X-ray Generated Recombination Exciplexes of Substituted

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X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems Anatoly R. Melnikov, Maria P. Davydova, Peter S. Sherin, Valeri V. Korolev, Alexander A. Stepanov, Evgeny V. Kalneus, Enrico Benassi, Sergei F. Vasilevsky, and Dmitri V. Stass J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11634 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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The Journal of Physical Chemistry

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems Anatoly R. Melnikov,†,‡ Maria P. Davydova,† Peter S. Sherin,§,‡ Valeri V. Korolev,† Alexander A. Stepanov,† Evgeny V. Kalneus,† Enrico Benassi,*,≠,¶ Sergei F. Vasilevsky,† Dmitri V. Stass*,†,‡ †

Institute of Chemical Kinetics and Combustion SB RAS, 3, Institutskaya Str., 630090 Novosibirsk, Russian Federation

‡

Novosibirsk State University, 2, Pirogova Str., 630090 Novosibirsk, Russian Federation §

International Tomography Center, 3a, Institutskaya Str., 630090 Novosibirsk, Russian Federation

≠

School of Science and Technology, Nazarbayev University, 53, Qabanbay Batyr Ave., 010000 Astana, Kazakhstan

¶

University of Oklahoma, 660 Parrington Oval, 73019 Norman, The United States of America

ACS Paragon Plus Environment

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ABSTRACT. Customizable and technology-friendly functional materials are one of the mainstays of the emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiationgenerated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) mono-substituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.


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INTRODUCTION A number of aromatic compounds containing ethynyl fragment exhibit unique structural, electronic, and luminescent properties that have been used in various fields of science for a long time.1-5

The

simplest

examples

of

such

molecules

are

1-(phenylethynyl)benzene

(diphenylacetylene, DPA) and 1,4-bis-(phenylethynyl)benzene. The rigid linear structure of the triple bond and the ability to participate in conjugations allow to create various electron conducting polymers for molecular wires,6-9 molecular photonic devices,10-13 light emitting diodes,14-15 and solar cells16-17 based on these molecules. DPA is also used as a building block in macrocycles,18 dendrimers19-21 and other molecules,22-24 as a polymer-based metal ion sensor,25-28 and as a ligand for organometallic π-complexes.29 Additionally, diphenylacetylene-based molecules were suggested to be light-activated reagents for live-cell imaging30-32 and cancer therapy.33-34 Photophysical properties of DPA have been extensively studied both experimentally35-39 and theoretically.35,40-44 Unlike highly conjugated diphenylacetylene-based poly(p-phenyleneethynylene)s,45-50 DPA is only weakly fluorescing. Picosecond transient absorption37-38 and fluorescence measurements36,51 together with quantum chemical calculations40,43-44 and fluorescence measurements in a supersonic free jet39 revealed unusual excited state dynamics of DPA. According to a previously suggested scheme of DPA photophysics, the initial electronic excitation of the DPA molecule proceeds via the transition 1B1u ← 1A1g in linear D2h symmetry. The decay channels of the excited 1B1u (D2h) state include (i) fluorescence via the allowed 1B1u → 1

A1g (D2h) transition and (ii) population of a second excited state 1Au with trans-stilbene like

geometry in bent C2h symmetry.40,43-44 The 1Au (C2h) state is expected to be the dark state52 and play a precursor role for unusually fast intersystem crossing into the lowest 3B1u triplet state


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again in linear D2h symmetry.38,42 The energy minimum of 1Au (C2h) state is slightly lower than the energy minimum of 1B1u (D2h) state. This means that potential energy curves of the fluorescent 1B1u (D2h) and the dark 1Au (C2h) states are crossed along the trans-stilbene like motion of the DPA molecule.40,43 As a result of this bending-related energy relaxation pathway, DPA has unique photophysical properties, namely, short fluorescence lifetime, low and temperature dependent fluorescence quantum yield, fast intersystem crossing, etc. In addition to DPA, the literature also provides luminescent properties of certain mono-,37,53 and various disubstituted diphenylacetylenes,35,47,54-56 as well as some substituted diphenylacetylenes with high fluorescence quantum yields,55 which indicates possible drastic changes in photophysics upon introduction of functional groups in the diphenylacetylene core. Despite being a rather poor fluorophore itself, DPA was recently found to be a convenient charge acceptor for efficient generation of light-emitting exciplexes under X-irradiation in nonpolar solutions via radical ion pair recombination.57 In the traditional scheme of radiation induced processes in non-polar liquids58 X-irradiation initially results in ionization of solvent molecules (S) to produce the primary radical ion pair S+●/e-. If suitable electron donor (D) and electron acceptor (A) molecules are added into the solution charge transfer reactions can occur with formation of a secondary radical ion pair D+●/A-●. Recombination of the secondary radical ion pair produces electronically excited states of molecules A or D, which, if singlet, can result in fluorescence. This recombination fluorescence is commonly used to study radiation-induced processes involving spin-correlated radical ion pairs in non-polar solutions.58-60 However, immediately after the recombination event the excited molecule and its partner happen to be next to each other, which provides excellent conditions to form excited complexes.57,61-62 In contrast to standard exciplex generation in solution via optical excitation followed by bulk diffusion-


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controlled quenching of an electronically excited molecule by its partner,63-68 this recombination channel of exciplex formation does not depend on the lifetime of excited state of the acceptor molecule. Efficient exciplex formation occurs even in systems with short-lived excited states, which significantly widens the class of possible exciplex-forming systems. Radiation-generated recombination exciplexes have been readily obtained in solutions for donor-acceptor systems with exciplexes optically inaccessible due to short excited state lifetimes, such as p-terphenyl – N,N-dimethylaniline (τF, pTP = 0.95 ns)69 and diphenylacetylene – N,N-dimethylaniline (τF, DPA

= 8 ps)38 in n-dodecane.57,70 N,N-dimethylaniline (DMA), functioning as electron donor D in

these settings, can also be substituted for other tertiary amines, e.g., N,N,N′,N′-tetramethyl-pphenylenediamine

(TMPD)

or

4,4′,4′′-tris[phenyl(m-tolyl)amino]triphenylamine

(m-

MTDATA). A similar recombination channel of generating local electronically excited states and exciplexes is also possible in the conditions of electrogenerated chemiluminescence (ECL).71-74 A further crucial advantage of the diphenylacetylene molecule is the relative ease of obtaining substituted analogues from halogenated aromatic precursors and phenylacetylenes or acetylides via cross-coupling reactions of Sonogashira and Castro, respectively.75-76 In contrast, in the case of classic exciplex-forming systems (e.g., pyrene – DMA, naphthalene – DMA) synthesizing a series of substituted molecules with systematically varied substituent position is rather difficult. Every molecule of a substituted polycyclic aromatic hydrocarbon in fact requires a dedicated synthetic strategy, as no simple and universal reaction of substitution in aromatic ring is known. Introduction of a substituent in a molecule can have a pronounced effect on its electronic, steric, and luminescent properties. Thus, by varying the position, type, and number of substituents introduced in one and the same base molecule, a set of luminophores with emission bands


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covering the entire visible range has been produced.77-78 An exciplex, being an excited charge transfer complex, is even more sensitive to changes in the properties of molecules it is formed from. By choosing a convenient donor, such as DMA, and studying radiation-generated exciplexes in its mixtures with various substituted DPAs, it is experimentally possible to determine the effect of the substituents on the properties of the substituted molecule and study the influence of local luminescence, steric, and energetic factors on exciplex formation while keeping the same donor. Recombination exciplexes in donor-acceptor systems of substituted diphenylacetylenes with aromatic amines can potentially be used to create small-molecule based organic light emitting diodes (OLEDs) with a tunable position of emission maximum and better conversion efficiency due to a red shift of the exciplex emission band away from absorption bands of the individual components. Formation of exciplexes between individual molecules has indeed been explored as a means to improve efficiency and spectrally tune OLED emission,79-82 up to ‘white’ light generation.83 As previously discussed,84 reactions that in the conditions of X-irradiation lead to recombination fluorescence are completely analogous to processes occurring in OLEDs upon charge injection. This opens the possibility to study model systems mimicking real OLED functional layers in more convenient solution-based conditions, which may be especially useful to screen a large number of similar systems to find the ones that have the required properties. The suggested model systems and X-irradiation based experiment help avoid additional complications related to the complex structure of real OLEDs and charge injection from electrodes and concentrate on the charge recombination and light-emitting aspects of the device, while significantly extending the range of probed systems beyond those accessible by optical excitation.


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One further advantage of the suggested diphenylacetylene-based exciplexes is their tunable deep-blue emission with maxima that can be moved to shorter than 400 nm, covering the most problematic for high-efficiency OLEDs shorter-wavelength part of the visible region. Together with polymer LEDs85-86 they may become a viable alternative option for deep-blue OLEDs87 or the blue-range duties in multi-emitter electroluminescent devices, in which the longerwavelength range is covered by triplet-harvesting phosphor-based88-92 or thermally-activated delayed fluorescence (TADF) based emitters.93-100 This may prove useful despite the recent impressive progress in the development of blue phosphors101-103 and TADF emitters,104-108 as it provides a very cost-effective and straightforward way to overcome the major problem of the blue phosphors/TADF emitters, the necessity to ensure the transport of high-energy triplet excitations over the host while keeping voltage drop over the device as low as practical. Although it has also seen explosive development in recent years,109-115 it is not yet a mature technology. While highly optimized systems have already demonstrated performance that rivals fluorescence tubes,116-119 they are rather complex and delicate, and a possible simpler and robust alternative for deep-blue emission, provided by exciplexes from chemically convenient small molecules as suggested in this work, may deserve consideration for this emerging high volume highly competitive and cost-sensitive market. In this work we report a study of luminescent properties, the efficiency of forming recombination exciplexes and their emission properties (position of emission band maximum and lifetime) of a series of prototypic substituted diphenylacetylenes with a systematically varied position of a donor or acceptor group, using methoxy or trifluoromethyl groups, respectively. The chemical structures of the studied molecules, 1-methoxy-2-(phenylethynyl)benzene (1OMe), 1-methoxy-3-(phenylethynyl)-benzene (2OMe), 1-methoxy-4-(phenylethynyl)benzene


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(3OMe),

1-(phenylethynyl)-2-(trifluoromethyl)benzene

Page 8 of 81

(1CF3),

1-(phenylethynyl)-3-

(trifluoromethyl)-benzene (2CF3), 1-(phenylethynyl)-4-(trifluoromethyl)benzene (3CF3), are shown in Table 1. Although these compounds have been known before and characterized in organic chemistry, their luminescent and exciplex-forming properties have not been studied previously, and for the purposes of this work they were synthesized in the lab via cross-coupling reactions mentioned above with conditions optimized as necessary. For all compounds we first obtained fluorescence spectra and picoseconds kinetics and evaluated the quantum yield and lifetime of intrinsic fluorescence, and then obtained the spectra of radiation-generated luminescence that yielded the emission spectra and lifetimes of their recombination exciplexes with DMA. Despite widely different luminescent properties of the molecules themselves, they all readily formed recombination exciplexes with DMA with similar properties, covering the wavelength range of 360-500 nm with maxima at 385 to 435 nm, straddling the position of the emission maximum of the parent DPA/DMA exciplex at about 405 nm, with the expected blue shift for donor and red shift for acceptor substituent. It was also found that for all seven systems the experimentally observed position of exciplex emission maximum linearly correlates with quantum chemically calculated gas phase electron affinity of the substituted diphenylacetylene, which can potentially be used for fast computational prescreening of prospective compounds without their actual synthesis.


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Table 1. Structures of the Compounds for This Work


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RESULTS AND DISCUSSION A. Synthesis of Substituted Diphenylacetylenes Table 2. Scheme of Synthesis of Diphenylacetylenes with -CF3 Substituent Group

Entry

Ar

Time, hr

Conditions

Product

Yield, % a

1

2-CF3C6H4

48

[Cu(acac)2], NaI, K2CO3, TBAB, 140 °C, DMSO

1CF3

43

2

3-CF3C6H4

5

[Pd(PPh3)2Cl2], CuI, PPh3, DABCO, 85 °C, toluene

2CF3

31

3

4-CF3C6H4

1

[Pd(PPh3)2Cl2], CuI, PPh3, NEt3, 70 °C, toluene

3CF3

57

a

Isolated yield after purification; acac – acetylacetonate; TBAB – tetra-N-butylammonium bromide; DMSO – dimethyl sulfoxide; DABCO – 1,4-diazabicyclo[2.2.2]octane.

The target -CF3 substituted DPAs (1CF3, 2CF3, 3CF3) were prepared by Sonogashira crosscoupling reaction75 of the corresponding aryl bromides with phenylacetylene as shown in Table 2. For synthesis of 1CF3 a palladium-free Sonogashira protocol was used because of a lower amount of 1,4-diphenylbutadiyn formed in the reaction (about 3% instead of 10% for palladium catalyzed reaction, see 2CF3, 3CF3). As 1,4-diphenylbutadiyn and 1CF3 have practically identical Rf values on silica gel, it is difficult to purify the target compound if there is a substantial amount of 1,4-diphenylbutadiyn in the mixture, so the procedure with the lowest amount of formed 1,4-diphenylbutadiyn should be chosen. 1,4-Diazabicyclo[2.2.2]octane was used as the base for the synthesis of 2CF3, since the reaction with triethylamine again led to the


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formation of 1,4-diphenylbutadiyn, instead of the target substituted diphenylacetylene. The protocol for 3CF3 is the standard Sonogashira cross-coupling.

Table 3. Scheme of Synthesis of Diphenylacetylenes with -OMe Substituent Group

Entry

Ar

Time, hr

Product

Yield, % a

1

2-OMeC6H4

6

1OMe

62

2

3-OMeC6H4

7

2OMe

48

3

4-OMeC6H4

7

3OMe

40

a

Isolated yield after purification.

In the case of donor substituted iodoarenes the Pd/Cu-catalyzed cross-coupling Sonogashira protocol may be unsuitable because of the possibility of reductive deiodination, accompanied by homocoupling of the terminal acetylene.120 So the -OMe substituted DPAs (1OMe, 2OMe, 3OMe) were prepared by standard Castro–Stephens cross-coupling reaction76 of the corresponding iodoanisoles with copper (I) phenylacetylide as shown in Table 3. This protocol allows to avoid undesirable formation of the homocoupling product (1,4-diphenylbutadiyn) and leads to the target diphenylacetylenes with reasonable yields.


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B. Luminescent Properties of Diphenylacetylenes with -CF3 or -OMe Substituent Group B

A 3OMe

250

300

350

400

3OMe

2OMe

2OMe

1OMe

1OMe

3CF3

3CF3

2CF3

2CF3

1CF3

1CF3

DPA

DPA

450

500

250

300

350

400

450

500

Wavelength, nm

Wavelength, nm

Figure 1. Normalized absorption (the left curve of each pair) and luminescence spectra (the right curve) for all DPAs under study in CHX (A) and ACN (B).

Figure 1 shows an overview of the spectra of optical absorption and luminescence of all compounds of this work and the parent unsubstituted diphenylacetylene in cyclohexane (CHX, Figure 1A) and acetonitrile (ACN, Figure 1B). The spectra in more detail can be found in the ESI. The spectra of all the compounds are generally similar. The red shift, as well as the loss of vibrational structure in ACN, originate from the solvent relaxation of the S1 excited state of the solute molecule by polar solvent.121 Absorption spectra for 1OMe and 2OMe show vibrational structure that somewhat differs from the structure of unsubstituted diphenylacetylene. Absorption spectra of all other substituted diphenylacetylenes are identical to the spectrum of DPA, except for a slight spectrum shift. According to quantum chemical calculations,40 DPA absorption is due to transition → , where is the first excited electronic state (ππ*) of the diphenylacetylene molecule in linear D2h symmetry. The similarity of the presented spectra demonstrates that the introduction of a simple electron donating or withdrawing substituent in


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any position of one of the phenyl rings generally preserves the nature and relative position of the HOMO/LUMO states accounting for the transition. These are also the states that determine exciplex formation. Spectra of luminescence of the substituted diphenylacetylenes in CHX show mirror symmetry with the corresponding absorption spectra and are similar in shape to the spectrum of DPA. As mentioned in the Introduction, the nature of DPA luminescence and its photophysical properties have been studied in detail and reported in refs.35-36,38,40,43-44 The main features of DPA luminescence are very low fluorescence quantum yield (0.00336 in 3-methylpentane at 25 °C),44 rising with decreasing temperature56 (up to 0.5 at 77 K in diethyl ether/2-methylbutane/ethanol glass),44 as well as very short fluorescence lifetime (8 ps in n-hexane at 25°C).38 According to picosecond transient absorption measurements37-38 and quantum chemical calculations,40,43-44 the lowest-energy excited state in the linear D2h symmetry of the ground-state molecule is 1B1u (ππ*) state, whereas in bent C2h symmetry there is a lower-lying practically nonfluorescent excited 1Au (πσ*) state. This gives rise to a crossing of the potential energy curves for the fluorescent ππ* state and the dark πσ* state upon changes in the ≡ − angle in the excited diphenylacetylene molecule.40,43 As a result, DPA molecule emits almost exclusively and very briefly from the short-lived ππ* state, which explains the very low quantum yield and short lifetime of its fluorescence. Furthermore, the presence of a small energy barrier for transition from the ππ* into the πσ* state (evaluated as 0.4 eV)43 explains the thermally activated quenching of fluorescence in solution.43-44 The lifetime of the dark πσ* state for DPA is about 200 ps in n-hexane according to picosecond transient absorption measurements.38 The works37,52,122 report that introduction of acceptor groups, such as -CN or -COOCH3, in the paraposition of one of the phenyl rings in DPA significantly raises the energy of the πσ* state and


13


moves the ππ*/πσ* intersection farther away from the region of freely available ≡ − angles. This should increase both the quantum yield and the lifetime of acceptor-substituted DPA fluorescence. On the contrary, para-substitution with donor groups, such as -OMe or -NH2, has been reported52,122 to enhance the state switch from the initially excited ππ* state to the πσ* state, and should further quench the local fluorescence of the para-substituted diphenylacetylene.

1.2

Normalized Intensity

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

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1 3

1.0

304 nm 314 nm

4

0.8 2

0.6 0.4 0.2 0.0 250

5 6

300

350

400

450

500

Wavelength, nm

Figure 2. Normalized absorption (1 – in ACN), luminescence excitation (in ACN; 2 – λem = 315 nm; 3 – λem = 350 nm; 4 – λem = 450 nm) and luminescence spectra (λexc = 270 nm; 5 – in ACN; 6 – in CHX) of 7·10-6 M solution of 2CF3. Absorption and luminescence excitation spectra are normalized to the intensity of the first vibronic band. Luminescence spectra are normalized to the maximum.

The luminescence spectra for the compounds of this work can be divided into three groups: DPA itself, 2CF3 and 3OMe (group 1, showing double-band short-lived emission as the parent DPA); 1CF3 and 3CF3 (group 2, showing DPA-like short-lived emission with second band appearing only in the more polar solvent); 1OMe and 2OMe (group 3, showing normal emission


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without the peculiarities of the parent DPA). They will now be discussed using one compound of each group as an example. The spectra for all compounds of this work may be found in the ESI (Figures SI2.1-SI2.7). Figure 2 shows in more detail the spectra of luminescence for 2CF3 in CHX and АСN, as well as absorption and excitation spectra for this compound in ACN. As Figures 2, SI2.6 and SI2.7 demonstrate, the spectra of luminescence for DPA, 2CF3 and 3OMe in CHX and ACN comprise two emission bands. Excitation spectra for the observed emission at any wavelength are identical and coincide with the corresponding absorption spectra, which is the case for all molecules of this study. Similar to the unsubstituted DPA (vide supra), the short wavelength emission of 2CF3 and 3OMe can be attributed to fluorescence from the short-lived ππ* state. In polar ACN the short wavelength emission bands for DPA, 2CF3 and 3OMe lose their vibrational structure and shift bathochromically. The weak long wavelength emission in the spectra of luminescence for DPA in methylcyclohexane and other non-polar solvents at room temperature was described earlier36 and attributed to emission from the dark πσ* state. In contrast to the initially excited ππ* state, which is fluorescent, the πσ* state formed by the state switching is expected to be very weakly emissive or non-emissive due to several reasons.52 First, the radiative transition between the bent πσ* state and the ground state is only weakly allowed. Second, the radiationless decay of the πσ* state to the ground state is predicted to be highly efficient. Third, the intersystem crossing to the ππ* triplet state is expected to be highly favored. As Figure SI2.7 shows, the spectrum of luminescence for DPA in a more polar ACN also has a long wavelength emission band which is identical to the band in CHX and most likely belongs to emission from the dark πσ* state.


15


Similar to unsubstituted DPA, the observed long wavelength emission for 2CF3 and 3OMe both in CHX and in ACN can also be attributed to emission from the πσ* state. Both molecules were found to have very low quantum yields and very short lifetimes of fluorescence (see Table 4 below), which indicates efficient state switching from the fluorescent ππ* state to the dark πσ* state. It can also be noted that the long wavelength emission in the polar ACN becomes more pronounced in comparison with CHX (cf. Figure 1). For 3OMe the contribution of the long wavelength emission in ACN dominates the integral luminescence intensity, which may be related to an enhancement in the state switch from the initially excited ππ* state to the πσ* state in ACN.

1.2 2


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

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1.0

4

307 nm 325 nm

3

0.8 1

0.6 0.4

5

0.2 6

0.0 250

300

350

400

450

500

Wavelength, nm

Figure 3. Absorption (1 – in ACN), luminescence excitation (in ACN; 2 – λem = 305 nm; 3 – λem = 350 nm; 4 – λem = 470 nm) and luminescence spectra (λexc = 270 nm; 5 – in ACN; 6 – in CHX) of 6·10-6 M solution of 3CF3. Absorption and luminescence excitation spectra are normalized to the intensity of the first vibronic band. Luminescence spectra are normalized to the maximum.


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Turning to group 2, Figure 3 shows that the spectrum of luminescence of 3CF3 in CHX does not comprise the long wavelength emission from the πσ* state, while a very low quantum yield and a very short lifetime of fluorescence (see Table 4 below) indicate the ππ*/πσ* intersection. However, the spectrum of luminescence of 3CF3 in ACN, in addition to the structureless and bathochromically shifted (as compared to CHX) short wavelength emission similar to the short wavelength emission for DPA, 2CF3 and 3OMe, contains a pronounced long wavelength emission band. A similar weak long wavelength tail of the dominant locally excited state emission in polar solvent was previously obtained for other mono- and disubstituted diphenylacetylenes and interpreted as emission from an intramolecular charge transfer state (ICT).55,123-124 In was further noted in ref.52 that in several disubstituted diphenylacetylenes the bent πσ* state is a precursor of the planar ICT state. The same long wavelength tail was also obtained in the spectrum of luminescence of the solution of 1CF3 in ACN (Figure SI2.1). This substituted diphenylacetylene has a higher quantum yield in comparison with 2CF3 and 3CF3 both in ACN and in CHX (see Table 4). An increased quantum yield of fluorescence indicates that state switching from ππ* to πσ* for 1CF3 is less efficient. The spectrum of luminescence of 1CF3 in CHX is a vibrationally-resolved emission from the ππ* state similar to unsubstituted diphenylacetylene.


17


1.2


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

323 nm 338 nm

1.0 0.8 3

0.6

Page 18 of 81

2 4 5

0.4 0.2 0.0 250

6

300

350

400

450

500

Wavelength, nm

Figure 4. Absorption (1 – in ACN), luminescence excitation (in ACN; 2 – λem = 315 nm; 3 – λem = 380 nm; 4 – λem = 450 nm) and luminescence spectra (λexc = 270 nm; 5 – in ACN; 6 – in CHX) of 6.5·10-6 M solution of 1OMe. Absorption spectrum is normalized to the intensity of the luminescence excitation spectrum at 295 nm. Luminescence excitation spectra are normalized at 280 nm. Luminescence spectra are normalized to the maximum.

Figure 4 shows the spectra for 1OMe, a member of group 3 that demonstrates substantially higher quantum yields of fluorescence (see Table 4), which, similar to 1CF3, indicates a less efficient ππ* → πσ* state switch in these molecules. The spectra of luminescence in CHX are a vibrationally-resolved emission from the ππ* state, while in more polar ACN the emission spectra lose their vibrational structure and shift bathochromically, similar to the short wavelength emission of the other studied diphenylacetylenes. No signs of the long wavelength emission band can be found in the spectra. Using the spectra of luminescence shown in Figure 1, quantum yields of fluorescence were determined for all molecules of this study in CHX and ACN. The results are given in Table 4 below, refractive-index correction125 was applied. As Table 4 demonstrates, the obtained


18

Page 19 of 81

quantum yields of fluorescence vary with the nature and position of the substituent. Diphenylacetylenes of group 1, DPA, 2CF3 and 3OMe, whose spectra of luminescence contain the characteristic long wavelength emission from the dark πσ* state, have rather low quantum yields in the range 0.004 to 0.009 in CHX. For these molecules the yield was determined only for the short wavelength emission band (the spectrum of luminescence was integrated up to 410 nm). A low quantum yield of 0.015 in CHX was also obtained for 3CF3, which, as mentioned earlier, is likely connected with an efficient switching from the linear ππ* state to the bent πσ* state. The remaining diphenylacetylenes 1CF3, 1OMe and 2OMe, for which the ππ* → πσ* state switch is believed to be less efficient, have much higher quantum yields of fluorescence, up to 0.17 and 0.44 in CHX for 1CF3 and 1OMe, respectively.

4

10

2 3 1

3

Counts, 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


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2

10

1

10

0

2

4

6

8

10

12

14

16

18

Time, ns

Figure 5. (1) Luminescence decay profile of 7·10-6 М solution of 1OMe in ACN, excitation at 270 nm and detection at 330 nm. (2) Instrument response function of the excitation source. (3) Fitting of the luminescence decay profile by an exponential function with characteristic time 0.96 ns.


19


Page 20 of 81

For all diphenylacetylenes of this work picoseconds kinetics of luminescence in ACN were taken, however, due to very short emission lifetimes reliable results could be obtained only for 1OMe and 2OMe. Figure 5 shows the kinetics for 1OMe, from which fluorescence lifetime was evaluated as 0.96 ns. Similar kinetics for 2OMe are given in the ESI (Figure SI2.11); their evaluation produced fluorescence lifetime 0.5 ns. For all other compounds of this work the emission lifetime was too short for the experimental kinetics to be limited by the instrument response function, and only upper bounds could be estimated in the range 200 to 300 ps, as listed in Table 4. The estimated time resolution with the used LED is 200 ps and is limited by the Full Width at Half-Maximum (FWHM) of the instrument response function (IRF) of the LED of 800 ps and the possibility to extract lifetimes down to 25% FWHM of IRF from the standard deconvolution analysis of experimental kinetics using experimental IRF and an exponential function. A typical kinetics of luminescence for a diphenylacetylene with short excited state lifetime, in this case 3CF3, is also given in the ESI as Figure SI2.12. Similar to DPA discussed above, the obtained lifetimes most likely belong to the excited πσ* state. No signs of the shortlived ππ* states were found in the kinetics of luminescence for the five poorly luminescing diphenylacetylenes due to insufficient time resolution. Thus, most of the substituted diphenylacetylenes of this work have luminescence properties similar to the parent DPA, demonstrating very short lifetimes of excited states and low quantum yields of fluorescence. Similar to DPA, the unusual photophysics of these compounds must probably be attributed to crossing of the potential energy curves for the fluorescent ππ* state and the dark πσ* state upon changes in the ≡ − angle in the diphenylacetylene frame. Looking slightly ahead, for the purposes of generating tunable emission via recombination exciplexes such compounds turned out to be more convenient: as shown below, in steady-state experiments


20

Page 21 of 81 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


in solution under X-irradiation such substituted diphenylacetylenes yield practically no intrinsic luminescence, while producing readily emitting exciplexes. The emission spectrum is dominated by the exciplex band, with a minor contribution from the intrinsic luminescence of the second partner, DMA, and has no contamination from the diphenylacetylene itself. However, in certain cases, e.g., for 1OMe and 2OMe, introduction of the substituent drastically increases the lifetime and quantum yield of the intrinsic fluorescence, which must reflect significant changes in the electronic structure of the molecule and the resulting inefficient ππ*/πσ* intersection. In order to provide a quantitatively accurate quantum mechanical support to the interpretation of the experimental observations and to explore the electronic structure of these systems, a rather conspicuous amount of advanced calculations would be required; this is an interesting computational task beyond the scope of this work. C. X-Ray Generated Recombination Exciplexes of Substituted DPAs with DMA B

A

250

300

350

400

450

500

3OMe

3OMe

2OMe

2OMe

1OMe

1OMe

3CF3

3CF3

2CF3

2CF3

1CF3

1CF3

DPA

DPA

550

600

250

300

350

400

450

500

550

600

Wavelength, nm

Wavelength, nm

Figure 6. A: Normalized X-ray generated luminescence spectra for the mixtures of 6·10-3 M of the corresponding diphenylacetylene and 1·10-2 M DMA in DDC. B: Normalized emission spectra of the corresponding diphenylacetylene – DMA exciplex obtained from emission spectra of the same sample solution of 6·10-3 M of the corresponding diphenylacetylene and 1·10-2 M


21


Page 22 of 81

DMA in DDC recorded in the presence and absence of dissolved oxygen (see text and ref.126 for details).

Introduction of the methoxy or trifluoromethyl substituent in the diphenylacetylene molecule significantly changes its intrinsic luminescence and photophysical properties, but does not prevent the formation of recombination exciplexes with DMA under X-irradiation. Exciplexes for these compounds under X-irradiation can be formed only via recombination of the radical ion pair, since all diphenylacetylenes of this work have short lifetimes of excited states, both ππ* and πσ* (maximum 0.96 ns for 1OMe). The short lifetimes also make these exciplexes optically inaccessible in solution. Figure 6A shows an overview of the spectra of radiation-generated luminescence of the mixtures of the seven compounds with DMA in n-dodecane (DDC). In all cases the spectrum can be seen to comprise two emission bands. As shown earlier for mixtures of DPA or 3CF3 with DMA,57,127 the longer wavelength luminescence in such systems belongs to exciplex emission. The exciplex emission band dominates or has comparable intensity to the shorter wavelength emission band for all studied diphenylacetylenes except for 1OMe and 1CF3 that have the highest intrinsic fluorescence quantum yields (see Table 4). Figure 6B shows the extracted exciplex emission spectra, obtained by taking pairs of luminescence spectra from the same sample with removed dissolved oxygen and equilibrated with atmosphere, normalizing them by the maximum of the short wavelength band, and subtracting from each other. This procedure takes advantage of the much longer emission lifetime, and thus more efficient quenching, of exciplexes in comparison with intrinsic emission producing the short wavelength band, and was


22

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suggested by reviewer to work.126 More details on it will be given below when discussing exciplex lifetime measurements. Figure 6 demonstrates how the wavelength of the exciplex emission band maximum depends on the nature and position of the substituent introduced into the diphenylacetylene core. Introduction of the electron-accepting trifluoromethyl group (1CF3, 2CF3, 3CF3) shifts the exciplex emission band to longer wavelength relative to the parent DPA by 15-30 nm depending on the position of the -CF3 group. This shift of exciplex emission results in an almost complete separation of the two bands in the spectrum of luminescence. This made the mixture of 3CF3 with DMA a very convenient probe to estimate the fraction of spin-correlated radical ion pairs,127 as the exciplex emission band is generated exclusively via the pair recombination and is free from background contributions that abound under X-irradiation. Introduction of the electrondonating methoxy group into DPA in the case of 3OMe produces a blue shift of about 15 nm, while for other positions of the substituent (1OMe and 2OMe) the exciplex emission band maximum remains practically at the same wavelength as for the parent DPA. Earlier the effect of the nature of the substituent (donor or acceptor) on the position of the exciplex emission band was demonstrated on several examples for optically generated exciplexes in the pair of substituted naphthalene – triethylamine.128 The shorter wavelength emission band with maximum in the range 335-350 nm depending on the specific diphenylacetylene is due to either intrinsic luminescence of DMA (DPA, 2CF3, 3CF3, 3OMe), which is a satisfactory luminophore itself (fluorescence quantum yield 0.11, τF = 2.4 ns),69 intrinsic luminescence of the corresponding substituted diphenylacetylene (1CF3, 1OMe), or intrinsic emission of both DMA and substituted diphenylacetylene (2OMe). The excited states of diphenylacetylenes and DMA are generated primarily by direct energy transfer


23


Page 24 of 81

from excited solvent molecules forming in abundance under X-irradiation in n-alkanes127 (for ndodecane fluorescence quantum yield is 0.0055,129 τF = 4.2 ns).130 If desired, this emission channel can be completely suppressed under X-irradiation by taking an alkane solvent with zero quantum yield of fluorescence, e.g., isooctane (2,2,4-trimethylpentane),131 and it will be eliminated if radical ions recombining into exciplexes are generated directly by charge injection. Lifetimes of recombination exciplexes for all diphenylacetylenes of this work with DMA in ndodecane were estimated using a universal CW-based approach suggested earlier for radiationgenerated exciplexes.126 Thus obtained lifetimes are given in Table 4 and amount to about 60 ns for all three acceptor-substituted diphenylacetylenes 1CF3, 2CF3, 3CF3, about 45 ns for the parent DPA, 1OMe and 2OMe, and about 35 ns for 3OMe. Lifetimes and positions of the exciplex emission band maxima were found to qualitatively correlate with relative luminescence efficiencies of exciplexes in the corresponding systems evaluated as described below. The exciplex luminescence efficiencies are given in Table 4 and are about 120 rel. u. for all trifluoromethyl-substituted diphenylacetylenes 1CF3, 2CF3, 3CF3, about 100 rel. u. for DPA, 1OMe and 2OMe, and 60 rel. u. for 3OMe. Thus, exciplexes formed in the mixtures of trifluoromethyl-substituted diphenylacetylenes with DMA have longer lifetimes, higher luminescence efficiencies and red-shifted exciplex emission in comparison to the parent DPA – DMA system. The 1OMe/DMA and 2OMe/DMA exciplexes have practically the same properties as the DPA/DMA exciplex, whereas the para-substituted 3OMe molecule gives rise to recombination exciplexes with DMA that have shorter lifetime, lower luminescence efficiency and blue-shifted emission band relative to the DPA – DMA system.


24

15 2

Luminescence 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



Page 25 of 81

A

12 1

9 6 3 0 250

300

350

400

450

500

550

600

50 4

B

40 30 1

20

2

10 0 250

3

300

350

400

450

500

550

600

Wavelength, nm

Wavelength, nm

Figure 7. А: X-ray generated luminescence spectrum of the mixture of 6·10-3 M 2CF3 and 1·102

M DMA in DDC (1). Normalized to the long wavelength tail of the spectrum (1) emission

spectrum of 2CF3 – DMA exciplex (2). The absolute area under exciplex emission band is 120 ± 12 rel. u. B: Decomposition of the X-ray generated luminescence spectrum for 1OMe – DMA mixture at concentrations 6·10-3 M and 1·10-2 M, respectively, in DDC (points, 1) into two (1OMe, exciplex – (2), (3), respectively) constituent emission components indicated by dashed lines. The obtained reconstruction of the spectrum shape of the mixture as the sum of two components is shown by solid line (4). Only two numerical parameters, the amplitudes of the emission bands for exciplex and 1OMe were varied. Decomposition was done in wavenumber representation ~ ∙ .132 The absolute area under exciplex emission band is 110 ± 11 rel. u.

The exciplex luminescence efficiencies quoted above were calculated as the absolute areas under the exciplex emission band of the corresponding spectra of radiation-generated luminescence taken in identical experimental conditions for samples of nominally equal concentrations. This is similar to calculating quantum yields in optically generated luminescence.


25


Page 26 of 81

However, in this case there is no reference for the absorbed excitation radiation, which allows to calculate absolute quantum yields (per single excited molecule) in photoluminescence. Although the absorbed X-ray doses can be determined, they have only very indirect connection to the amount of eventually forming electronic excitations due to the complex cascade of processes in radiation chemistry, and thus are of limited usefulness. Furthermore, when a direct comparison of photoluminescence and radiation-generated luminescence for a series of similar systems is possible, the ordering of fluorescence quantum yields/luminescence efficiencies in the series is not necessarily the same for optical and radiation generation. This was noted and discussed84 for three oxyquinolinates, Alq3, Inq3, Gaq3, and was attributed to the additional stages of charge capture, transport and recombination under X-irradiation. It should be stressed that the ordering of radiation-induced emission efficiency observed for the three Meq3 coincided with the ordering of their electroluminescence efficiencies, which reflects the similarity between electrogenerated and radiation-induced luminescence.84 Regarding radiation-generated exciplex emission, for luminophores that produce both substantial intrinsic emission and exciplex emission, e.g., anthracene in mixtures with DMA, it is possible to use the intrinsic emission as an internal reference for the number of excited states and normalize exciplex emission to it,57,70,131 but it is only useful for comparing the effects of different excitation sources (optical vs. radiation) on the same system, and not to compare different systems to each other. The ‘area under the emission band’ metric for radiation-induced luminescence efficiency was recently introduced and verified for solid samples producing both photo- and radiation-induced luminescence,133 and has been adopted here as well. Technically, for all diphenylacetylenes, except for 1OMe and 3OMe, as the exciplex luminescence efficiency was taken the absolute area under exciplex emission band normalized to the long wavelength tail of the experimental


26

Page 27 of 81

luminescence spectrum of the corresponding mixture. For the mixtures of 1OMe and 3OMe with DMA, where the intrinsic and exciplex luminescence bands substantially overlap, as the exciplex luminescence efficiency was taken the absolute area under exciplex emission band obtained from decomposition

of

the

luminescence

spectrum

into

the

corresponding

substituted

diphenylacetylene, DMA and exciplex constituent emission components (details see ESI section 3.8). Figure 7 shows examples of a normalized 2CF3 – DMA exciplex emission spectrum used to directly determine the exciplex luminescence efficiency, and a decomposition of the radiationgenerated luminescence spectrum for 1OMe – DMA system. Similar figures for other diphenylacetylenes under study are shown in the ESI (Figures SI3.29-SI3.35).

21

1.0

3

18

A

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


15 12 9 4

6 2

3 0 250

300

350

400

450

500

550

600

B

0.8 0.6 0.4 0.2 0.0 250

300

350

400

450

500

550

600

Wavelength, nm

Wavelength, nm

Figure 8. А: X-ray generated luminescence spectra of degassed (1) and equilibrated with normal atmosphere (2) mixtures of 6·10-3 M 1CF3 and 1·10-2 M DMA in DDC. The integral quenching of the exciplex emission band I0/I − 1 in the indicated wavelength range is 3.6 ± 0.2. X-ray generated luminescence spectra of degassed solution of 6·10-3 M 1CF3 in DDC (3) and 1·10-2 M DMA in DDC (4) are also shown for comparison. B: Emission spectrum of 1CF3 – DMA exciplex illustrating the procedure uses to obtain the exciplex emission spectra of Figure 6B.


27


Page 28 of 81

Recombination exciplex lifetime discussed above was estimated by the relative quenching of exciplex emission by dissolved oxygen in comparison to quenching of a standard luminophore with close excited state lifetime (naphthalene, τF = 96 ns)69 in the same experimental conditions. An example of the typical spectra of radiation-generated luminescence for a degassed sample and the same sample equilibrated with atmosphere for a mixture of 1CF3 and DMA is shown in Figure 8A. The figure also shows the spectra of radiation-generated luminescence for solutions of individual components, 1CF3 and DMA, in n-dodecane for comparison. Figure 8B shows the emission spectrum for the exciplex of 1CF3 and DMA in n-dodecane, obtained by subtracting the luminescence spectrum for the equilibrated with atmosphere sample from the spectrum for the same degassed sample after normalization by the maximum of the short wavelength emission band (spectra (1) and (2) from Figure 8A), the procedure that was used to obtain the spectra of Figure 6B above. Similar figures for all compounds of this work are given in the ESI (Figures SI3.3-SI3.9). Figure SI3.11 of the ESI compares the normalized emission spectra of the exciplexes from the substituted diphenylacetylenes of this work with DMA with the spectrum for the parent DPA. Figures SI3.12-SI3.18 of the ESI additionally show the exciplex emission spectra in three standard wavenumber representations: plain wavelength , only with transformation ~ 1/; wavenumber representation ~ ∙ 132 that preserves area under the spectrum; and the more advanced transition dipole moment representation

~ ⁄ .134 Integral quenching was evaluated by integrating the long wavelength side of the total emission band corresponding to the exciplex emission without contribution from the intrinsic luminophore emission, as indicated in Figures 8A and SI3.3-SI3.9, and taking the ratio of the two integrals.126


28

Page 29 of 81

For most spectra with well separated exciplex and intrinsic emission bands this was done directly on experimental spectra, like spectra (1) and (2) from Figure 8A. For the mixtures of 1OMe or 3OMe with DMA, where the two emission bands substantially overlap (Figures SI3.6 and SI3.8 of the ESI), the spectra were decomposed into the sum of spectra of the individual emission components using the approach of refs.70,131 The relevant details are given in ESI section 3.8. An example of decomposed luminescence spectrum for 1OMe – DMA mixture into 1OMe and exciplex constituent emission components is shown in Figure 7B.

14 2; x 0.5

12

A

10 8 6 4 2

1

0 300

350

400 450 500 550 600 650 700



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


14 12

B 2

10 8 6 4

1

2 0 300

350

Wavelength, nm

400

450

500

550

600

650

700

Wavelength, nm

Figure 9. А: X-ray generated luminescence spectrum of the mixture of 6·10-3 M DPA and 1·102

M TMPD in DDC (1). X-ray generated luminescence spectrum of the solution of 1·10-2 M

TMPD in DDC multiplied by 0.5 (2) is shown for comparison. B: X-ray generated luminescence spectrum of the mixture of 6·10-3 M DPA and 1·10-2 M m-MTDATA in 1-chlorobutane (1). Xray generated luminescence spectrum of the solution of 1·10-2 M m-MTDATA in 1-chlorobutane (2) is shown for comparison.


29


Page 30 of 81

As was mentioned in the Introduction, other tertiary amines can also be readily used instead of DMA as the electron donor for generation of recombination exciplexes with DPAs. Figure 9 shows the spectra of radiation-generated luminescence of the mixtures of DPA with two widely used aromatic tertiary amines, TMPD and m-MTDATA. The spectra of radiation-generated luminescence of pure TMPD and m-MTDATA are also shown for comparison. TMPD is typically employed as a convenient molecule for radical cation generation in ECL experiments,135-137 while m-MTDATA is used as a hole-transporting layer and a hole injection layer material in OLEDs.138-142 It should be noted that m-MTDATA is poorly soluble in ndodecane, so more polar 1-chlorobutane (ε = 7.28 at 293 K)143 was used as a solvent in which recombination luminescence can still be detected. As can be seen from Figure 9, both TMPD and m-MTDATA form recombination exciplexes with DPA. Emission bands of DPA – TMPD and DPA – m-MTDATA exciplexes are substantially shifted bathochromically as compared to the DPA – DMA exciplex (see Fig. 6A). In case of DPA – TMPD mixture intrinsic TMPD luminescence predominates exciplex emission, which can be due to both a higher fluorescence quantum yield of TMPD (quantum yield 0.18, τF = 4.3 ns),69 as compared to DMA, and a lower efficiency of exciplex generation in comparison to the DPA – DMA system. The intensity of exciplex emission in DPA – m-MTDATA system is comparable with intrinsic luminescence of m-MTDATA, but still about three times lower than that for DPA – DMA in n-dodecane. This fact may be related to a decrease in the efficiency of exciplex generation, but it is more likely that the observed low intensity of exciplex emission is due to the use of a more polar 1-chlorobutane in which the efficiency of recombination of the secondary radical ion pairs may be substantially lower than in non-polar n-dodecane (ε = 2.01 at 293 K).143 This is supported by a


30

Page 31 of 81 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


significant, three-fold, decrease in the intensity of the radiation-generated luminescence in the DPA – DMA system in 1-chlorobutane as compared to n-dodecane (Figure SI3.10 of the ESI). Thus, introduction of a single methoxy of trifluoromethyl substituent in the diphenylacetylene molecule does not prevent formation of recombination exciplexes with DMA that demonstrate prominent

emission

irrespective

of

the

luminescent

properties

of

the

substituted

diphenylacetylene, and results in a shift of the emission band and changes in the emission lifetime that correlate with exciplex luminescence efficiency. The effects of the electronaccepting and electron-donating substituent are opposite. Trifluoromethyl group, for any position in the phenyl ring of the DPA (ortho-, meta- , para-), lengthens the exciplex lifetime, increases exciplex luminescence efficiency and shifts its emission band to the red relative to the DPA/DMA exciplex. Methoxy-substituted diphenylacetylenes, depending on the substituent position, either form recombination exciplexes with properties practically identical to the DPA/DMA exciplex (ortho- and meta- substituted DPA, 1OMe, 2OMe), or yield exciplexes with a shorter lifetime, lower exciplex luminescence efficiency and blue-shifted emission in comparison to the DPA/DMA exciplex (para- substituted DPA, 3OMe). Radiation-generated recombination exciplexes can also be formed in the mixtures of diphenylacetylenes with other tertiary amines in nonpolar or weakly polar solvents.

D. Quantum Chemical Calculation of Electron Affinities of DPAs under Study As noted in the preceding section, the position of exciplex emission band maximum varies with the type and position of the substituent in the diphenylacetylene core. It would have being very convenient to be able to predict, or at least reproduce, this correlation in calculations. However, sufficiently accurate quantum chemical calculations to describe exciplex formation


31


Page 32 of 81

and the transition energy of exciplex emission as a function of the type and position of substituents are rather tricky and computationally expensive; this task is beyond the scope of this work. Calculation of the properties of individual molecules requires a more reasonable effort, and a much simpler correlation between the observed position of exciplex emission maximum and single molecule properties that can be calculated, at least within the set of diphenylacetylenes of this work, was found and will be now discussed. Exciplex by its nature is an excited complex with partial charge transfer,64,144 with the systems of this work DMA functioning as electron donor and diphenylacetylene being electron acceptor. In the exciplex, diphenylacetylene therefore carries a partial negative charge. It is reasonable to expect the position of the exciplex emission band to depend on the electron affinity of the specific substituted diphenylacetylene. The higher (more positive) the electron affinity of the substituted diphenylacetylene molecule, the lower is the energy of the photon released upon luminescence, ℏ

!

. This has been well established and verified experimentally in the field of

organic electroluminescence (see, e.g., review),145 and can be formally expressed by the simple relation: ℏ

!

~ "# − $%& − $ ,

where "# , $%& , $ = ) -4+ , indicate the ionization potential of donor (e.g., DMA), electron affinity of acceptor (e.g., DPA), and Coulomb stabilization energy with some effective distance r, respectively. If in a series of systems the donor remains unchanged (DMA), and the acceptors of the pairs are not widely different so we may expect that the Coulomb term does not vary much, the energy of the emitted photon (that we shall assume to coincide with the position of the maximum of the emission band) may linearly correlate with electron affinity of the acceptor, EAA. To test this hypothesis we performed quantum chemical calculations of electron


32

Page 33 of 81 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


affinity for all the diphenylacetylenes investigated in this work. The calculations were performed at the Density Functional Theory (DFT) level, using the long-range corrected hybrid functional CAM-B3LYP146 coupled with the double-ζ 6-31+G(d,p) basis set, as described in more detail in the Experimental section. The method was chosen on the basis of preliminary benchmarking investigations. Molecules 1OMe and 2OMe show several stable rotamers with slight differences in energy; for these cases the most stable rotamer was considered. The energies and geometries of the rotamers are shown in part 4.2 of the ESI. The calculated gas phase electron affinities are given in Table 4.

Table 4. Intrinsic Fluorescence Quantum Yields (λEX = 270 nm) in Acetonitrile (ФACN) and Cyclohexane (ФCHX), Fluorescence Lifetime in Acetonitrile (τF, ACN), and Calculated Electron Affinities (EA) of Diphenylacetylenes under Study, and Maximum of X-Ray Generated Exciplex Emission (λMAX), Exciplex Luminescence Efficiency (εEXC), and Position of the Exciplex Lifetime (τEXC) for Their Exciplexes with DMA

Compound

./01 ∙ 234

.056 ∙ 234

78, /01 , ps

7960 , ns b

:960 , rel. u.

;

X-ray Generated Recombination Exciplexes of Substituted

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