Symmetry-Breaking Charge Transfer in Boron Dipyridylmethene

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Symmetry Breaking Charge Transfer in Boron Dipyridylmethene (DIPYR) Dimers Jessica H. Golden, Laura Estergreen, Tyler M Porter, Abegail Tadle, Daniel Sylvinson M. R., John W. Facendola, Clifford P. Kubiak, Stephen E. Bradforth, and Mark E Thompson ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00214 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 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

ACS Applied Energy Materials

Symmetry Breaking Charge Transfer in Boron Dipyridylmethene (DIPYR) Dimers Jessica H. Golden,1 Laura Estergreen, 1 Tyler Porter,2 Abegail C. Tadle,1 Daniel Sylvinson M.R.,1 John W. Facendola,1 Clifford P. Kubiak,2 Stephen E. Bradforth,1 and Mark E. Thompson*1 1

Department of Chemistry, University of Southern California, Los Angeles, California, 90089, United States 2

Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, United States Keywords: dipyridylmethene, charge transfer, symmetry-breaking, dimers, fluorophores, transient absorption

ABSTRACT We recently reported the photophysical properties of boron dipyridylmethene (DIPYR) dyes; a class of intensely fluorescent pyridine-based chromophores which are structural analogues of both acenes and BODIPYs. In this work, we endeavored to explore the properties of DIPYR dimers. The synthesis and characterization of two novel homoleptic meso-linked dimers of boron dipyridylmethene dyes, bis-DIPYR and bis-α-DIPYR, are herein reported. Their structural, electrochemical, and photophysical properties have been probed using both steady-state and time-resolved techniques including femtosecond and nanosecond transient absorption spectroscopies. Of particular focus are the excited state photophysical dynamics of the dimers, which are studied in several solvents of varying polarity, from methylcyclohexane to acetonitrile. It was found that both dimers undergo symmetry-breaking charge transfer within 3 ps of photoexcitation, forming a radical anion and radical cation which were observed using transient absorption and confirmed by spectroelectrochemical characterization. Further, it was found that 1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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

Page 2 of 39

the emitting species is the symmetry-broken state, which is stable for several nanoseconds before radiative recombination to the ground state occurs. The efficiency and rapidity of symmetry breaking, even in nonpolar media, is highly promising for application of these materials to optoelectronic technologies requiring charge transfer from an excitonic state. INTRODUCTION Understanding the charge transfer (CT) and charge separation processes in molecular systems is imperative to inform the design of efficient organic electronic devices. In organic optoelectronic devices where photoexcitation of a molecular chromophore produces a bound electron-hole pair (excitonic state), efficient CT presents a significant technological challenge to high device efficiency. In such systems, the energy required to separate the exciton into a free electron and hole is large, on the order of 0.5 to 1 eV.1 An energetic driving force for charge transfer is often provided for by engineering donor-acceptor (D/A) interfaces with large LUMO energy offsets; however, this method limits the open-circuit voltage and, in turn, overall device efficiency. Thus the design of highly efficient devices requires a better understanding of CT mechanisms and the development of materials which rapidly and efficiently transfer charge without large D/A energy losses. Symmetry breaking charge transfer (SBCT) has received significant attention due to its role in charge separation within photosystem II of the photosynthetic reaction center.2-5 SBCT can be generally defined as an excited state process wherein a symmetrical excited state undergoes electron transfer between two electronically degenerate states, producing a desymmetrized CT state. This process can occur either intermolecularly, as is the case in natural photosynthetic systems, or intramolecularly, as has been studied in an array of chromophoric dyads. Materials capable of SBCT have been applied to artificial photosynthesis,6,

7

photovoltaics,8,

9

and

2 ACS Paragon Plus Environment

Page 3 of 39 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


photonics10 where rapid formation of photo-oxidized and photo-reduced products are necessary. The rate of SBCT varies depending on the strength of chromophoric coupling and the solvent environment.7, 11 In dyads with weak electronic coupling between adjacent chromophores, such as acene,12-14 perylene,15-17 and BODIPY dimers,18 as well as homoleptic metallodipyrrins,8, 19 SBCT occurs on the order of hundreds of femtoseconds to tens of picoseconds in polar solvents, and is disfavored in nonpolar media. In the opposite case, if the ground-state geometry involves significant wavefunction overlap between chromophores (i.e. if it exhibits strong electronic coupling), exciton coupling predominates and the relaxed excited state lacks CT character.20 Significant population of the SBCT state in nonpolar media is exceptional, only occurring in systems where the electronic coupling between chromophores is strong enough to induce ultrafast charge transfer but weak enough to stabilize a symmetry-broken state.21 Recently we have investigated boron dipyridylmethenes (DIPYRs); a class of intensely fluorescent (PLQY = 0.2 – 0.8) pyridine-based chromophores that are structural analogues of both BODIPYs and acenes.22 Few reports have explored the properties of DIPYR dyes, and none, to our knowledge, have reported the properties of DIPYR dimers.23-25 In this work, the synthesis and photophysical properties of meso-linked DIPYR dimers (bis-DIPYRs), a new class of visible-light absorbing SBCT dyads, are described. The rate of formation of the SBCT state in polar and nonpolar solvents and its decay pathways are probed by a combination of steady state spectroscopies as well as femtosecond- and nanosecond transient absorption spectroscopies.

RESULTS AND DISCUSSION Synthesis and Structural Characterization



Page 4 of 39

Scheme 1. (a) 2 eq. Cu(OAc)2·H2O, dimethylacetamide, 120 ˚C, 24 h. (b) 4 eq. BF3OEt2, 1,2-dichloroethane, reflux 2h, N,N-diisopropylethylamine N

N

N

a N

F2 B

N

b

N N

N

N

B F2

N

bis-DIPYR

N

N N

N b

a N

F2 B

B F2

N

N

bis-α α -DIPYR +

N N

N N

N

N

B F2

N

monoborylated bis-α -DIPYR

The

dimers

were

prepared

by

a

modified

two-step

procedure

wherein

the

diheteroarylmethane ligand was coupled via direct C-C bond coupling using copper (II) acetate as a sacrificial oxidant, followed by borylation of the tetraheteroarylethane ligand (Scheme 1) by the same method we developed for synthesis of the monomers. Both tetraheteroarylethane ligands have been previously reported in the literature, but, to our knowledge, this is the first preparation of tetraquinolylethane by this method.26,

27

The synthesis of bis-DIPYR from

tetrapyridylethane is fairly tolerant of impurities in the ligand, whereas the synthesis of bis-αDIPYR from tetraisoqiunolylethane is not.

Achieving a yield over 30% for bis-α-DIPYR


Page 5 of 39 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


requires multiple recrystallizations of the tetraquinolylethane ligand. In cases where the starting material is not pure, the monoborylated bis-α-DIPYR predominates. The dimers were analyzed by single-crystal X-ray diffraction using crystals grown from the slow diffusion of hexanes into a saturated dichloromethane solution (Figure 1, see SI for crystal packing diagrams). Bis-DIPYR crystallized readily into P 21/c crystals with four bis-DIPYR molecules comprising the unit cell, whereas bis-α-DIPYR crystallized into small red needles packed into a C 2/c space group with n-hexane and two bis-α-DIPYR molecules incorporated into the asymmetric unit. The measured structure of bis-α-DIPYR is inherently more disordered due to the presence of hexane in the crystal structure, resulting in a poor R-factor of 0.12. Thus, while the structure of bis-DIPYR gives bond distances and angles with low estimated standard deviation values, the structure of bis-α-DIPYR can be used to verify connectivity and gross structure, but the bond lengths and angles are not accurate. Bis-DIPYR exhibits more structural distortion of the DIPYR chromophoric units than is observed in the crystal structure of the monomer. This quality was quantified by measuring the angle between the planes formed by the carbons in each of the two flanking pyridine rings of the DIPYR moiety. In bis-DIPYR, the angle between pyridyl planes, which would be 0° in a perfectly flat chromophoric unit, is 21° in bis-DIPYR; in the monomer DIPYR, this value is 4.7°, considerably closer to the ideal value. In contrast, the structure of bis-α-DIPYR shows an angle between the pyridyl planes of 13º, close to the value of 7º seen for α-DIPYR. The extra puckering in bis-DIPYR is likely due to intermolecular crystal packing forces in the dimer, which exhibits considerable π-π stacking between adjacent pyridine rings. The monomer, which packs in a herringbone fashion, does not experience the same degree of steric repulsion as is observed in the crystal structures of the dimer. Of considerable importance in the photophysical 5 ACS Paragon Plus Environment


Page 6 of 39

characterization of dilute solutions is the degree wavefunction overlap between coupled chromophores. The dihedral angles between coupled chromophores in both dimers, measured as the angle between the planes defined by the meso carbons and the two flanking carbons on each chromophoric unit, are 89.9° and 86° in bis-DIPYR and bis-α-DIPYR, respectively. Meso-bound dimers with nearly orthogonal DIPYR units are expected to have minimal wavefunction overlap and thus negligible excitonic coupling between the DIPYR units.

Figure 1. ORTEP structures of bis-DIPYR (left) and bis-α-DIPYR* (right) with hydrogen atoms displayed as white spheres with atomic radius set to 0.15 A, and thermal ellipsoids for carbon (grey), nitrogen (blue), boron (pink), and fluorine (yellow) atoms set to 50% probability. *bis-α-DIPYR crystallized in two distinct conformations with nhexane in the asymmetric unit. The conformer depicted is the one packed closest to hexane. See SI for complete crystal packing diagrams.

Steady-State Photophysical Properties The photophysical properties of the dimers in a range of solvents of varying polarity are shown in Figure 2 and tabulated in Table 1. The molar absorptivities (ε) of bis-DIPYR and bisα-DIPYR at λmax in methylcyclohexane are 4.1x104 and 2.6x104 L·mol-1·cm-1 while the molar absorptivities of their respective monomers are 2.9x104 and 1.1x105. The absorption spectra of the dimers are broadened relative to the monomers (FWHM = 3148 and 844 cm-1 bis-DIPYR and bis-α-DIPYR, compared to 2057 and 222 cm-1 in the monomers).22 The absorption spectrum of bis-DIPYR is not solvent dependent, and has a similar lineshape to absorption spectra of the


Page 7 of 39

monomer DIPYR in polar solvents. The visible absorption spectra of bis-DIPYR and its parent monomer DIPYR are characterized by two nearly degenerate transitions: S0-S1 and S0-S2.22 In DIPYR, the lowest energy absorption maximum (S0-S1) corresponds to an electronic transition along the long-axis of the DIPYR chromophore, which contains no net permanent dipole in the ground state. The S0→S2 electronic transition, which is approximately degenerate with the S0 v=0 → S1 v=1 transition, occurs along the short, permanent dipole-containing axis of the chromophore and is therefore enhanced in polar media. In the dimer, which is symmetric about both axes, the S0-S2 transition involves no net permanent dipole and therefore is not affected by a change in solvent polarity. Consequently, the S0-S2 transition in the dimer is equally stabilized in polar and nonpolar solvents; i.e. the relative population of the S1 and S2 states are not changed by varying solvent polarity in bis-DIPYR. The result is an absorption spectrum wherein the firstand second-lowest energy maxima do not change in intensity with a change in solvent polarity, which contrasts with the monomer DIPYR which exhibits significant changes in the shape of the

1.0

Absorbance/Emission (arb. units)

absorption spectrum with changes in solvent. Absorbance/Emission (arb. units)

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


MeCyHex Toluene THF DCM MeCN

bis-DIPYR

0.8 0.6 0.4 0.2

1.0

bis-α α-DIPYR

MeCyHex Toluene THF DCM MeCN

0.8 0.6 0.4 0.2 0.0

0.0 300

400

500

600

700

300

400

500

600

700

Wavelength (nm)

Wavelength (nm)

Figure 2. UV-vis absorption (solid) and fluorescence emission (dashed) spectra of DIPYR dimers in various solvents.



Page 8 of 39

The emission spectra of both bis-DIPYR and bis-α-DIPYR are broad and featureless, exhibiting large Stokes shifts (1290 cm-1 and 887 cm-1, respectively), Figure 2. This is in contrast with the monomeric species, which have structured emission spectra and almost no Stokes’ shift (130 and 40 cm-1).22 It is interesting to note that the monoborylated bis-α-DIPYR exhibits absorption and emission features which are nearly identical with the monomer α-DIPYR, indicating that the broadening and bathochromic shifts of absorption and emission spectra in the dimers is not correlated with the substitution of a C-H bond at the meso position with a C-C bond. Rather, it appears that the source of emission broadening is related to the dimerization of the chromophores, indicating either that there is significant distortion of the S1 excited state relative to the ground state or that emission occurs from another state. There is a small bathochromic shift in the emission maximum with increasing solvent polarity of bis-DIPYR, whereas no significant shift in the emission spectrum of bis-α-DIPYR is observed. In both dimers, the photoluminescent quantum yields (ΦPL) and radiative rates (kr) decrease commensurate with an increase in the non-radiative decay rate (knr) as the solvent polarity is increased (Table 2). The radiative rates decrease linearly with solvent polarity, as estimated by the ET(30) solvent polarity index28 (Figure 3). However, plotting radiative rates versus dielectric constant yields a poor fit, indicating that solvation power (i.e. the specific solvation sphere of the excited state) has a greater influence on the emissive state than does the bulk dielectric of the solvent.28,

29

Further, the linearity of the trend with solvent polarity

indicates that the same state is responsible for emission in all solvents. Dilute thin films (1% in polystyrene) of both dimers were prepared from toluene solution on quartz to examine the effect of rigidification on excited state decay dynamics. The absorption features of the thin films show no significant differences from absorption in solution. Likewise, 8 ACS Paragon Plus Environment

Page 9 of 39

the emission spectra of the films are very similar to emission from methylcyclohexane solution (Figure 4), showing a slight narrowing at the red edge of the emission band and no significant difference in the emission onset or maximum. These data suggest that the geometry of the relaxed excited state is similar to that of the ground state, i.e. that the chromophoric units remain orthogonal in the emissive state. Table 1. Solvent dependent steady-state photophysical properties of DIPYR dimers Solvent ET(30) absorption emission ΦPL τ (ns) λmax (nm) λmax (nm) MeCyHex 30.9 492 523 0.23 3.1 bis-DIPYR Toluene 33.9 494 528 0.17 2.8 THF 37.4 492 532 0.12 2.5 DCM 40.7 492 530 0.08 1.9 MeCN 45.6 489 537 0.03 0.96 Polystyrene 492 524 0.16 2.3 30.9 543 570 0.57 4.6 bis-α α-DIPYR MeCyHex Toluene 33.9 543 576 0.45 4.0 THF 37.4 541 573 0.44 4.5 DCM 40.7 541 574 0.37 3.9 MeCN 45.6 537 572 0.31 4.1 Polystyrene 541 567 0.47 4.8

kr (108 s-1) 0.75 0.61 0.49 0.42 0.31 0.70 1.2 1.1 0.98 0.95 0.78 0.98

knr (108 s1 ) 2.5 3.0 3.5 4.8 10. 3.6 0.97 1.4 1.2 1.6 1.7 1.1

1.2

Radiative Rates (108 S-1)

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


1.0 0.8 0.6 MeCyHex Toluene THF DCM MeCN

0.4 0.2 0.0 30

35

40

45

Solvent Polarity Index

Figure 3. The relationship between radiative rate and solvent polarity index in both bis-DIPYR (black) bis-α-DIPYR (red) is linear, which is characteristic of emission from excited states with large dipole moments.



Page 10 of 39

In frozen solution (methylcyclohexane at 77 K), the emission spectra are narrowed (Figure 4) and have discernable vibronic features which are nevertheless significantly more broad than are observed in the monomer models. This rigidochromism indicates that the excited state is slightly distorted relative to the ground state. The fluorescence lifetimes at 77 K (3.25 ns for bis-DIPYR, 4.44 ns for bis-α-DIPYR) are similar to those observed at room temperature. Interestingly, new bands at 593 nm and 657 nm in the 77 K spectrum of bis-DIPYR and bis-α-DIPYR, respectively, are observed. Gated detection enhances these new bands relative to fluorescence features, and they are thus assigned as phosphorescence emission from the T1 state. Phosphorescence of the same magnitude is observed in the DIPYR monomer under similarly mild conditions (77 K in methylcyclohexane), as the triplet is readily accessed in DIPYR via intersystem crossing from the S1 to the T2 state. However, phosphorescence is not readily observed in α-DIPYR; in order to discern the phosphorescence features in the monomer, it was necessary to treat with methyl iodide to facilitate intersystem crossing. Even then, phosphorescence was not discernable from the fluorescence features and the more rigorous method of gated detection was required to resolve it. We were therefore surprised to observe phosphorescence from bis-α-DIPYR at 15% of the fluorescence intensity at 77 K in methylcyclohexane. Intersystem crossing (ISC) from the S1 to the T1 state in bis-α-DIPYR is both a spin- and a symmetry-forbidden transition. While the observation of a phosphorescence band in bis-DIPYR is expected due to the juxtaposition of state energies facilitating ISC, the observation in bis-α-DIPYR is striking and suggests the presence of an intermediate state at or below the energy of the S1 state, which facilitates ISC to the T1.


300 1.0

400

500

600

700

400

500

600

700

0.5 1.0

bisDIPYR

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


bis-α-DIPYR

Page 11 of 39

0.5 0.0 300

Wavelength (nm) Figure 4. DIPYR dimer absorption (dashed) and emission (solid) in methylcyclohexane at room temperature (black) and 77 K (blue) and in a dilute polystyrene thin film (red).

Electrochemistry and Spectroeletrochemistry The electrochemical properties of the dimers were explored using a combination of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (see SI for CV and DPV traces). Electrochemistry was performed in acetonitrile solutions using a three-electrode set-up consisting of a polished 3 mm glassy carbon working electrode, a Pt wire auxiliary electrode and a Ag/AgCl reference electrode. The CV of bis-α-DIPYR shows two fully reversible, 1 electron reductions at -1.81 and -2.02 V vs Fc+/0 corresponding to the reduction of each chromophore in the dimer. Upon sweeping to oxidative potentials, a quasi-reversible oxidation is observed at 0.45 V vs Fc+/0 that becomes more reversible at faster scan rates. The CV of bis-DIPYR is characterized by an irreversible first reduction and a quasi-reversible second reduction at 2.31 and 2.55 V vs Fc+/0 respectively. Analogous to bis-α-DIPYR, bis-DIPYR displays an irreversible oxidation at 0.19 V vs Fc+/0 that gains reversibility at faster scan rates. The differential pulse voltammograms were used to obtain the redox potentials enumerated in Table 1, and these values were used to estimate the HOMO and LUMO energies of both dimers.30, 31 11 ACS Paragon Plus Environment


Page 12 of 39

Table 2. Red./Ox. potentials and corresponding HOMO/LUMO levels. Electrochemical values are in V vs. ferrocenea and HOMO/LUMO values are in eVb.

DIPYR α-DIPYR bis-DIPYR bis-α α-DIPYR

Ox. 0.14 0.40 0.16 0.45

Red. 1 -2.32 -1.95 -2.29 -1.80

Red. 2 -2.56 -2.01

(Eox – Ered) 2.46 2.35 2.45 2.25

HOMO -4.80 -5.16 -4.82 -5.23

LUMO -2.01 -2.46 -2.05 -2.64

a

Measurements acquired by cyclic voltammetry, with a scan rate of 0.1 V/s in acetonitrile and referenced to the Fc+/0 redox couple. b The HOMO and LUMO energies were derived from the electrochemical redox potentials.

A bathochromic shift in the absorption maxima of bis-DIPYR and bis-α-DIPYR relative to their respective monomer models is observed (∆ṽ = 465 and 815 cm-1 in methylcyclohexane), suggesting some degree of interaction between chromophores, possibly due to an acceptor effect exerted by the polar BF2 group para to the meso-bridge. The redox gap of bis-DIPYR is only 10 mV smaller than that of its parent monomer, DIPYR, consistent with the small bathochromic shift in the absorption spectrum. A larger bathochromic shift is observed for the absorption of bis-α-DIPYR relative to the monomer α-DIPYR, correlating well with a 100 mV decrease in the redox gap between the two. Notably, the S1 absorption energies of both dimers (2.51 eV and 2.28 eV, respectively for bis-DIPYR and bis-α-DIPYR) are slightly higher in energy than their respective redox gaps (2.45 V and 2.25 V). The SBCT state of covalent dimers is formed when the lowest energy localized excited state transfers an electron from one chromophore to another in the dimer. It is thus characterized by a cation, localized on one (oxidized) chromophore, and an anion, localized on the other (reduced) chromophore. The cation and anion are expected to have unique absorption signatures which can be detected by transient absorption spectroscopy. The spectroscopic signatures of the cationic and anionic forms of the dyes will be needed to interpret the transient absorbance studies


Page 13 of 39

discussed in the next section. Spectroelectrochemical measurements were made of each dimer alternatively under oxidizing and reducing conditions (Figure 5). 2

2

1 Absorption (a.u.)

Absorption (a.u.)

0 -2 -4 Cation Oxidation Neutral

-6

350

0 -1 -2 -3

-8

Cation Oxidation Neutral

-4 400

450

500

550

600

650

350

400

Smoothed X1

500

550

600

650

1.0 0.5

0.0

0.0 Absorption (a.u.)

-0.4 -0.8 -1.2 -1.6

Anion Reduction Neutral

-2.0 350

450

Wavelength (nm)

0.4

Absorption (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


400

450

500

550

Wavelength (nm)

600

650

-0.5 -1.0 -1.5 -2.0 -2.5 350

Anion Reduction Neutral

400

450

500

550

600

650

Wavelength (nm)

Figure 5. Spectroelectrochemistry traces for bis-DIPYR (left) and bis-α-DIPYR (right). Oxidation and reduction traces were obtained by acquiring a 10 s exposure UV/Vis spectrum through a platinum mesh working electrode cycling at 3 V/s about the redox wave of interest. The cation and anion traces (red) are generated from subtracting the normalized neutral absorption spectrum in acetonitrile (blue - · -) from the oxidation and reduction traces (black), respectively.

A solution of each dimer was alternatively cycled at 3 V/s at a platinum mesh working electrode about the oxidation and reduction waves to mitigate the formation of decomposition products (Figure 5, solid lines). The UV/Vis spectrum was acquired through the electrode during cycling using a 10 second exposure, resulting in a bleach corresponding to the neutral compound and the growth of peaks corresponding to the oxidized/reduced species. At the end of the experiment, the solution was grounded and recovery of the neutral species was observed. The 13 ACS Paragon Plus Environment


Page 14 of 39

resulting spectroelectrochemical absorptions were overlaid with the inverted spectrum of the neutral compound, normalized to the neutral compound bleach, and the anion/cation spectra were obtained by subtracting neutral absorption spectrum from the spectroelectrochemical spectrum. This method provides good qualitative information about the shape and energy of anion and cation absorption features, which were applied to our assignments of the transient absorption data. In the case of an irreversible redox process such as is observed in the oxidations of bisDIPYRs, it can be assumed that some of the oxidized or reduced species decomposes, and that the measured absorption spectrum will contain a sum of the absorption signatures of the radical anion/cation as well as the decomposition product. To deconvolute redox absorption signatures from decomposition features, we repeated spectroelectrochemical measurements under rigorously air-free bulk electrolytic conditions, then grounded the system and observed a bleach of the redox peaks and recovery of the neutral compound absorption bands (Figure S5). The remaining features, which were localized predominately in the UV and violet portions of the spectrum, were attributed to decomposition products, likely caused by decomposition of the radical cation which is observed by CV to be unstable. These species were not observed to grow in during photophysical measurements. Time Resolved Photophysical Properties The nature of the emissive state was not immediately clear, even from detailed examination of the steady-state photophysics. Two possibilities appear plausible, given the steady-state photophysical properties; the emissive state could be a highly distorted S1 state characterized by a large change in permanent dipole moment upon excitation. This explanation, however, does not provide a likely mechanism for formation of the triplet state in bis-α-DIPYR, which is observed in cryogenic emission studies. It also fails to account for the similarity


Page 15 of 39 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


between emission from frozen and fluid methylcyclohexane solutions, which indicates that the emitting state is largely unchanged upon freezing into the Frank-Condon excited state compared to its relaxed excited state. This observation is consistent with the dilute film emission, which even more closely resembles solution state emission, since significant distortion (i.e. twisting) is prohibited in a rigid matrix. The second possibility is that the emissive state is an SBCT state formed via decay of the S1 excited state into a radical cation and a radical anion, localized on either chromophore in the dimer. Support for this hypothesis is provided by comparing the energy of the S1 excited states of the dimers (2.51 eV and 2.28 eV, respectively for bis-DIPYR and bis-α-DIPYR) to their associated redox gaps (2.44 V and 2.25 V). The excited state energies of both dimers are slightly higher than their redox gaps, making the possible formation of an SBCT state an exothermic process. Further, the close proximity of the directly linked chromophores, and their mutual orthogonality as observed by X-ray crystallography suggest that the wavefunction overlap between chromophores in the dimers may be both strong enough to allow for charge transfer and weak enough to stabilize the resulting symmetry-broken state. Further support for this hypothesis is given by the observation of triplet phosphorescence at 77 K in bis-α-DIPYR; it is well documented that CT states facilitate intersystem crossing.32, 33 The time-correlated single photon counting (TCSPC) fluorescent lifetime traces for both bis-DIPYR dimers, however, are fitted well with a mono-exponential function independent of detected photon wavelength, suggesting that a single radiative process is responsible for the observed emission reported in Table 1. The instrument response function of the TCSPC spectrophotometer used to obtain this data does not allow for resolution of kinetic processes occurring in under 1.2 ns, which suggested that if an intermediate CT state is responsible for the observed emission, it must be populated within the



Page 16 of 39

first 1200 ps after excitation. To directly observe the excited state decay processes and characterize the nature of the emissive state, it was necessary to use ultrafast transient absorption spectroscopy to obtain kinetic resolution in the 150 fs to 1 ns range. Transient absorption spectra of each dimer were obtained in cyclohexane, THF, and acetonitrile, and are reported in Figure 6. The femtosecond DIPYR dimer and monomer transient absorption experiments were performed by pumping the S1 state in each sample and probing with a white light continuum set at magic angle with respect to the pump. The spectra were collected with an instrument response of 150 fs between -2 ps and 1 ns. Within the first picosecond after excitation in both nonpolar and polar solvents, the TA traces of bis-DIPYR show excited state absorption (350 – 450 nm) and stimulated emission (525 – 650 nm). Notably, the stimulated emission (SE) trace at short time delays (from 150 fs to 1 ps) do not closely resemble the steadystate emission spectrum; there is no significant Stoke’s shift and the SE appear to have vibronic features. Decay of these excited state absorption and SE features follow in the first 10 ps after excitation in all solvents. The decay in the excited state absorption corresponds to a growth in a new absorption feature centered at 513, 523, and 506 nm in cyclohexane, THF, and acetonitrile respectively. These new spectral features correspond to the cation features observed spectroelectrochemically (Figure 5), and are assigned to growth of the CT state. The rate of population of the CT state was determined by global analysis of the transients, and yielded values of 3.3 ps in cyclohexane, 1.7 ps in THF, and 330 fs in acetonitrile (Table 3). The faster rate of SBCT in the more polar solvents is consistent with known SBCT dimers.7, 8, 11, 13, 16, 18-21 The fsTA spectra of bis-DIPYR was also measured in a PMMA matrix to observe the decay kinetics of species locked into the ground state geometry; the rate of SBCT in this system was slowed by two orders of magnitude relative bis-DIPYR in solution (Table 3 & SI). These


Page 17 of 39 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


measurements were attempted in less polar polystyrene films, but bis-DIPYR photo-bleaches in the presence of polystyrene, making it impossible to take a sufficient number of scans before the signal decays completely. The TA of bis-α-DIPYR likewise shows evidence for SBCT in both nonpolar and polar media. Within the first picosecond after excitation, both excited state absorption and stimulated emission are observed. In the next 10 ps, the intense excited state absorption feature at 510 nm decays, while a new structured absorption feature with local maxima at

~460 and 480 nm in

all three solvents grows in. This new peak is assigned to the growth of the CT state, and resembles features observed in the anion and cation spectra of bis-α-DIPYR. Also resolved is an apparent narrowing of the red edge of the ground-state bleach which occurs on the same timescale as the decay of the 510 nm S1 band; this narrowing corresponds well to a growth in the anion absorption as determined by spectroelectrochemistry, as well as decreased stimulated emission from the S1, which is replaced by a red-shifted stimulated emission feature that better resembles the steady-state emission spectrum. Fitting of these features by global analysis yielded rates of population of the CT state equal to 2 ps, 2 ps, and 1.4 ps in cyclohexane, THF, and acetonitrile, respectively. These rates are remarkable both for how quickly they occur, and for the fact that the CT state appears to be nearly as stable in nonpolar solvent as it is in polar solvents. Femtosecond TA traces of bis-α-DIPYR in dilute polystyrene thin films were acquired (SI). Consistent with steady-state photophysical measurements in dilute films, which indicate that the emitting species is the same in the solid state as in solution, SBCT was observed with rates two orders of magnitude slower in the film than in solution (Table 3).



bis-α α-DIPYR in Cyclohexane

bis-DIPYR in Cyclohexane

0.4

0.0

-0.4

∆Abs

∆Abs

0.0 0.5 ps 1 ps 10 ps 100 ps 500 ps 900 ps Abs. Em.

-0.8

350

400

450

500

550

600

0.3 ps 1 ps 10 ps 100 ps 500 ps 900 ps Abs. Em.

-0.4

-0.8

-1.2 350

-1.2 650

400

450

500

550

600

650

Wavelength (nm)

Wave length (nm)

bis-α α-DIPYR in THF

bis-DIPYR in THF

0.4

0.0

-0.4 0.5 ps 1 ps 10 ps 100 ps 500 ps 900 ps Abs. Em.

-0.8

-1.2 350

400

450

500

550

600

650

∆Abs

∆Abs

0.0

-0.4

-0.8

-1.2 350


400

450

Wavelength (nm)

0.4

bis-DIPYR in Acetonitrile

-0.8

-1.2 350

550

600

650

bis-α α-DIPYR in Acetonitrile

0.4

0.0 ∆Abs.

-0.4

500

Wavelength (nm)

0.0 ∆Abs.

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

Page 18 of 39


400

-0.4

-0.8

450

500

550

600

650

-1.2 350


400

450

500

550

600

650

Wavelength (nm)

Wavelength (nm)

Figure 6. Femtosecond absorption transient spectra of bis-DIPYR (left) and bis-α-DIPYR (right), in cyclohexane (top), THF (middle), and acetonitrile (right). The steady state absorption and emission spectra are overlaid, and black arrows indicate features corresponding to the growth of the CT state. The above spectra were normalized to their ground state bleach (GSB) minima so that the spectral features which are independent of Sn → S1 decay are shown more prominently, the raw spectra are shown in the SI.


Page 19 of 39 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


The CT state persists well past the experimental timeframe of the femtosecond transient absorption, so excited state decay processes were probed by nanosecond transient absorption spectroscopy. We were unable to acquire nanosecond TA spectra of bis-DIPYR due to poor energy matching between the excitation source and the absorption manifold. Decay of the CT state in bis-α-DIPYR, however, was analyzed by nanosecond TA in cyclohexane and in THF (Figure 7) and fit via global analysis to a four-state model. In both solvents, the CT state decays by two competing mechanisms. The first and most predominant is recombination to the S0 ground state, occurring in 4.8 ns, 4.8 ns, and 4.2 ns in cyclohexane, THF, and acetonitrile, respectively. These recombination rates are consistent with the fluorescence emission lifetimes measured by TCSPC (Table 2), and thus fluorescence emission in the dimers is assigned to direct CT→S0 emission. Two important structural analogues of DIPYR dimers, 9,9’-bianthryl, and 10,10’-dicyano-9,9’-bianthryl, are also characterized by bright SBCT states (0.22 and 0.29, respectively, in acetonitrile).34 Like we observe in the DIPYR dimers, 10,10’-dicyano-9,9’bianthryl also displays significant CT character in cyclohexane (55% of the excited state population),

35

while 9,9’-bianthryl has considerable less CT character in nonpolar solvent,

showing partial CT in heptane.36 In the case of DIPYR dimers, it appears that 100% of S1 excitons decay to an SBCT state. This appears to be a unique phenomenon in nonpolar media and may be due to exceptional polarizability in the DIPYR dimers. As evidenced by the steady-state phosphorescence study, the second SBCT state decay pathway involves the formation of a triplet state. According to global analysis, a four-state model best describes the decay dynamics in the nanosecond decay traces, suggesting that an intermediating state between the SBCT state and the ultimate triplet is formed. This state is tentatively assigned as a triplet SBCT state pending further investigation. The assigned states and



their associated rate constants in THF are depicted in the Jablonski diagram in Figure 8; the full set of rate constants for excited state decay processes are summarized in Table 3. 6 bis-α α-DIPYR in Cyclohexane

bis-α α-DIPYR in THF

4

4

2

2

0

∆Abs. (mOD)

∆Abs. (mOD)

6

-2 -4 -6 -8 -10

0.3 ps 1 ps 10 ps 100 ps 500 ps 1 ns 5 ns

50 ns 100 ns 510 ns 1 µs 5 µs 11 µs

0 -2 -4 -6 -8

-12 350

400

450

500

550

600

-10 350

650

0.3 ps 1 ps 10 ps 100 ps 500 ps 1.6 ns 5 ns

400

Wavelength (nm)

1.0

50 ns 150 ns 510 ns 1.05 µs 21 µs 51 µs

450

500

1.0

bis-α α-DIPYR in Cyclohexane

600

650

bis-α α-DIPYR in THF S1

S1

(ICT)1 (ICT)3 T1

0.6 0.4

0.8 Concentration

(ICT) (ICT)3 T1

0.8

550

Wavelength (nm)

1

Concentration

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

Page 20 of 39

0.6 0.4

0.2

0.2

0.0

0.0

10-2 10-1 100 101 102 103 104 105 106 107

10-2 10-1 100 101 102 103 104 105 106 107 Time (ps)

Time (ps)

Figure 7. Combined femtosecond and nanosecond transient absorption of bis-α-DIPYR in cyclohexane (top left) and THF (top right), and (bottom) time-dependent concentration of the species-associated decay spectra.

The transient absorption spectra of the monomers were measured for comparison to the dimers. Our recent description of the juxtaposition of state energies in DIPYR established that the monomer absorbs efficiently into the S1 and S2 states.22 Radiative emission from the S1 state in DIPYR competes kinetically with symmetry-allowed intersystem crossing (ISC) into the T2 state, which is slightly lower in energy than the S1 state, followed by rapid internal conversion to the T1, leading to steady-state observations of both a low fluorescence QY that decreases with 20 ACS Paragon Plus Environment

Page 21 of 39 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


increasing polarity and easily discernible phosphorescence at 77 K, when non-radiative processes are stifled.22 The TA spectrum of DIPYR is largely characterized by direct relaxation from the S1 to the S0 state, leading to fine resolution of isosbestic points which are indicative of A → B kinetic processes. Also observed in the TA is a rapid increase in population around the GSB, which we attribute to the predicted population of the triplet state which has an absorption energy close to that of the ground state. Population of the triplet state occurs in about 2 ps in acetonitrile whereas the rate of population of the triplet state is about 30 ps in methylcyclohexane, indicating that ISC is more favorable in the more polar solvent, which is consistent with our understanding of the polar T2 state being better stabilized in polar media. Table 3. Rates of excited state transitions in DIPYR dimers in various media. kSBCT (s-1) kISC (s-1) kIC (s-1) bis-DIPYR

PMMA cyclohexane

bis-α-DIPYR

5x109 3x10

11 11

-

krec (s-1)

knr (s-1)

-

2x108

-

-

8

-

8

3x10

THF acetonitrile polystyrene

6x10 3x1012 2x1010

-

-

4x10 1x109 2x108

-

cyclohexane

5x1011

7.7x106

1.2x108

2.1x108

3x105

THF

5x1011

8.3x106

2.9x107

2.1x108

3x104

acetonitrile

7x1011

-

-

2.4x108

-

Rate of SBCT determined by global analysis of fsTA. Rates of ISC, IC, and non-radiative decay determined by a global analysis of nsTA. Rate of recombination determined by TCSPC and applied to global analysis fitting of nsTA.

The case of α-DIPYR is much simpler, as both the S2 and T2 states are destabilized by the addition of benzene rings to the DIPYR core, so the excited state photophysics are dominated by direct S1-S0 radiative and non-radiative transitions. Indeed, in the TA spectrum of α-DIPYR we observe only the expected two-state kinetics; after laser excitation, the S1 state shows weak absorption and stimulated emission, followed by a decay in the absorption of the S1 state correlating to a decay of the ground state bleach. 21 ACS Paragon Plus Environment


bis-DIPYR bis-α-DIPYR

2.5

1

S1

Energy (eV)

CT

kSBCT =

2.0

5 x 10 11

3

CT

kIC = 2.9 x 10

kISC = 8.3 x 106

7

T1

1.5 Abs.

1.0

Emission krec = 2.1 x 108

knr = 3 x 10 4

0.5 0.0

Figure 8. Jablonski diagram summarizing state energies of bis-DIPYR (black) and bis-α-DIPYR (red). Rate constants derived from femtosecond and nanosecond transient absorption spectra of bis-α-DIPYR in THF.

4

DIPYR in Methylcyclohexane

DIPYR in Acetonitrile

2 2

1 0

-2 1 ps 10 ps 100 ps 400 ps 600 ps 800 ps 900 ps

-4 -6 -8

∆Abs. (mOD)

∆Abs. (mOD)

0

-1 1 ps 10 ps 100 ps 400 ps 600 ps 800 ps 900 ps

-2 -3 -4 -5

-10 350

400

450

500

550

600

350

650

400

450

500

550

600

650

Wavelength (nm)

Wavelength (nm)

α-DIPYR in Methylcyclohexane

2

α -DIPYR in Acetonitrile

1

0

-2 0.5 ps 1 ps 10 ps 100 ps 500 ps 900 ps

-4

-6 400

450

500

550

600

650

∆Abs. (m OD)

0 ∆Abs. (m OD)

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

Page 22 of 39

-1 0.5 ps 1 ps 10 ps 100 ps 500 ps 900 ps

-2 -3 -4 -5 400

450

Wavelength (nm)

500

550

600

650

Wavelength (nm)

Figure 9. Femtosecond transient absorption spectra of DIPYR and α-DIPYR.


Page 23 of 39 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


Molecular Modeling The monomers DIPYR and α-DIPYR exhibit relatively simple excited state photophysics. After photon absorption, radiative and non-radiative decay process occur directly from the S1 and T1 states to the S0 state. In contrast, the S1 state for the dimers rapidly decays into the symmetrybroken charge transfer state, in both polar and nonpolar solvents. The SBCT state then decays by both radiative recombination to the ground state and formation of a triplet state, via an intermediate 3SBCT state. We found it surprising both that the SBCT state is formed rapidly, even in cyclohexane, and that the SBCT is a bright state, responsible for the observed fluorescence emission. In order to elucidate the source of this anomalous photophysical behavior, the orbital configurations of both dimers were explored with Density Functional Theory (DFT) at the B3LYP/6-31G** level. The resulting optimized structures and their molecular orbital configurations were validated by comparison to X-ray crystal structures and measured electrochemical and photophysical properties. The calculated geometry of bis-DIPYR exhibits dihedral angles between the chromophores of 85.9 degrees, compared to 86.0 degrees according to crystal structure. The BF2 groups are puckered out of plane by an average of 0.42 Å in the calculated structure, compared to 0.51 Å in the crystal. These small differences can be explained by crystal packing forces, as optimized geometries were calculated for molecules in the gas phase. In bis-α-DIPYR, computational modeling results in a dihedral angle of 89.8 degrees between the planes of the chromophores, compared to 89.9° from the crystal structure of bis-α-DIPYR. The calculated singlet excited states (Table 4), determined by TD-DFT of the optimized geometries, are 0.25 and 0.14 eV higher than experimental values in bis-DIPYR and bis-α-DIPYR, respectively; error margins of this magnitude are typical for cyanine-like dyes on account of significant



Page 24 of 39

multireference character in their excited singlet states.37 These errors are smaller than those obtained from TD-DFT for the parent structures.22 Furthermore, the excited states of bis-DIPYR were computed using the multi-reference XMCQDPT2 (extended multi-configuration quasidegenerate second order perturbation theory) method38 and the computed S1 energy (2.36 eV) is in good agreement with the experimental value. However, performing these calculations for bisα-DIPYR was found to be prohibitively expensive. The triplet energy was determined by the delta self-consistent field approach within DFT, wherein the triplet state geometry was optimized and the energy of the optimized ground state was subtracted from the energy of the optimized triplet state.39 This method provided a very good estimation of the triplet energy, within 100 meV of experimental values. Table 4. Calculated and experimental (λmax abs) excited state energies. S1 calc. S1 exp. T1 calc. bis-DIPYR bis-α α-DIPYR

a

449 nm (2.76/2.36 eV) 513 nm (2.42 eV)

493 nm (2.51 eV) 543 nm (2.28 eV)

596 nm (2.08 eV) 685 nm (1.81 eV)

T1 exp. 593 nm (2.09 eV) 657 nm (1.89 eV)

a

XMCQDPT2(14,13)/6-31G(d)

The frontier molecular orbital configurations of both dimers are localized on the dipyridylmethene portion of the dimers, with very little orbital density on the outer benzene rings of bis-α-DIPYR, again similar to what is observed in the monomers (Figure 10). Both dimers exhibit localization of the LUMO orbitals on each chromophore, while the HOMO orbitals are characterized by significant electron density at the meso position, which bridges the two chromophores. For this reason, it is reasonable to expect that there is some electronic coupling between the two chromophores, even in the ground state, and that this is the source of the bathochromic shifts observed when comparing the UV-Vis absorption spectra of the dimers to their corresponding monomers. It is interesting to note that the orbital configuration of these


Page 25 of 39 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


bis-DIPYRs is opposite to that observed in bis-BODIPYs, which are characterized by a node at the meso position of their HOMO orbitals and chromophoric bridging in the LUMOs.

Figure 10. bis-DIPYR (left) and bis-α-DIPYR (right) HOMO (bottom, solid) and LUMO (top, mesh) orbitals.

The natural transition orbitals (SI) of the calculated S1 excited states in both dimers are characterized by a hole at the HOMO, localized at the meso position, and an electron at the LUMO, delocalized across the chromophore. The rate of formation of the SBCT state is thought to be proportional to the degree of wavefunction overlap between coupled chromophores.7 The rapidity and near solvent independence of charge transfer rates in both DIPYR dimers is possibly



Page 26 of 39

due to the orbital configurations between coupled chromophores. We believe that the mechanism for formation of the SBCT state in DIPYR dimers involves hole localization at the meso-carbon of one of the DIPYR moieties after excitation, which then spreads to the adjacent chromophore but remains insulated from solvent near the meso C-C bridge, and that CT may involve the transfer of partial charges.36,

40

This process is likely facilitated by the highly polarizable

zwitterionic chromophores, which self-stabilizes the induced dipole. In such a system, it is possible that small changes in local solvent density could further facilitate stabilization of the SBCT state.35, 41 Insulation of charge density on the interior of these relatively bulky systems could be an explanation for the lack of solvatochromism in the fluorescence spectra. We acknowledge, however, that further probing the mechanism of SBCT state formation, perhaps by transient infrared spectroscopy, is necessary to develop a clearer picture of this unique phenomenon. CONCLUSION Two novel bis-DIPYR compounds, homoleptic meso-linked dimers of DIPYR and α-DIPYR were synthesized, and show strong UV- and visible-light absorption and fluorophorescence. Their excited state photophysical properties were explored using a combination of steady-state and time-resolved spectroscpic methods. It was discovered that the dimers couple such that the chromophore units are mutually orthogonal, with the HOMO localized at the meso position and bridging the two chromophores, and the LUMO delocalized across each chromophore. This coupling facilitates rapid and efficient symmetry-breaking charge transfer in both polar acetonitrile (330 fs in bis-DIPYR and 1.4 ps in bis-α-DIPYR) and nonpolar cyclohexane (3 ps in bis-DIPYR and 2 ps in bis-α-DIPYR) solvents. We found it both exciting and surprising that


Page 27 of 39 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


these dimers form stable, highly emissive (ΦPL = 0.03 to 0.57), symmetry-broken states even in nonpolar solvents. The structural likeness between bis-DIPYRs, 9,9’-bianthryl,11, 13, 34 and bis-BODIPY18 merits a brief consideration of their photophysical similarities and differences. All three structure classes are known to undergo symmetry breaking charge transfer, forming a radical cation localized on one chromophore, and a radical anion localized on the other chromophore shortly after photoexcitation. This process has been shown to occur in bis-BODIPY in 170 fs in acetonitrile, in 4.5 ps in the considerably less polar toluene and to not take place in cyclohexane (BODIPY fluorescence is observed). The faster rate of SBCT in bis-BODIPY may be due to the relative rearrangement of HOMO and LUMO orbital densities in the BODIPY dimer compared to the DIPYR dimers. In bis-BODIPY, the HOMO is delocalized across the chromophore, while the LUMO is localized at the meso position bridging the two chromophores. In both DIPYR dimers, as discussed above, the opposite arrangement is observed. The localization of excited state electron density at the meso position in bis-BODIPY may be the source of the faster rate of SBCT in bis-BODIPY in acetonitrile. However, the rate of SBCT in bis-α-DIPYR remains nearly the same in nonpolar solvents as it is in acetonitrile, whereas it is significantly slowed in bis-BODIPY in toluene. This suggests that the mechanism for charge transfer in DIPYR dimers is different than it is in bis-BODIPY, possibly involving migration of the hole in DIPYRs and migration of the electron in bis-BODIPY. Moreover, it suggests that once the symmetry-broken state is formed in bis-DIPYRs, non-radiative deactivation is not the primary mechanism of energy loss as it is in many other SBCT dimers; rather, DIPYRs remain in the symmetry broken state for several nanoseconds before either emitting a photon to recombine to the ground state (ΦPL up to 0.57 and 0.23 in bis-α-DIPYR and bis-DIPYR, respectively) or intersystem crossing



Page 28 of 39

to form a triplet state, which then decays predominately non-radiatively due to lack of sufficient spin-orbit coupling to facilitate ISC back to the ground state. Likewise in 9,9’-bianthryl, SBCT has been shown to occur efficiently in solvents of strong polarity, within 390 fs in acetonitrile, and to a lesser extent (about 10% of the excited state population) in 510 fs in cyclohexane.35 The rate of SBCT is enhanced in 10,10’-dicyano-9,9’bianthryl relative to 9,9’-bianthryl in acetonitrile (150 fs) and in cyclohexane (400 fs), where the equilibrium population of the CT state relative to the localized excited state, is also increased in cyclohexane from 10% in 9,9’-bianthryl to 55% in the more quadrupolar and polarizable cyanoderivative. We propose that the rate of symmetry breaking charge transfer in mutually orthogonal dimers is enhanced by (a) increasing the polarizability of the dimer thereby allowing greater selfsolvation effects to stabilize CT transitions, (b) localizing HOMO or LUMO density at the bridging position between chromophores, and (c) allowing for either rapid torsional (via steric decongestion) or solvent (via an increase in intermolecular interaction) relaxation to induce the charge transfer event. The discovery of a new structure class capable of rapid and efficient charge transfer to form a stable SBCT state which persists for several nanoseconds is highly promising for application to new energy conversion technologies. Investigation of bis-DIPYRs as active layer materials in organic photovoltaics is actively underway. Applications of bis-DIPYRs to the generation of solar fuels, to artificial photosynthesis, and to photonics are also possible.

EXPERIMENTAL Methods and instrumentation


Page 29 of 39 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


UV-visible spectra were recorded on a Hewlett‑Packard 4853 diode array spectrometer. Photoluminescence spectra were measured using a QuantaMaster Photon Technology International phosphorescence/fluorescence spectrofluorimeter. Quantum yield measurements were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating

sphere

and

model

C10027

photonic

multi-channel

analyzer

(PMA).

Photoluminescence lifetimes were measured by time-correlated single-photon counting using an IBH Fluorocube instrument equipped with an LED excitation source. Molar extinction coefficients were obtained by plotting solutions at four concentrations between 0.1 and 0.9 on a Beer’s Law plot, with y-intercept set to zero. Line fitting for all samples provided R2 values greater than 0.98. Excitation, emission, photoluminescence quantum yield, and lifetime measurements were acquired from solutions at maximum optical densities between 0.1-0.2 to minimize the effects of solute-solute interactions and inner filter effects. Room temperature photophysical measurements were recorded in indicated solvents and cryogenic photophysical measurements were carried out in methylcyclohexane at 77 K. NMR spectra were recorded on a Varian 400 NMR spectrometer and referenced to the residual proton resonance of chloroform (CDCl3) solvent at 7.26 ppm. Matrix assisted laser desorption/ionization (MALDI) mass spectroscopy data was acquired on a Bruker Autoflex Speed LRF. Spectroelectrochemical samples were prepared at an optical density of 0.6-0.8 in an acetonitrile solution inside a glass Hcell with tetrabutylammonium hexafluorophosphate as the electrolyte. One cuvette of the H-cell, containing the working (glassy carbon) and reference (silver) electrodes was placed in the path of the UV-Vis excitation source, and the other, containing the counter electrode (platinum) was separated by a medium frit designed to minimize mixing of oxidized and reduced analyte



Page 30 of 39

between the two cells. The UV-Vis was blanked to the neutral solution, and then chronoamperometry at 100 mV overpotentials for the desired redox process was begun.

Femtosecond Transient Absorption Pump and probe pulses were obtained from the output of a Ti:Sapphire regenerative amplifier (Coherent Legend Elite, 1 kHz, 3.2 mJ, 35 fs). The pump pulses were generated by pumping a type II OPA (Spectra Physics OPA-800C) with 10% of the 800 nm amplifier output and mixing the OPA signal with the 800 nm residual on a type II BBO to generate 500 nm and 485 nm. 500 nm was used to pump the DIPYR dimers and α-DIPYR monomer while 485 nm was used to pump the DIPYR monomer. A white light super continuum (320-950 nm) was generated by focusing a small amount of the 800 nm on a rotating CaF2 disk. The supercontinuum probe was collimated and focused by a pair of off-axis parabolic mirrors onto the sample, while the pump was focused before the sample with a 25 cm CaF2 lens. The probe was set to magic angle (54.7 ̊) with respect to the pump to avoid any contribution from reorientational motion. The supercontinuum probe was dispersed using a spectrograph (Oriel MS127i) onto a 256-pixel silicon diode array (Hamamatsu) for multiplexed detection of the probe. Samples containing bis-DIPYR and bis-α-DIPYR dissolved in cyclohexane and THF were placed in a closed capped 1mm quartz cuvette. The DIPYR monomer and α-DIPYR monomer were dissolved in methylcyclohexane and acetonitrile and also placed in a 1mm capped quartz cuvette. The samples were made such that the optical densities were between 0.2


Page 31 of 39 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


and 0.4 at the pump wavelength. During data collection the samples were moved on a motorized stage perpendicular to the pump to decrease photodamage caused by the pump. Nanosecond Transient Absorption Samples were prepared on a Schlenk line where the samples were purged with N2 and dissolved in dry solvents such that the maximum absorbance did not exceed OD=0.5. The samples were sealed in a capped 1 mm quartz cuvette which was oscillated on a stage. Nanosecond pump generation was performed using a 532 nm output from a Nd:YAG laser (Alphalas, 532 nm, 1 kHz, 700 ps) which is externally triggered and synchronized with the femtosecond amplifier. The pump pulse is delayed with respect to the femtosecond supercontinuum probe using a delay generator DG 645 (Stanford Research Systems). Computational Methods All DFT calculations reported here were performed using Jaguar (version 9.5, release 11) program on the Schrödinger Materials Science Suite (v2017-3), Schrödinger, LLC, New York, NY, 2017. Gas phase geometry optimizations were calculated using B3LYP functional with the 6-31G** basis set as implemented in Jaguar. Single point XMCQDPT238 calculations were performed on the B3LYP/6-31G** optimized structure using the 6-31G(d) basis set and an active space of 14 electrons in 13 orbitals (14,13) with state-averaging over 8 states. The XMCQDPT2 calculations were performed using the Firefly quantum chemistry package.42

General Synthesis All reagents were purchased from Sigma Aldrich and used without purification. Anhydrous 1,2-dichloroethane was purchased from EMD Millipore. 31 ACS Paragon Plus Environment


Page 32 of 39

A 15 mM solution of 1,1,2,2-tetra(pyridin-2-yl)ethane or 1,1,2,2-tetra(quinolin-2-yl)ethane ligand in dry 1,2-dichloroethane was prepared in an N2-purged schlenk flask equipped with a magnetic stir bar and fitted with a reflux condenser. The flask was submerged in a preheated oil bath and brought to reflux, at which time 4.0 eq. boron trifluoride diethyl etherate were added dropwise, at which time a precipitate was formed. The solution was stirred for 2 hours at reflux, then cooled to room temperature and treated with 10 eq. N,N-diisopropylethylamine, causing the precipitate to dissolve. The solution was washed with water and the aqueous layer was separated and extracted three times with dichloromethane. The organic layers were combined, dried over sodium sulfate, filtered, and reduced to a polycrystalline solid by rotary evaporation. The products were purified by silica gel flash chromatography with the gradient eluent 0-50% dichloromethane in hexanes, followed by recrystallization from a concentrated dichloromethane solution layered with hexanes. bis-DIPYR: 0.668 g (31% yield) bright orange crystals. 1H NMR (500 MHz, Chloroform-d) δ 8.11 (d, J = 6.1 Hz, 4H), 7.28 – 7.21 (m, 4H), 6.74 (d, J = 9.0 Hz, 4H), 6.63 (t, J = 6.8 Hz, 4H). 13

C NMR (101 MHz, Chloroform-d) δ 149.39, 137.43 (t J = 1.8 Hz), 136.92, 119.34, 112.07,

88.21. Anal. Calcd for C22H16B2F4N4: C, 60.88; H, 3.72; N, 12.91. Found: C, 60.99; H, 3.81; N, 12.61. MS (MALDI-TOF) m/z calcd for C22H16B2F4N4+ 434.150 [M+]; found 633.892. bis-α-DIPYR: 0.105 g (32% yield) magenta needles. 1H NMR (400 MHz, Chloroform-d) δ 8.79 – 8.73 (m, 4H), 7.68 (ddd, J = 8.9, 7.1, 1.7 Hz, 4H), 7.55 (d, J = 9.1 Hz, 4H), 7.52 (dd, J = 7.8, 1.6 Hz, 4H), 7.33 (ddd, J = 7.8, 7.0, 0.9 Hz, 4H), 7.01 (d, J = 9.3 Hz, 4H). 13C NMR (101 MHz, Chloroform-d) δ 150.53, 140.30, 138.23, 131.07, 128.23, 124.97, 124.22, 121.91 (t, J = 8.5 Hz), 119.72, 94.25. Anal. Calcd for C38H24B2F4N4: C, 71.96; H, 3.81; N, 8.83. Found: C,


Page 33 of 39 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


69.60; H, 4.81; N, 7.78. MS (MALDI-TOF) m/z calcd for C38H24B2F4N4+ 634.212 [M+]; found 634.031. Supporting Information: Single crystal X-ray diffraction data, cyclic and differential pulse voltammograms, photophysics

of

monomers,

additional

spectroelectrochemistry,

transient

absorption

spectroscopy, and TD-DFT data and figures are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information: Corresponding Author: [email protected] The authors declare no competing financial interest. Acknowledgements: Financial support from the Office of Basic Energy Sciences at the Department of Energy (DE-SC0016450) and the National Science Foundation (NSF CBET-1511757 and NSF CHE– 1461632) are gratefully acknowledged. We thank Dr. Ralf Haiges for his help in acquiring and refining reported crystal structures. REFERENCES 1. Schlenker, C. W.; Thompson, M. E., The Molecular Nature of Photovoltage Losses in Organic Solar Cells. Chem. Commun. 2011, 47, 3702-3716. 2. Meech, S. R.; Hoff, A. J.; Wiersma, D. A., Evidence for a very Early Intermediate in Bacterial Photosynthesis. A Photon-Echo and Hole-Burning Study of the Primary Donor Band in Rhodopseudomonas sphaeroides. Chem. Phys. Lett. 1985, 121, 287-292. 3. Lockhart, D. J.; Boxer, S. G., Magnitude and Direction of the Change in Dipole Moment Associated with Excitation of the Primary Electron Donor in Rhodopseudomonas sphaeroides Reaction Centers. Biochemistry 1987, 26, 664-668. 33 ACS Paragon Plus Environment


Page 34 of 39

4. Lathrop, E. J. P.; Friesner, R. A., Simulation of Optical Spectra from the Reaction Center of Rb. sphaeroides. Effects of an Internal Charge-Separated State of the Special Pair. J. Phys. Chem. 1994, 98, 3056-3066. 5. Thompson, M. A.; Schenter, G. K., Excited States of the Bacteriochlorophyll b Dimer of Rhodopseudomonas viridis: A QM/MM Study of the Photosynthetic Reaction Center That Includes MM Polarization. J. Phys. Chem. 1995, 99, 6374-6386. 6. Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R., Combining Light-Harvesting and Charge Separation in a Self-Assembled Artificial Photosynthetic System Based on Perylenediimide Chromophores. J. Am. Chem. Soc. 2004, 126, 12268-12269. 7. Vauthey, E., Photoinduced Symmetry-Breaking Charge Separation. ChemPhysChem 2012, 13, 2001-2011. 8. Bartynski, A. N.; Gruber, M.; Das, S.; Rangan, S.; Mollinger, S.; Trinh, C.; Bradforth, S. E.; Vandewal, K.; Salleo, A.; Bartynski, R. A.; Bruetting, W.; Thompson, M. E., SymmetryBreaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics. J. Am. Chem. Soc. 2015, 137, 5397-5405. 9. Sisson, A. L.; Sakai, N.; Banerji, N.; Fürstenberg, A.; Vauthey, E.; Matile, S., Zipper Assembly of Vectorial Rigid-Rod π-Stack Architectures with Red and Blue Naphthalenediimides: Toward Supramolecular Cascade n/p-Heterojunctions. Angew. Chem. Int. Ed. 2008, 47, 3727-3729. 10. Collet, E.; Lemée-Cailleau, M.-H.; Buron-Le Cointe, M.; Cailleau, H.; Wulff, M.; Luty, T.; Koshihara, S.-Y.; Meyer, M.; Toupet, L.; Rabiller, P.; Techert, S., Laser-Induced Ferroelectric Structural Order in an Organic Charge-Transfer Crystal. Science 2003, 300, 612615. 11. Mataga, N.; Yao, H.; Okada, T.; Rettig, W., Charge-Transfer Rates in Symmetric and Symmetry-Disturbed Derivatives of 9,9'-Bianthryl. J. Phys. Chem. 1989, 93, 3383-3386. 12. Grozema, F. C.; Swart, M.; Zijlstra, R. W. J.; Piet, J. J.; Siebbeles, L. D. A.; van Duijnen, P. T., QM/MM Study of the Role of the Solvent in the Formation of the Charge Separated Excited State in 9,9‘-Bianthryl. J. Am. Chem. Soc. 2005, 127, 11019-11028. 13. Rettig, W.; Zander, M., Broken Symmetry in Excited States of 9,9′-Bianthryl. Ber. Bunsen-Ges. Phys. Chem 1983, 87, 1143-1149. 14. Lueck, H.; Windsor, M. W.; Rettig, W., Pressure Dependence of the Kinetics of Photoinduced Intramolecular Charge Separation in 9,9'-Bianthryl Monitored by Picosecond Transient Absorption: Comparison with Electron Transfer in Photosynthesis. J. Phys. Chem. 1990, 94, 4550-4559. 15. Holman, M. W.; Yan, P.; Adams, D. M.; Westenhoff, S.; Silva, C., Ultrafast Spectroscopy of the Solvent Dependence of Electron Transfer in a Perylenebisimide Dimer. J. Phys. Chem. A 2005, 109, 8548-8552. 34 ACS Paragon Plus Environment

Page 35 of 39 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


16. Wu, Y.; Young, R. M.; Frasconi, M.; Schneebeli, S. T.; Spenst, P.; Gardner, D. M.; Brown, K. E.; Würthner, F.; Stoddart, J. F.; Wasielewski, M. R., Ultrafast Photoinduced Symmetry-Breaking Charge Separation and Electron Sharing in Perylenediimide Molecular Triangles. J. Am. Chem. Soc. 2015, 137, 13236-13239. 17. Sung, J.; Nowak-Król, A.; Schlosser, F.; Fimmel, B.; Kim, W.; Kim, D.; Würthner, F., Direct Observation of Excimer-Mediated Intramolecular Electron Transfer in a CofaciallyStacked Perylene Bisimide Pair. J. Am. Chem. Soc. 2016, 138, 9029-9032. 18. Whited, M. T.; Patel, N. M.; Roberts, S. T.; Allen, K.; Djurovich, P. I.; Bradforth, S. E.; Thompson, M. E., Symmetry-Breaking Intramolecular Charge Transfer in the Excited State of Meso-Linked BODIPY Dyads. Chem. Commun. 2012, 48, 284-286. 19. Trinh, C.; Kirlikovali, K.; Das, S.; Ener, M. E.; Gray, H. B.; Djurovich, P.; Bradforth, S. E.; Thompson, M. E., Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc Dipyrrins. J. Phys. Chem. C 2014, 118, 21834-21845. 20. Cook, R. E.; Phelan, B. T.; Kamire, R. J.; Majewski, M. B.; Young, R. M.; Wasielewski, M. R., Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers. J. Phys. Chem. A 2017, 121, 1607-1615. 21. Giaimo, J. M.; Gusev, A. V.; Wasielewski, M. R., Excited-State Symmetry Breaking in Cofacial and Linear Dimers of a Green Perylenediimide Chlorophyll Analogue Leading to Ultrafast Charge Separation. J. Am. Chem. Soc. 2002, 124, 8530-8531. 22. Golden, J. H.; Facendola, J. W.; Sylvinson M. R, D.; Baez, C. Q.; Djurovich, P. I.; Thompson, M. E., Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores. J. Org. Chem. 2017, 82, 7215-7222. 23. Douglass, J. E.; Barelski, P. M.; Blankenship, R. M., Diazaboracyclic cations. III. A Homomorph of 9,10-Dihydroanthracene. J. Heterocycl. Chem. 1973, 10, 255-257. 24. Sathyamoorthi, G.; Soong, M.-L.; Ross, T. W.; Boyer, J. H., Fluorescent Tricyclic βAzavinamidine–BF2 Complexes. Heteroat. Chem 1993, 4, 603-608. 25. Kubota, Y.; Tsuzuki, T.; Funabiki, K.; Ebihara, M.; Matsui, M., Synthesis and Fluorescence Properties of a Pyridomethene−BF2 Complex. Org. Lett. 2010, 12, 4010-4013. 26. Itoh, M.; Hirano, K.; Satoh, T.; Miura, M., Copper-Catalyzed α-Methylenation of Benzylpyridines Using Dimethylacetamide as One-Carbon Source. Org. Lett. 2014, 16, 20502053. 27. Scheibe, G.; Friedrich, H. J., Synthese und Reaktionen der Chinolylmethane. Ber. 1961, 94, 1336-1345. 28. Reichardt, C., Solvatochromic Dyes as Solvent Polarity Indicators. Chemical Reviews 1994, 94, 2319-2358.



Page 36 of 39

29. Kumpulainen, T.; Lang, B.; Rosspeintner, A.; Vauthey, E., Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution. Chem. Rev. 2017, 117, 10826-10939. 30. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Measurement of the Lowest Unoccupied Molecular Orbital Energies of Molecular Organic Semiconductors. Org. Electron. 2009, 10, 515-520. 31. W. D’Andrade, B.; Datta, S.; R. Forrest, S.; Djurovich, P.; Polikarpov, E.; Thompson, M., Relationship Between the Ionization and Oxidation Potentials of Molecular Organic Semiconductors. Org. Electron. 2005, 6, 11-20. 32. Dance, Z. E. X.; Mickley, S. M.; Wilson, T. M.; Ricks, A. B.; Scott, A. M.; Ratner, M. A.; Wasielewski, M. R., Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine−Anthracene Molecules with Perpendicular π Systems. J. Phys. Chem. A 2008, 112, 4194-4201. 33. Liu, Y.; Zhao, J.; Iagatti, A.; Bussotti, L.; Foggi, P.; Castellucci, E.; Di Donato, M.; Han, K.-L., A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing. J. Phys. Chem. C 2018, 122, 25022511. 34. Piet, J. J.; Schuddeboom, W.; Wegewijs, B. R.; Grozema, F. C.; Warman, J. M., Symmetry Breaking in the Relaxed S1 Excited State of Bianthryl Derivatives in Weakly Polar Solvents. J. Am. Chem. Soc. 2001, 123, 5337-5347. 35. Kovalenko, S. A.; Pérez Lustres, J. L.; Ernsting, N. P.; Rettig, W., Photoinduced Electron Transfer in Bianthryl and Cyanobianthryl in Solution: The Case for a High-Frequency Intramolecular Reaction Coordinate. J. Phys. Chem. A 2003, 107, 10228-10232. 36. Asami, N.; Takaya, T.; Yabumoto, S.; Shigeto, S.; Hamaguchi, H.-o.; Iwata, K., Two Different Charge Transfer States of Photoexcited 9,9′-Bianthryl in Polar and Nonpolar Solvents Characterized by Nanosecond Time-Resolved Near-IR Spectroscopy in the 4500−10 500 cm−1 Region. J. Phys. Chem. A 2010, 114, 6351-6355. 37. Charaf-Eddin, A.; Le Guennic, B.; Jacquemin, D., Excited-states of BODIPY-cyanines: ultimate TD-DFT challenges? RSC Adv. 2014, 4, 49449-49456. 38. Granovsky, A. A., Extended Multi-Configuration Quasi-Degenerate Perturbation Theory: The New Approach to Multi-State Multi-Reference Perturbation Theory. J. Chem. Phys. 2011, 134, 214113-1-214113-14. 39. Gavnholt, J.; Olsen, T.; Engelund, M.; Schiøtz, J., Delta Self-Consistent Field Method to Obtain Potential Energy Surfaces of Excited Molecules on Surfaces. Phys. Rev. B: Condens. Matter 2008, 78, 075441-1-075441-10.


Page 37 of 39 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


40. Takaya, T.; Saha, S.; Hamaguchi, H.-o.; Sarkar, M.; Samanta, A.; Iwata, K., Charge Resonance Character in the Charge Transfer State of Bianthryls: Effect of Symmetry Breaking on Time-Resolved Near-IR Absorption Spectra. J. Phys. Chem. A 2006, 110, 4291-4295. 41. Ivanov, A. I.; Dereka, B.; Vauthey, E., A Simple Model of Solvent-Induced SymmetryBreaking Charge Transfer in Excited Quadrupolar Molecules. J. Chem. Phys. 2017, 146, 164306-1-164306-7. 42. Granovsky, A. A. Firefly version 8. http://classic.chem.msu.su/gran/gamess/index.html (accessed Jan 18, 2018).



Page 38 of 39


Page 39 of 39 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


Symmetry Breaking Charge Transfer in bis-alpha-DIPYR 82x44mm (300 x 300 DPI)

ACS Paragon Plus Environment

Symmetry-Breaking Charge Transfer in Boron Dipyridylmethene

Recommend Documents