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Phosphorescence in Bromobenzaldehyde Can Be Enhanced Through Intramolecular Heavy Atom Effect Sunandan Sarkar, Heidi Phillips Hendrickson, Dongwook Lee, Francis Devine, Jaehun Jung, Eitan Geva, Jinsang Kim, and Barry D. Dunietz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12027 • Publication Date (Web): 22 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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The Journal of Physical Chemistry

Phosphorescence in Bromobenzaldehyde can be Enhanced through Intramolecular Heavy Atom Effect Sunandan Sarkar,†,§ Heidi P. Hendrickson,‡,§ Dongwook Lee,¶ Francis DeVine,‡ Jaehun Jung,¶ Eitan Geva,‡ Jinsang Kim,∗,¶ and Barry D. Dunietz∗,† Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA, Department of Chemistry, University of Michigan, Ann Arbor, MI, and Materials Science and Engineering, University of Michigan, Ann Arbor, MI Received January 13, 2017; E-mail: [email protected]; [email protected]

Abstract: A synthetic route to achieve high phosphorescence quantum yield in a purely organic material was achieved by doping a crystal containing heavy bromine atoms with a molecule that contains a triplet producing aromatic carbonyl group. The enhanced phosphorescence originated from intermolecular nonbonding interactions between the bromine and the carbonyl oxygen. In this study we employ a computational approach to design molecules containing both structural motifs which exhibit enhanced phosphorescence through intramolecular nonbonding interactions between bromine and carbonyl groups.

the triplet producing site (C=O group) increases SOC due to the heavy atom effect, which enhances intersystem crossing (ISC) between the singlet and triplet excited states through intramolecular interactions. Specifically, we provide a combined computational and experimental analysis of a series of molecules 2-, 3-, and 4-bromobenzaldehydes (ortho-Br, meta-Br, and para-Br, respectively) for which experimental spectra are available. Our computational protocol is first tested against experimental spectra and is then used to establish design principles for achieving optimal phosphorescence in purely organic materials.

INTRODUCTION High phosphorescence quantum yields at ambient conditions was recently reported in metal-free purely organic materials. 1 Such materials can serve as alternatives to organometallic materials used in light emitting diode (OLED) applications. 1–12 Bolton et al. provided evidence that interactions between bromine and aromatic carbonyl groups contribute significantly to the phosphorescence. In particular, crystals of mixed 2,5-dihexyloxy4-bromobenzaldehyde (Br6A) and 2,5-dihexyloxy-1,4dibromobenzene (Br6) were shown to function as bright metal-free organic phosphors. 1 The enhanced phosphorescence in these organic materials is achieved by coupling an aromatic carbonyl group to a heavy bromine atom via intermolecular halogen bonding. The triplet generation by the aromatic carbonyl is enhanced through intermolecular halogen bonding (Br· · · O), which increases the spin orbit coupling (SOC) due to the heavy atom effect. In this report, we identify a family of molecules that possess these structural design characteristics and thereby facilitate enhanced phosphorescence via intramolecular nonbonding interactions. Importantly, this is achieved in a metal free molecule through the introduction of a bromine atom that interacts with an adjacent carbonyl site. This is different than the previous report 1 where the bromine interactions are intermolecular and are present only in the crystal. We show that introducing the bromine atom close to † Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA ‡

Department of Chemistry, University of Michigan, Ann Arbor,

MI ¶

Materials Science and Engineering, University of Michigan, Ann Arbor, MI §

Contributed equally in this work

Figure 1. Schematic of key singlet and triplet states involved in the excitation and relaxation processes in benzaldehyde molecules. Absorption (black solid arrows) can occur from the ground state (S0 ) to the excited singlet states (S1−3 ). Nonradiative relaxation pathways are shown by gray dashed arrows. Once the system relaxes to the lowest excited singlet state, intersystem crossing (kisc ) to an excited triplet state can occur. The triplet excited state then relaxes to the ground state via phosphorescence (kp ). The phosphorescence in benzaldehyde occurs from the 3 (nπ ∗ ) state. The phosphorescence of 13 (ππ ∗ ) state is found to be negligible. (Phosphorescence strength is indicated by the size of the solid arrows designating emission).

METHODS Computational Details The triplet based emission involves two key steps: (i) Spinforbidden non-radiative (intersystem crossing [ISC]) transition between the lowest excited singlet and nearby triplet excited states, and (ii) Radiative transition from the lowest excited triplet to the singlet ground state (phosphorescence [P]). These two processes are schematically illustrated in Figure 1 with the relevant states in benzaldehyde indicated. Ideally both steps should be faster than other competitive processes to achieve bright triplet emission. The first step, the ISC, essentially determines the phosphorescence quantum yield and conversion efficiency, where fast ISC reduces energy losses due to competing relaxation processes. The

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second step is the actual phosphorescence, whose rate determines the number of phosphorescence photons emitted per unit time, i.e., the power. Singlet-triplet transitions require the coupled excited states to have different spatial character (e.g., between (nπ ∗ ) and (ππ ∗ )) as laid out by the El-Sayed’s rules. 13,14 The rates of the two key processes are determined as follows: (i) The ISC rate constant (kisc ) from lowest singlet excited state to the triplet manifold is calculated following the Marcus expression for a thermally activated process: 15–17 soc h (λ + ∆E)2 i V(T ,S ) 2 r π m 1 kisc = exp − , ~ kB T λ 4λkB T (1) soc where V(T ≡ hT |H | S i and H are the m 1 SO SO m ,S1 ) spin-orbit coupling coefficient and Hamiltonian, respectively. λ is the reorganization energy (difference between the triplet state energy at the S1 geometry and at the triplet state geometry) and ∆E is the free energy difference between the populating triplet state (Tm ) and the lowest singlet excited state (S1 ). (ii) The triplet emission rate constant (kpi ) from a particular spin sublevel of lowest triplet state to the ground state (Ti1 → S0 ) can be expressed by the perturbation theory expression: 18–20 2 64π 4 3 X i (2) ν kpi = Ma (T1 ) , 3 3hc a where Ma (T1i ) is defined as: X hSn |HSO | T1i i s,n Ma (T1i ) = µa ETnS n +

i X hTm |HSO | S0 i ti,m µa . m EST m

(3)

Therefore, the average phosphorescence lifetime (τp ) can be observed as: 1 1 1 X i = kp = kp , τp 3 i=−1

(4)

The key parameters in these equations include (i) the frequency of the emitted light, ν, (ii) the SOC mai trix elements hTm |HSO | S0 i and hSn |HSO | T1i i, (iii) the energy gap between singlet and triplet states, m where ETnS ≡ E(T1 ) − E(Sn ) and EST ≡ E(S0 ) − E(Tm ), and (iv) the transition dipole moment ini tegrals µs,n ≡ hSn |µa | S0 i and µti,m ≡ hT1i |µa | Tm i, a a where µa is the electron dipole moment operator with the a summation over the spatial coordinates. In this work, we use density functional theory (DFT) to calculate ground state properties, and the Tamm-Damcoff approximation (TDA) of time-dependent density functional theory (TDDFT) to address excited states. We confirm earlier studies, showing that TDA corrects the tendency of TDDFT to underestimate triplet excited state energies. 21–24 We employ the range-separated hybrid (RSH) ωB97X-D 25–27 exchange-correlation functional, which accounts for long-range Coulomb interactions as well as dispersion forces. All DFT and TDDFT calculations were performed using Q-Chem 4.4 program suite. 28 These calculations utilized the 6-311+G(d,p) basis set.

Experimental Details The chemical compounds used in the measurements were purchased from Sigma-Aldrich. Absorption spectra were measured on a Varian Cary50 UV/Vis spectrophotometer and photoluminescence spectra were obtained using a Photon Technologies International Quanta master. Phosphorescent lifetimes were recorded using time correlated single photon counting technique. The photo physical properties of the compounds were measured in chloroform solution of 10−5 M concentration. Benchmark Calculations We initially benchmarked our computational protocol on well-studied aromatic carbonyl systems (which is the key structural feature for triplet generation). In molecules such as benzaldehyde and acetophenone, the low-lying S1 excited state corresponds to a n-π ∗ transition, between an oxygen non-bonding p-orbital and a carbonyl anti-bonding π ∗ -orbital. The next low-lying S2 and S3 states are of ππ ∗ nature involving the carbonyl and aromatic ring (see their detachment and attachment densities in Figure S1). Two semi-degenerate triplet states 13 (ππ ∗ ) and 3 (nπ ∗ ) are lower in energy than the S1 state. 29–32 Efficient ISC has been attributed to a three-state crossing region between the 1 (nπ ∗ ), 13 (ππ ∗ ), and 3 (nπ ∗ ) excited states of aromatic carbonyls. 33–36 Recently Huix-Rotllant et al. reported that the triplet state photochemistry of acetophenone and the threestate crossing between the 1 (nπ ∗ ), 13 (ππ ∗ ), and 3 (nπ ∗ ) excited states are well reproduced using TDDFT. 36 The authors compared multiple levels of TDDFT to second-order extended multi-configuration quasi-degenerate perturbation theory (XMCQDPT2) calculations, which provide a high quality benchmark, though more expensive, treatment of dynamical correlation. Our TDDFT approach agrees well with the XMCQDPT2 benchmark values 35,36 for key structural changes of acetophenone at various excited state minimum energy configurations.i The TDDFT excitation energies for acetophenone also agree well with the values reported using the CAM-B3LYP RSH functional (comparison provided in SI, Table S2). We find that our approach overestimates the singlet excitation energies by ∼0.7 eV when compared to both XMCQDPT2 and measured values. 35,37 We ascribe this substantial difference to a two electron character of the excitations that is not addressed in DFT/TDDFT calculations. 36 The excitation energies of acetophenone at the local minima of ground and excited states involved in the relaxation process are shown in Figure 2, where the lowest singlet excited state, S1 (1 (nπ ∗ )), is indeed close in energy to the two lowest-lying triplet states, T1 and T2 (3 (nπ ∗ ) and 13 (ππ ∗ ) respectively). The three-state crossing region highlighted in Figure 2 implies that either triplet state may be involved in the phosphorescence process. In fact, the surrounding molecular environment can affect the energy ordering of the triplet states, which can result in phosphorescence from either the 13 (ππ ∗ ) state or the 3 (nπ ∗ ) state. 38–41 An increased stability of 13 (ππ ∗ ) state compared to the 3 (nπ ∗ ) may also weaken the observed radiative decay due to mixing between the 13 (ππ ∗ ) and 3 (nπ ∗ ) states. 40 i The triplet equilibrium geometry is used in place of the 13 (ππ ∗ ) geometry due to difficulties in converging the TDDFT excited state geometry.


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Table 1. Measured and calculated absorption energies, calculated oscillator strengths, dominant molecular orbital replacements, and the deviation of the calculated absorption energies from the measured absorption spectral peaks (∆ ≡ ECalc. − EExp. ).

Transition

Exp

Calc

Strength

MO involvement

∆

Acetophenone 1

(nπ ∗ )

−

4.03

0.0002

H → L (0.81)

−

11 (ππ ∗ )

4.22

5.16

0.0174

H-2 → L (0.70)

0.94

21 (ππ ∗ )

5.17

5.73

0.2705

H-1 → L (0.69)

0.56

ππ ∗ Gap

0.95

0.57

−

−

0.38

(nπ ∗ )

−

3.92

0.0001

H-1 → L (0.94)

−

11 (ππ ∗ )

4.40

5.11

0.0202

H-2 → L (0.77)

0.71

Benzaldehyde 1

21 (ππ ∗ )

5.00

5.68

0.3004

H → L (0.86)

0.68

ππ ∗ Gap

0.60

0.57

−

−

0.03

−

3.91

0.0001

H-1 → L (0.92)

−

5.08

0.0144

H-2 → L (0.77)

0.37

5.34

0.4969

H → L (0.94)

0.63

−

−

0.11

Para-Br 1

(nπ ∗ )

11 (ππ ∗ ) 21 (ππ ∗ )

4.71

ππ ∗ Gap

−

0.26

1

(nπ ∗ )

−

3.91

0.0001

H-1 → L (0.93)

−

11 (ππ ∗ )

4.20

4.89

0.0309

H → L (0.80)

0.69

21 (ππ ∗ )

4.98

5.60

0.1465

H-2 → L (0.72)

0.62

ππ ∗ Gap

0.78

0.71

−

−

0.07

Meta-Br

Ortho-Br 1

(nπ )

−

3.80

0.0001

H-1 → L (0.93)

−

11 (ππ ∗ )

4.13

4.80

0.0564

H → L (0.88)

0.67

21 (ππ ∗ )

4.94

5.50

0.1722

H-2 → L (0.83)

0.56

ππ ∗ Gap

0.81

0.70

−

−

0.11

∗

The energy gap between the two low lying π − π ∗ absorbing states is also listed (ππ ∗ Gap). Energies are in eV.

In order to obtain bright phosphorescence, the nonradiative transition from lowest singlet excited state to the triplet manifold must be faster than the radiative transition (fluorescence) to the ground state, which typically occurs on the ns timescale. The calculated non-radiative ISC rate constant (kisc ) of acetophenone for the 1 (nπ ∗ )-13 (ππ ∗ ) transition is 9.92×1011 s−1 using Equation 1, where the parameters λ and ∆E are 0.291 and -0.334 eV respectively. This is on the same order as a previously reported value (1011 s−1 ). 33,42,43 This relatively fast ISC stems from both the strong SOC (46.96 cm−1 ) and the close proximity in energies (∼40 meV) of the 1 (nπ ∗ ) and 13 (ππ ∗ ) states at the 1 (nπ ∗ ) geometry. To complete our computational benchmarking, we list the different components that determine the subsequent triplet emission. Importantly, we compare the terms affecting the phosphorescence rates in Table 2 and the rates listed in Table 3 from either of the possible triplet states. We note that soc in Table 2 we list the magnitudes of the SOC terms (V(S i n ,T1 ) soc and V(T i ,S0 ) ) and the electron dipole moment (µs and µt ), m where the vector components that are used in Equation 3 are listed in SI Table S5. The averaged rates are listed in Table 3, where the rates of the three triplet spin sublevels are listed in SI Table S6 (see Equation 4). We consider both optimal geometries of the two possible triplet emitting states:

Figure 2. Calculated excitation energies for acetophenone at the 1 (ππ), 1 (nπ ∗ ), 3 (nπ ∗ ), and 13 (ππ ∗ ) geometries. The TDDFT approach in this study successfully captures the three-states crossing region between 1 (nπ ∗ ), 3 (nπ ∗ ), 13 (ππ ∗ ) states, where the nearly degenerate states are highlighted by gray circles. Emission of the 3 (nπ ∗ ) state is emphasized whereas the 13 (ππ ∗ ) emission is negligble due to symmetry. (Excited states are color-coded: green - 3 (nπ ∗ ), orange - 13 (ππ ∗ ), blue - 1 (nπ ∗ ), tan - 23 (ππ ∗ ), red 11 (ππ ∗ ), purple - 21 (ππ ∗ ).)

At the 3 (nπ ∗ ) minimum energy structure, the n=0, 3 and m=1 terms dominate the summation in Equation 2. Although these terms have large denominator due to large energy differences (ET S or EST ), they involve high SOC between the triplet emissive state and singlet state and possess a large transition dipole moment. At the 13 (ππ ∗ ) minimum energy structure, the terms associated with n=1 and m=2 involve high SOC values. However, these terms are not associated with large transition dipole moments and as a result, the rate constant is small. Consequently, we find a higher phosphorescence rate at the 3 (nπ ∗ ) minimum energy structure than at the 13 (ππ ∗ ) structure (see Table 3). The calculated phosphorescence rate (kp = 3.26 × 102 s−1 ) at the 3 (nπ ∗ ) geometry agrees well with the measured radiative decay rate (kp = 1.2 × 102 s−1 ) of triplet acetophenone in the gas phase. 44

RESULTS AND DISCUSSION Absorption Next we consider the absorption spectra of benzaldehyde and the series of brominated molecules that are shown in Figure 3. The spectra are normalized based on the value of the absorption peak of the higher intensity. The two absorption peaks are located around 280-310 nm and 245-265 nm. The lowest energy peak at around 280-310 nm is red shifted upon bromination, with the shift increasing as the bromine is positioned closer to the carbonyl group. The higher energy peak is much less affected by the bromination site, except in the case of para-Br. For the para-Br system, the measured peak is broad around 265 nm with a tail extended to around



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Table 2. The guiding parameters for phosphorescence rate constant of acetophenone at the optimal geometry of both triplet states (see Equation 3). Geometry 3

soc V(S i n ,T )

|ETnS |

µs

m

soc V(T i ,S ) 0 m

m |EST |

0

84.63

2.87

3.87

µt

1

0.30

0.50

2

3.77

2.08

0.10

1

84.63

2.87

2.23

1.20

2

0.57

3.34

3

27.48

0.08

2.55

4.41

3

0.71

4.11

0

0.00

2.02

2.81

3.64

1

33.72

0.74

0.09

1

2.02

2.81

4.20

2

0.33

2.07

2.47

2

77.93

3.09

0.21

n

1

(nπ ) ∗

13 (ππ ∗ )

3 0.96 2.23 4.32 3 1.13 4.07 0.09 1 −1 2 2 soc 0 2 1 2 V(S ≡ |hS |H | T i| + |hS |H | T i| + |hS |H | T i| n n n SO SO SO i 1 1 1 n ,T1 ) 1 2 soc 0 1 −1 V(T ≡ |hTm |HSO | S0 i|2 + |hTm |HSO | S0 i|2 + |hTm |HSO | S0 i|2 i m ,S0 ) 1 2 µs ≡ |hSn |µx | S0 i|2 + |hSn |µy | S0 i|2 + |hSn |µz | S0 i|2 1 2 µt ≡ |hT1 |µx | Tm i|2 + |hT1 |µy | Tm i|2 + |hT1 |µz | Tm i|2 The n and m indices indicate the singlet and triplet states respectively. The V soc , E and µ are in cm−1 , eV and Debye, respectively. (The vectors components are provided in the SI.)

Table 3. The average phosphorescence lifetimes (τp in ms) and rate constants (kp in s−1 ) of the two lowest triplet states are compared. The phosphorescence frequency (ν in s−1 × 1014 ) and 2 P P MT ≡ 31 i a Ma (T1i ) in (statC.cm)2 × 10−40 are also tabulated. Values are calculated at optimal geometries.

Geometry 3

(nπ )

3

∗

1 (ππ ) ∗

ν

MT

τp

kp

6.94

0.84

3.07

3.26×102

6.79

0.004

721

1.39×100

290 nm. The maxima of the broad absorption peak of paraBr is red-shifted in comparison to the maxima of the higher energy peak of the other molecules. The calculated excitation energies are in excellent agreement with the measured spectral trends, although the absolute values tend to be overestimated (similar to the case of acetophenone). Both the calculated vertical excitation energies and the maxima of the measured absorption peaks are listed in Table 1. For para-Br, the two absorbing states are closely spaced, which agrees with the broad absorption peak in the measured spectra (Figure 3). For all systems, the lower energy peak is associated with the 11 (ππ ∗ ) excitation and the higher energy peak is assigned to 21 (ππ ∗ ). Importantly, the calculated 11 (ππ ∗ ) oscillator strengths follow the measured trend of the spectral peak heights. The calculated oscillator strengths for the higher energy 21 (ππ ∗ ) states are one order of magnitude greater than those of the lower energy 11 (ππ ∗ ) states. The calculated energy differences between the two states also captures the experimental trends. The measured gap between absorbing states increases upon bromination from para- to ortho-Br (Table 1 and Figure 3). In the ortho-Br and meta-Br systems, the 11 (ππ ∗ ) and 21 (ππ ∗ ) states are red-shifted relative to those of benzaldehyde (with calculated shifts of ∼ 0.3 and 0.2 eV, respectively for ortho-Br, and of ∼ 0.2 and 0.1 eV, respectively for meta-Br). The greater redshift of the 11 (ππ ∗ ) state increases the energy gap between absorbing states compared to benzaldehyde. In the para-Br system, the 21 (ππ ∗ ) state is red-shifted compared to that of benzaldehyde (with a calculated shift of ∼ 0.3 eV). The 11 (ππ ∗ ) state remains relatively unchanged, which decreases the energy gap between absorbing states.

Figure 3. Measured absorption spectra of benzaldehyde and bromobenzaldehydes in chloroform solution of 10−5 M concentration. The curves are normalized each against the higher energy peak.

The effect of Br on the energy difference between the two states can be explained by the Br contributions to the frontier molecular orbitals (FMOs) involved in each excitation. The bromination lifts the semi-degeneracy of the highest occupied MOs (HOMO, HOMO-1 and HOMO-2) in benzaldehyde (see the orbitals illustrated with their energies noted in Figure 4). Each lowest singlet excited state is dominated by an orbital transition of one of the highest occupied orbitals to the LUMO. (The attachment and detachment densities are illustrated in Figure S1.) In bromobenzaldehydes the pz -orbital of the Br atom couples to the HOMO π-orbital. This coupling results in a smaller fundamental orbital gap because the HOMO (LUMO) energy is raised (lowered) by ∼ 0.27 eV with respect to benzaldehyde. The smaller orbital gap due to Brcoupling explains the red-shift of the (π − π ∗ ) excitations dominated by the HOMO to LUMO transition in bromobenzaldehydes (see Table 1). On the other hand, the excitations that exhibit a smaller red-shift do not involve a large contribution from MOs that involve Br-coupling. The 1 (nπ ∗ ) is a symmetry forbidden excited state with a negligible oscillator strength. However as the lowest singlet excited state it plays a crucial role in the relaxation process leading to triplet population. The 1 (nπ ∗ ) excitation in all benzaldehydes is dominated by the transition between HOMO-1 and LUMO (H-1 → L), with a 0.1 eV red-shift of the state in the case of the ortho-Br system (Table 1). This red-shift depends on the proximity between the Br atom and carbonyl group, where the localized Br px -orbital couples most effectively with the non-bonding MO of the carbonyl group (Figure 4). As a result the HOMO-1 energy level is raised in the ortho-Br case, and thus lowers the energy of the transition itself compared to the other bromobenzaldehydes. Emission Phosphorescence in benzaldehyde occurs after ISC from the 1 (nπ ∗ ) to the triplet manifold, which takes place upon relaxation from the absorbing π −π ∗ states or following direct absorption to 1 (nπ ∗ ). 45–48 According to the measured spectra shown in Figure 5, the phosphorescence intensity increases as the bromine position becomes closer to the carbonyl group. We analyze each step of the emission process to explain the enhanced phosphorescence observed in the ortho-Br case of the brominated series of molecules. The first step in the relaxation process leading to phospho-


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Figure 4.


The frontier molecular orbitals of benzaldehyde and bromobenzaldehydes at the ground 1 (ππ) state equilibrium geometry.

Figure 5. Measured emission spectra of benzaldehyde and bromobenzaldehydes at 77 K in chloroform solution.

1

rescence is the ISC that is expected to occur at the (nπ ) geometry. Just as in the case of acetophenone, the energy gap between 1 (nπ ∗ ) and 13 (ππ ∗ ) is smaller at the 1 (nπ ∗ ) geometry than at the ground state geometry (shown in Figure 6). We now consider the effect of the bromination site on the kisc which essentially determines the quantum yield and therefore the overall efficiency of the phosphorescence. The kisc calculated at the 1 (nπ ∗ ) geometry is largest for the ortho-Br, with a value of 3.15 × 1012 s−1 , and is smallest for para-Br, with a value of 2.10 × 1011 s−1 . The kisc is determined by the SOC factors and the energy gap between the singlet and triplet states, which are provided in Table 4. At the 1 (nπ ∗ ) geometry, the ortho-Br molecule has both the largest SOC value and the smallest energy gap between 1 (nπ ∗ ) and 13 (ππ ∗ ) states of those corresponding to the other molecules. Thus, the proximity of the Br to the carbonyl enhances these two factors, resulting with the fastest ISC for the ortho-Br molecule. Next, we consider the second key step of the overall relaxation process; the phosphorescence step which determines the power that can be extracted from the conversion. After ISC to the 13 (ππ ∗ ) state, relaxation within the triplet man∗

Figure 6. Calculated excitation energies for benzaldehyde, ortho-Br, meta-Br, and para-Br at the 1 (ππ), 1 (nπ ∗ ), 3 (nπ ∗ ), and 13 (ππ ∗ ) optimal geometries. Excited states are color-coded: green - 3 (nπ ∗ ), orange - 13 (ππ ∗ ), blue - 1 (nπ ∗ ), tan - 23 (ππ ∗ ), red - 11 (ππ ∗ ), purple - 21 (ππ ∗ ). Table 4. Calculated ISC rate constants (kisc in s−1 ) from 1 (nπ ∗ ) to 13 (ππ ∗ ) transitions at the 1 (nπ ∗ ) minimum energy structure. The SOC magnitudes between the 1 (nπ ∗ ) and 13 (ππ ∗ ) states at 1 (nπ ∗ ) soc geometry ( V(T = hT2 |HSO | S1 i) are reported in cm−1 . The 2 ,S1 ) reorganization energies (λ) and the energy difference between states (∆E) are reported in eV.

Molecule

soc V(T 2 ,S1 )

λ

∆E

kisc

Benzaldehyde

47.01

0.237

-0.355

6.55×1011

Para-Br

43.44

0.266

-0.468

2.10×1011

Meta-Br

51.1

0.291

-0.394

8.69×1011

Ortho-Br

80.49

0.245

-0.289

3.15×1012

ifold eventually leads to the phosphorescence from the lowest triplet state. Similar to acetophenone and other aromatic carbonyls, 33–36 the 1 (nπ ∗ ), 13 (ππ ∗ ), and 3 (nπ ∗ ) energies are close, indicating a three-state crossing region that provides



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for fast relaxation from 13 (ππ ∗ ) to 3 (nπ ∗ ). The triplet emission rate constant calculated at the 3 (nπ ∗ ) geometry for benzaldehyde (kp =2.91×102 s−1 ) is similar to the rate constant of acetophenone (kp =3.25×102 s−1 ). In all bromobenzaldehyde systems, the 3 (nπ ∗ ) state is also lower in energy than the 13 (ππ ∗ ) state. The emission rate from the 13 (ππ ∗ ) state in ortho-Br is three orders slower than that from the 3 (nπ ∗ ) state. While we analyze below in detail only the emission from the 3 (nπ ∗ ) state, we list, for completeness, also the rate constants for the processes originating from both possible triplet states of ortho-Br in Table S10 and Table S11. Table 5. The guiding parameters for phosphorescence rate constant (see Equation 3). n and m indicate the singlet and triplet state respectively. Values are calculated at the 3 (nπ ∗ ) optimal geometry. Molecule

n

soc V(S i n ,T )

|ETnS |

µs

Benzaldehyde

0

84.96

2.84

4.16

1

0.08

0.53

2

2.34

2.07

3

26.15

0

85.06

1 2

Para-Br

Meta-Br

Ortho-Br

m

soc V(T i ,S ) 0

0.09

1

84.96

2.84

1.41

1.23

2

0.65

3.28

0.06

2.56

4.51

3

0.52

4.05

0.00

2.83

2.92

0.09

0.53

0.09

1

85.06

2.83

3.17

1.22

2.10

1.04

2

0.31

3.18

0.05

3

19.61

2.27

5.62

3

2.93

4.15

0.00

0

85.91

2.80

2.65

1

0.27

0.54

0.08

1

85.91

2.80

2.93

2

24.78

1.85

1.53

2

0.99

3.23

0.05

3

27.78

2.57

3.68

3

2.15

3.89

0.01

0

75.00

2.73

4.10

1

0.4

0.52

0.07

1

75.00

2.73

1.60

2

90.16

1.85

2.09

2

1.04

3.22

0.07

3

56.75

2.54

3.63

3

2.80

3.82

0.00

1

m

m | |EST

µt

See Table 2 for definition of parameter symbols.

We investigated the effect of the bromination site on the phosphorescence rate and parameters that determine kp (based on Equation 4) are provided in Tables S7-S10 and the rate constants in Tables 6 and S11. The magnitude of the SOC and the electronic dipole moments provide insights into the origin of the trends observed in the measured phosphorescence rates and are reported in Table 5. The calculated rate constants agree well with the experimental trends, where ortho-Br has the largest calculated rate constant, kp = 1.56 × 103 s−1 , and para-Br and benzaldehyde molecules have the smallest rates, 3.08 × 102 s−1 and 2.91 × 102 s−1 , respectively. These trends reflect the larger SOC in the ortho-Br molecule compared to the other molecules in the series. For all molecules in this study, the SOC between the lowest singlet state (S1 ) and lowest triplet state (T1 ) is small because both states are of the same character (n − π ∗ ). On the other hand, both the S2 (11 (ππ ∗ )) and S3 (21 (ππ ∗ )) states are of different character than the lowest triplet state (3 (nπ ∗ )) and therefore give rise to a substantial SOC. Indeed the SOC between S2 and T1 is substantial for the ortho-Br case, with a magnitude of 90.16 cm−1 . The SOC strength gradually decreases as the bromine site is positioned further away from the carbonyl group. The SOC is substantially smaller in the case of para-Br and benzaldehyde. The hS3 |HSO | T1 i matrix element also provides significant SOC contributions for all molecules, however the electric dipole moment is also a dominant factor for the S3 (21 (ππ ∗ )) state. The largest SOC of 56.75 cm−1 is found for the ortho-Br molecule due to the proximity of the Br to the carbonyl group.

Table 6. Calculated average phosphorescence lifetime (τp in ms) and rate constant (kp in s−1 ) are compared. The phosphorescence 2 P P frequency (ν in s−1 × 1014 ) and MT ≡ 13 i a Ma (T1i ) , in (statC.cm)2 × 10−40 are also tabulated. Values are calculated at the 3 (nπ ∗ ) optimal geometry.

Molecule

ν

MT

τp

kp

Benzaldehyde

6.87

0.77

3.44

2.91×102

Para-Br

6.84

0.83

3.24

3.09×102

Meta-Br

6.77

0.97

2.85

3.50×102

Ortho-Br

6.59

4.67

0.64

1.56×103

Overall, these results suggest that it should be possible to obtain enhanced phosphorescence in organic materials via intramolecular interactions between bromine and an aromatic carbonyl. Furthermore, these results indicate that in order to achieve an enhanced phosphorescence rate, the contribution of the heavy atom to the transition rates of the states involved in the relaxation pathways could be optimized.

CONCLUSIONS In summary, we present a synthetically viable route to enhance the phosphorescence quantum yield and power output in organic materials by introducing intramolecular interactions between bromine and a carbonyl group. This is in contrast to an earlier work where enhanced phosphorescence in metal free system was made possible via, arguably less controllable, intermolecular interactions in a mixed crystal phase. 1 We find that the proximity of heavy bromine atom to the triplet producing carbonyl group determines the trend of the phosphorescence observed in a series of bromobenzaldehyde isomers. Indeed the ortho isomer is found to possess the highest SOC, which results from interactions between the Br p-orbital and a non-bonding orbital of the carbonyl group. We find that both the kisc and kp rates are enhanced in the ortho-Br case. These computational results explain the experimentally observed trend, where the ortho bromination site features a relatively enhanced phosphorescence compared to the other molecules in the series and highlights the prospect of using ortho-Br in related optoelectronic applications. Acknowledgement B.D.D. acknowledges support from NSF grant CHE-1362504. We are also grateful to generous resource allocations on the Ohio Supercomputer Center and the Kent State University, College of Arts and Sciences Computing Cluster. E.G. would like to acknowledge support for this project by the National Science Foundation through Grant No. CHE-1464477. J.K. acknowledges the financial support from NSF (DMREF DMR 143965). Supporting Information Available: Detachment and attachment densities, key bond lengths at different excited states, excitation energies (acetophenone), absolute and normalized oscillator strengths, parameters for the phosphorescence rate constants and averaged phosphorescence rate constant and lifetime. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Bolton, O.; Lee, K.; Kim, H.-J.; Lin, K. Y.; Kim, J. Activating efficient phosphorescence from purely organic materials by


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