yield, lower environmental impact and easier up-scaling of the synthesis. It is hoped that emphasis on such structures will stimulate further synthetic chemistry oriented towards the reduction of the cost and environmental impact of active materials and thus contribute to a future industrial development of photovoltaic cells. Considering all the above, it is possible to determine that, structurally, compounds as fluorene, aniline and thiophene, not only display photophysical properties in their derivatives and polymers but also excellent charge transfer and electrochemical properties, most of them being used in photovoltaic systems[11e15]. In the current work, the synthesis is carried out and the subsequent characterization is accomplished by

*Corresponding author.

**Corresponding author.

E-mail addresses:[email protected](F.R. Díaz),[email protected] (J.C. Bernede).

Contents lists available atScienceDirect

Journal of Molecular Structure

j o u r n a l h o m e p a g e : h t t p : / / w w w . e l se v i e r . c o m / l o c a t e / m o l s t r u c

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UVeVis, FT-IR, mass, fluorescence and NMR spectroscopy and elemental analysis of 4 dyes, M8-1, M8-2, M8-O1 and M8-O2 (Fig. 1, Table 1). Dyes molecular characteristics were compared by grouping them into two families (alkyl- or alkoxy-substituted), in order to characterize their behavior and establish which modification is the most appropriate. As for the chemical structures, the synthesized products are based on“molecule 4”skeleton reported by Chih-Hsin Chen et al.[16], which was subjected to modifications, mainly in the areas close to the only existing nitrogen of the molecule, in order to determine how the photophysical behavior is modified. Both the experimental and theoretical photophysical studies were conducted using 8 solvents, sorted in ascending order according to their polarity index: dichloromethane (DCM), tetrahydrofuran (THF), ethyl acetate (ETA), 1,4-dioxane (DIO), acetone (ACT), acetonitrile (ACN), dimethylformamide (DMF) and dimethylsulfoxide (DMSO). It is noteworthy that in DCM and ACN, studies were just qualitative as the dyes were not fully soluble.

2. Material and methods

The molecules indicated previously are shown below,

All solvents used in both syntheses and subsequent analyses were high purity anhydrous, and purchased from Aldrich. Synthesis methods are shown at the end in“dyes synthesis”section, where both the synthesis itself and the results of the characterization of each of the new prepared products are detailed. NMR spectra were recorded on a Bruker 400 MHz Spectrometer (Spectra available on thesupplementary material). Subsequently, measurements of solutions of each dye, concentrations between 0.5 and 3m^{mol L}¹^, were used for quantum yield determinations. UVeVis spectra were

recorded on an Agilent spectrophotometer andfluorescence on a HORIBA Scientific Fluoromax-4spectrofluorometer. FT-IR spectra were recorded on a Bruker, VECTOR 22 model spectrophotometer (Spectra available on the supplementary material). The samples have been prepared with KBr (for solid ones), and KBr liquidfilm (for oils), the resolution used is 16 cm¹with 32 scans and a range of measurement of 600e4500 cm¹.

For mass spectrometry, a Triple Quad, 4500 AB SCIEXmass spectrometer was employed. Theoretical calculations were performed with the Gaussian 09 program, utilizing the density functional theory (DFT), speciﬁcally, the B3LYP method and 6e31 g(d) basis set for optimizations. As for photo-physical, calculations were performed using time dependent methods, TD-DFT, to obtain three of the most important dyes intensities[16].

3. Results and discussion

The most relevant results of this systematic study, grouped according to the type of study,i.e. synthesis (3.1), photo-physical (3.2) and theoretical (3.3), are described and discussed below.

3.1. Synthesis and characterization of dyes

The synthesis route consists of several steps, based mainly on ref.[16], however, some modifications, detailed at the end of the corresponding section, have been carried out to improve purity and yield. The molecules studied here have three distinct parts: the charge donor centered on the nitrogen, the spacer chain (or bridge) is the moiety that includesfluoren-vinyl thiophene structure and finally, the acceptor is the acrylic acid, which acts as anchor group

Fig. 1.Synthesized products: (a) alkyl-substituted series, M8-n; (b) alkoxy-substituted series, M8-On. n¼1(Me), 2(Et).

Table 1

Synthesizeddyesnomenclature.

Dye IUPAC name Codenumber

(E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(methyl(phenyl)amino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid M8-1 (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-(ethyl(phenyl)amino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid M8-2 (E)-2-cyano-3-(5-((E)-2-(9,9-diethyl-7-((4-methoxyphenyl)(methyl)amino)-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid M8-O1 (E)-2-cyano-3-(5-((E)-2-(7-((4-ethoxyphenyl)(ethyl)amino)-9,9-diethyl-9H-fluoren-2-yl)vinyl)thiophen-2-yl)acrylic acid M8-O2

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in the chemical adsorption on the TiO2 surface. The synthetic method has many steps (Fig. 2), and they were carried out by: 1) a SN2 reaction wherein the hydrogens at the thiophene sp3 carbon act as acid compared to KOH (M1)[17]; 2) an addition-elimination reaction (M2); 3) a reduction with NaBH4(M4); 4) alcohol reaction with triphenylphosphoniumbromide forming the phosphonium salt (M5), and 5) formation of a pair of E/Z isomers using the Wittig reaction (M6) [16]. Puriﬁcation of the mixture is conducted by exposure to I2(Fig. 3). An important detail that should be empha- sized in this reaction is that whenever the solution is diluted, the corresponding stereomutation of the Z compound is brought about,

however, when dealing with concentrate solutions, a different process occurs, which produces a solid with other properties, different to the goal of the synthesis proposed herein. This is explained considering that the stereomutation mechanism possesses a cationic intermediate[18]that produces an attack on the double bond of another neighboring molecule, yielding other structures. NMR spectrum of the mixture exhibits a pair of double signals for Z isomer, chemical shift of 6.72 and 6.63 ppm and coupling constant 11.9 Hz (in chloroform-D), which disappears after exposure to I₂. Continuing with the synthetic pathway (Fig. 4) in 6), a PdCl2catalyzed Buchwald-Hartwig coupling reaction of C-N, Fig. 2.Synthetic route to“M5”compound: i) KOH, KI, CH3CH2Br, DMSO; ii) n-BuLi, DMF, THF,78C; iii) NaBH4, MeOH, THF; iv) PHPh3Br, CHCl3, reﬂux; v) thiophen-2- carbaldehyde,t-BuOK, reﬂux, toluene.

Fig. 3.Z to E isomer stereomutation reaction.

Fig. 4.Synthetic route to“M5”ﬁnal products: vi) secondary amine, PdCl2, (t-bu)3PHBF4,t-BuOK, toluene; vii) n-BuLi, DMF, THF,78C; viii) cyanoacetic acid, ammonium acetate, acetic acid, 120C.

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using (t-Bu)3PHBF4, was performed. This ligand is deprotonated in solution, by complexing with the metal, which catalyzes the reaction. This compound was selected because it is more environ- mentally stable and improves yields (M6) [19e21]; in 7) an addition-elimination reaction in thea-hydrogen of the thiophene fraction (M7) occurs. Subsequently, a condensation reaction between the aldehyde and cyanoacetic acid (M8)[16,22]takes place in 8). Thus, the four compounds, all solid, dark red, partially soluble in DCM and ACN and completely soluble in THF, DMSO, DIO, ETA, DMF and ACT, were produced.

3.2. Photophysical properties of synthesized dyes 3.2.1. UVevis spectrometry

In general, the dyes exhibit two absorption bands in the UVeVis range (example for M8-1 in all solvents in Fig. S1), the highest wavelength at 300 nm (band I) and the lowest, at about 450 nm (band II). Band I is ascribed to n-p*-type localized transitions of high energy[16]. This band tends to move to the low wavelength zone, being confused in the experimental noise Band II, also corresponds to a n-p*-type transition, as demonstrated by the hyp- sochromic shift (Table S1) of the visible band of the dyes in the tested solvents [23]. Furthermore, this shift evidences a charge transfer mechanism, which, as usually, occurs in the amino- benzene derivatives[23]. It should be noted that the compounds have average solubility in DCM and ACN, but studies were conducted all the same, because these solvents are classic in this kind of study. In solvents where the compounds are solubles, the molar extinction coefﬁcients,ε0(Table S1) was determined. In each case, in the different studied solvents, a large value of ε0 was found indicating that these molecules have high absorption at itslmax

compared with other chromophores[24,25]. Therefore, it is noteworthy that low concentrations of these substances (in the order of

m^{mol L}¹), enable the measurement of the optical characteristics.

Furthermore, in all cases, using between 1 and 3 mg in 50 mL of the corresponding solvent, saturate any optical instrument, consequently 25e100-fold dilution will be required in order to satisfac- torily conduct each measurement. Thus, concentrations for measuring ranged between 0.5 and 3 mmol L-1. Likewise, most compounds used for DSSCs exhibit high eo values, as the main requirement for these molecules is to absorb as much energy as possible, to subsequently transform it into electricity[26,27].

3.2.2. Fluorescence spectrometry of synthesized dyes

3.2.2.1. Solvent effect on emission patterns. Spectra of each dye showed different patterns, from which 1 to 4 bands were found.

Consequently, two patterns for two different wavelengths were obtained. Each maximum were analysed using gaussian functions (Fig. S2), to separate and obtain those values exactly. Theﬁrst,l1,is the maximum absortion wavelength. The second,l2, is the wavelength at which the absortion spectra of both, the sample and the standard, are intersected. Thus, this wavelength will be use for quantum yield calculations later in this paper. The different bands are related to different excited states, each one was labeled (or- dered from lowest to highest energy), S1, S2, S3and S4. The shown bold number corresponds to the intensity of each one, 1 being the highest and 4 being the lowest. When excited at a wavelength corresponding tolmax(Fig. 5[a, b] for M8-1 andFigs. S3, S4, S5[a, b]), a shouldered band is usually observed. When excited at a wavelength different fromlmax(Fig. 5[c, d] for M8-1 andFigs. S3, S4, S5[c, d]), a separation of the previous pattern is observed, in two or more bands, depending on the solvent. In summary, it is possible to observe, in general, that the energy absorbed atlmax, is subsequently emitted in two different ways: S1and S2to S0. On the other hand, when excited at a wavelength different from lmax, energy is emitted through S₁, S₂, S₃S₄to S₀.

Fig. 5.M8-1 emission spectra in the solvents indicated in the insets, exciting at: (a) and (b)lmax; (c) and (d)lslmax

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Table 2(andTables S2, S3, S4) summarizes maximum emission wavelengths for all the samples (emissionﬁgures summary). As mentioned above, these bands are assigned to the four singlet states, S1atca. 640 nm, S2at 600 nm, S3at 500 nm and S4at 450 nm (approximately). In general, ACT quenches theﬁrst state S₁. An explanation for this would be that in acetone some form of

“quenching”is favored, such as in the most classic in the case of donor-acceptor dipolar molecules namely, the Forster resonance energy transfer (FRET) effect, where charge transfer from donor to acceptor occurs, decreasing thefluorescence intensity[28]. In this particular case, the A-p-D molecules are dipolar, which would explain that, when excited at the maximum absorbance wavelength, thefirst state intensity be lower than the second. Another explanation would be that acetone quenches the intensity of the first state due to some effect on the of the excited-state complex, also possible in this case[29].

3.2.2.2. Stokes shift of the studied dyes. The difference between each maximum, the so-called“Stokes shift”(Table S5), is useful to determine differences in dipole moment of the molecules under survey, between their ground and excited states, after light absorption [30]. In this instance, a polarity decrease occurs for all products, because n-p* transitions depolarize the chemical system, as usually happens in the anilines[31,32]and, therefore, the ground state is more polar than the excited state.

3.2.23. Lippert equation and calculation of polarity difference in ground and excited state of the synthesized dyes. To determine the value of the difference in the dipole moments of the compounds, the Lippert equation (Eq.(1)) was utilized:

vabsvem¼2

m

e

m

g

₂ cha³

"

2ðεs1Þ 2εsþ1 2

n²1 2n²þ1

#

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Where“Stokes shift”orDn, corresponds to the difference between the frequencies (cm¹) of the absorption and emission maxima. The polarity difference (me-mg) is the difference between the excited and ground state dipole moment, respectively. C is the light speed (approximately 3$10⁸ ms^-1). h is Planck's constant (6.63$10³⁴J s). a³is the volume of solvent box around theﬂuo- rophore.εis the dielectric constant of the solvent and h^{it's the} refractive index, whose mathematical relationship corresponds to the solvent dipolar factor.

Thus, by plotting the“Stokes shift”value in all studied solvents vs. the dipolar solvent facto, known as Lippert-Mataga plot, the difference in dipole moments for each dye can be obtained from the respective slope[33,34]. Thus, the absolute value of the difference between absorption and emission maxima, in frequency units (cm¹),vs. solvent dipolar factor was plotted (Fig. 6). A large slope, as inferred from Eq.(2), indicates that the dye presents a greater difference between its ground and excited state, meaning that

donation of the nitrogen non-bonding electron pair is highly shared with the rest of the molecule. On the other hand, for the polarity estimation of the excited state, the Onsager radius (a), is assessed as the ninth of the distance between the carbon from the carbonyl and nitrogen[34],i.e. 17.87Å. The difference of dipole moment in this case corresponds to the difference between the ground and the excited state. Due to the type of transition occurring in this kind of molecules, the ground state is more polar than the excited one[35].

On relation with the graphics, the points tend to diverge, moreover in many cases they do not follow exactly the straight line. However, this is not an isolated fact. Many compoundspconjugated have been studied resulting in different behaviours. For example, there are compounds that follow the linearity (with some divergences) [36e43], others have different tendencies inside one plot[44e47]

and an asymmetric amine[48], which has a close behavior with the type of compound reported here. In addition, compounds like ketocoumarins[49]shown this behavior. In general, the aim is to compare, then this plot has been made to obtain some information related with substituents (qualitative information). Values for ground states were estimated by performing theoretical calculations using the Gaussian 09 program and optimizing the chemical structures[16,50]. This way, comparing the values of the difference of dyes dipole moment (Table 3) the greater difference corre- sponded to M8-2. Ethyl group has a greater effect on the transition than the methyl group and this is evident when comparing M8-1 with 2. Nevertheless, the inverse effect it has been seen for M8- O1 compared with O2. As for the comparison of M8-2 with M8-1 and O1 with O2, in the former the methoxyl group decreases the difference of the dipole moment, whereas in the second, ethoxyl decreases even more this difference.

3.2.2.4. Dyes quantum yield (FF). A comparative method reported by Williams et al.[51,52], employing standard zinc phthalocyanine (ZnPc) in 1% pyridine in toluene, was used for determining the quantum yields according to Eq.(2):

F

x¼

F

ST

Grad_X Grad_ST

h

²_X

h

²_ST

!

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whereFXstands for quantum yield of the sample andFSTfor the quantum yield of the reference, in this case 0.30. Grad_xis the slope of integrated emission intensity vs. sample absorbance plot and GradSTis the standard slope.hXandhSTare the refractive indexes of sample and standard solvents, respectively. For solvents wherein the solubility is not complete, as DCM and ACN, the above method was employed but as an equation with unknowns, not as an equation derived from a plot, according to Eq.(3):

F

S¼Abs_R Abs_SArea_S

Area_Rn_S

n_R

F

R (3)

Table 2

M8-1lemexcitedatlmaxandlslmax. The numbers indicated are related to intensity.

Solvent lmax(nm) lem(nm) lslmax(nm) lem(nm)

Estado S1 S2 Estado S1 S2 S3 S4

DCM 481.0 636 (2) 621 (1) 387.0 639 (2) 621 (1)

THF 464.0 638 (2) 606 (1) 384.0 639(2) 600 (1) 474 (3)

ETA 460.0 637 (2) 599 (1) 383.7 631 (3) 569 (1) 474 (2)

DIO 466.0 639 (2) 616 (1) 391.8 638 (3) 586 (2) 542 (1) 444 (4)

ACT 463.0 638 (2) 627 (1) 390.8 598 (2) 495 (1) 443 (3)

ACN 439.0 641 (1) 627 (2) 387.0 653(1) 617 (2)

DMF 432.0 639 (2) 612 (1) 385.6 635 (2) 610(1) 513 (3)

DMSO 430.0 643 (1) 629 (2) 381.0 638 (3) 625 (2) 515 (1)

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whereFSandFRstand for sample and reference quantum yields.

AreaSand AreaRare the integrated emission intensity for sample and reference, respectively. Abs_Rand Abs_Scorrespond to reference and sample absorbance.hSandhRare, respectively, the refractive indexes of sample and reference solvents,

As for the quantum yields values (Table 4), no linear relationship with the polarity of the solvent was found, but some trends are likely to be observed. Taking into account the main factors affecting ﬂuorescence,e.g. chemical structure, structural stiffness, temperature, solvent, pH and concentration effect. For the measurements, parameters such as temperature (ambient), structural rigidity, pH (not applicable) and low concentrations (less than 0.1 absorbance

units) were kept constant. The chemical structure changes and the modifications were mentioned in the Introduction. The solvent was changed, in order to observe the different responses. Comparison of M8-1 and 2 yields with M8-O1 and O2 yields demonstrated that the latter are largely overcome,i.e. the solely nitrogen alkyl modification increases the quantum yield, but not the alkoxy modification.

As for the length chain, in most solvents, M8-2 exhibits higher yield consequently, the substituent length affects it proportionally. In the alkoxy-substituted series, the opposite is true,i.e. an inverse proportional relationship is found.

Another important point is that, on average, yields are higher in nonpolar solvents DCM, THF, ETA and DIOvs. polar solvents ACT, ACN, DMF and DMSO (Table 5), which will be discussed in the

“theoretical approximation of the molecular properties”section. It is noteworthy that the yields are particularly low for ACT and ACN.

In the former, this is due to the total quenching of the S₁ state, Fig. 6.Dyes Lippert-matagaplots: (a) M8-1. (b) M8-2. (c) M8-O1. (d) M8-O2.

Table 3

Dipole moment from Lippert plots.

Dye Slope Dipolemomento difference(D) Ground state dipole moment (D) Excited state dipole moment (D)

M8-1 24162.04 3.69 11.47 7.778

M8-2 21356.95 3.47 14.24 10.767

M8-O1 17790.75 3.17 11.40 8.236

M8-O2 12774.43 2.69 9.93 7.246

Table 4

Dyes quantum yield in different solvents.

Solvent DCM THF ETA DIO ACT ACN DMF DMSO

Polarity index 3.1 4.0 4.4 4.8 5.1 5.8 6.4 7.2

Viscosity 0.40 0.51 0.41 1.15 0.31 0.37 0.77 2.13 Dye

M8-1 20.5 32.7 61.4 98.8 10.9 8.2 56.1 17.6

M8-2 18.4 47.1 34.2 62.5 26.3 25.5 60.2 31.7

Average 19.5 39.9 47.8 80.7 18.6 16.9 58.2 24.7

M8-O1 0.9 16.2 7.4 34.5 1.4 1.2 7.1 2.7

M8-O2 5.2 10.6 5.8 33.9 0.8 1.0 7.1 5.1

Average 3.1 13.4 6.6 34.2 1.1 1.1 7.1 3.9

Table 5

Average quantum yield in polar and non-polar solvents.

Dye Non-polar solventsaverage Polar solventsaverage General average

M8-1 53.4 23.2 38.3

M8-2 40.6 35.9 38.2

M8-O1 14.8 3.1 8.9

M8-O2 13.9 3.5 8.7

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discussed in emissions section. In contrast, in ACN, is due only to the general quenching of the signals and the intensity is only partly diminished. On the other hand, in DIO yields are high, since in most dyes the appearance of the contribution of the four states is observed and all display high intensity. In addition, it can be seen that, on average (Table 4),“low”viscosities (ACT) negatively affect yields; the same also occurs for“high”viscosities (DMSO).

Then, intermediate viscosities as DIO particularly favor this property.

Another important point is the relationship with the“Stokes shift”: in this case, M8-2 has a value greater than that of M8-1 and, therefore, yields are higher for M8-2. As for M8-O1 and O2, yields are higher for the former, but in general tend to the same value.

Consequently, a large difference in polarity between ground and excited state leads to an increase of quantum yield.

3.3. Theoretical approach to molecular properties of dyes

In the search for answers to the observed phenomena, theoretical calculations were conducted to determine whether there is any relationship between molecules geometry and experimental changes.

3.3.1. Dihedral angles and their relationship to the quantum yield Another way toﬁnd an answer forFFvalues is to compare the values of the dihedral angles. In this respect, the angle formed between the carbonyl acceptor and the amine donor is compared, as an approximation to the lining up of these two groups (Fig. 7).

This takes place because both LUMO and HOMO orbitals settle around these functional groups (Fig. S6[a, b, c, d])

In this comparison, two angles are possible: Ar-CO (aryl to carbonyl), which refers to that formed between ring-nitrogen and carbonyl, and the other, Al-CO (alkyl to carbonyl), to that formed between aliphatic chain-nitrogen and carbonyl. Thep^{-bridge is} considered as linear and it does not show any important change on these values.

3.3.1.1. M8-n vs. M8-On series. It can be veriﬁed (Table S6) that the Ar-CO angle is less for M8-n, in turn, the Al-CO angle is less for

M8-On. This allows explaining that a high quantum yield is associated with lower values of Ar-CO and higher values of Al-CO, as in the M8-n series (Table 5). The alkyl group also has the function of orienting the aromatic ring with the remainder of the conjugated chain. In addition, both angles are inversely proportional for both families. Comparing the effect of the substituent length,ﬁrstly between M8-1 and 2, both possess similar yields, M8-1 being greater, on average, by only 0.1%. This relates to the fact that the Ar-CO dihedral angle is always greater and Al-CO lower for M8-1. Besides, between M8-O1 and O2 they also exhibited similar yields, M8-O1 being higher, on average, by 0.2%.

Opposite to the previous series, in this case Ar-CO is lower and Al- CO higher for M8-O1. In summary, the alkoxy group distorts the geometry of the molecule so that to maintain the quantum yield trend in its series, but decreasing it by 77.0% on average. InTable 4, difference in yields measured in nonpolar and polar solvents is observed; their relationship with the geometry of the molecules will be discussed later.

3.3.1.2. Effect of nonpolar solvents. In this respect, it is possible to appreciate (Tables S6 and S7) that in series M8-n the Ar-CO angle increases and Al-CO decreases with polarity. Furthermore, Ar-CO values are always higher for M8-1 (Table 6) and Al-CO is always greater for M8-2. Relating this to quantum yields in nonpolar solvents (Table S4), a greater Ar-CO angle favors higher values of the quantum yield. Although yield (Table 4) is proportional to the Ar-CO angle for M8-1, this is not true for M8-2 wherein the trend between DCM-ETA and THF-DIO is perceived. In the M8-On series Ar-CO decreases and Al-CO increases with polarity. In addition, Ar-CO values are always higher for M8-O2 (Table 6) and Al-CO is always greater for M8-O1. Relating this to quantum yields in nonpolar solvents, an Ar-CO lower angle favors higher quantum yield values.

In this case, the yield (Table 4) is inversely proportional to Ar-CO angle for both dyes, but the trend between DCM-ETA and THF- DIO do exists.

3.3.1.3. Effect of polar solvents. Table S7, shows that in the M8-n series the Ar-CO angle decreases or remains constant and Al-CO increases with polarity. Furthermore, Ar-CO values are always higher for M8-1 (Table 6) and Al-CO is always greater for M8-2, ascribing this to quantum yields, in polar solvents (Table 5), a lower Ar-CO angle favors higher values of quantum yield. The yields (Table 4) cannot be explained hereby the dihedral angles, as they remain relatively constant, however, between ACT-DMF and ACN- DMSO, a slight angle decrease is observed, which allows inferring that the yield is inﬂuenced by these geometrical changes. In the M8-series, Ar-CO increases and Al-CO slightly decreases or remains constant with polarity. In addition, Ar-CO values are always higher for M8-O2 (Table 6) and Al-CO is always greater for M8-O1. Relating this to quantum yields in nonpolar solvents, a greater Ar-CO angle favors higher values of quantum yield. In this respect, the yield (Table 4) increases linearly between ACT-DMF and ACN-DMSO, which is consistent with the fact that the Ar-CO angle increases between these solvents,i.e. the quantum yield is affected by geo- metric changes.

Fig. 7.Approximate structures for the determination of dihedral angles of: a) M8-1 and M8-2. b) M8-O1 and M8-O2.

Table 6

Angles grouped according nonpolar and polar solvents.

Dye/Angle Averageinnonpolarsolvents Average in polar solvents

An-CO Al-CO An-CO Al-CO

M8-1 45.5 122.7 44.1 124.8

M8-2 21.3 170.0 19.7 171.5

M8-O1 38.3 127.0 39.3 126.3

M8-O2 62.9 90.1 64.8 88.3

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In summary,Table 4shows that quantum efficiencies are greater in nonpolar solvents than in polar solvents. In the case of M8-1 and 2 molecules, because the ring is more aligned with the acceptor, which would favor conjugation from the benzene ring, partici- pating thus in thefluorescence. On the other hand,fluorescence is affected in M8-O1 and O2, where the ring, because of the alkyl group, cannot achieve this structure and, consequently, thefluorescence becomes diminished. Finally, for M8-1 and 2 in polar solvents, the Ar-CO angle decreases and the Al-CO increases and, on the other hand, for M8-O1 and O2, the Ar-CO angle increases and Al-CO decreases, corroborating that the structure where the acceptor isflatter with the substituent ring, favorsfluorescence in thefirst family.

3.3.2. Theoretical photophysics, assignments comparison

To corroborate assignments patterns in theﬂuorescence spectra, theoretical calculations were made, as mentioned in experimental section. Most transitions occur between the frontier orbital and other close orbitals. HOMO (H), the orbital before HOMO (H-1) and that prior to this (H-2) appear in this case. As for LUMO (L), the following to LUMO (Lþ1) and the next to this (Lþ2) also appear.

Both HOMO and LUMO orbitals, for both families, are depicted in Fig. 4. The other orbitals are observed inFigs. S6 and S7. In general,

“Stokes shifts”are explained by the Franck-Condon principle, which also explains that at two different excitation lengths (l1andl2), where the former has lower energy than the second, some or all excited states are displayed[39]. In general in all studied molecules, the theoretical approximation (Tables S8 and S9) is accurate enough to predict theﬁrst excited state (maximum error near to 6.7% from the real value). The second and third states are predicted but not accurate. The fourth excited state is not predicted by this method in any case.

Summarizing, these results would validate the TD-DFT calculations, even though it is true that predict only 3 of the 4 states, the most likely state is always predicted with fairly good accuracy. As for the Stokes Shift, M8-2 presents the largest difference between the ground and excited state, which is directly related to the fact that this molecule possesses the largest quantum efﬁciency in most of the studied solvents. Thus, this systematic study, both experimental and theoretical, will enable adequately design the use of the best dyes for assembling solar cells, to corroborate its usefulness in the development of these devices, to be investigated in the near future.

4. Conclusions

The synthesis and characterization of four A-p-D-type dyes, in two series: alkyl- and alkoxy-substituted, M8-n and M8-On, were respectively completed. The reaction yields are 10.0% (M8-1), 7.8%

(M8-2), 7.4% (M8-O1) and 8.8% (M8-O2). Two absorption bands, at 300 nm and 450 nm range, were observed. Four different states have been found, S1(640 nm), S2 (600 nm), S3 (500 nm) and S4

(450 nm). State S₁is usually moderately quenched by FRET effect.

TD-DFT calculations predict only 3 of 4 states. Theﬁrst is more precisely predicted, at 650 nm, while the state S2at 600 nm, is not predicted. As for the Stokes Shift, M8-2 has the largest difference between the ground and excited state, which is directly related to the fact that this molecule shows the highest quantum yield in many solvents. Quantum yields are higher for the M8-n series in all solvents. However, the yields are always higher in non-polar solvents due to planarity of the aromatic ring (amine) respect to the acceptor carbonyl moiety. In polar solvents, the alkyl group must be planar. Maximum yield was achieved with (E)-2-cyano-3-(5-((E)-2- (9,9-diethyl-7-(methyl(phenyl)amino)-9H-ﬂuoren-2-yl)vinyl)thiophen-2-yl)acrylic acid in 1,4-dioxane, reaching a yield of 98.8%, that

allows projecting the utility of this dye in the fabrication of photovoltaic cells. Products reported here in will shortly be tested to corroborate these results in practice.

Acknowledgements

This work was made possible thanks to the contribution of FONDECYT 1141158 and Ecos-Conicyt C14E05 grants.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://

dx.doi.org/10.1016/j.molstruc.2017.04.019.

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