Theoretical Investigation on Excited-State Cyclization Reactions of

Mar 3, 2015 - singlet excited state when the open-form dithienylethene is irradiated ... and platinum-terpyridyl segment to form a complex through an ...
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Theoretical Investigation on Excited-State Cyclization Reactions of Platinum-Sensitized Dithienylethene Complexes Li Li, Fu-Quan Bai,* and Hong-Xing Zhang* State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: The ring-closing reaction commonly occurs at the lowest singlet excited state when the open-form dithienylethene is irradiated at about 300 nm. A lower-energy light, at 425 nm, can also elicit this ringclosing reaction when there is a connection between the dithienylethene and platinum-terpyridyl segment to form a complex through an ethynyl linker or an ethynyl-ether linker. Through the calculation of the energy levels, we propose the ring-closing process as follows. The light absorbed by the platinum-terpyridyl unit excites the molecule to a singlet excited state. Meanwhile, this electronic state of the molecule transfers to the lowest triplet excited state through intersystem crossing and internal conversion. When energy is asborbed from the environment, this state goes up to a higher triplet state around the dithienylethene part, where the ring-closing reaction takes place. Moreover, different patterns of linkers bring about different efficiency of the reaction, and a direct shared linker may facilitate the ring-closing process. In addition, the conjugated linker also causes the maximum wavelength of the complex to red shift because the energy gap between the involved frontier molecular orbitals becomes lower.

1. INTRODUCTION Photoisomerization 1,2 caused by photoexcitation is an important molecular property in which there is structural change between the isomers. Both reversible and irreversible photoisomerization reactions exist. However, the word “photoisomerization” usually indicates the reversible process. Typically, the photoswitchable molecule,3−5 such as dithienylethene (DTE),6−13 which can be reversibly shifted between two stable structures, presenting different optical and electronic properties that can be induced by light irradiation with appropriate wavelengths, has attracted more and more attention due to its unique photosensitive reversible characters in applied functional electronic components. The nearly insulating open form of DTE undergoes a ring-closure reaction under UV irradiation, whereas the ring-opening of the strongly delocalized closed form occurs under visible light irradiation (Scheme 1).6,12 In addition, the open and closed isomers of DTE are different not only in absorption spectra but also in

many other physical and chemical properties, such as photoluminescence, refractive index, electrical carrier injection, and transport characters. This makes the DTE highly attractive for its applications in the field of all-optical information processing and storage, among others. In the field of organometallic photoresponsive materials,13−17 platinum-sensitized complexes are considered to be the most popular photoresponsive materials.18,19 It is well-known that the phosphorescent Pt(II) complexes play an important role because their strong spin−orbit coupling (SOC), caused by the transition metal atom, makes intersystem crossing (ISC) between the singlet and triplet excited states occur efficiently. The rich photophysical behaviors of phosphorescent Pt(II) complexes have also been extensively studied for applications in biological sensing and photocatalysis, and in color-based sensors.20−22 In particular, square-planar platinum(II) terpyridyl [Pt(trpy)] complexes23−25 (Figure 1) can exhibit longlived emissions and large quantum yields with respect to other nonplanar complexes,26−31 and the absorptions involved may arise from metal-to-ligand charge transfer (MLCT), intraligand charge transfer (ILCT), or ligand-to-ligand charge transfer (LLCT). Thus, when introducing the DTE as a substitution to the Pt(trpy) complex, the reversible DTE isomers may modify the

Scheme 1. Photochromism of Dithienylethene

Received: December 23, 2013 Revised: January 22, 2015 Published: March 3, 2015 © 2015 American Chemical Society

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because according to the Grotthuss−Draper law, only that light which is absorbed by a system can bring about a photochemical change, which means normally only the light absorbed by the DTE can result in itself the ring-closing reaction. Thus, there may be a process that under irradiation B, the platinumsensitized part is first excited, and then the energy shifts to the DTE part through the linker, where the reaction may occur at the triplet excited state instead of mostly at singlet excited state. In addition, though the ring-closing reaction of the DTE component in 2o exists by the visible light irradiation (425 nm), it is much less efficient than that in 1o.33 In this paper, we have made a series of theoretical calculations on electronic structures, molecular orbitals, absorption and emission properties of complexes 1, 2, Pt(trpy) and DTE for comparison. We have investigated the different cyclization mechanisms of DTE to prove the possibility and validation of the reaction occurring at the triplet state, and we have also explained the different efficiencies of ring-closing in 1o and 2o.

Figure 1. Structures of Pt(trpy) complexes.

lifetime of the excited state of this organometallic complex, and the photoresponsive properties of the DTE may also be influenced. Especially, the combined molecule will bring some amazing characteristics not found in either organic or inorganic species alone and can offer numerous new optical and electronic properties which may be applied in groundbreaking photoelectron field. Recently, two kinds of such molecules containing both Pt(trpy) and DTE moieties have been synthesized, one is directly linking the DTE group to the Pt(trpy) complex (molecule 1 in Scheme 2), and the other is combined the two parts through a nonconjugated ether linker (molecule 2 in Scheme 2).32,33 It is reported that the ring closing of 1o and 2o can both be triggered either directly by irradiation with UV light (λ = 302 nm, E = 4.105 eV), which is absorbed by the DTE chromophore (irradiation A in Scheme 2), or indirectly by selective excitation with visible light (λ = 425 nm, E = 2.917 eV), which is sensitized by the Pt(trpy) component (irradiation B in Scheme 2). The comparison of irradiation A and B reveals the significant change of the conversion energy of DTE adding Pt(trpy) group. It is particularly amazing that the lower-energy irradiation may also trigger the ring-closing reaction of DTE,

2. COMPUTATIONAL DETAILS First, for simplicity, the p-tolyl groups on the terpyridine ligand of these complexes are truncated in our calculations. Take 1o, for examplewe compared the molecular characters with a ptolyl group and without a p-tolyl group (Table S1 and Figure S1). The result shows this kind of truncation is acceptable and will not affect the calculations. For these conjugation extended molecules, the long-range corrected functional CAM-B3LYP34,35 was employed to optimize the ground-state structures. The “double-ξ” basis set LANL2DZ36,37 consisting of 18 valence electrons associated with the pseudopotential was utilized for Pt atom and the 631G(d) basis sets38 was utilized for hydrogen, carbon, nitrogen, oxygen, and fluorine atoms, respectively. For the absorption

Scheme 2. (a) Conversions between Isomers of Molecule 1; (b) Conversions between Isomers of Molecule 2a

a

The subscript o represents ring-opening form, and c represents ring-closing form. 2820

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The Journal of Physical Chemistry A energy calculation of such organometallic complex containing a DTE group, CAM-B3LYP, B3LYP,39 and M0640 are considered as the most three adaptive and appropriate functionals. So the above three were tested, and the M06 was identified as the best in this study, from the comparative results shown in Table S2. On the basis of the optimized ground-state structures, the vertical transition energies to the first 40 singlet and the first 10 triplet excited states were determined under the timedependent density functional theory (TD-DFT)41−43 methods with functional M06. The calculated singlet excited states energies were transformed to the nanometer unit (nm), which is equal to the wavelength scale, and then simulated as absorption spectra, to allow comparisons easily with experimental data.33 To be consistent with the experimental measurements, the above simulations were performed in CH3CN solution using the polarizable continuum model (PCM).44,45 All the calculations were accomplished by using the Gaussian 09 program package.46

Figure 2. Simulated absorption spectra of individual molecules by TDM06 method in CH3CN solution (Gaussian curves, peak half-width at half-height is 0.333 eV).

3. RESULTS AND DISCUSSION 3.1. Ground-State Geometries. As shown in Scheme 2, each of the combined complexes (1o, 1c, 2o, and 2c) can be separated into two individual molecules, Pt(trpy) and DTE. In addition, the DTE molecules with substituents in different conditions are named, respectively, as DTE1o, DTE1c, DTE2o, and DTE2c. The ground-state geometry optimizations of these combined complexes were performed, together with the structures of the individual molecules for comparison. Figure S2 shows the optimized structures of these complexes. Table S3 tabulates the selected bond lengths and dihedral angles results. The symbol R (C1−C6) refers to atomic distances of the two reactive carbons, D1 (C1−C2−C3-C4) and D2 (C3−C4−C5-C6) are the dihedral angels in each DTE, and all of the values in the complexes are almost the same to those in reported simple DTE molecules, which indicates that the addition of the Pt(trpy) will not considerable change the structure of DTE. D3 is the dihedral angel between the Pt(trpy) plane and the left thiophene plane. Overall, these values of the four complexes are in this order: 1c < 1o < 2c < 2o. That means the two planes at the ends of the direct shared linker in 1o and 1c are more planar than those at the ends of a nonconjugated ether linker in 2o and 2c; moreover, the ring-closing may also reduce the D3 values. 3.2. Frontier Molecular Orbitals Analysis and Absorption Properties. The frontier molecular orbitals and the absorptions of these individual simple molecules were investigated first for comparison. Figure 2 describes the simulated absorption spectra, and Table 1 gives the main absorption data in terms of the transition states, excitation energies, oscillator strengths, and the related transition molecular orbitals with the maximum CI coefficients. The calculated absorption band of Pt(trpy) is at 409 nm, which is in accordance exactly with the experimental value,23−25 and the main excitation is assigned to the transition from HOMO to LUMO (Figure 3). It is classified as a MLCT excitation, because the molecular orbital composition analysis shows that the population of the Pt atom in HOMO is about 50%, whereas that in LUMO is less than 10%. The absorptions of DTE1c and DTE2c are also induced by the first electronic excitation from HOMO to LUMO. In the ring-opening forms DTE1o and DTE2o, the oscillator strengths of the first two excitations (S0 to S1, and S0 to S2) are relatively tiny, and the lowest-energy

Table 1. Main Absorptions of Individual Molecules in CH3CN Solution molecule

state

Enm(eV)

oscillator

excitation

coeff.

Pt(trpy) DTE1o

S1 S1 S2 S3 S1 S2 S3 S1 S1

409(3.034) 324(3.823) 293(4.234) 285(4.345) 347(3.577) 320(3.878) 295(4.204) 537(2.309) 575(2.157)

0.059 0.081 0.071 0.635 0.043 0.019 1.074 0.287 0.538

H→L H→L H-1→L H→L+1 H→L H-1→L H→L+2 H→L H→L

0.70 0.69 0.69 0.69 0.69 0.69 0.62 0.70 0.70

DTE2o

DTE1c DTE2c

Figure 3. Topology of HOMO and LUMO of Pt(trpy).

absorption peaks are mainly attributed to the S0 to S3 excitation, or rather, owing to the electronic transition from HOMO to LUMO+1 for DTE1o, and from HOMO to LUMO+2 for DTE2o. With respect to the energy values of these lowestenergy absorption peaks, the one of DTE1o is slightly larger than that of DTE2o, and the value of DTE1c is markedly larger than that of DTE2c. This means adding a conjugated benzene ring on DTE may reduce the transition energy of the longestwavelength absorption band, irrespective of the open or closed form. Figure 4 describes the first three electronic transitions for DTE1o, together with the topology of relevant molecular orbitals. The two black dashed arrows stand for the two weak excitations from HOMO to LUMO and from HOMO−1 to LUMO, respectively, and the red solid arrow stands for the 2821

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state. Here, the conversion mainly acts from the S3 state to S1 state, where the electron is just localized on the LUMO orbital with a bonding character between the reactive carbon atoms, and then the ring-closing reaction happens. The mechanism of the ring-closing reaction of DTE2o is considered to be like the one of DTE1o. Next, the frontier molecular orbitals and the absorptions of the combined complexes were investigated in detail. Figure 6

Figure 4. Selected electronic transitions and correlative molecular orbitals of DTE1o.

strongest excitation from HOMO to LUMO+1. These excitations are all attributed to ππ* electronic transitions. The HOMO−1 orbital primarily occupies the left thiophene ring. The HOMO and LUMO+1 are mostly localized on the right thiophene and benzene rings, and the large orbital overlap of the two orbitals exactly corresponds to the strong oscillator strength of the transition between them. The LUMO orbital mainly distributes at the central six carbon atoms, and in particular, the highlighted red frame in Figure 4 indicates the neighboring wave function phases on the two reactive carbon atoms (the lower part of C1 and the upper part of C6) are the same (both in blue color), which presents the bonding character between the two carbon atoms. This means the cyclization reaction probably happens when the electron is just excited to the LUMO orbital in DTE1o system.47 So the mechanism of the ring-closing reaction of DTE1o was conjectured, as described in Figure 5. The black arrows represent the first three excitations between electronic states when the molecule absorbs lights, of which the thickest one means the main absorption at 285 nm, and the red dashed arrow describes the process of internal conversion (IC), which represents a quick transition from a higher to a lower electronic

Figure 6. Energy level diagrams of partial molecular orbitals for complexes.

gives the energy level diagrams of partial molecular orbitals. (The related values are listed in Table S4, and the topologies of several important molecular orbitals of 1o, 2o, 1c, and 2c are revealed in Figure S3−S6.) Here, every complex can be divided into four types of contributions, Pt atom, trpy part, ethinyl linker, and DTE unit. (The division way is shown in Figure S7.) The compositions of these molecular orbitals are collected in Table 2. From Figure 6, the HOMOs of 1o and 2o almost have the same energy levels, but in fact, their characters are different. The HOMO of 1o has a hybrid composition of 15% Pt, 4% trpy, 24% ethinyl linker, and 57% DTE unit, whereas the HOMO of 2o is absolutely located on the DTE unit. Meanwhile, the HOMOs of 1c and 2c are also mainly localized on their own DTE parts. The LUMOs of 1o, 2o, 1c, and 2c are similarall of them contain a large percentage of trpy ligand (about 90%) and a small percentage of Pt atom (about 6%), just like the character of the LUMO of model complex Pt(trpy). Furthermore, their LUMO+1 orbitals all mainly occupy the trpy part, whereas the LUMO+2 orbitals are mostly localized on the DTE part. Especially, for 1o and 2o, the characters of LUMO+2 orbital are the as same as that of LUMO in DTE1o, in which the two active atoms (C1 and C6) have a bonding trend (the highlighted red frames in Figure S3 and S4). This means when the photons absorbed by 1o or 2o, the electron can be excited to the LUMO+2 orbital to reach the excited state, where the ring-closing reaction of the DTE group is likely to happen. Table 3 reveals the selected absorption data of these complexes, with the spectra described in Figure 7. As shown in Table 3, most of the calculated absorptions by M06 functional are in good agreement with the experimental results.33 Only the energy values of the lowest-energy absorptions in 1o and 1c are a little smaller, because the two absorptions are both assigned to a major long-range electronic

Figure 5. Jablonski diagram exhibiting the conjectural mechanism of the ring-closing reaction of DTE1o. 2822

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The Journal of Physical Chemistry A Table 2. Compositions of Several Molecular Orbitals of 1o and 2o 1o L+4 L+3 L+2 L+1 LUMO HOMO H-1 H-2 H-3 H-4

2o

Pt

trpy

ethinyl

DTE

Pt

trpy

ethinyl

DTE

3% 1% 2% 6% 15% 2% 31% 4% -

97% 99% 1% 98% 91% 4% 1% 12% 1% -

1% 24% 3% 41% 3% -

99% 2% 57% 94% 16% 92% 100%

3% 1% 2% 6% 43% 2% 33%

97% 99% 98% 93% 8% 1% 14%

1% 41% 2% 43%

100% 100% 100% 8% 95% 10%

Pt

trpy

ethinyl

DTE

Pt

trpy

ethinyl

DTE

3% 1% 2% 2% 5% 2% 6% 28% 33% 2%

97% 99% 9% 98% 82% 5% 13% -

4% 2% 5% 6% 20% 41% 2%

85% 11% 93% 88% 47% 13% 96%

3% 1% 2% 6% 2% 39% 3% 35%

97% 99% 98% 93% 7% 14%

1% 2% 37% 2% 45%

100% 100% 96% 17% 95% 6%

1c L+4 L+3 L+2 L+1 LUMO HOMO H-1 H-2 H-3 H-4

2c

Table 3. Absorption Characters of Complexes in CH3CN Solution, Together with the Experimental Values complex

state

Enm (eV)

oscillator

excitation

assignment

coeff.

exptl/nma

1o

S1 S4 S18 S2 S16 S22 S1 S4 S6 S16 S22 S26 S1 S4 S6 S14 S22 S25

516(2.402) 394(3.146) 305(4.064) 423(2.932) 311(3.992) 295(4.198) 655(1.892) 404(3.071) 380(3.262) 323(3.833) 312(3.979) 295(4.200) 574(2.159) 422(2.939) 381(3.251) 338(3.667) 310(3.992) 303(4.079)

0.212 0.012 0.489 0.110 0.371 1.263 0.438 0.097 0.080 0.425 0.396 0.156 0.589 0.100 0.060 0.457 0.352 0.295

H→L H-2→L H→L+7 H-2→L H-11→L H→L+7 H→L H-1→L H-2→L H-2→L+2 H-8→L H-4→L+2 H→L+2 H-2→L H-1→L+2 H→L+5 H-10→L H-5→L+2

LLCT/MLCT MLCT/LLCT ILCT(DTE) MLCT/LLCT MLCT/ILCT(trpy) ILCT(DTE) LLCT LLCT LLCT/MLCT LLCT/MLCT MLCT/ILCT(trpy) ILCT(DTE) ILCT(DTE) MLCT/LLCT LLCT ILCT(DTE) ILCT(trpy)/MLCT ILCT(DTE)

0.69 0.67 0.37 0.69 0.67 0.59 0.67 0.60 0.52 0.60 0.62 0.57 0.70 0.66 0.60 0.53 0.67 0.67

465 404 287 417 336 286 602 404 382 328 310 286 592 416 380 336 314 286

2o

1c

2c

a

from ref 33.

transition from the DTE end to the Pt(trpy) end, whereas the M06 functional does not include long-range corrections. From Figure 7, every complex seems to exhibit two main absorption peaksone appears at low-energy region and the other exists at high-energy region with a larger intensity. For 1o, the lowest-energy absorption appears at 516 nm, which is assigned to a HOMO to LUMO transition. This electronic transition is classified as a mixture of LLCT (from DTE to trpy) and MLCT (from Pt to trpy). The second absorption band at 394 nm arising from HOMO−2 to LUMO excitation is mostly attributed MLCT. The 305 nm absorption with the largest oscillator strength from HOMO to LUMO+7 transition belongs to ILCT in DTE unit. This light corresponds to the

reaction condition for DTE1o (285 nm), which can directly turn the ring-opening form into the ring-closing one. For 2o, the lowest-energy absorption is at 423 nm. It is originated from a HOMO−2 to LUMO transition, which is mainly assigned to the MLCT (from Pt to trpy) character, and the peak at highenergy region (295 nm) is due to a ILCT entirely in DTE unit. This light may directly result in the ring-closing of DTE because the energy matches well with the reaction condition for DTE2o. The lowest-energy absorption of 1c at 655 nm, results from the HOMO to LUMO transition which has a LLCT (from DTE to trpy) character. The lowest-energy absorption band of 2c appears at 574 nm, which is remarkably blue-shifted compared to the value of 1c, because this absorption arises 2823

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ring-closing reaction in 1o or 2o in principle. But actually, upon this light irradiation, the ring-closing reaction occurs in both 1o and 2o, although the efficiency in 2o is very low. 3.3. Cyclization Mechanism of DTE, Spin−Orbit Coupling, and Triplet Excited State Properties. Then, our investigation focuses on how the 425 nm light gives rise to the cyclization of DTE ring. The experimenter provided a simple mechanism (see Figure 9) of this process.33 They have

Figure 7. Absorption spectra of the complexes by TD-M06 method in CH3CN solution (Gaussian curves, peak half-width at half-height is 0.333 eV).

Figure 9. Direct conjectured cyclization of DTE based on the experimental results.

from a HOMO to LUMO+2 transition in DTE unit, whose energy difference is larger than the one of the transition in 1c. Then, the relationships of the energy levels of the involved molecular orbitals between the ring-opening complexes and their individual molecules were examined. As shown in Figure 8, for the molecular orbitals of 1o and 2o, the contributions from Pt(trpy) part are marked in green color, and the contributions from DTE part are marked in red color (Table S5 listing the detailed values). The black arrows represent the electronic excitations in sensitized Pt(trpy) part when the complexes absorb the selected light. The calculated absorptions in individual molecule Pt(trpy) is at 409 nm, and the values in 1o and 2o are, respectively, at 394 and 423 nm, close to the irradiation condition for cyclization (about 425 nm). From Figure 8, the light absorbed by the Pt(trpy) part arouses a transition from HOMO−2 to LUMO in both 1o and 2o. Figures S3 and S4 show that, for both ring-open complexes, the LUMO and LUMO+1 orbitals mainly occupy the Pt(trpy) unit, and the LUMO+2 orbital locates on the central DTE unit, which has a bonding character between the reactive atoms. This is similar to the LUMO of DTE1o (Figure 4). According to our previous discussion, the ring-closing reactions of 1o and 2o will take place only when the electron is excited to the LUMO+2 orbital. So, the 425 nm light (in experiment) cannot induce the

considered that first the complex is directly excited to its 1CT state mainly locating on the metal atom by 425 nm irradiation, then the singlet excited state transfers to a lower triplet excited state (3CT) through intersystem crossing. After that, if the temperature is above 140 K, the 3CT state may convert to a 3IL state on the DTE part, where the ring-closing take place.33 If the temperature is below 125 K, the 3CT state may fall to the ground state through energy loss in a phosphorescence emission way. As the spin−orbit coupling plays an important role in the ISC process,16,17,48 so first the SOC interactions of 1o and 2o were calculated.49−54 The energy levels of the lower Franck− Condon singlet excited states and Franck−Condon triplet excited states in 1o and 2o were calculated and collected in Table S8 and Table S9, respectively. The SOC data of 1o and 2o at ground state are listed in Table 4 and Table 5, respectively (the computational detail is provided in Supporting Information). In 1o, there is a largest SOC between S4 and T6, whose value is 216 cm−1. In 2o, the largest one is 157 cm−1, generated between S2 and T5. Actually, the two singlet excited states (S4 for 1o and S2 for 2o) both result from the photons absorbed by the Pt(trpy) part (the experimental irradiation light is at about 425 nm). Because of the different SOC values,

Figure 8. Relationships of the molecular orbitals between the complexes and their component molecules. 2824

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The Journal of Physical Chemistry A Table 4. Spin−Orbit Coupling (cm−1) between Triplet and Singlet States of 1o at Its S0 Geometry T1 S1 S2 S3 S4

T2

T3

T4

T5

10 64

T6

T7

20 216

36

Then the lowest triplet excited-state (T1) geometries of 1o, 2o, and Pt(trpy) were optimized for comparison (Figure S8). Several important structural parameters are listed in Table S6. Compared to the ground-state values (Table S3), the excitedstate ones of R, D1, and D2 all become slightly larger, which means the DTE parts in excited 1o and 2o have a trend of expansion and that is why the ring-closing is not believed to react at T1 state. Although the values of D3 change much more, from about 20° to almost 90° for 1o and from about 30° to near 0° for 2o, indicating that the two planes at the both ends of the linker in 1o become vertical and in 2o become planar when the complexes are excited. The calculated emission data are listed in Table 6, in good agreement with the experimental ones.33 Pt(trpy) can be

T8

Table 5. Spin−Orbit Coupling (cm−1) between Triplet and Singlet States of 2o at Its S0 Geometry T1 S1 S2 S3 S4

T2

T3

T4

T5

T6

T7

T8

157

Table 6. Calculated Emission Data of Complexes, Together with the Experimental Values

the ISC process may take place more easily in 1o than in 2o. The excited-state characters and the ISC process of 1o and 2o were described in Figure 10 and Figure 11, respectively. The a

complex

state

λcal./nm

energy/eV

λexptl/nm

Pt(trpy) 1o

T1 T1

557 610

2.225 2.031

550a 608b

From ref 22. bFrom ref 33.

applied in green phosphorescent organic light-emitting diodes due to its phosphorescence emission at 557 nm. Obviously, the addition of a DTE on the Pt(trpy) complex makes the emission wavelength red-shifted and highly reduces the luminescence efficiency of green light, which may be a guide for design of the optical materials. We also calculated the energy levels of the vertical triplet transitions on the T1-optimized structures of 1o and 2o, and the data are listed in Table 7. From Table 7, for both 1o and Table 7. Several Vertical Triplet Transitions on the T1Optimized Structures of 1o and 2o 1o

Figure 10. Energy levels of vertical excited states on the optimized S0 structure of 1o.

2o

state

energy/eV

transition

T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6

2.03 2.11 2.38 2.44 2.47 2.61 1.98 2.01 2.29 2.49 2.52 2.61

HOMO→LUMO HOMO→LUMO+2 HOMO−2→LUMO HOMO→LUMO HOMO−1→LUMO+7 HOMO−5→LUMO HOMO→LUMO HOMO→LUMO+5 HOMO−2→LUMO HOMO−1→LUMO+2 HOMO→LUMO+1 HOMO−7→LUMO

2o, the T1 state is assigned to the electronic transition from HOMO to LUMO, and the LUMO is completely localized on the Pt(trpy) part (Figure 12). The T2 state in 1o results from the electronic transition from HOMO to LUMO+2. This LUMO+2 orbital has a bonding character between the two active carbon atoms (Figure 12), just like the LUMO orbital in DTE1o molecule (Figure 4). So when the 1o complex gets its T2 state, the ring-closing reaction of DTE is likely to take place. The whole process is concluded in Figure 13. First, the 425 nm light excites 1o to its S4 state. The S4 state transfers to T6 state through ISC and then falls to T1 state by IC process. Meanwhile the complex in T1 state quickly relaxes to its lowestenergy T1-optimized structure. In this condition, when the

Figure 11. Energy levels of vertical excited states on the optimized S0 structure of 2o.

green lines represent these states whose excited electron locates on the Pt(trpy) part, and the red lines represent the states whose excited electron locates on the DTE part. In the two figures, the 425 nm light can both irradiate 1o and 2o to their own triplet excited states. 2825

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Figure 14. Conjectured ring-closing process of DTE in 2o under the 425 nm irradiation. Figure 12. Several molecular orbitals of 1o and 2o on their optimized T1 structure.

Figure 15. Ring-closing process of DTE under different irradiations (302 and 425 nm).

arrows), the complex is directly excited to the 1IL state, localized on the DTE part. In this state, the cyclization reaction can be induced quickly. Upon irradiation with 425 nm light (described in blue arrows), only the 1MLCT state on Pt(trpy) unit is populated. This state transfers to a corresponding triplet excited state 3MLCT by ISC and then through IC converts into a stable 3MLCT state (T1). The complex absorbs energy from the environment so that the 3MLCT state (T1) jumps to the 3 IL state localized on the DTE chromophore, where the ringclosing reaction takes place. In addition, if the temperature is low enough (below 125 K in experiment), the 3MLCT state (T1) cannot transfer to the 3IL state and then it will fall into ground state by energy loss in a phosphorescence emission way.

Figure 13. Conjectured ring-closing process of DTE in 1o under the 425 nm irradiation.

temperature is above 140 K, the excited molecule may get energy from the environment and jumps to its T2 state (the energy gap between T1 and T2 is only 0.08 eV), where the ringclosing reaction occurs. The whole process proves the conjectured mechanism by the experimenter,33 and our calculation provides more details. In Figure 13, another light at 470 nm can promote 1o to its S1 state, but this light cannot evoke the ring-closing of DTE, because there is almost no SOC between S1 and triplet states. This phenomenon matches well with the experimental result that 1o has a absorption band at 465 nm, but this light cannot induce the ring-closing of DTE.33 For 2o, the T4 state has a similar character as T2 in 1o, whose outmost electron locates on the LUMO+2 orbital. Then, like the ring-closing process in 1o, when 2o absorbs energy from environment and rises to its T4 state, the cyclization reaction of DTE may happen. However, because of the large energy gap between T1 and T4 (about 0.51 eV), it is hard for 2o to rise to T4 state from T1, which leads to the lower efficiency of the ringclosing. The conjectured ring-closing process is described in Figure 14. In summary, we succeeded to explain how the two kinds of lights (at 302 and 425 nm, respectively) can both arouse the ring-closing reaction of the DTE part of the complex (see Figure 15). When excited with 302 nm light (described in red

4. CONCLUSIONS At ground state, the structures of DTE have no outstanding changes when adding to the Pt(trpy) complex by whichever linkers. However, different linkers of the complexes may result in different lowest-energy absorptions, and the direct shared linker makes the wavelengths red-shifted markedly. The ringclosing of the individual molecule DTE is believed to react at S1 state only when it is irradiated by the light at 302 nm. Although in complex 1o, the two irradiations at respective 302 and 425 nm can both bring the ring-closing of the DTE unit. The irradiation at 302 nm, which is directly absorbed by the DTE unit in the complex, elicits the cyclization reaction at singlet excited state just like that in the DTE molecule alone. The irradiation at 425 nm first excites the molecule to a singlet 2826

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The Journal of Physical Chemistry A excited state on Pt(trpy) part (1MLCT), and then through ISC and IC, the electronic state reaches the lowest triplet excited state (3MLCT). Subsequently, by absorbing heat from the environment, the 3MLCT state jumps to the 3IL state localized on the DTE part, where the ring-closing reaction takes place. Moreover, because the cyclization in 2o requires more energy, its efficiency is much lower than that in 1o, which matches well with the experimental result.33 Finally, it may be an useful phenomenon that adding a DTE to the Pt(trpy) through different linkers can cause very different excited-state properties and modify chemical behavior of the platinum complex. We hope our work will be helpful in the design of new optical devices.

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ASSOCIATED CONTENT

S Supporting Information *

Functionals test (CAM-B3LYP, M06, and B3LYP) for absorption simulation, selected structural parameters of the singlet ground state, triplet excited states for 1o, 2o, 1c, and 2c, Cartesian coordinates of the structures reported in the paper and their whole calculated electronic energies through singlet and triplet vertical transitions, and the details of the SOC calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-431-8849-8966. Fax: +86-431-8894-5942. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant No. 21003057 and 21173096) and the State Key Development Program for Basic Research of China (Grant No. 2013CB834801) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20110061110018).

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