Photobehavior and Nonlinear Optical Properties of PushâPull

Article pubs.acs.org/JPCC

Photobehavior and Nonlinear Optical Properties of Push−Pull, Symmetrical, and Highly Fluorescent Benzothiadiazole Derivatives Federica Ricci,† Fausto Elisei,† Paolo Foggi,†,‡,§ Assunta Marrocchi,† Anna Spalletti,† and Benedetta Carlotti*,† †

Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN) University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy ‡ LENS (European Laboratory for Non-Linear Spectroscopy), via Nello Carrara 1, Sesto Fiorentino, 50019 Firenze, Italy § INO−CNR, Istituto Nazionale di Ottica−Consiglio Nazionale delle Ricerche, Largo Fermi 6, 50125 Firenze, Italy S Supporting Information *

ABSTRACT: Three quadrupolar D−π−A−π−D compounds, bearing alkoxy phenyls as mild electron donors and a benzothiadiazole (A), two benzothiadiazoles (B), or a benzothiadiazole linked to two thiophenes (C) as the acceptor units, are collectively the object of this study. They proved to be efficient yellow/orange/red fluorophores, respectively, with fluorescence being their preferred deactivation pathway. These systems exhibited positive fluorosolvatochromism and a noticeable decrease of the fluorescence quantum yield in the most polar solvent (with the quenching following the trend B > A > C). These findings point to an intramolecular charge transfer (ICT) nature of the emitting state, whose photoinduced dynamics was investigated by femtosecond-resolved transient absorption and fluorescence upconversion. Remarkable values of hyperpolarizability were estimated by the solvatochromic method. The significant two-photon absorption cross sections measured for A−C (whose trend nicely parallels that of ICT efficiency), coupled with the intense emission, make them promising for applications as environment-sensitive probes in two-photon excited fluorescence imaging.

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INTRODUCTION The development of organic materials showing high twophoton absorption (TPA) has attracted great interest in the last years due to a wide range of emerging applications in photonics and optoelectronics.1−3 All these applications benefit from the following advantages of the two-photon excitation: population of high-energy excited electronic states by means of low-energy infrared radiation and quadratic dependence of the absorption on the light intensity, which allows for more control and enhanced spatial resolution. In view of biological applications, such as imaging, these benefits play an important role as they lead to better resolution and in depth tissue penetration combined with reduced photodamage.4−6 Besides large TPA, significant fluorescence quantum yields, good photostability, and proper response wavelengths are desired molecular attributes to meet the required needs for use in fields such as fluorescence microscopy. In particular, the commonly used fluorophores are mainly blue or green emitters, and efficient orange/red emission by organic molecules has been rarely observed. However, the 600−800 nm wavelength region is a window widely recognized as very attractive for bio-analysis and in vivo imaging.7,8 Recently, chemists have been paying increasing attention to organic systems bearing electron donor (D) and acceptor (A) groups linked by π-conjugated bridges for their outstanding © 2016 American Chemical Society

nonlinear optical (NLO) properties. Many factors play an important role in achieving large TPA cross sections including the efficiency of intramolecular charge transfer (ICT),9 conjugation length, planarity of the molecule, and donating and withdrawing abilities of the D/A portions.3 Among all the possible molecular arrangements, the quadrupolar D−π−A−π− D structure is highlighted by its excellent response.10−12 Among various aromatics and heteroaromatics, benzothiadiazole has attractive physical and chemical properties making it an appropriate electron acceptor building block for the construction of functional organic materials. It has been successfully employed not only in organic semiconductors for field effect transistors13−15 but also in highly efficient donor/ acceptor polymers16−18 and low-dimensional molecular architectures for organic photovoltaic cells.19,20 Moreover, benzothiadiazole has been effectively included in asymmetric push− pull compounds21,22 and also resulted in the best-performing central chromophore in series of structurally analogous symmetric D−π−A−π−D molecules,23,24 in terms of ICT efficiency and TPA response. In some cases, benzothiadiazoleReceived: July 20, 2016 Revised: September 27, 2016 Published: September 30, 2016 23726

DOI: 10.1021/acs.jpcc.6b07290 J. Phys. Chem. C 2016, 120, 23726−23739

Article

The Journal of Physical Chemistry C Chart 1. Molecular Structures of the Investigated Compounds (with R = C6H13)

The fluorescence spectra, corrected for the instrumental response, were measured by a FluoroMax-4P spectrofluorimeter of HORIBA Scientific operated by FluorEssence. Dilute solutions (absorbance < 0.1 at the excitation wavelength, λexc) were used for fluorimetric measurements. The fluorescence quantum yield (ϕF, uncertainty ±10%) was determined at λexc corresponding to the maximum of the first absorption band. Tetracene (ϕF = 0.17 in aerated CH)34 was used as fluorimetric standard. Fluorescence lifetimes were measured using the single photon counting method through an Edinburgh Instrument 199S spectrofluorimeter, equipped with a LED source centered at 460 nm, with a 0.2 ns temporal resolution. The experimental setups for femtosecond transient absorption and fluorescence upconversion measurements have been widely described elsewhere.35−37 In particular, the 400 nm excitation pulses of ca. 40 fs are generated by an amplified Ti:sapphire laser system (Spectra Physics). The transient absorption setup (Helios, Ultrafast Systems) is characterized by temporal resolution of ca. 150 fs and spectral resolution of 1.5 nm. Probe pulses are produced in the 475−750 nm range by passing a small portion of 800 nm light through an optical delay line (with a time window of 3200 ps) and focusing it into a 2 mm thick sapphire window to generate a white-light continuum. In the upconversion setup (Halcyone, Ultrafast Systems), the 400 nm pulses excite the sample, whereas the remaining fundamental laser beam plays the role of the “optical gate” after passing through a delay line. The fluorescence of the sample is collected and focused onto a BBO crystal together with the delayed fundamental laser beam. The upconverted fluorescence beam is focused into the entrance of a monochromator by a lens, and it is then detected by a photomultiplier connected to a photon counter. The temporal resolution of the upconversion equipment is about 250 fs, whereas the spectral resolution is 5 nm. Ultrafast spectroscopic data were fitted by global and target analysis using the Glotaran software.38 The two-photon excited fluorescence setup employs a Nd:YAG pump laser (Surelite II-Continuum) at 355 nm and an optical parametric oscillator (OPO-Surelite P/N 996−0210,

based quadrupolar systems have interestingly displayed large TPA coupled with intense emission.25−30 In the present work, three quadrupolar benzothiadiazole derivatives (A−C in Chart 1) have been considered, displaying triple bonds as π-linkers which ensure strong photostability. They exhibit highly symmetric D−π−A−π−D (compounds A and C) or D−π−A−π−A−π−D (compound B) molecular structures where the mild electron donors are in all cases alkoxy substituted phenyl rings. These molecules have been designed keeping in mind that a very efficient ICT is often accompanied by a dramatic fluorescence quenching and that employment of weak donor groups might be helpful with respect to the goal of obtaining strongly fluorescent materials. Moreover, molecular structures featuring multiple conjugative carbon−carbon triple bonds (so-called oligoyne/polyynes), such as compound B, have been reported to display very attractive NLO and electronic properties.31,32 Introduction of additional thiophenes in compound C allowed the effect of increased conjugation and molecular dimensionality to be investigated. This study has been carried out by means of stationary and time-resolved (with nanosecond and femtosecond resolution) spectroscopies both in absorption and in emission, supported by density functional theory (DFT) calculations. The effects of solvent on the spectra, photophysics, and excited state dynamics have been deeply investigated to obtain information about the occurrence of ICT. Knowledge about NLO properties (hyperpolarizability and TPA) of A−C has been also gained and discussed in connection to their ICT efficiency and structural features.

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EXPERIMENTAL SECTION Chemicals. The investigated compounds (A−C, Chart 1) were synthesized following previously reported procedures.13,14,33 Measurements were performed in various solvents (Fluka, spectroscopic grade): cyclohexane (CH), toluene (Tol), anisole (An), chloroform (CHCl3), ethyl acetate (EtAc), dichloromethane (DCM), 1,2-dichloroethane (DCE), and dimethylformamide (DMF). Experimental Techniques. A PerkinElmer Lambda 800 spectrophotometer was used for the absorption measurements. 23727


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The Journal of Physical Chemistry C

diameter) of the optimized structures. This procedure was chosen on the basis of the results reported in a previous paper51 where a was calculated by integration of the solvent accessible surface using both the Hartree−Fock and DFT-optimized geometries and was found to be 60% of the CT diameter. Equation 1 may be written as

Continuum) which can be manually tuned to produce radiation between 410 and 2200 nm (signal between 410 and 750 nm; idler between 820 and 2200 nm). The fluorescence light is collected on a 1/4 m monochromator equipped with a 1200 grooves/mm grating. Subsequently, there is a photomultiplier tube (Hamamatsu R3788), powered by a high-voltage power supply (PS-310, SRS), connected to an oscilloscope (LeCroyWave Runner-LT322, 500 MHz, 200 MS/s, DSO) where the fluorescence intensity is read in mV. The TPA cross section was determined through the comparative method that uses a standard substance (fluorescein in buffered water at pH 11) with a well-known cross section (σ = 26 GM at 930 nm).39 Computational Details. Quantum−mechanical calculations were carried out using Gaussian 09 package.40 DFT based on the CAM-B3LYP method was used to optimize the geometry and to obtain the properties of the substrates in the ground state, while the lowest excited singlet states were characterized by time-dependent (TD)-DFT CAM-B3LYP excited-state calculations.41,42 In both cases, a 6-31G(d) basis set was employed. DCE solvation effects were included in the calculations by means of the conductor-like polarizable continuum model (CPCM).43 Calculations were carried out on a molecular structure with the C6H13 groups replaced by the CH3 ones. TPA transition tensors and cross sections of A−C and of the reference compound (fluorescein in its bianionic form) in vacuum were computed by use of the CAM-B3LYP/6-31G* method, Dalton2016.1 package,44,45 according to the formulas given by McClain et al.46,47 The absorption depends on the light polarization; a linearly polarized light source is considered. The rather good agreement obtained in the case of the reference sample fluorescein (the computed TPA cross section is of ca. 16 GM; the corresponding experimental value is of 26 GM) constitutes a validation of the employed computational method. Derivation of Hyperpolarizability. The experimental results on the fluorosolvatochromism allowed information on the difference between the excited ICT and ground state quadrupole moments (Qe − Qg) to be obtained by using eq 1, as derived on the basis of McRae’s theory,48,49 and eqs 2 and 350 that define the quadrupolar moment:

y = Ax + B where y = Δυ = υA − υF, x = f (ε , n2) =

(

ε−1 ε+2

−

n2 − 1 n2 + 2

),

8d 2Δμ2

and B = δA + δF. A= hca5 From the slope A, Δμ of part of the molecule (responsible for the observed solvatochromism) was then derived. The hyperpolarizability was then calculated by the Oudar formula:49,52 βCT = βzzz =

υeg2 reg2 Δμ 3 × 2h2c 2 (υeg2 − υL2)(υeg2 − 4υL2)

(5)

where reg is the transition dipole moment, υeg is the transition frequency (assumed to be the maximum of the bathochromic absorption band), and υL is the frequency of the reference incident radiation (Nd:YAG at 1064 nm) to which the β value would be referred. The reg value is related to the oscillator strength (f) by r2eg =

3e 2 h 8π 2mc

×

f υeg

= 2.13 × 10−30 ×

f , υeg

(f being

obtained from the absorption integrated band as f = 4.32 × 10−9 ∫ ε(υ) dυ).53 Methods based on solvatochromism to obtain physical properties of the solute have been used for long time (Lippert−Mataga approach)54,55 and improved over time (McRae,48 Bakhshiev,56 Bilot−Kawaski)57 also applying multilinear correlation using three- or four-parameter solvents scales (Kamlet−Taft58 and Catalan).59,60 It has to be noted that despite the methods based on the solvent effect on the spectra containing several approximations, thus allowing only an approximate estimation of β, a good agreement has been reported for several compounds between the β values thus estimated and those experimentally determined by more refined techniques (the well-known electric field induced second harmonic, EFISH, and hyperRayleigh scattering, HRS)61 and those theoretically calculated.62 The solvatochromic methods offer the advantage of simplicity and easy availability over EFISH generation and HRS. This method has recently gained popularity and a large number of studies in the past few years have employed it for an estimation of the hyperpolarizability of organic molecules (see refs 63−65). Being based on the production of the ICT state in polar solvents, the method here used gives βCT dominant contribution (corresponding to the βxxx component of the β tensor when related to the CT transition). Moreover, being referred to the Nd:YAG exciting laser frequency, the described method to calculate βCT allows a direct comparison with the value measured by EFISH. The static hyperpolarizability, whose value is instead frequency independent, can be defined as follows:66

Δυ = υA − υF 2(Q e − Q g)2 ⎛ ε − 1 n2 − 1 ⎞ − = (δA − δ F) + ⎟ ⎜ ⎝ε + 2 hca5 n2 + 2 ⎠ (1)

Q = 2μd

(4)

(2)

(Q e − Q g)2 = (2μe d − 2μg d)2 = 4d 2(μe − μg )2 = 4d 2Δμ2 (3)

where υA − υF is the Stokes shift (cm−1), δA and δF are the differences in the vibrational energy (cm−1) of the molecule in the excited and ground state for absorption and emission, respectively, h is the Planck’s constant (erg × s), c is the speed of light in vacuum (cm s−1), a is the cavity radius within Onsager’s model (cm), ε is the relative dielectric constant, n is the static refractive index of the solvent, and Q is the quadrupole moment that in the case of centrosymmetric molecules can be described by a simple model of two opposite dipoles separated by a distance d. The a value was estimated as 60% of the calculated diameter along the CT direction (CT

β0 = 23728

reg2 Δμ 3 × 2h2c 2 υeg2

(6) DOI: 10.1021/acs.jpcc.6b07290 J. Phys. Chem. C 2016, 120, 23726−23739

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Figure 1. Normalized absorption and emission spectra of A−C in solvents of different polarity.

Table 1. Spectral Properties of A−C in Solvents of Different Polarity λabs (nm)

a

2 a

solvent

f (ε, n )

A

CH Tol Tol/An 30:70 Tol/An 70:30 An CHCl3b EtAc DCM DCE DMF

−0.00206 0.0242 0.0996 0.178 0.224 0.293 0.4000 0.474 0.497 0.6668

446 448 448 446 448 447 440 445 444 443

B 470 472 472 472 472 469 471 468

Δυ (cm−1)

λF (nm) C

A

507 500 503 502 502 504 496 500 501 500

504 537 556 547 561 580 550 577 581 600

B 528 545 534 555 563 582 591 638

C

A

B

600 620 635 626 636 648 635 649 650 666

2580 3699 4335 4139 4496 5129 4545 5140 5310 5906

2337 2837 2459 3168 3424 4139 4310 5693

C 3057 3870 4132 3945 4276 4409 4413 4591 4575 4984

f (ε, n2) = [(ε − 1)/(ε + 2)] − [(n2 − 1)/(n2 + 2)]. bε (M−1 cm−1): A = 31 500, B = 64 100, C = 37 000.

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the case of A and B, the fluorescence spectrum appears structured in the least polar solvents (e.g., cyclohexane and toluene) and becomes a bell-like band upon increasing the solvent polarity. Moreover, a significant redshift of the emission maximum is observed on going from nonpolar to highly polar solvents. This bathochromic shift is rather small in the case of C, intermediate for compound A, and very apparent when B is considered. Positive fluorosolvatochromism is quite usual for neutral donor−acceptor fluorophores67,68 and may be due to an intramolecular charge transfer (ICT) character of the excited

RESULTS AND DISCUSSION

Solvatochromism. Figure 1 shows the normalized absorption and emission spectra of A−C in several solvents of different polarity. In all cases, the absorption band is not much affected by the solvent, with its maximum position being influenced by the solvent polarizability (the bathochromic spectrum is observed for the three compounds in anisole, the solvent characterized by the highest refractive index; see Table 1). In contrast, the emission undergoes a large solvent effect. In 23729


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intermediate polarity (chloroform), and a highly polar solvent (dimethylformamide) are considered as representative examples. In all the investigated media, the absorption spectrum of A is blue-shifted with respect to those of B and C, with the presence of additional benzodiathiazole and π-bridges (B) or thiophene rings (C) being responsible for an increase in conjugation and a consequent bathochromic shift of the absorption. It is noteworthy that the addition of thiophenes is particularly effective in inducing a red-shifted absorption, and compound C is thus very appealing for applications in organic photovoltaic solar cells where exploitation of the red part of the solar radiation is usually difficult, which is an open challenge. In previous works25,26,28,29 concerning molecular structures slightly different from those of A and C, the same effect of red-shifting the absorption and emission spectra was observed when benzothiadiazole is coupled to thiophenes, which remarkably agrees with our findings regarding compound C. As for the absorption coefficients of the investigated compounds, their values are significant (31 500 M−1 cm−1 for A, 64 100 M−1 cm−1 for B, 37 000 M−1 cm−1 for C in chloroform) and increase with conjugation. This experimental finding strengthens the interest for these compounds as potentially useful solar absorbers in organic photovoltaic cells. The absorption coefficient of compound B is double the value obtained in the case of compound A. As for the emission bands, the fluorescence of C is interestingly located at longer wavelengths (red emission) with respect to those of A and B in all the investigated media. Compounds A and B show emission bands in roughly the same spectral regions even though the emission of the latter undergoes a larger solvent effect and results in a more effectively red-shifted and enlarged spectrum in polar DMF (Figure 2). Fluorescence Properties. Fluorescence quantum yields were measured for compounds A−C in several solvents of different polarity, and the results are reported in Table 2. The observed fluorescence is relevant, with the efficiency of the radiative pathway being higher than 50% in most of the investigated solvents. In nonpolar media (e.g., toluene), compound A shows a slightly enhanced emission yield with respect to that of B and C (0.71 vs 0.58 and 0.55), probably because of an increased competition of intersystem crossing (ISC) in the latter two33 which bear heavy atom containing additional aromatics. However, in all cases fluorescence is the largely prevalent decay pathway in nonpolar media, with the ISC being responsible for the dissipation of only a small fraction (5−10%) of the absorbed quanta.33 Fluorescence quantum yields of A−C undergo a progressive reduction upon increasing solvent polarity (Table 2). However, the fluorescence quenching is apparent in dimethylformamide which is the most polar solvent among those under consideration (ϕF is 0.06 for A, 0.006 for B, and 0.24 for C). On going from toluene to dimethylformamide, the fluorescence efficiency is thus reduced by 1 order of magnitude in the case of compound A, by 2 orders of magnitude for B, and is only halved in the case of C. The quenching of emission in the most polar solvents is a typical behavior for push−pull compounds and is a hint of their excited state ICT character.67,68,73 If compared with other push−pull benzothiadiazole derivatives previously studied in the literature,25,26 then compounds A−C show the advantage of high fluorescence quantum yields observed in a larger range of solvent polarity because of a less effective quenching in the more polar media due to the weakly electron donating alkoxy groups. The ICT emitting state might get stabilized in the most

states of the quadrupolar compounds object of the present study.69 As a consequence of this positive fluorosolvatochromic behavior, an increase of the Stokes shift with the solvent dielectric constant is observed (Table 1), suggesting that the emission takes place from a polar excited state. For the three compounds in all the investigated solvents, no wavelength effect was retrieved on the fluorescence spectra, and a good overlap of the fluorescence excitation with the absorption band could be observed (Figure S1). These findings suggest that there is just one species responsible for light absorption in solution and that the observed changes in the emission should be related to solvent-dependent excited state processes. It is noteworthy that the FWHM is significantly enhanced with the solvent polarity (Table S1), with the enlargement being more apparent for B, intermediate for A, and less important for C. The expected increase of this parameter in polar solvents when a ICT process is operative due to the increase in the solvent and low-energy modes reorganization energy70 can be little discriminating or even inverted if opposite factors play a role.71,72 The latter could be the narrowing of the population distribution around the equilibrium position of a twisted geometry when rotation around the single bonds between the side groups and the ethyne bridges leads to a charge separation decoupling the donor/acceptor moieties (TICT states).71 However, for all the investigated compounds a net growth of FWHM2 with the solvent function f(ε, n2) is observed (Figure S2). This experimental evidence thus points to a planar geometry of the emitting state of ICT nature.67 Figure 2 shows the effect of molecular structure on the spectral properties, that is, a comparison of absorption and emission spectra of A−C considered in the same solvent. A low polarity solvent (toluene), a solvent characterized by

Figure 2. Normalized absorption and emission spectra of A−C in Tol, CHCl3, and DMF. 23730


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The Journal of Physical Chemistry C Table 2. Fluorescence Properties of A−C in Solvents of Different Polarity ϕF solvent

A

CH Tol Tol/An 30:70 Tol/An 70:30 An CHCl3 EtAc DCM DCE DMF

0.80 0.71 0.54 0.61 0.57 0.60 0.71 0.50 0.46 0.06

B 0.58 0.49 0.51 0.43 0.49 0.36 0.27 0.006

kF (108 s−1)

τF (ns) C

A

B

C

A

B

C

0.54 0.55 0.47 0.49 0.48 0.43 0.46 0.35 0.34 0.24

4.0 4.3

1.9

4.4 4.5

2.0 1.6

3.1

1.2 1.2

4.5 4.9 5.0 4.8 4.7 3.7

1.2 1.0 1.3 0.86 0.86 0.65

4.8 5.9 5.4 5.8 5.4 1.0

polar solvents (as pointed out also by the redshift of the emission spectrum), and the reduced S1−S0 energy gap might induce a more efficient internal conversion (IC) to the ground state in these media. In fact, the ISC yield decrease parallels the fluorescence quenching with solvent polarity.33 The different entities of the fluorescence quenching for the studied compounds may therefore give some guidance of the extent of ICT character of their excited states (B > A > C). The lessimportant ICT character observed for compound C with respect to that of A is likely due to the increased distance between the donor and the acceptor portion in the former which is known to have a negative effect on the ICT rate constant.74,75 Fluorescence lifetimes for A−C were measured through nanosecond-resolved single photon counting, and the results obtained in different solvents are reported in Table 2. They generally exhibit values around 5 ns in the case of A and C and smaller values (around 2 ns) for compound B. The fluorescence lifetimes of the three compounds show a certain solvent dependence; it generally gets longer on going from toluene to chloroform and then undergoes a certain quenching with solvent polarity reaching the smallest value in the most polar dimethylformamide. The fluorescence rate constants (Table 2, calculated as kF = ϕF/τF) exhibit values of the order of 108 s−1, which indicate a fully allowed radiative transition. The kF values of A−C are reduced upon increasing the solvent polarity. These experimental findings are in agreement with a change in the nature of the emitting state on going from nonpolar to highly polar solvents. Ultrafast Spectroscopic Investigation. The fluorescence properties of these quadrupolar systems were also investigated with a femtosecond time resolution through the fluorescence upconversion technique. Figure 3 shows the results of the ultrafast emission investigation for compound A in dimethylformamide (the lower solubility of B and C in this solvent prevented the upconversion measurements to be successfully carried out for all the investigated compounds). The timeresolved emission spectrum (Figure 3B) centered at ca. 570 nm at early delays from the excitation undergoes a large redshift in time (up to ca. 610 nm) and then decays to zero in a longer time scale. Fluorescence kinetics recorded on the blue side of the spectrum clearly exhibit a fast decay, whereas those recorded on the red side show a fast rise and then a subsequent slower decay. The spectral evolution in time and the rise−decay dynamics clearly point to an excited state process occurring after light absorption. The global fitting of the acquired data revealed the presence of three exponential components

2.2 2.6 2.5 2.0 0.1

1.1 0.88 0.91 0.72 0.73 0.65

Figure 3. Fluorescence upconversion spectroscopy of A in DMF (λexc = 400 nm): (A) Contour plot of the experimental data, (B) timeresolved emission spectra recorded at increasing delays after the laser pulse (inset: decay kinetics at meaningful wavelengths, with a linear scale for the first picoseconds and a log scale for longer times), and (C) species-associated spectra (SAS) calculated by target analysis.

characterized by lifetimes of 0.61, 3.0, and 830 ps and by species-associated spectra (Figure 3C) centered at 550, 580, and 610 nm, respectively. The first two components are compatible with the occurrence of inertial and diffusive solvation76,77 in dimethylformamide, whereas the longer living transient can be assigned to the fully relaxed S1 state which surely bears a certain ICT character in this polar solvent. Time-resolved area-normalized emission spectra (TRANES) analysis78−80 (Figure 4) allowed a deeper understanding of the upconversion results to be reached. In fact, a continuous redshift of the calculated TRANES may be indicative of the taking place of mere relaxation processes (such as for instance solvation). However, the presence of an isoemissive point in the TRANES may suggest that population dynamics phenomena (such as ICT) are occurring, processes which are generally difficult to distinguish from solvation because they usually occur in similar time scales. The TRANES calculated for A in DMF unambiguously show an isoemissive point in the time-resolved 23731


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stimulated emission appears structured in toluene, whereas it is broad and bell-like shaped in anisole (similarly to the stationary fluorescence spectra in Figure 1). Interestingly, it shows a significant dynamic redshift within the first tens of picoseconds. In the more polar dichloroethane, no stimulated emission could be detected in agreement with the fluorescence rate constant decrease with solvent polarity. In this solvent, the transient spectra exhibit an additional excited state absorption band located at ca. 530 nm. Figure 6 shows the molecular structure effect on the ultrafast absorption dynamics of A−C; in fact, in this figure the results of the femtosecond pump−probe measurements carried out for the three compounds in the same solvent (dichloroethane) are shown. In all cases, the spectra are dominated by positive signals of excited state absorption, even though the tail of a stimulated emission band is clearly visible below 500 nm for B and below 550 nm for C in agreement with its red-shifted fluorescence (see Figure 2). The absorption spectra recorded at early delays exhibit only one band located between 650 and 700 nm depending upon the molecular structure (Figure 6B). This band undergoes a fast decay within the first 10 ps (see kinetics in the inset) which parallels the rise of a second absorption band at 510 nm for A, 530 nm for B, and 570 nm for C on a similar time scale. The formed transient spectrum thus characterized by two absorption bands subsequently decays on a longer time scale. The global fitting of the data collected in dichloroethane resulted for all compounds in the observation of four exponential components: The first two components (hundreds of femtoseconds and few picoseconds) are compatible with the taking place of solvent relaxation, the third component (several tens of picoseconds) may reflect to some kind of relaxation of the excited states, possibly of geometrical nature or implying the cooling of vibrationally hot excited molecules,81−83 and the fourth component (a few nanoseconds, in fair agreement with the lifetime measured by nanosecond-resolved single photon

Figure 4. TRANES analysis of the fluorescence upconversion data of A in DMF: Concentration profiles of the transients obtained by target analysis (upper panel) and TRANES evolution over time (lower panels) in the 0.22−1.0 ps delay time interval (lower left panel) and in the 3−10 ps delay time interval (lower right panel).

spectra corresponding to the conversion of the first transient (0.61 ps) into the second one (3.0 ps). These results indicate that mixed with the occurring of inertial solvation there is the decay of a distinct emissive state (the locally excited state populated by light absorption, 1LE*) which then evolves to give the fully relaxed 1ICT* state. Femtosecond transient absorption measurements were carried out for compounds A−C in several solvents of different polarity. Figure 5 shows the solvent effect on the time-resolved absorption spectra obtained in the case of compound B as a representative example. In a nonpolar solvent such as toluene, the spectra show a region of negative signal due to the stimulated emission below 640 nm and a positive broad band of excited state absorption centered around 700 nm. The

Figure 5. Pump−probe absorption spectroscopy (λexc = 400 nm) of B in three solvents. Left graph: contour plot of the experimental data; right graph: time-resolved absorption spectra recorded at different delays after the laser pulse. 23732


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Figure 6. Pump−probe absorption spectroscopy of A−C in DCE (λexc = 400 nm): (A) Contour plot of the experimental data, (B) time-resolved absorption spectra recorded at different delays after the laser pulse (inset: decay kinetics recorded at meaningful wavelengths) and (C) speciesassociated spectra obtained by target analysis.

Figure 7. Frontier molecular orbitals of compounds A−C at the optimized S0 geometry.

counting) reflects the decay of the fully relaxed fluorescent S1 state of ICT character. It is likely that together with diffusive solvation or vibrational cooling of the photoexcited chromophore the 1LE* → 1ICT* transition takes place which might be slowed down in this solvent characterized by a lower dielectric constant with respect to that in dimethylformamide. Similar lifetimes were retrieved by fitting the fluorescence upconversion kinetics recorded for A−C in dichloroethane (Figure S3), with

the only exception being the shorter lifetime probably due to the decreased temporal resolution of the ultrafast emission setup. Analogous results were obtained from the ultrafast transient absorption investigation in other solvents of intermediate polarity (anisole, chloroform, and dichloromethane; see Table S2). In the least polar among the investigated solvents (toluene), a smaller time-resolved spectral evolution was observed, and the fitting revealed the presence of 23733


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underestimated by the computations. In the literature, excited state symmetry breaking has been associated with positive fluorosolvatochromic behavior of quadrupolar systems.69 Hyperpolarizability. The solvent effect on the absorption and emission spectra of compounds A−C was exploited to obtain information on their hyperpolarizability. The difference between the excited ICT and ground state quadrupole moment was calculated on the basis of the McRae’s theory (eq 1) by plotting the Stokes shift versus the solvent function f(ε, n2) (see Figure 8). By considering the A−C quadrupoles as two

only three exponential components (Table S2) assigned to inertial and diffusive solvent relaxations and decay of the relaxed S1. Being that the emission observed in this solvent is structured, it is likely that the low polarity inhibits the occurrence of any excited state charge rearrangement and that the fluorescence takes place from the locally excited state reached by light absorption. Quantum−Mechanical Calculations. DFT calculations gave deep insight into the structural and electronic properties of the ground and excited states of compounds A−C. Dichloroethane solvent effects were included into the calculations by means of an explicit solvation model. Planar geometries were predicted by the calculations for both the ground and the excited states of A−C. This result is in agreement with the substantial fluorescence quantum yields measured for the investigated compounds and with the monotonic trends of their emission FWHM with the solvent polarity function (Figure S2).70−72 In fact, it is well-known that twisted geometries usually prefer nonradiative excited state deactivation to the ground state.84 Conformational rearrangements have been found in some cases to help a significant charge separation in donor−acceptor systems, but this is not probably the case for compounds A−C which bear alkoxy groups as mild electron donors. The lowest excited singlet state S1 is mainly described by the HOMO−LUMO configuration of π,π* nature in all cases (Figure 7). The HOMO → LUMO transition implies a charge displacement from delocalized π-electron-rich portions of the molecules toward the central strong electron acceptor benzothiadiazoles (difference of electron density between ground and first excited singlet state shown in Figure S4 gives additional information about the charge movement occurring during the S0 → S1 transition). The calculated absorption and emission spectra in dichloroethane for the three compounds (Tables S3−S5) are in remarkable agreement with those experimentally measured. The electronic properties of A−C in their ground and excited states were further investigated by means of the Mulliken charge analysis (Table S6) and calculation of their dipole and quadrupole moments (Tables S6 and S7). Due to the high symmetry of the molecular structures, the dipole moment values calculated for A−C are very low in the ground state and only slightly increased in excited state. However, the Mulliken charge analysis clearly indicates that a certain charge separation is present in the ground state (with the negative charge concentrated on the central benzothiadiazoles and the positive charge on the alkoxy substituted phenyl portions), which is changed upon reaching Frank−Condon and relaxed S1 states. An enhancement of the Qxx component of the quadrupole moment tensor is observed on going from the ground to Frank−Condon and relaxed S1 states (Table S7). These findings point to a charge rearrangement occurring during the excited state deactivation, which agrees with the results of the ultrafast spectroscopic investigation. A certain asymmetry in the charge distribution of the relaxed lowest excited singlet state might be present particularly in the most polar solvent (dimethylformamide) where peculiar effects have been experimentally revealed for A−C.85,86 It has been reported that TD-DFT calculations may fail in providing a correct description of charge transfer excited states.2 It might be particularly the case when considering neutral compounds bearing weak donors. The dipolar character of the fully relaxed S1 state of A−C in highly polar media might be somehow

Figure 8. Trend of the Stokes shift with the solvent polarity for A−C according to eq 1.

opposite point dipoles μ separated by a distance d (sharing the acceptor pole, in the case of A and C), it has been possible to define the quadrupole moment as Q = 2μd. In this way, information about the difference between the excited ICT and ground state dipole moment (Δμ) of half of the molecule (responsible for the observed solvatochromism) could be obtained from the slopes of the linear best fittings in Figure 8. The obtained values of Δμ (24 D for A, 33 D for B, and 18 D for C) are in perfect agreement with the extent of excited state ICT character of these compounds (B > A > C) previously speculated on the basis of their fluorescence properties. The hyperpolarizability coefficients (βCT) could be then calculated from the Oudar equation, and the results obtained for A−C are described in Table 3. The obtained values are generally higher than those measured with other techniques for push−pull compounds considered model NLO materials, such as 4-nitro4′-dimethylamino-stilbene (450 × 10−30 esu−1 cm5).87 Moreover, the βCT values calculated for these quadrupolar compounds are larger than those previously estimated through 23734


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The Journal of Physical Chemistry C Table 3. Calculated Parameters for Compounds A−C and Experimental Parametersa A B C

a (Å)

d (Å)

Δμ (D)

υeg (cm−1)

f

βCT (10−30 esu−1 cm5)

β0 (10−30 esu−1 cm5)

13.7 19.3 17.1

8.1 17.6 13.4

24 33 18

22371 21186 19841

0.63 1.27 0.68

460 2100 1580

110 360 125

Onsager cavity radius, a; frequency of the absorption maximum in CHCl3, υeg; oscillator strength, f; change of dipole moment when ICT is operative, Δμ; and hyperpolarizability coefficient, βCT and β0. Experimental parameters derived by their fluorosolvatochromism using eqs 5 and 6.

a

the solvatochromic method for dipolar systems bearing strong electron donor/acceptor groups (at most a value of 1400 × 10−30 esu−1 cm5 was observed).67 This finding suggests an enhanced NLO response of quadrupolar with respect to dipolar systems as extensively reported in the literature. It is very interesting to analyze the hyperpolarizabilities obtained for A− C by a comparison with their structural properties. The presence of an additional benzothiadiazole unit and the peculiar D−π−A−π−A−π−D molecular structure (compound B) favor a stronger excited state ICT character which is associated with the largest βCT value in this series of structurally analogous molecules. When comparing the hyperpolarizability of compounds A and C, it has to be noted that the red-shifted maximum position of the absorption of C is crucial in determining its enhanced β with respect to that of A despite the lower calculated Δμ. An analogous trend (B > C > A) can be observed when considering the static hyperpolarizability, frequency-independent β0 values (see Table 3). Two-Photon Absorption. Two-photon excited fluorescence (TPEF) excitation spectra were measured for compounds A−C in chloroform by tuning the exciting infrared radiation in the 820−1200 nm spectral range. Unfortunately, the optical parametric oscillator used to produce the excitation light shows a drop in effectively generating an intense idler below 820 nm, whereas substantial signal generation is observed only below 750 nm. For this reason, the spectral window just below 820 nm could not be explored for the investigated samples. In Figure 9, the nonlinear fluorescence excitation spectra measured for A−C are overlapped with that of the corresponding one-photon absorption. These spectra feature the onset of a new intense band in the region around 800 nm, indicating the presence of new transitions allowed by different selection rules as often observed for symmetric compounds.88,89 The maximum of this intense absorption could be successfully observed at 830 nm in the case of compound B. This would be in agreement with a one-photonforbidden, two-photon-allowed S0 → S2 transition, predicted by the calculations to take place at 416 nm for compound B (see Table S4). The same transition is foreseen at 328 and 369 nm for compounds A and C, respectively, (see Table S3 and S5) which would indeed imply TPA bands peaked below 820 nm. A perfect overlap between the one- and two-photon S0 → S1 absorptions can be observed in the case of the centrosymmetric compound B, whereas a different superimposition is observed for A and C exhibiting a C2v symmetry. These findings suggest that the transition probabilities are likely dependent upon molecular symmetry. The TPA cross sections (σ) obtained by the comparative method exhibit values of ca. 40 GM for compound A, 125 GM for B, and 20 GM for C at wavelengths corresponding to their S0 → S1 transition (see Figure 9). Interestingly, the observed trend of TPA response (B > A > C) parallels the trend observed for the excited state ICT character of these compounds, as often reported in the literature.9 The significant

Figure 9. Quantitative two-photon excited fluorescence (TPEF) excitation spectra measured for A−C in CHCl3, compared with the corresponding one−photon absorption (OPA).

TPA cross sections (hundreds of GM) measured in the case of compound B and expected for those of A and C for the onephoton-forbidden S0 → S2 transition indicate that they are promising novel organic NLO materials. In fact, theoretical calculations of the TPA cross sections, which provide values in nice agreement with the experimentally observed intensity of the S0 → S2 transition in the case of compound B, predict enhanced response for the higher energy transitions not accessible from the experimental point of view (see Tables 4 and S8). In particular, cross sections of thousands of GM are computed for the S0 → S2 transition of compounds A and C, whereas an even more important TPA is foreseen for the S0 → S4 transition in the case of compound B. The lack of quantitative agreement of the σ computed for the S0 → S1 absorption of the three investigated chromophores with the ones measured by TPEF is probably due to the tail of the intense S0 → S2 transition determining an enhancement of the experimentally observed TPA at longer wavelengths. Remarkable TPA response was previously observed in symmetric benzothiadiazole derivatives bearing diphenylamino groups as strong electron donors.25−30 In the present work, these interesting properties have been surprisingly revealed even in benzothiadiazole quadrupolar systems containing mild electron donors such as alkoxy groups. 23735


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relaxation of these compounds which becomes particularly fast in the most polar solvent whereas is inhibited in nonpolar media. The DFT calculations have indeed revealed a charge separation between the central acceptor and the lateral donors, which is modified upon light absorption and excited state relaxation. This is reflected in a photoinduced increase of the component of the quadrupolar moment tensor along the direction of the charge separation. Planar geometries are predicted by the computations for both the ground and excited states in agreement with the experimentally measured intense fluorescence. Information about the NLO properties of compounds A−C could be obtained by applying the solvatochromic method to estimate their hyperpolarizabilities and by two-photon excited fluorescence measurements carried out with our newly assembled homemade setup. Remarkable values of hyperpolarizability coefficients were obtained, particularly for compound B, higher than those reported in the literature for model NLO compounds and those previously estimated by the same method for dipolar systems. This is in agreement with an enhanced NLO response of quadrupolar versus dipolar molecules. TPA cross sections show values which are in a trend (B > A > C) perfectly matching the one retrieved for the excited state ICT character of these molecules. This finding interestingly points out that a close correlation exists between the two. The significant TPA response of these compounds coupled with their high emission efficiency make them really promising for applications as environment-sensitive fluorescent probes for two-photon excited fluorescence imaging.

Table 4. Transition Energies, Cross Sections (σ), and Polarization Ratios (R) Computed for the Two-Photon Absorption to the Lowest Excited Singlet States of Bianionic Fluorescein (Fluo) and Compounds A−C in Vacuum transition

energy (eV)

σ (GM)

R

Fluo

S0 S0 S0 S0

→ → → →

S1 S2 S3 S4

3.16 3.63 3.89 4.05

3.81 15.8 0.0250 0.0012

1.50 1.39 1.50 1.49

A

S0 S0 S0 S0

→ → → →

S1 S2 S3 S4

2.92 3.90 4.34 4.55

3.75 2870 36.9 22.8

1.50 0.68 1.50 1.50

B

S0 S0 S0 S0 S0 S0

→ → → → → →

S1 S2 S3 S4 S5 S6

2.70 3.17 3.70 3.78 3.86 4.23

4.86 346 0.00745 19600 16.6 61.1

1.50 0.67 1.50 0.67 1.50 1.50

C

S0 S0 S0 S0 S0 S0

→ → → → → →

S1 S2 S3 S4 S5 S6

2.58 3.44 3.84 4.14 4.21 4.43

0.129 5290 60.3 8460 257 11.9

1.50 0.66 1.50 0.67 1.50 0.84

sample

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CONCLUSIONS Three quadrupolar D−π−A−π−D (A and C) or D−π−A−π− A−π−D (B) compounds have been the object of this study, where the mild electron donors are alkoxy substituted phenyls, the acceptor portion consists of a benzothiadiazole (A), two benzothiadiazoles (B), or a benzothiadiazole linked to two lateral thiophenes (C), and the π-bridge is a triple bond. These symmetric systems have shown a significant positive fluorosolvatochromism in solvents of different polarity accompanied by a change in shape of the emission spectrum, which appears structured in nonpolar solvents and becomes a broad bell-like band in the most polar media. Fluorescence is the preferred excited state deactivation pathway for these molecules (efficiency higher than 50%). The investigated compounds are thus efficient fluorophores of great interest for a large variety of applications, which might exploit their intense emission as well as their solvatochromic properties. A substantial fluorescence quenching is observed in the most polar solvent: Fluorescence quantum yields are decreased by 2 orders of magnitude for B, by 1 order of magnitude for A, and by half for compound C. This finding suggests a certain ICT character of the emissive state, with the extent of ICT being in the trend B > A > C. The presence of an additional benzothiadiazole and the peculiar D−π−A−π−A−π−D structure (compound B) thus favor the ICT, whereas the thiophenes have the effect of significantly red-shifting the absorption and emission spectra of compound C. The latter are thus particularly appealing as absorber of the red portion of the solar spectrum in organic photovoltaic cells and as efficient emitter displaying an interesting red fluorescence. The ultrafast spectroscopic investigation (both in absorption and in emission) has unambiguously revealed the occurrence of a population dynamics process (ICT) during excited state

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07290. Details of the spectral properties of the investigated compounds, the ultrafast spectroscopic data and the results of the quantum mechanical calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Italian “Ministero per l′Università e la Ricerca Scientifica e Tecnologica”, MIUR (Rome, Italy) under the FIRB “Futuro in Ricerca” 2013, no. RBFR13PSB6. Dr. Rebecca Flamini is kindly acknowledged for previous experimental work and useful discussions that inspired this work.

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REFERENCES

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Photobehavior and Nonlinear Optical Properties of PushâPull

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