Femtosecond Fluorescence Upconversion Study of a Naphthalimide

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Femtosecond Fluorescence Upconversion Study of a NaphthalimideBithiophene-Triphenylamine Push-Pull Dye in Solution Valentin Maffeis, Romain Brisse, Vanessa Labet, Bruno Jousselme, and Thomas Gustavsson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05177 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Femtosecond Fluorescence Upconversion Study of a Naphthalimide-Bithiophene-Triphenylamine Push-Pull Dye in Solution Valentin Maffeis,a,b Romain Brisse,b Vanessa Labet,c Bruno Jousselmeb and Thomas Gustavsson*,a a

LIDYL, CEA, CNRS, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France

b

Laboratory of Innovation in Surface Chemistry and Nanosciences (LICSEN), NIMBE, CEA,

CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette Cedex, France c

Sorbonne Universités, UPMC Univ. Paris 06, CNRS, De la Molécule aux Nano-objets :

Réactivité, Interactions et Spectroscopies - UMR 8233 (MONARIS), 75005 Paris, France *Corresponding author Thomas Gustavsson Tel: +33 169 089 309. Email: [email protected]

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ABSTRACT There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photo-excited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophenenaphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC), evolves towards a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher non-radiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in non-polar solvents, hinting that the excited state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. Finally, the highly stabilized S1/CT state undergoes a fast nonradiative quenching. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.


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INTRODUCTION For more than 20 years, dye-sensitized solar cells (DSSCs) have been put forward as a promising technology to achieve cheap and environment-friendly solar energy conversion.1–3 Using tailored dyes, the efficiency of n-type based solar cells is today exceeding 10 %.4–6 A substantial improvement in DSSC efficiency could be obtained using tandem cells combining n- and p- type electrodes with complementary dyes.7–10 However, such tandem cells are limited by the weak current generated at the p-type electrode.11 The poor performances of p-type DSSCs are intimately related to the intrinsic properties of the dye molecule. For this reason, it is not surprising that the development of new dye molecules, with improved characteristics, is a particularly rich field. However, tailoring new dyes with, on the one hand, solar light absorption characteristics and, on the other hand, fast charge separation and hole injection and slow charge recombination performances, good enough to improve the final efficiency of the solar cell, has turned out to be a considerable task. Among the various molecules proposed for organic photovoltaic materials (OPVs), πconjugated donor-acceptor (push-pull) dyes are particularly popular.11 By tuning the level of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the constituents, the vertical absorption transition may trigger an intramolecular charge transfer (ICT) between the electron rich (donor) group and the electron poor (acceptor) group.12,13 The charge transfer (CT) nature of the S1 state at the geometry corresponding to the Franck-Condon transition can be enhanced by the presence of a π-conjugated spacer which boosts the charge delocalization.13 The organic dyes obtained using this synthetic strategy are currently used in both n-type3,5 and p-type DSSCs.9,14–24 Numerous studies have addressed the characterization of solar cells using, among others, timeresolved spectroscopy19,22,25–30 and computational approaches such as time-dependent density


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function theory (TD-DFT).31–34 Processes like charge transfer,27,32–34 charge injection,26,31,35,36 geminate charge recombination37 or dye regeneration35 have therefore being studied in depth. While femtosecond transient absorption spectroscopy gives very rich information, the many overlapping contributions at early times may render the analysis very difficult. For this purpose, femtosecond fluorescence spectroscopy is a very attractive alternative, since it only monitors the allowed emission transitions thereby simplifying the analysis enormously. The photophysics of some π-conjugated push-pull dyes have been studied using ultrafast timeresolved fluorescence measurement.38–40 These studies have not only elucidated the dynamics of the excited CT state but also shown the role of conformational relaxation processes such as photoisomerization41 or the formation of a twisted intramolecular charge transfer (TICT) state.42 Moreover, several ultrafast fluorescence studies have specifically aimed dye molecules designed for n-type DSSCs when in solution43,44 or when sensitized on a semiconductor surface.45–51 To our knowledge, no femtosecond fluorescence studies have been conducted on push-pull dyes used in p-type DSSCs. We recently synthesized a push-pull dye based on a triphenylamine (TPA) donor group linked by a bithiophene (2T) π-bridge to a naphthalimide (NI) acceptor group.52 It is specifically designed for p-type sensitization having anchoring groups on the donor side.7,53 Since our purpose is to characterize its photophysical properties, the carboxyl anchor groups were protected by t-butyl ester groups in order to increase the solubility of the dye and avoid aggregation. The structure of the dye, hence denoted TPA-2T-NIp, is depicted in Figure 1. The molecule is similar in design to several dyes already studied in the literature. The push-pull effect arising from the association of a bithiophene and a perylene imide (PMI, close to naphthalimide in properties and structure) groups was already studied by Fron et al.39 whereas the association of bithiophene with a triphenylamine moiety was found to produce a large red-


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shifted fluorescence.54 Gudeika et al. studied the properties of dyes resembling TPA-2T-NIp with no bridge,55 or a butadiene chain as π-bridge.56 An important polarity dependent red-shift of the fluorescence associated with a drop in fluorescence quantum yield was already shown by Inari et al. for a push-pull molecule closely resembling TPA-2T-NIp but with a bridge constituted by one single thiophene unit denoted Ph2NPh-TNI.57 This was assigned to an increase in the non-radiative relaxation rate. Various push-pull molecules based on a TPA moiety were developed using a thienothiophene bridge and different acceptor groups by Mastodonato et al.58 Concerning p-type tailored dyes, perylene imide has been used as an acceptor group by Wang et al.59 in a molecule sharing the design of our dye named TPA-T2PMI. This was the base of several dyes with varying π-bridge. Nattestad et al. added thiophene unit to the chain showing increasing recombination times9 whereas Weidelener et al. added an ethylene unit to obtain a 30% increase in the open-circuit current density.19 Then, a “double acceptor” structure was used by Click et al.21 in order to obtain an extraordinarily high molar absorption coefficient. Therefore, studying TPA-2T-NIp photophysics could improve our understanding of a large class of push-pull molecule.

Figure 1. Molecular structure of the triphenylamine-bithiophene-naphthalimide push-pull dye is the dihedral angle defined by atoms (1, 2, 3, 4) inside the TPA (TPA-2T-NIp). Angle moiety. Angle is the dihedral angle defined by atoms (5, 6, 7, 8) between the TPA and 2T is the dihedral angle defined by atoms (8, 9, 10, 11) between the two moieties. Angle thiophenes. Angle is the dihedral angle defined by atoms (11, 12, 13, 14) between the 2T and NI moieties. Here, we present a spectroscopic study, using steady-state absorption and fluorescence spectroscopy as well as time-resolved fluorescence spectroscopy of TPA-2T-NIp in solution. In addition, quantum chemistry calculations have been performed to determine the geometry


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and the electronic structures of the ground state. The role of solvation is specifically addressed by comparative measurements in a series of solvents of different polarities. A particular effort has been consecrated to the characterization of the dynamical spectral evolution in order to compare with solvation dynamics. The resulting fluorescence lifetimes, dynamical spectral shifts and time-zero fluorescence anisotropies point toward a rapid solvent dependent evolution from the S1/FC state towards a relaxed configuration with an increased charge transfer character, henceforth denoted S1/CT. Moreover, this relaxed S1/CT state is efficiently quenched in polar solvents. EXPERIMENTAL SECTION Synthesis and chemicals. The synthesis of the TPA-2T-NIp dye was described previously.16,52 Two different batches were used. One was specifically purified for the steady-state fluorescence and TCSPC experiments using a preparative High-Performance Liquid Chromatography (HPLC, see Figure S2). This was required due to the very low fluorescence quantum yield of TPA-2T-NIp. For practical reasons, another batch containing a small amount of precursor was used for upconversion measurements. This has no influence on the measured FU signals (for more details see the Supporting Information). Toluene, dichloromethane (DCM), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) of spectroscopic grade were purchased from Sigma Aldrich and used without further treatment. Steady-state measurements. Steady-state absorption spectra were recorded with a PerkinElmer Lambda 900 spectrophotometer. Steady-state fluorescence spectra were obtained using a SPEX Fluorolog 3 and corrected for the sensitivity of the detection system. Solutions were placed into 10 mm × 10 mm quartz cells. The absorbance of the samples at the excitation wavelength was kept below 0.05. All experiments were performed at room temperature (22 °C). Fluorescence spectra were fitted by a log-normal function (see below), as shown in (Figure 2).


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The fluorescence quantum yields (ɸ ) were calculated from the following equation:

ɸ =ɸ where

(1)

represents the area under the fluorescence spectrum curve,

solution and (ɸ

the absorbance of the

the refractive index of the solution. A solution of degassed C153 in ethanol

= 0.38) was used as reference.60

Time-resolved fluorescence measurements. Time-resolved fluorescence measurements were performed using the fluorescence upconversion (FU) and time-correlated single photon counting (TCSPC) techniques.61 The same excitation source was used for the two kinds of experiments: the second harmonic (450 nm) of a mode-locked Ti-Sapphire laser (Coherent MIRA 900), delivering ~120 fs pulses whose repetition rate was 76 and 4.75 MHz for FU and TCSPC respectively (in the latter case set by a Coherent 9000 pulse-picker). For the TCSPC experiments, a Becker & Hickl GmbH SP-630 PC card was used.62 The fluorescence from a standard quartz cell was collected and focused onto the entrance slit of a small monochromator (Jobin-Yvon HR250), equipped with a photomultiplier detector. Fluorescence decays were recorded every 50 nm from 550 nm to the detection limit of our setup, 750 nm. For the FU measurements, a home-built setup, already described in detail elsewhere,63 was used. In this setup, a 1.0 mm type I BBO sum-frequency crystal was used for mixing the fluorescence and the fundamental laser pulse, providing an instrumental response function of about 350 fs (full width at half maximum, fwhm). We judge that the time resolution of our FU setup is better than 200 fs after deconvolution, depending on the signal-to-noise ratio.64 The average excitation power at 450 nm used was about 10 mW. The spectral correction of the detection line was obtained by recording the fluorescence spectra of two reference compounds


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= 722 nm)) 200 ps after excitation, where

(Coumarin 153 in EtOH and RK165 in THF (

all spectral relaxation is finished, and scaling them to the corresponding corrected steady-state fluorescence spectra. The time-resolved experiments (FU and TCSPC) were performed either at magic angle or under successive parallel (

#$

%&') and perpendicular (

#

# %&')

excitation/detection conditions,

defined as follows. Since only the vertical component of the emission was detected, in FU by the phase matching conditions of the crystal and in TCPSC by placing a Glan-Thomson polarizer in front of the monochromator, the parallel and perpendicular components are defined only by the polarization of the excitation beam. This was set to be either vertical or horizontal using a zero-order half-wave plate, mounted on a motorized rotation unit, allowing an easy adjustment. From the measurements of the parallel and perpendicular components, the total fluorescence (%&' and the fluorescence anisotropy )%&' were calculated from the formulae: (%&' =

#$

%&' + 2G

#

,-. %/' 0 ,- - %/' -. %/'1 0 ,- - %/'

)%&' = ,

# %&'

(2)

(3)

The transmission of the excitation beam was found to be identical under parallel and perpendicular conditions so the correction factor G was put to unity for TCSPC and FU measurements. Spectral analysis. Fluorescence and absorption spectra, recorded on a wavelength scale, were scaled to a wavenumber scale using a λ² scaling factor. A simplified log-normal function63 (or a sum of several of them) was fitted to the obtained spectra. %2' =

3

4 5

6

7 . 9: 8

(4)


8

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When needed, the peak wave number ;# and the mean wave number ;

Femtosecond Fluorescence Upconversion Study of a Naphthalimide

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