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A: Spectroscopy, Photochemistry, and Excited States

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds Domantas Peckus, Tomas Matulaitis, Marius Franckevi#ius, Viktorija Mimait#, Tomas Tamulevi#ius, Jurate Simokaitiene, Dmytro Volyniuk, Vidmantas Gulbinas, Sigitas Tamulevicius, and Juozas Vidas Grazulevicius J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00981 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds

Domantas Peckus1*†, Tomas Matulaitis2†, Marius Franckevičius3, Viktorija Mimaitė2, Tomas Tamulevičius1, Jūratė Simokaitienė2, Dmytro Volyniuk2, Vidmantas Gulbinas3, Sigitas Tamulevičius1, Juozas Vidas Gražulevičius2* 1

Institute of Materials Science of Kaunas University of Technology, K. Baršausko Str. 59, LT-51423 Kaunas, Lithuania

2

Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilėnų Rd. 19, LT-50254 Kaunas, Lithuania 3

Center for Physical Sciences and Technology, Saulėtekio Ave. 3, LT-10257 Vilnius, Lithuania

Abstract: Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. Low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and its strong 1 ACS Paragon Plus Environment

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dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experience excited state twisting, however weakly sensitive to the solvent polarity.

1. Introduction

Star-shaped molecules are widely used for development of organic electronic and optoelectronic devices. They have advantages over other organic materials like polymers or small molecules because of their increased solubility and better film-forming properties. They also have other useful properties such as high solid-state photoluminescence quantum yield or high nonlinear optical response.1–4 One of the most intriguing properties of such C3 symmetry molecules is delocalization or localization of their excited states and their migration between molecular arms.1,3,5,6 These processes were studied for a number of branched star-shaped organic molecules with different chemical structures using experimental and theoretical methods, however clear understanding of the excited state dynamics and how it relates to the molecule structures is still lacking.1,3,5 These molecules have a nonpolar ground state, therefore no solvatochromism is expected in their absorption spectra. Fluorescence, in contrary, usually shows a significant bathochromic shift with increasing solvent polarity.6 Photon absorption usually creates excitons delocalized over the whole molecule, while fluorescent states are typically localized on a single arm due to conformational changes in excited states.1 Fluorescence of star-shaped molecules dissolved in polar solvents usually originates from excited states localized on one of the arms, however fluorescence from delocalized states in nonpolar solvents was also reported.3 The localized excitation may also be transferred between molecular arms by means of a Forster-type resonant energy transfer (FRET) or even Dexter-type transfer.3,5 Star-shaped compounds containing electron donor (D) and acceptor (A) groups in their core and arms exhibit particularly intriguing properties. Generally, organic molecules possessing electron donating and 2 ACS Paragon Plus Environment

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accepting groups linked by π-conjugated bridges, commonly referred to as “push-pull” systems and experiencing excited state internal charge transfer (ICT), attract considerable interest due to the versatile range of their applications.7–9 During the last few decades, the excited state dynamics of such systems has been the subject of many experimental and computational studies. After photoexcitation, such compounds can relax to a different conformational state with a stronger charge transfer character. Molecules in the ICT state can be twisted or planar.8 An electron transfer process and subsequent twisting of organic molecular fragments is called twisted intramolecular charge transfer (TICT), while electron transfer leading to the molecule planarization is designated as planar intramolecular charge transfer (PICT).10,11 The TICT phenomenon is particularly interesting because of many possible applications, like environmental polarity sensing, microenvironmental viscosity sensing, chemical species sensing, photoswitches, organic light emitting diodes (OLEDs), nonlinear optics (NLO), solar energy conversion.8,12,13 These applications, however, require cleverly engineering of donor, acceptor, and pendant groups. Such application-driven chemistry can only be performed well if the basic photophysical properties of TICT reaction and its dependence on the molecular structure are clearly understood.12 Triphenylamino (TPA) groups have been widely used in star-shaped molecules, as an electron donating cores,14–16 but they have not been used as branches of the star-shaped molecules yet. In this work, we present the synthesis and comparative study of three new star-shaped compounds, possessing cores with a different electron accepting strengths, which are linked to TPA side-arms by means of double bonds. The influence of the properties of the core on thermal, optical, photophysical and TICT formation properties of the starshaped compounds was thoroughly investigated. We demonstrate that the solvent polarity controls TICT state formation of the compound with an electron accepting core, while compound with identical core and arms also experiences excited state twisting, however, weakly dependent on the solvent polarity.

2. Experimental Materials and synthesis. The chemical structures of the studied molecules are presented in Figure 1, S1. All the target materials were synthesized according to the convergent growth strategy by coupling corresponding cores with triphenylamine-based side moieties. Detailed synthetic procedures can be found in the

Supporting

Information

(SI).

For

the

synthesis

of

tris(4-(4-(di(43

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methoxyphenyl)amino)phenylethenyl)phenyl)amine

(TPA-TPA)

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and

2,4,6-Tris(4-(4-(di(4-

methoxyphenyl)amino)phenylethenyl)phenyl)benzene (TPA-TPB), the Heck reaction was selected as the main

coupling

reaction,

while

for

the

synthesis

of

2,4,6-Tris(4-(4-(di(4-

methoxyphenyl)amino)phenylethenyl)phenyl)-1,3,5-triazine (TPA-TRZ) Wittig reaction was used.

Figure 1. Chemical structures of the synthesized compounds.

Structure characterization. Nuclear magnetic resonance spectra of solutions of the synthesized compounds in deuterated chloroform were recorded with a “Varian Gemini−2000” (300 MHz (1H), 75.4 MHz (13C)) spectrometer. All the data are given as chemical shifts in δ (ppm), multiplicity, integration down field from (CH3)4Si as the internal standard. Mass spectra (MS) were obtained on “Waters ZQ 2000”. Elemental analysis was performed on a EuroEA Elemental Analyzer. Infrared (IR) spectra were recorded using “Perkin Elmer Spectrum GX II FT−IR System”. Differential scanning calorimetry (DSC) measurements were carried out with a TA Instruments “DSC Q100” calorimeter. The samples were heated at a scan rate of 10 °C/min in a nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a “Mettler TGA/SDTA851e/LF/1100”. The samples were heated at a rate of 20 °C/min.

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Cyclic voltammetry (CV) measurements were carried out with a glassy carbon working electrode in a three−electrode cell using a µ−Autolab Type III (EcoChemie, Netherlands) potentiostat. A platinum wire and Ag/AgNO3 (0.01 mol/l in acetonitrile) were used as counter and reference electrodes, respectively, and Bu4NBF6 in dichloromethane (0.1 M) was used as an electrolyte. The data were collected using General Purpose Electrochemical System (GPES) software. Electrochemical measurements were conducted at room temperature at a potential rate of 100 mV/s. The reference electrode was calibrated versus ferrocene/ferrocenium redox couple. The solid state ionization potential energy (IpCV) was estimated from the onset oxidation potential by using the relationship IpCV = 4.8 + Eox, where the potential is related to that of ferrocenium/ferrocene. The electron affinity (EASS) values were obtained from the reduction potential using the approximation EACV = 4.8 + Ered. The theoretical calculations were carried out using a Gaussian 09 quantum chemical package.17 Full geometry optimizations of the compounds in their electronic ground state were performed with DFT using the rB3LYP18,19 functional with the 6-31G (d) basis set in a vacuum. The energies of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals were obtained from single point calculations in the framework of DFT B3LYP/6-31G (d) approach in a vacuum. Absorption spectra were simulated from the oscillator strengths of singlet transitions calculated by the TD-DFT B3LYP/6-31G(d) method in vacuum. Steady-state spectroscopy. Absorption spectra of 10-4 M solutions of the compounds in quartz cells were recorded using Perkin Elmer Lambda 35 and Avantes Avaspec 2048XL spectrometers. Photoluminescence (PL) spectra of 10-5 M solutions of the compounds were recorded using Edinburgh Instruments’ FLS980 Fluorescence Spectrometer. Fluorescence quantum yields (η) of the solutions were estimated using the integrated sphere method.20 An integrating sphere (Edinburgh Instruments) coupled to the FLS980 spectrometer was calibrated with two standards: quinine sulfate in 0.1 M H2SO4 and rhodamine 6G in ethanol. Each quantum yield measurement was repeated 3 times, and the error corridor was estimated. Time-resolved fluorescence. Fluorescence decay curves of the samples were recorded using a time−correlated single photon counting technique with a Picoquant LDH-D-C-375 laser with an excitation wavelength of 374 nm, the duration of laser pulses was ~50 ps. Investigations with higher time-resolution of 5 ACS Paragon Plus Environment


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about 3 ps were performed with a Hamamatsu streak camera operating in a synchoscan mode. A femtosecond Yb:KGW oscillator (Light Conversion Ltd.) generating 80 fs duration pulses at 1030 nm, which were frequency tripled to 343 nm (HIRO harmonics generator, Light Conversion Ltd.), was used for the sample excitation. The laser pulses were attenuated and focused into ∼100 µm spot on the sample, resulting in an average excitation power of about 1 mW/mm2. Transient absorption (TA). During transient absorption measurements, samples were excited with ultrafast Yb:KGW laser Pharos (Light Conversion) with a regenerative amplifier generating 200/3 kHz repetition rate 290 fs duration pulses at 1030 nm wavelength. The excitation wavelength was tuned to 400 and 435 nm with a collinear optical parametric generator Orpheus and harmonic generator Lyra (Light Conversion). Excitation intensities were 28 and 20.8 µJ/cm2 for 400 and 435 nm respectively. Transient absorption spectra in the 361-668 nm range were measured using white light continuum generated by the second harmonic pulses, while measurements in the 478-785 nm range were performed with continuum generated by fundamental laser radiation. Total TA spectra were obtained by combining spectra measured in both spectral ranges. The excitation beam was focused to a spot of about 700 µm in diameter, while the probe white light diameter was of about 500 µm.

3. Results 3.1 Structure and physical properties Experimental structure characterization. The chemical structures of the target compounds were identified by mass spectrometry, as well as by IR and NMR spectroscopies and elemental analysis. The data were found to be in good agreement with the proposed structures. For all the compounds, the characteristic singlet signal corresponding to protons of a methyl group of the methoxy moiety was found at 3.80-3.81 ppm in 1H NMR spectra. Meanwhile, carbon atoms of the triazine heterocycle of TPA-TRZ were characterized by 171.3 ppm signals in 13C NMR spectrum. Since all the compounds possess three stilbene fragments, there is a possibility of the appearance of four isomers. A small contribution of cis-isomers in 1H NMR spectra appearing at 6.60-6.80 ppm was observed for all the compounds. Moreover, only a small contribution of cispeaks appearing at 860-870 cm-1 and quite large intensity trans-peaks at 950-970 cm-1 were observed in IR 6 ACS Paragon Plus Environment

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spectra. We assume that target materials are mainly trans-isomers. All the compounds were found to be well soluble in common organic solvents, such as chloroform, THF, methylene chloride, toluene, ethyl acetate. Thermal properties. Thermal characterization of the target compounds was performed by the combination of DSC and TGA (Figure S2). The values of the glass transition temperatures (Tg), and the decomposition temperatures, at which the initial loss of mass (5 %) was observed (Tdec-5), are summarized in Table 1. All the compounds were isolated as amorphous solids, what can be seen from the DSC experiment (Figure S2a). Usage of star-shaped molecular architecture and relatively large molecular weight proved to be a successful choice in achieving amorphous materials. All the target materials exhibit only a glass transition, and no melting or crystallization. Glass transition temperatures are rather high (>115 °C) and similar because of the similarity of the structures. However, the nature of the core was found to influence thermal degradation temperatures. TPA cored material possesses the lowest Tdec-5, then it is followed by TRZ and the highest thermal stability was observed for TPB. Its 5 % weight loss temperature is as high as 435 °C (Figure S2b). High thermal degradation is consistent with other TPB-based star-shaped compounds, reported previously.21,22 Electrochemical characterization. Cyclic voltammetry (CV) was applied to elucidate electronic energy levels of molecules. Moreover, it also gives information about the electrochemical stability of the material upon applying a voltage to the solution during the CV experiment. CV graphs of the synthesized materials are presented in SI Figure S3. Since all para-positions of TPA moieties are protected, all the compounds undergo reversible oxidation processes. The shape of CV spectra did not change even after repeating the oxidation scans for multiple times, what shows that these compounds are capable of forming stable radical cations. Four TPA moieties bearing TPA-TPA showed double reversible oxidation. Since only in case of TPA-TRZ, LUMOs of the first two excitations are localized on the core, while LUMOs for other materials from the series are delocalized through stilbene units (Figure 2a), reduction peak during the CV experiment was observed only for the TPA-TRZ. The solid-state ionization potential energy values were estimated from the onset oxidation potential and are in the close range of 4.95-4.98 eV for all the compounds. The electron affinity for TPA-TRZ was estimated from the onset reduction potential. Table 1 summarizes thermal, optical and electrochemical properties. 7 ACS Paragon Plus Environment


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Figure 2. (a) Frontier orbitals of TPA series compounds and (b)theoretical absorption spectra together with major contributions of transitions of the first two excited states.


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Table 1. Summary of thermal, optical, electrochemical and photoelectrical characteristics of TPA series derivatives.

Compound

Tdec-5, TG, o (a) C o (b) C

IpCV, eV(c)

EACV, EHOMO, ELUMO, QYToluene. (krad; knr, ns-1) eV(c) eV(d) eV(d)

QYTHF, (krad; knr, ns-1)

TPA-TPA

300

115

4.95

-

-4.69

-1.81

0.36 (0.24; 0.42)

0.32 (0.18; 0.38)

TPA-TPB

435

122

4.96

-

-4.94

-1.90

0.86 (0.45; 0.07)

0.81 (0.31; 0.07)

TPA-TRZ

376

117

4.98

2.48

-4.99

-2.43

0.64 (0.28; 0.15)

0.19 (0.11; 0.46)

(a) 5% weight loss temperature obtained from TG curves; (b) Glass transition temperatures were obtained from DSC curves; (c) ionization potential and electron affinity data from CV experiment; (d) theoretically calculated HOMO and LUMO energy values;

Quantum chemistry calculations. Ground state geometries were calculated using DFT B3LYP/6-31G(d) approach in a vacuum. The optimized structures and some geometrical parameters are presented in SI Figure S1. Variation of different cores yielded different ground state geometries for all the molecules. Usage of strong electron acceptor triazine core ensures a high degree of conjugation, thus making the molecule almost entirely flat. TPA-TPA molecule is composed of four TPA moieties and is expected to have a higher level of torsion of all the bonds, since TPA is not only propeller shaped, but also an electron donor, which eliminates push-pull system within the molecule. All C-N torsion angles were found to be typical for TPA moiety,23 however, the overall planarity of stilbene moiety was found to be 8.6°. The use of weaker electron donor benzene core was found to reduce the torsion of stilbene moiety, as compared with TPA-TPA molecule. However, the biphenyl C-C bond torsion makes overall twisting of TPA-TPB similarly as for TPA-TPA. Electronic states of the target molecules were studied by the TD-DFT rB3LYP method with the basis set 6-31G(Gd) in a vacuum. From the vertical transitions simulation, it can be deduced that most of the allowed transitions with the highest oscillator strength in all the molecules are of S0→S1-2 type (Figure 2b, S1-S3). Such high values of oscillator strength mean that there is an efficient conjugation between cores and arms. Analysis of excitation contributions to the S0→S1-2 transitions in TPA-TRZ has revealed the origin of these transitions to be of an intramolecular charge transfer (ICT) character since HOMOs were found to be localized on electron donating arms, while LUMOs are localized on electron acceptor triphenyltriazine core. 9 ACS Paragon Plus Environment


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When neutral TPB moiety was used, the involvement of stilbene units in HOMOs was observed, while in TPA-TPA comprised only of donors, HOMOs are delocalized within the entire molecule. Since there is no notable electron accepting unit in both TPA-TPA and TPA-TPB, their LUMOs are mainly localized on stilbene units.

3.2 Optical properties

Absorption spectra. Optical characterization of all investigated compounds was performed in low to high polarity organics solvents at concentrations of about 10-2 mmol/L. The absorption spectra presented in Figure 3a slightly differ between compounds but do not show a significant dependence on solvent polarity. They exhibit two pronounced bands: the higher energy absorption band at 280-320 nm should be attributed to the local excitation of the TPA moiety,24 and the low energy absorption band originating from the S0→S1 transition, according to the quantum chemistry calculations, involves entire molecules. Positions of the lowest energy band of the three compounds are slightly different. The largest bathochromic shift observed for the TPA-TRZ containing electron accepting TRZ moiety is typical for TPA-based conjugated donoracceptor systems.14,25,26 The S0→S1 band slightly red-shifts in polar solvents, but the weak shift indicates that there is no significant dipole moment in the ground state in all the studied compounds, which is not surprising taking into account C3 symmetry of the molecules.


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Figure 3. (a) UV/Vis absorption and fluorescence spectra (λex= 310 nm) of dilute hexane, toluene, THF, and DMF, (b) Lippert-Mataga plots for the investigated compounds.

Fluorescence spectra. Figure 3a also shows fluorescence spectra of molecules in various polarity solvents. Vibronic structure of the fluorescence spectra of all compounds is strongly pronounced in nonpolar solvent hexane, where inhomogeneous spectral broadening is weak. Meanwhile, upon increase of the solvent polarity, fluorescence spectra experience a significant bathochromic shift, which is particularly strong for the TPA-TRZ bearing D-A character. Solvent-dependent red-shift of the fluorescence spectra of these compounds indicates the existence of highly polar excited states. The Stokes shift for the TPA-TRZ increases to 6436 cm-1 in THF and the fluorescence intensity becomes strongly suppressed. The PL intensity of TPA-TRZ in highly polar DMF was even too low to measure. Strong solvatochromism and emission 11 ACS Paragon Plus Environment


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quenching indicate strong conformation changes taking place in the excited state. Based on the quantum chemistry calculations, the S0→S1 optical transition causes electron density shift from side TPA groups to the molecule core. Strong fluorescence spectra dependence on the solvent polarity indicates that the S1 state acquires large dipole moment. We can formally express the Stokes shift by the Lippert-Mataga equation:27,28

∆

∆ ν = − + .

(1)

Here h is the Planck constant, c is the speed of light, a is the radius of Onsager cavity, µE and µG are dipole moments in the excited and ground states, respectively and ∆f characterizes the solvent orientation polarizability. Lippert-Mataga plot, in Figure 3b shows dependences of the Stokes shifts of all compounds on the orientation polarizability. From the slopes of the linear approximations and taking into account that the ground state dipole moments are small, we conclude that excited state dipole moments increase in the order TPA-TPA, TPA-TPB, and TPA-TRZ approximately as 0.4, 0.8 and 1. Surprisingly, TPA-TPA compound composed only from donor moieties also possesses significant excited state dipole moment.

3.3 Excited state dynamics Time-resolved fluorescence. Time-correlated single photon counting (TCSPC) technique was applied to measure the fluorescence decay kinetics. Figure 4 shows the decay kinetics for all the compounds in toluene and THF solutions. The fluorescence decayed quite similarly for all compounds with time constants ranging from 1.6 to 2.5 ns. Fluorescence decay for TPA-TPB is close to exponential in both solvents. Two other compounds show slightly nonexponential decays, which also become slower at longer wavelengths (see SI Figure S4). It suggests the existence of some minor species (dimers, excimers or some stabile conformers) radiating at longer wavelengths. From the fluorescence decay rates and quantum yields, we determine the radiative and nonradiative relaxation rates: krad = kfl × QY; kfl = krad + knr. These values are presented in Table 1. The radiative relaxation rates of all compounds are lower in THF solution, but particularly 12 ACS Paragon Plus Environment

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drastically, almost three times, krad drops for the TPA-TRZ compound. Moreover, the nonradiative relaxation rate for this compound also increases about three times in THF.

Figure 4. Fluorescence relaxation kinetics for all compounds in toluene (blue) and THF (red) measured at the maxima of fluorescence bands by TCSPC (kinetics are normalized and vertically shifted).

To address faster excited state relaxation processes, we used a Streak-camera with a time resolution of about 3 ps. Figure 5 shows the time-resolved fluorescence spectra for the TPA-TRZ solution in THF at different delay times after excitation.

Figure 5. Time-resolved fluorescence spectra of TPA-TRZ solution in THF at different detection times measured with Streak-camera. Inset picture shows Streak camera plot.



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The spectra reveal a very fast fluorescence red-shift and simultaneous PL intensity decay. Time-resolved spectra for other two compounds in toluene and THF are presented in SI. From this data, we calculated time evolution of the integrated PL intensity and the mean fluorescence frequency: = ⁄ ,

!

= ⁄ . Division by ν3 here was used to eliminate fluorescence intensity decay

related to the red-shift, as follows from the expression of the Einstein coefficient for spontaneous emission. These time dependences are presented in Figure 6.

Figure 6. Time dependences of the integrated fluorescence intensity and mean florescence frequency for all compounds: TPA-TRZ (a), TPA-TPB (b), TPA-TPA (c) in THF, toluene and cyclohexane. Average wavenumber dependence on decay time for all compounds: TPA-TRZ (d), TPA-TPB (e), TPA-TPA (f) in THF, toluene and cyclohexane. 14 ACS Paragon Plus Environment

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During initial tens of picoseconds, the fluorescence of all compounds in THF experiences a significant decrease in intensity and simultaneously shifts towards low energy region. The strongest shift and the fastest decay were found for TPA-TRZ compounds, however they are less expressed in weakly polar toluene and particularly nonpolar cyclohexane solutions. Meanwhile, for TPA-TPA molecules, both processes are more pronounced in nonpolar solvents. Thus, these data show that the dynamic spectral shift and intensity decay for TPA-TRZ and TPA-TPB strongly increase with the solvent polarity, whereas, for TPA-TPA these features weakly depend on the solvent polarity. Transient differential absorption. Figure 7 presents transient absorption (TA) spectra and decay kinetics for TPA-TRZ molecules measured in toluene and THF solutions under excitation at 435 nm.



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Figure 7. Transient absorption spectra of TPA-TRZ solutions in toluene (a) and THF (b), measured under excitation at 435 nm. Inserts show transient absorption decay kinetics at various probe wavelengths.

The initial TA spectra of both solutions are almost identical, with the negative absorption band in the blue spectral region caused by the absorption bleaching, stimulated emission band in 475-550 nm region, and two positive excited state absorption bands in the green-red spectral region. However, subsequent TA evolutions are significantly different. Very similar TA kinetics at different wavelength and the presence of the isobestic point at about 540 nm for the toluene solution show that the TA spectrum does not experience significant modifications in time, except that the stimulated emission band decays slightly faster. Similar TA and fluorescence decay kinetics (Figure 6a) indicate that both of them are mainly determined by the excited state relaxation. However, stimulated emission probed at 450 nm has a weak faster decay component (see inset in Figure 7a) which is absent in decay of the absorption bleaching at 450 nm and excited state absorption at 600 nm. It suggests that fast conformation excited state relaxation diminishes the stimulated emission intensity. TA spectra of the TPA-TRZ in THF experience significant modifications: the stimulated emission band is replaced with the induced absorption band during 10 ps, while the absorption bleaching during this time experiences only minor decay, probably also mainly caused by the decay of the stimulated emission, which apparently initially contributes to the negative TA band. Excited state absorption spectrum also experiences significant modifications during this time. These TA evolution features together with the fast fluorescence decay taking place on the same time scale indicate that transition to another excited state with significantly reduced transition dipole moment to the ground state takes place in TPA-TRZ solution in THF during several initial ps. TA spectra of TPA-TPB in toluene and THF show quantitatively similar dynamics as those of TPA-TRZ, with clearly expressed stimulated emission band decay in THF. These spectra are presented in SI (Figure S5).


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Figure 8. Transient absorption spectra and kinetics (insets) of TPA-TPA solutions in toluene (a) and THF (b) measured under excitation at 400 nm. TA spectra of TPA-TPA solutions in toluene and THF are presented in Figure 8. They show qualitatively different dependences on the solvent polarity. The stimulated emission band of this compound is less red shifted, therefore, stronger overlap with the absorption bleaching band and not so clearly distinguishable. Nevertheless, they sufficiently clearly show that the stimulated emission band in both solutions in the 425475 nm region decays much faster during initial ~10 ps than the absorption bleaching and induced absorption bands. This is in agreement with the fast initial fluorescence decay observed in both solutions, indicating that relaxation to the excites state with lower fluorescence transition dipole moment takes place in both solutions. However, these changes for TPA-TPA are slightly slower than for TPA-TRZ or TPA-TPB in THF solution, and the transition dipole moment decreases less significantly. 17 ACS Paragon Plus Environment


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Absorption and fluorescence anisotropy. Fluorescence and transient absorption anisotropy is an additional important parameter giving information about properties of the excited states. Figure 9a shows the fluorescence anisotropy kinetics for all compounds in cyclohexane measured with a streak camera. The initial anisotropy values are of about 0.2, and the anisotropy decays on a several nanoseconds time scale, which apparently corresponds to the rotational diffusion of entire molecules. The anisotropy of the TPATPA compound has initial hundreds of ps decay component. These relatively low initial anisotropy values suggest that it probably has partly decayed faster than the time resolution of our measurements.

Figure 9. Fluorescence anisotropy kinetics for all compounds in cyclohexane measured by Streak-camera spectrometer (a) and TA anisotropy of TPA-TRZ in solutions of THF and toluene at 445 nm (b).


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Transient absorption measurements have better time resolution, however, they also did not reveal ultrafast anisotropy decay. Transient absorption shows significantly lower anisotropy values of about 0.1 in comparison with the fluorescence anisotropy (Figure 9b). However, this is expected for this type of molecules. The difference between fluorescence and absorption bleaching anisotropies is related to the fact that fluorescence anisotropy is determined by S1-S0 transition localized on one of the three differently oriented transitions, while excited molecule experiences bleaching of all degenerate electronic transitions. Therefore, the absorption bleaching anisotropy is expected to be equal to about 1/7, which is maximal theoretical value for compounds with the transition dipole moment distributed within disk-like molecules.

4. Discussion Excitation delocalization over degenerated excited states, its migration and localization processes in C3 symmetry molecules were discussed in several publications.1,3,5,6 Because of the C3 symmetry of TPA compounds, they are expected to have two degenerated bright excited states,29 which may be optically excited with different light polarizations relatively to the molecule skeleton. According to the quantum chemistry calculations, each optical transition involves a combination of several transitions between different electronic states. For ideal C3 symmetry molecules, the initial fluorescence anisotropy is expected to be as high as 0.7, while after excitation localization randomly on one of the arms, the anisotropy is expected to drop to 0.1.5,30,31 On the other hand, conformational motions of the molecule arms may cause symmetry breaking in the ground state and localization of absorption transition on one of the molecule chains. In such a case, the polarization properties of the star-shaped molecules would be identical as those of rod-like molecules, with well-known initial anisotropy value of 0.4. Being so, the anisotropy is expected to decay relatively slowly due to Forster energy transfer between molecule arms and rotational diffusion of the entire molecule. The initial fluorescence anisotropy values of about 0.2 observed for all the compounds are in between the two above discussed extreme cases. These values, however, allow us to rule out both extreme cases of a total initial excitation delocalization and total symmetry breaking of molecules in the ground state. Unfortunately,



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the anisotropy data give insufficient information to make more clear conclusions regarding the excitation delocalization/localization processes. Faster initial anisotropy decay for TPA-TPA compound is most likely caused by the excitation migration between excited states with different orientations of transition dipole moments. However, in case of other two compounds, where more polar and stronger localized excited states are created, this energy migration is apparently less efficient or does not take place. Noteworthy, that efficient excitation trapping takes place even in case of TPA-TRZ and TPA-TPB in nonpolar solvents, when fluorescence spectral shift and intensity decays are insignificant, which shows that solvation processes play no important role in the excitation trapping. Consequently, the excited state dynamics revealed by the fluorescence and transient absorption investigations should be related to the evolution of localized excited states. This conclusion agrees with previous investigations of similar compounds by Raišys at al. who observed no significant difference between fluorescence properties between C3 symmetry compounds and those lacking one or even two side arms.24,32 Based on a previous discussion, the red-shift of the fluorescence spectra and fluorescence intensity decay can be attributed to the evolution of the localized excited states. Absorption and initial fluorescence spectra of all compounds only weakly depend on the solvent polarity. This is not surprising, because the investigated compounds have C3 symmetry, therefore they have no static dipole moments, consequently, no polar solvation shell. Thus, initially excited Franc-Condon state also has no polar solvent shell even in polar solvents. At longer times, the dependence of the fluorescence spectra on the solvent polarity becomes obvious indicating that the relaxed state is polar. The fluorescence shift may be partly caused by solvation, which in low viscosity solvents typically takes place on a several ps timescale. However, the simultaneous decay of the fluorescence intensity, observed particularly clearly in polar solvents, indicates that formation of new relaxed excited state takes place, as it was already discussed. Spectral and dynamical properties of the new states enable their attribution to some conformational changes of molecules. Such states associated with the conformational motions of some molecular fragments and simultaneous electron density redistribution are known as TICT or PICT states. Since formation of the new state reduces emission oscillator strength, it shall be more likely attributed to the TICT state type, where 20 ACS Paragon Plus Environment

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twisting of the molecule breaks the conjugation. TPA-TRZ and TPA-TPB molecules containing fragments with electron donor and acceptor properties show clear solvation-assisted TICT state behavior: the TICT states are formed in polar solvent, while TICT formation in nonpolar solvents does not take place. As follows from comparison of the radiative relaxation rate in toluene and THF (Table 1) and fluorescence intensity decay during several initial ps, TICT state formation reduces oscillator strength of the radiative transition by about 1.5 times for TPA-TPB and almost 3 times for TPA-TRZ. In case of TPA-TPA molecules, there is no substantial difference between behavior in polar and nonpolar solvents. Although this compound also acquires significant dipole moment in relaxed excited state, the molecule twisting is apparently decoupled from the charge transfer and takes place without solvation assistance. Therefore, excited state reaction taking place in TPA-TPA compound shall be considered just as excited state twisting. Schematic representation of excited state relaxation in TPA compounds is shown in Figure 10.

Figure 10. Schematic representation of the excited state relaxation processes in TPA compounds.

This scheme explains a strong dependence of spectroscopic properties of the investigated compounds on the properties of their central group and solvent polarity. Electron-accepting central groups of TPA-TRZ and TPA-TPB compounds cause formation of polar TICT states in polar solvents, and as a consequence, complex excited state dynamics, large Stokes shift, and reduced fluorescence yield in polar solvents. While TPA-TPA compounds composed only from electron-donating moieties, although have similar absorption



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spectra, experience much simpler excited state dynamics and much weaker influence of the solvent polarity on their fluorescence properties.

5. Conclusions We have investigated excited state dynamics of three C3 symmetry molecules composed of the central core with different electron accepting properties and electron donating tripenylamine side arms. Linearlypolarized light excites only one of two degenerate bright optical transitions and excitation is trapped on central core and one of the side arms. The initial fluorescence anisotropy values of ca. 0.2 observed for all the compounds allow us to rule out both extreme cases of a total initial excitation delocalization and total symmetry breaking of molecules in a ground state. Two of the compounds composed of central electron accepting groups and electron donating arms show twisted internal charge transfer type excited state dynamics where electron transfer state with reduced fluorescence oscillator strength is formed, but only in polar solvents, thus requiring solvation assistance. The molecule composed of identical tripenylamine central and arm groups also experiences excited state twisting, however, the twisting is slower, less reduces fluorescence oscillator strength and weakly depends on the solvent polarity, because of much weaker charge transfer character of the excited state. Conformational relaxation of this compound is decoupled from intramolecular charge transfer and should be considered as excited state twisting. By choosing rational molecular design, we are can tune optical, thermal, and electrochemical properties. Application of the star-shape C3 symmetry design and rational selection electron donating/accepting central and arm groups enables the design of compounds with widely tunable by solvent polarity fluorescence spectra and almost polarity independent absorption. Careful selection of appropriate donor-acceptor system allows one to harvest the best combination of desired properties of the materials.

ASSOCIATED CONTENT


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Supporting Information Additional information about materials, synthesis, geometry optimization, thermal characteristics, computational data and optical properties are included in the supporting information. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

† These authors contributed equally to the work.

Acknowledgments: This research is/was funded by the European Regional Development Fund according to the supported activity ‘Research Projects Implemented by World-class Researcher Groups’ under Measure No. 01.2.2-LMT-K-718.

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