âThough It Be but Little, It Is Fierceâ: Excited State Engineering of

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"Though It Be but Little, It Is Fierce" - Excited State Engineering of Conjugated Organic Materials by Fluorination Begoña Milián-Medina, and Johannes Gierschner J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02495 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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The Journal of Physical Chemistry Letters

Perspective

'Though It Be but Little, It Is Fierce' - Excited State Engineering of Conjugated Organic Materials by Fluorination Begoña Milián-Medina,1,2,* Johannes Gierschner2,* 1

Department for Physical Chemistry, Faculty of Chemistry, University of Valencia, Avda. Dr.

Moliner 50, 46100 Burjassot (Valencia), Spain. 2

Madrid Institute for Advanced Studies, IMDEA Nanoscience, Calle Faraday 9, Campus

Cantoblanco, 28049 Madrid, Spain. * e-mail: [email protected], [email protected] Keywords: optoelectronics, electronic properties, optical properties, photophysics, quantumchemistry

Abstract Fluorination is frequently used to significantly change the properties of conjugated organic materials due to fluorine's exceptional properties; well known is the impact on the electronic structure, but also on the geometry despite fluorine's small size. Less known, the changes in the electronic and geometrical properties may provoke drastic changes of the excited state properties like batho- and hypsochromic shifts of absorption and emission bands (inter alia leading to excited state switching), hypo- and hyperchromic effects, spectral broadening and changes of the non-radiative deactivation pathways. The state of the art on these issues is summarized in the current perspective to stimulate further discussions on this intriguing subject. TOC graph

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Over the years organic optoelectronics have become an important field in materials science due to a number of advantages over their inorganic counterparts. In fact, organics offer lowcost, sustainable, thin, flexible, large-area, light-weighted, semi-transparent devices, and e.g. for OLEDs superior radiation characteristics. In particular however, the versatility of the organic route is highly attractive,1-3 being able to precisely adapt the materials properties such as

chain

packing,

polarizability,

lowest/highest

un-/occupied

molecular

orbital

(LUMO/HOMO) levels, energy and strength of optical transitions, photophysics (in particular luminescence efficiency), as well as exciton and charge transport to the specific requirements of a given application. A particular intriguing strategy of manipulating organic optoelectronic materials properties is fluorination,4-9 being all related to its exceptional position in the periodic table. In fact, fluorine exhibits the smallest van der Waals radius (1.47 Å) of all nonmetals besides H (1.20 Å) and the highest electronegativity (EN; Pauling: 3.98), resulting in the smallest polarizability (3.76 a.u.) asides He. The pronounced ionic nature of the C-F bond induces a large dipole moment , and makes the bond short (1.35 Å) and exceedingly strong.4 The large  directs in particular the solid state arrangement; attached to an aromatic core, the charge distribution is inverted compared to the non-fluorinated compound,5,10,11 which has a strong impact on solid state organization (vide infra). Furthermore, the HOMO (and the LUMO) of conjugated organic compounds is considerably stabilized by fluorine's high EN, resulting in reduced oxidation potentials; this is true for direct fluorination,12-18 but even more for fluoroalkyl-substitution.18 For these reasons, fluorine substitution is used as a successful strategy for air-stable organic semiconductors,19,20 photoswitches,21,22 and (fluorescent) probes in biology.23-25 The lowering of the HOMO also allows to achieve higher open circuit voltages in organic solar cells (OSCs) when based on F-containing low bandgap donoracceptor (DA) copolymers.25-28 The lowering of the LUMO facilitates electron injection,29 and was capitalized to improve the acceptor ability in DA copolymers.30,31 Fluorination was further employed as a successful strategy to turn hole into electron transporting materials for organic field effect transistors (OFETs),32-35 due to a number of factors. This includes not only better electron injection, lower polarizability and stability issues,36 but also changes of the modality of charge transport.37-39 In fact, fluorination tends to convert herringbone (i.e. edge-to-face) into -stack (face-to-face) arrangements,11,40-52 driven by dipolar or quadrupolar interactions,10,53 but also by dispersive contributions.38,39,54,55 Additionally, weak FH-C interactions may additionally direct the self-assembly.8,56 2 ACS Paragon Plus Environment

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Modulation of chain packing, and thus of modus, path and efficiency of charge transport is considered as a further benefit of using fluorinated low bandgap polymers in OSCs.26,27 In these devices moreover, reduced recombination losses at the interface between the fluorinated polymer and PCBM were ascribed to the fluorine-induced large  which persists in the excited state.57 The directing forces of the C-F bond were further used to generate mixed stack DA charge-transfer co-crystals.54,58-65 Yet not only the intermolecular coordinates are substantially changed upon fluorination, but also the intramolecular ones despite fluorine's small radius, sometimes planarizing conjugated molecular backbones (as often observed in thiophene-based materials),26,27,66-69 or twisting them (in particular in alkoxy- or unsubstituted phenyl-based hydrocarbons),14,42,70-74 due to electrostatic and/or sterical interactions with neighboring atoms and functional groups.66,75 Fluorine-induced planarization is held responsible for increased aggregation of low bandgap DA co-polymers, being an additional benefit for the use of such materials in OSCs26,27 and OFETs;76 on the contrary, reduced aggregation was reported for poly-fluorinated cyanines, being advantageous for fluorescent bio-probes.25 Importantly, fluorination also may significantly (and sometimes dramatically) change energies, strengths, orientations and spectral shapes of the optical transitions in conjugated organic molecules and materials, as well as the fate of the initially created excited states. In view of envisaged optoelectronic applications, understanding and control of these effects might be equally important to the already mentioned electronic effects. In the present report, we will thus reflect the current state of the art on excited state engineering through fluorination concentrating on a few showcases where the effects are exceptionally pronounced to prompt further discussions on this intriguing matter. Hypso- and Bathochromic Shifts. Different to conventional substituents, fluorination of conjugated compounds can induce either hypso- (blue) or bathochromic (red) shifts of UV/Vis absorption spectra, depending on the molecular backbone.8,77 For instance, bathochromic shifts are found for ring substitution in oligoacenes (PFnAc),18,43,78 rubrene,79 porphyrins,80 or polymers based on pyrrole81 and phenylenevinylene repetition units (PPV).82 Conversely, (substantial) hypsochromic shifts are observed upon direct fluorination of oligothiophenes fluorenes,

85

(PFnT),42,83

arene-thiophenes,46 73

and oligophenylenes,

dithieno-benzothiadiazole,84

oligo-

upon fluorination in the vinylene unit of p-

distyrylbenzene (DSB),86 as well as of MEHPPV-type oligomers and polymers,70,87,88 or upon 3 ACS Paragon Plus Environment


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perfluoroalkyl-substitution of polythiophene (PT)89 and PPV.19 Small hypsochromic or even negligible shifts were observed for cyanine dyes.25,90,91 Multiple fluorination of oligoacenes,92,93 azulene,94 hemicyanines,24 spirobifluorene,95 as well as ring-fluorination of PPV-type oligomers,11,40,77,96,97 or perfluoro-substitution of phenyl-porphyrins98 induces batho- or hypsochromic shifts depending on number and position of fluorine substituents. Strong modulations of the luminescence color upon fluorination are in particular observed for charge-transfer (CT) emissions due to a selective stabilization of the HOMO or LUMO levels by specific fluorination of the donor or acceptor moieties, respectively.97,100 For instance, in phosphorescent heteroleptic Ir (III) complexes,8,100-103 the lowering of HOMO of the donor ligand by fluorination can be used to shift the emitting 3MLCT/3LL'CT states towards the blue. Spectral shifts can be caused by geometrical and/or electronic effects, which from experiments alone are often all but impossible to disentangle; therefore, quantum chemistry is required to properly analyze the different contributions by calculating the optical properties of planar and non-planar conformations. Calculations are mostly done at the (time-dependent) density functional theory, (TD)DFT, level of theory, where the standard B3LYP functional turned out to perform sufficiently well to calculate even subtle changes in the absorption properties upon fluorination for medium size molecules.77 An extreme example for geometrical induced shifts is MEHPPV with fluorinated vinylene units, i.e. F-MEHPPV, giving a hypsochromic shift by 0.89 eV of the main absorption (ascribed to a transition from the ground to the first excited singlet state, S0S1),70 see Fig. 1. DFT calculations explain this by a virtually perpendicular situation between neighboring phenyl rings,70 whereas electronic contributions are very small.88 Raman measurements confirmed the strong twist compared to MEHPPV.71 The opening of the optical gap is already seen in the (de)stabilization of the HOMO (LUMO), which indeed constitute the electronic transition to S1.70 On the other side, the extension of the wavefunction hardly changes upon fluorination. This agrees with the analysis of the 'maximum conducive chainlength' NMCC,104 which can be graphically extracted from the chainlength evolution if oligomer absorption maxima are plotted against 1/N, where N = 3n + 2 represents the number of double bonds along the shortest path between the terminal C-atoms and n is the number of repetition units, see Fig. 1. Indeed, similar values are found for MEHPPV (NMCC = 30) and F-MEHPPV (NMCC = 27). For (close to) planar systems like MEHPPV, NMCC therefore agrees with the 4 ACS Paragon Plus Environment

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'effective conjugation length' NECL in H. Meier's original formulation.105 However, for strongly twisted systems the ECL has to be redefined. This is readily done via the intersection of the polymer value for F-MEHPPV and the curve of MEHPPV, see Fig. 1. This gives NECL  5 and thus nECL  1, i.e. strikingly different from nMCC  8.88 700 600 500

400

wavelength / nm 300 MEHPPV F-MEHPPV

1

absorbance (a.u.)

(a) fluorescence intensity (a.u.)

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


0

2.0

2.5

3.0

3.5

4.0 4.5 energy / eV

(b) vertical transition energies / eV

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4.5

F-MEHPPV

4.0 3.5 3.0 2.5

n

NMCC = 27

F-MEHPPV

MEHPPV NMCC = 30

2.0 n

1.5 0.0

NECL = 5.3

0.1

0.2

MEHPPV

1/N

Figure 1: Optical properties of MEHPPV and F-MEHPPV oligomers and polymers; R1 = 2ethylhexyl, R2 = methyl. (a) Polymer absorption and emission spectra in chloroform, (b) Vertical transition energies as a function of 1/N, where N is the number of double bonds between the terminal C-atoms. Adapted from Ref. 88 with permission of The Royal Society of Chemistry.

Also perfluorinated oligophenylenes PFnP were reported to be strongly twisted in S0, with torsional angles of about 59º,73 while twists in the non-fluorinated nP are about 42º. Here as well, the stronger twist upon fluorination induces a strong hypsochromic shift of the main absorption band of the PFnP oligomers against the nP ones as seen in Fig. 2 (while nPs are substantially hypsochromically shifted against the planar ladder-type pendants as shown earlier106). Likewise, hypsochromic shifts are observed for perfluorinated oligofluorenes against the non-fluorinated ones.85 Extrapolation of the PFnP series to the (hypothetical) polymer limit (PFPP) gives a shift of about 0.9 eV against the non-fluorinated counterpart (PP).73 Similar to F-MEHPPV, the sterical effects prevail over the electronic ones; the latter can be estimated to ca. 0.2 eV from calculations on planar backbones. The effective conjugation length of PFPP can be estimated from the available data73 to NECL  2 and thus 5 ACS Paragon Plus Environment


nECL  1, whereas for PP one finds NECL  4 and thus nECL  2. For the planar (nonfluorinated) ladder-type polymer on the other side s NECL  22 is obtained.104,106 It should be further stressed that also indirect fluorination, i.e. through perfluoroalkylsubstitution in the rings of PPV 19 or PT 89 induce (very) pronounced hypsochromic shifts of up to 1 eV against the alkyl-substituted pendants, which again were traced back mainly to geometrical effects.89

wavelength / nm 450 400

350

300

250 1

n=2

n=3

1 1

1 B1

1

1 B2 0

n=4

1

absorbance (a.u.)

0

fluorescence intensity (a.u.)

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0 1

n=6

0 2.5

3.0

3.5

4.0

4.5

5.0

energy / eV

Figure 2: Fluorescence and absorption of nP and PFnP oligomers in solution. Adapted from Ref. 73, copyright 2011, American Institute of Physics.

The electronic contribution of chemical substitution is qualitatively well understood, describing the net polar effect of a given substituent to inductive (I) and/or mesomeric (M) effects. The latter is associated with the substituent's ability to withdraw (-M) or donate (+M) electrons via free electron pairs or vacancies, and always leads to enlarged conjugation and thus a bathochromic (red) shift of the spectra. The inductive effect (±I) is related with the EN of the substituent relative to the sp2 hybridized carbon where substitution takes place. Substituents with a +I effect (i.e. with a smaller EN; e.g. CH3) increase the electron density of the -system and thus lead to a bathochromic shift, while introduction of -I substituents (larger EN; e.g. NO2) results in the opposite effect and induces a hypsochromic shift. If both effects are present, the M effect usually prevails over the I effect. The only notable exception is fluorine, whose -I effect due to the high EN is able to compete with its +M effect,5,107 6 ACS Paragon Plus Environment

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reflected in the low Hammett constant of p = 0.06.108,109 This allows for a subtle interplay of fluorine with the parent molecular backbone. Alternant hydrocarbons (i.e. molecules with a symmetry relation of all occupied and unoccupied MOs so that EHOMO-EHOMO-1 = -(ELUMOELUMO+1) and so forth) can be treated in a qualitative way by a simple Hückel picture coupled to 1st/2nd order perturbation, as demonstrated earlier for PF5Ac.77 Here, the -I effect leads to a symmetrical stabilization of HOMO and LUMO, while the +M effect destabilizes the HOMO stronger than the LUMO (see Fig. 3), in all narrowing the electronic, and thus the optical gap; this accounts e.g. for the observed bathochromic shift in PF5Ac in agreement with TD-DFT calculations.77,110 (a)

(b) 0

0

5Ac

-1

-10

+M

-20 -30 -40 -50

PF5Ac -I

-2

MO energy / eV

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au

L+1

-3 b1u

-4

L

-5 b3g

-6

H

-7 -60

b2g H-x

-8 -70

Figure 3: Optical and electronic properties of 5Ac and PF5Ac. (a) Normalized absorption spectra in solution; reproduced from the SI of Ref. 43 with the permission of the authors. (b) Frontier MO energies and topologies. Adapted from Ref. 77, copyright 2007, American Institute of Physics.

Remarkably, even indirect fluorination may lead to pronounced bathochromic shifts, which depends however on the specific topology of the frontier MOs. For instance, terminal fluoroalkyl-substitution in oligothiophenes hardly changes the (optical) gap.111 Differently, in ladder-type quaterphenyl (L4P) with perfluorination of all three methylene bridges, L4P-F6, a strong bathochromic effect of the S0S1 transition is observed, see Fig. 4. DFT calculations reveal that this is caused by hyperconjugation operative in the LUMO, which shifts the absorption by 0.2 eV upon fluorination in agreement with experiment. On the other hand, 7 ACS Paragon Plus Environment


partial fluorination (i.e. of only the outer or central methylene positions) does not lead to

600

500

wavelength / nm 300

400

12

S1

10 S2

L4P-F6

8

L4P

6 4

S2

S1

2 2.0

2.5

3.0

3.5

4.0

0 4.5

molar ext. coefficient M / M-1cm

(a)

-1

notable shifts.112

fluorescence intensity (a.u.)

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energy / eV

(b)

L4P-F6

L4P

-0.61 eV

LUMO +1 LUMO

-1.91 eV

-1.56 eV

-2.48 eV

-5.34 eV HOMO

-6.12 eV

-6.35 eV HOMO -1

-7.01 eV

Figure 4: Optical and electronic properties of LP4 and LP4-F6. (a) Absorption and emission spectra in solution. (b) Frontier MO energies and topologies. Adapted from Ref. 112 with permission of The PCCP Owner Societies.

For non-alternant hydrocarbons, the strength of the -I effect will depend on the relative LCAO coefficient for a given F-carrying C-atom in the considered MO, and will in general lead to unequal stabilization of HOMO, LUMO depending on the molecular backbone as well as position and number of fluorine substituents. However, simple qualitative predictions (for instance the meta-effect; based on resonance stabilization) are often only effective in simple cases. TD-DFT, on the other side, was shown to reproduce the hypsochromic shifts observed for the non-alternant hydrocarbon PF6T,15,77 as well as multiple fluorine substitution patterns in the phenyl rings of DSB and 5Ac.77,93 Such multiple substitution patterns are especially interesting as targeted design strategies to induce specific intermolecular arrangements in (co)crystals,11,40,41 vide infra. Combined geometrical and electronic fluorine-induced spectral shifts effects were used to specifically manipulate the n-* transition in the Z,E isomers of azobenzene,113 being an important class of photochromic switches. In fact, selective isomerization of the Z,E isomers 8 ACS Paragon Plus Environment

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of azobenzene in the visible is inhibited by strongly overlapping n-* bands of the two isomers, limiting the application e.g. in light-energy conversion. The reason for the small energetic separation between the n-* bands (ca. 0.1 eV) is the very similar destabilization of both MOs upon EZ isomerization, see Fig. 5. Upon fluorination in the ortho-positions of the rings, the n-type orbital is significantly stabilized in the Z-isomer due to the reduction of the n-electron density by the nearby -electron withdrawing fluorine substituents. In total, this leads to a hypsochromic shift of the n-* band in the Z-isomer upon fluorination, while in the E-isomer a hypsochromic shift is observed, so that in total the n-* bands of the Z,E isomers are indeed separated by up to 0.33 eV (50 nm).113 It is worth to mention in this context that fluorination can further specifically improve speed and efficiency of photochromic switches, along with better spectral splitting and fatigue resistance, as demonstrated over the years for diarylethene-based photoswitches.21,22,114 Z

E

Figure 5: DFT calculated MO topologies and energies of E- and Z-isomers of azobenzene and ofluoro-azobenzene; n-* transitions are indicated as arrows. Reproduced from Ref. 113 (modified), copyright 2012, American Chemical Society.

Hyper- and Hypochromic Effects. Bathochromic shifts of the main absorption band through fluorine substitution are usually accompanied by (rather moderate) color intensification (hyperchromic effect), i.e. by an increase of the oscillator strength due to effective extension of the conjugated path. This was indeed observed for a number of oligomers like PFnAc,77 as well as for low bandgap DA copolymers.27 Conversely, hypsochromic shifts as induced by a decrease of the effective conjugation, should provoke an hypochromic effect, i.e. reduced oscillator strength and thus of the molar extinction coefficient m. Indeed, this was found for instance in the strongly twisted PFnP oligomers and in F-MEHPPV.70,73,88 In any case, such simple picture will generally only hold in states with a monoconfigurational description, where the main absorption is described e.g. by a



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HOMOLUMO excitation. For transitions with more complex configuration interaction (CI), fluorine can generate surprisingly strong hyper- and hypochromic effects. This is due to the fact that, no matter whether the parent (i.e. non-fluorinated) molecule is an alternant hydrocarbon or not, fluorination will always break the symmetry of the MOs; i.e. changing the energy differences EH = EHOMO-EHOMO-1 vs. EL = ELUMO-ELUMO+1 and so forth. Here, EL becomes generally smaller compared to EH due to the directive +M effect of fluorine as discussed above; see Figs. 3b, 4b.73,77,93,112 This might have important consequences for the excited state description, depending on the underlying electronic situation. For instance, in L4P-F6, DFT gives EL of only 0.57 eV, while EH = 0.89 eV, see Fig. 4b.112 This mixes the HOMOLUMO+1 (6%) to the HOMOLUMO excitation in the CI description of the S0S1 transition, leading to a very pronounced hypochromic effect of the transition against L4P by a factor of more than three (despite the pronounced bathochromic shift, vide supra), see Fig. 4a. Differently, in PF5Ac, EL is much larger with about 1.4 eV (Fig. 3b);77 thus, for both, 5Ac and PF5Ac, the S0S1 transition is exclusively described by a HOMOLUMO excitation and the oscillator strength slightly increases upon fluorination, giving rise to a small hyperchromic effect only. The consequences of fluorine-induced symmetry breaking are even more pronounced for higher excited states due to their generally more complex CI description. For instance, in the alternant hydrocarbon 5Ac, the second symmetry allowed transition (S0S3; oriented perpendicular to S0S1) is described by a linear combination of (a) HOMOLUMO+1 and (b) HOMO-xLUMO at the TD-DFT level of theory, with almost equal CI coefficients but opposite sign; i.e. ca = +0.52 and cb = -0.47 due to the approximate symmetrical MO situation in 5Ac;77 this results in a very small oscillator strength f = 0.006 in agreement with experiment, see Fig. 3a. In PF5Ac, due to revocation of symmetry, the coefficients are ca = +0.62 and cb = -0.31, leading to a enormous hyperchromic effect of the transition with an increase of the oscillator strength to f = 0.325. A somewhat more complex situation is found for the S2 state of L4P-F6.112 The main CI contribution is HOMOLUMO+1 for both, L4P and L4P-F6 (86 and 85%). However, the other CI contributions differ significantly. Furthermore, the topology of LUMO+1 changes upon fluorination (see Fig. 4b), enhancing the LCAO coefficients by hyperconjugation through the F-carrying C-atoms by participation of fluorine p-type orbitals on cost of those in other parts of the molecules. In all, a very strong hyperchromic effect is observed, see Fig. 4a. 10 ACS Paragon Plus Environment

Orientation of Transition Dipole Moments. A complex CI description is found for the second symmetry allowed transition (S0S3) in DSB with multiple fluorinated benzene rings, i.e. 2Fc10Ft (Fig. 6). In DSB, the transition dipole moment (TDM) of S3 (21Bu) is oriented in the same direction as S1 (11Bu), i.e. approximately along the long axis of the molecule.44 In 2Fc10Ft, a pronounced hyperchromic effect is observed for 21Bu, and at the same time a large change in the direction of the TDM caused by the asymmetric substitution. It should be noted that such a change is permitted by symmetry within the C2h point group of the molecule, since the transitions are not defined along a specific axis, but only within the x,y-mirror plane. For that reason, no changes in the direction of the TDM can be observed for higher states in PF5Ac and L4P-F6 since their symmetries (5AC: D2h; L4P: C2v) are not perturbed upon per-fluorination, and the directions of the electronic transitions are defined in these point groups along the long (x) or short (y) molecular axes. DSB 11Bu ,21Bu wavelength / nm 600

500

400

300

“edge-to-face”

absorbance

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


fluorescence intensity

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“face-to-face” 11Bu

21Bu 2.0

2.5

3.0

3.5

4.0

4.5

energy / eV

2Fc10Ft 11Bu

21Bu

Figure 6: Emission (left) and absorption (right) spectra of solid state spectra (nanoparticle suspensions) of DSB (top: red solid lines) and 2Fc10Ft (bottom: blue solid lines); solution spectra are shown for comparison (black dashed lines). Adapted from Ref. 44, copyright 2005, American Institute of Physics. The insets show schematic edge-to-face (top) and face-to-face (bottom) arrangements. Arrows in the molecular structures indicate the directions of the transition dipole moments of the first two allowed singlet transitions (11Ag11Bu, 21Bu).



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Excited State Switching. The strong symmetry breaking effect of fluorine can evoke curious consequences; intriguing examples are the aforementioned perfluorinated oligophenylenes PFnP. The non-fluorinated counterparts nP are highly fluorescent with the exception of 2P; in the latter, the low intensity 11B2 state is located below the bright 11B1 state. However, 11B1 decreases much more rapidly in energy with n than 11B2, so that for n  2 the S1 state corresponds to the bright 11B1, see Fig. 2.115 Upon per-fluorination, the main absorption (i.e. 11B1) is strongly hypsochromically shifted as discussed further up. This is however not found for the 11B2 state; on the contrary, the latter is bathochromically shifted due to a strong symmetry breaking effect in the system. Thus, for PFnP, the lowest excited state S1 now corresponds to the low-intensity 11B2 state for all n (Fig. 2), i.e. giving rise to a low radiative rate kF which is a major reason for the observed low fluorescence quantum yield F;73 in any case, also nonradiative contributions play an important role for the observed low F, vide infra. Excited state switching upon fluorination was also observed within the triplet manifold of phosphorescent heteroleptic Ir(III) complexes;101 this can have significant impact on luminescence efficiencies100 through changes of admixing of metal d-type orbitals in the emitting state. However, also here non-radiative contributions might be non-negligible as reviewed earlier.116,117 Spectral Bandshapes and Excited State Deactivation. The optical spectra can be subject to substantial broadening upon fluorination. Such broadening can be due to (i) enlarged geometrical reorganization through coupling of high frequency (usually totally symmetrical) vibrational modes, (ii) thermal activation of low frequency modes, and (iii) solvent effects.118,119 Fluorination can in fact contribute to all three processes. A striking example for enhanced geometrical reorganization is L4P-F6, where both absorption and emission spectra are substantially broadened against L4P despite equally stiff molecular backbones, see Fig. 4.112 The reason is readily found in the geometrical changes of bonds length and angles in the central fluorinated methylene bridge upon electronic de-/activation, which activate localized vibrational modes within this unit. Consequently, the broadening is very small if only the outer methylene bridges are fluorinated. TD-DFT modeling indeed largely recovers the broadening for L4P-F6, although some extra-broadening is required for L4P-F6 vs. L4P, which is attributed to specific solvent effects.112 Enhanced reorganization might also account for the small but notable broadening in the absorption spectrum of PF5Ac (Fig. 3)77 and 2Fc10Ft;44 the latter exhibits a flexible, but planar equilibrium structure.


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Strong spectral broadening is observed for the emission spectra of F-MEHPPV (Fig. 1) and the PFnP series (Fig. 2) against their non-fluorinated counterparts. This effect can be traced back to substantial persistent twists in the first excited state S1, whereas the backbones of the non-fluorinated molecules are planar in S1.73,88 The substantial vibrational reorganization in S1 for these molecules directly promotes non-radiative deactivation knr via internal conversion,73,88 as reported previously for systems with strongly twisted S1 geometries like DSB with cyano-substituents in the vinylene units.120 This is the major reason for the low fluorescence quantum yields F = kF /(kF +knr) of F-MEHPPV solution (4%) vs. MEHPPV (30%). A minor contribution of the lower F in the fluorinated polymer originates from a lowering radiative rate kF due to the before mentioned hypochromic effect.88 Although the lowered knr caused by twisted S1 state is not fluorine-specific, it should be stressed that fluorine is surely the smallest possible distortion of the S1 state which can cause such effects. A similar effect is observed for the PFnP oligomers; however, here the decrease of kF, due to excited state switching as discussed before, prevails over the increase of knr, in all lowering F to 2-4%.88 Solid State Photophysics. Fluorination often induces substantial changes in the solid state optical and photophysical properties, which may not only be of intramolecular nature as discussed by now, but also of intermolecular nature as provoked by the change in packing induced by the fluorine atoms. In fact, as stated in the beginning, fluorination tends to convert edge-to-face (herringbone) arrangements (as commonly observed for unsubstituted rodshaped molecules)121 into face-to-face (-stacks) ones.11,40-51 Although a detailed discussion on aggregation phenomena in conjugated materials is beyond the scope of the current perspective, we would like to pin some noteworthy points with respect to the photophysics.121,122 Depending on molecular backbone and fluorine substituent pattern, the degree of --overlap can differ significantly; this is driven both by geometrical arguments such as non-/planarity of the molecular backbone,69,70 or sterical hindrance by additional substituents,50 but also by electronic arguments, i.e. quadrupolar interactions,10,53 directive dipolar effects,84 and dispersive contributions.38,39,54,55 An instructive example is given by poly-fluorinated DSB, see Fig. 6. Unsubstituted DSB crystallizes in an edge-to-face fashion,123,124 with side-by-side orientation of the long molecular axis; the latter coincides with the TDM responsible for the main absorption as well as for emission, and thus gives rise to (pronounced) H-aggregation.44,125 In contrast, fluorination leads to face-to-face stacks however with varying degrees of displacement along the short (y-slip), but in particular along 13 ACS Paragon Plus Environment


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the long molecular axes (x-slip),11,40,41 allowing for a continuous evolution from J- to Haggregates.121 For instance, DSB with perfluorinated central ring (4Fc) forms structures with strong x-slips (and small y-slips), so that the fluorinated central ring is found on top of the non-fluorinated terminal ring of the neighbor. Differently, in DSB with perfluorinated terminal rings (10Ft) or 2Fc10Ft,11,40 the x-slip is much smaller so that the central ring is located on top of the vinylene unit of the neighbor, giving rise to strong H-aggregation features.44 However, while edge-to-face packed DSB exhibits structured emission indicative of a relaxed exciton with only very small intermolecular vibronic coupling,126 the face-to-face packed structures exhibit strong intermolecular vibronic contributions by breathing and/or shearing modes due to pronounced CT contributions, which gives rise to structureless, redshifted, i.e. excimer-like emission features, see Fig. 6.44,64,127 Summary and Outlook. Introduction of fluorine is long known as a versatile method to enhance the stability of conjugated organic materials thanks to its high electronegativity. It was however only in recent years that other aspects of fluorine substitution found increasing interest, in particular in the quest for improved materials in OSCs, OLEDs, OFETs and photoswitches. This concerned especially the impact of electronic and geometrical changes through fluorination on charge transport but also the features and fates of excited states. In the present perspective, fluorine was shown to be an extraordinary auxochromic group for conjugated organic compounds, allowing to induce inter alia substantial batho- or hypsochromic shifts in the optical spectra due to geometrical and/or electronic factors, where the latter relies on the subtle interplay of similarly strong +M and -I effects of fluorine. This leads to excited state-switching in some cases, changing completely excited state deactivation. Furthermore, very pronounced hyper- or hypochromic effects can be generated through fluorine's symmetry breaking effect on the MO manifold. Activation of local C-F vibrations may broaden the optical spectra. Considerable de-/planarization effects were observed depending on the molecular backbone despite fluorine's small size. Backbone twisting can persist in the first excited state, not only broadening the emission spectra but consistently promoting non-radiative deactivation. Aggregation is significantly enhanced by fluorination in many cases, promoting -stacking of the molecular backbones with different degrees, depending on the nature of the backbone and number and position of fluorine atoms. This not only changes the transport of charges, but also the formation and transport of excitons. In particular, the strong -stacking tendency of fluorinated compounds gives often rise to excimer-like emission features. 14 ACS Paragon Plus Environment

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The examples in the current perspective were selected for their particularly pronounced impact in excited state manipulation, while it should be noted that in the majority of fluorinated compounds the effects will be less obvious. However, tracing back the geometrical and electronic reasons for the here reported extraordinary effects should help to pave the path towards unusual targeted design concepts based on the intriguing - 'little, but fierce' - fluorine atom. AUTHOR INFORMATION Begoña Milián-Medina obtained her PhD in Theoretical Chemistry in 2004 at the University of Valencia. After postdoctoral stays in Mons (2004-2007) and GeorgiaTech, Atlanta (2005), she returned to Valencia with a Juan de la Cierva grant of the Spanish Science Ministry. From 2011-2013 she was researcher at IMDEA Nanoscience. In 2014 she received a position at the University of Valencia. Her work is dedicated to computational approaches to understand the (opto-)electronic properties of conjugated (metal-)organic materials. Johannes Gierschner received his Ph.D. in Physical Chemistry in Tübingen in 2000. After stays in Tübingen, in Mons and at Georgia Tech, Atlanta, he joined IMDEA Nanoscience in 2008 as a senior researcher (Ramón y Cajal fellow 2008-13). In 2014 he habilitated at the University of Tübingen and holds an adjunct professor position there since then. His work integrates optical spectroscopy and computational chemistry to elucidate structure-property relationships in conjugated organic materials. www.nanoscience.imdea.org/ ACKNOWLEDGEMENTS The work at IMDEA and University of Valencia was supported by the Spanish Ministerio de Economía y Competitividad (MINECO; coordinated project MultiCrom, grant no. CTQ201458801), co-financed with FEDER funds. We especially thank our collaboration partners from synthesis and materials science, Gianluca M. Farinola (Bari), Mark D. Watson (Lexington), Guillermo C. Bazan (Santa Barbara), John E. Anthony (Lexington), Stefan Hecht (Berlin) and Soo Young Park (Seoul), for stimulating discussions on the impact of fluorination and fluoroalkylation over the years. The article title phrase is an altered version of William Shakespeare's quote "Though she be but little, she is fierce" from "A midsummer night's dream".



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