I measured TA (Transient Absorption) spectroscopy of my compound and obtained the UV-Vis absorption data of the T₁ excited state. There is a peak that I suspect corresponds to a triplet–triplet absorption (T₁ → Tₙ transition), and I would like to confirm whether this peak indeed arises from a T–T absorption using Q-Chem calculations. Is there a way to do this?
Such calculations are possible with Q-Chem with a few methods—please refer to the Q-Chem User Manual (7.1.1 Overview‣ 7.1 General Excited-State Features ‣ Chapter 7 Open-Shell and Excited-State Methods ‣ Q-Chem 6.3 User’s Manual). If you have already tried such a calculation with Q-Chem and your method of choice but have further questions, please tell us how you are setting up the Q-Chem calculation by posting a sample input using the preformatted text icon “</>”.
I have already optimized the T₁ state of my complex using the input file below. Now, I would like to calculate the triplet–triplet (T–T) absorption energy and strength.
$rem
Job_type opt
Method B3LYP
Basis def2-SVP
DFT_D D3_BJ
Symmetry False
Max_SCF_cycles 3000
Geom_opt_max_cycles 3000
mem_total 2000000
mem_static 2000
CIS_N_Roots 20
CIS_Singlets False
CIS_Triplets True
Set_state_deriv 1
Max_CIS_cycles 3000
THRESH 14
GEOM_OPT_TOL_GRADIENT 100
GEOM_OPT_TOL_DISPLACEMENT 400
GEOM_OPT_TOL_ENERGY 33
XC_GRID 3
$end
@@@
$molecule
read
$end
$rem
Job_type freq
Method B3LYP
Basis def2-SVP
DFT_D D3_BJ
Symmetry False
Max_SCF_cycles 3000
mem_total 1000000
mem_static 2000
CIS_N_Roots 20
CIS_Singlets False
CIS_Triplets True
Set_State_Deriv 1
CPSCF_NSEG 2
$end
@@@
$molecule
read
$end
$rem
Job_type sp
Method B3LYP
Basis def2-TZVP
DFT_D D3_BJ
Solvent_method SMD
Symmetry False
Max_SCF_cycles 300
mem_total 200000
mem_static 4000
CIS_N_Roots 40
CIS_Singlets True
CIS_Triplets True
Max_CIS_cycles 300
Print_Orbitals True
Molden_Format True
State_Analysis True
CIS_Ampl_Anal True
CHELPG True
$end
$smx
Solvent THF
$end
You should be able to do this with the keyword STS_MOM, see below. The molecule in this input file is 4-(dimethylamino)benzonitrile (DMABN). Its triplet-triplet absorption spectrum is known from spectroscopic measurements - it shows absorption at around 550 nm (2.3 eV). The calculation predicts an intense T1 → T5 transition at 2.5 eV.
$molecule
0 1
C -1.210008000 -1.233188000 -0.238719000
C 0.137726000 -1.240835000 -0.254259000
C 0.916264000 -0.009252000 -0.053578000
C 0.184187000 1.221130000 0.100746000
C -1.167829000 1.256803000 0.127849000
C -1.948305000 0.001647000 -0.010487000
N 2.295601000 -0.048042000 -0.108159000
C 3.041107000 -1.155993000 0.450034000
H 2.432578000 -2.065238000 0.480764000
H 3.363633000 -0.936485000 1.485953000
H 3.939712000 -1.355070000 -0.153268000
C 3.055470000 1.168846000 -0.267703000
H 4.067943000 0.918838000 -0.615050000
H 3.154022000 1.733662000 0.679206000
H 2.587794000 1.820797000 -1.016041000
H 0.731996000 2.155487000 0.230702000
H -1.770985000 -2.152270000 -0.413282000
H 0.659424000 -2.172526000 -0.475250000
H -1.704721000 2.194957000 0.263868000
C -3.338804000 0.006170000 0.058697000
N -4.507065000 0.014597000 0.116290000
$end
$rem
scf_guess sad
scf_algorithm diis_gdm
MAX_SCF_CYCLES 1000
JOBTYPE sp
UNRESTRICTED false
EXCHANGE CAM-B3LYP
BASIS def2-SVP
XC_GRID 000099000590
RPA false
SCF_CONVERGENCE 8
CIS_N_ROOTS 10
CIS_CONVERGENCE 8
max_cis_cycles 2000
STS_MOM TRUE
CIS_singlets False
CIS_triplets True
$end