Design and theoretical study of D – A –π– A' organic sensitizers with [1,2,5]oxa-, thia-or selenadiazoloazine fragment

A theoretical study and design of novel sensitizers based on D – A –π– A concept is fulfilled. The study shows a key role of central A-block for wavelength in UV-Vis spectra. Chalcogen of higher period as well as additional nitrogen in central A-block provide a noticeable red shift. The maximum wavelengths are observed for aliphatic/alicyclic D-blocks. The maximum oscillator strengths are observed for the planar D-block and increase along with its charge. Structure-spectral property relationships of sensitizers are studied using CRAQC (Correlation and Regression Analysis of Quantum Calculations). A method for chromophore determination is invented


Introduction
Heterocyclic chemistry of chalcogen-containing compounds is currently one of the most rapidly developing areas.2][3][4] For example, some of them containing 1,2,5-chalcogenadiazole ring are prospective high performance and low cost components of dye-sensitized solar cells, [5][6][7][8][9] anti-cancer and anti-HIV1 agents [10][11][12][13] and have drawn great interest both for industrial and academic specialists.Thus, a design of new 1,2,5chalcogenadiazole containing molecules that determine properties useful for medicine and engineering is of a great importance.Therefore, the goal of the paper is a theoretical study of the previously unknown heterocyclic compounds containing 1,2,5-chalcogenadiazole ring which may be of interest for materials with useful properties for components of small-molecule organic solar cells (SMOSCs).The mainstream for the design of chalcogen containing heterocyclic compounds for these aims is a construction of "donor -p bridge -acceptor" (D-p-A) configuration due to their convenient modulation of the intramolecular charge-transfer nature.Recently, it has been shown 7 that incorporation of additional electron-acceptors (such as benzothiadiazole, benzotriazole, quinoxaline, phthalimide, diketopyrrolopyrrole, thienopyrazine, thiazole, triazine, cyanovinyl, cyano-and fluoro-substituted phenyl) into the p bridge (termed the D-A-p-A) configuration, displays several advantages such as regulation of the molecular energy levels, red shift in UV-Vis spectrum, and distinct improvement of photovoltaic performance along with stability.Moreover, a new "D-A-π-A" 7 concept has been proposed for designing novel organic sensitizers, in which several kinds of electron-withdrawing units are incorporated into the π bridge to tailor molecular structures and to optimize energy levels.
It has been demonstrated that the incorporated electron withdrawing additional acceptor can be treated as an ''electron trap'', showing several distinguished merits such as: 1) essentially facilitating the electron transfer from the donor to the acceptor/anchor; 2) conveniently tailoring the solar cell performance with a facile structural modification on the additional acceptor; 3) improving circuit photovoltage with the nitrogencontaining heterocyclic group; 4) conveniently tuning the molecular energy gap, and modulating the response of the light-harvesting range with the new resulting absorption band; and 5) most importantly, being capable of greatly improving the sensitizer photo-stability. 7Organic sensitizers containing additional electron-withdrawing units (or D-A-π-A dyes) are reviewed with specific concern on the relationship between molecular structures and absorption, energy levels as well as photovoltaic performances.From the quantum viewpoint the electronic and spectral properties are dependent on the energy and distribution of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals so as dependent on the gap between their energies.Therefore, more specifically, our study is focused on fused [1,2,5]thiadiazolo [3,4-c]azine and [1,2,5]selenadiazolo [3,4c]azine derivatives incorporated into D-A-π-A configuration and their comparison with the similar [1,2,5]oxadiazolo [3,4-c]azine derivatives using DFT B3LYP calculations at the 6/311G(d,p) level of theory for the further design of new compounds with perspective properties.The specific interest to [1,2,5]thiadiazolo [3,4c]azine and [1,2,5]selenadiazolo [3,4-c]azine derivatives is concerned with the fact that these derivatives condensed with strong electronegative rings such as pyridine or pyridazine may be used in SMOSCs due to their capability to convert light into cheap electricity.
Scheme 1.General representation of the studied organic sensitizers.
In order to distinguish the acceptors at the right and left side of π-block we have introduced a somewhat another designation D-A-π-A'.Cyanoacrylic acid was selected as acceptor (A'-block in Scheme 2), bridged with a phenyl or thienyl (π-block in Scheme 2) since they are traditionally used in that role in a number of studies (e.g., 7,24 ).Substituted amines are currently used as donor groups and here we are also not original in their use but our list of donor groups (D-block) also includes some previously unexplored fragments.All these building blocks are shown in Scheme 2. Therefore, a total of 19 building blocks named from a to s are used in this work.These building blocks allow a combinatorial design of 126 different molecules listed in Table

The analysis of characteristics of the designed organic sensitizers
The geometry optimization along with conformational search has been performed for each molecule listed in Table 1 at the DFT B3LYP 6-311G(d,p) level of theory.Then, the HOMO and LUMO energies, excitation energies, oscillator strengths and wavelength for the minimum excitations has been calculated using TD SCF at the DFT B3LYP 6-311G(d,p).All these characteristics are represented in Table 1.The quantum computations were performed using GAMESS software, release May 2013 R1. 26,27 Preliminary, the capability of the DFT B3LYP 6-311G(d,p) features was tested using 18 molecules with experimentally measured wavelengths [28][29][30][31][32][33][34][35] under the same conditions in dichloromethane solutions.The molecules are the benzo [1,2,5]thiadiazole derivatives which are the D-A-π-A' analogs to the molecules studied in the current paper.A good agreement between calculated and experimental wavelengths was shown.The correlation coefficient between calculated and experimental wavelengths is 0.955 and the standard error of wavelength estimation is 19 nm.Therefore, the validity of the accepted level of theory for the study of UV-Vis properties was shown.
In general, all the elucidated quantum characteristics differ significantly for pyridine and pyridazine containing compounds (Table 4).The presence of an extra nitrogen in the pyridazine containing molecules compared to pyridine containing ones reduces both HOMO and LUMO levels but the latter is decreased stronger.Then, it yields to a reduction of the LUMO-HOMO gap and in turn to an excitation energy reduction, and a red shift in UV-Vis spectra.This is not a surprise since the additional nitrogen allows an additional low energy n -π* transition.Moreover, a pyridazine ring actually is a cyclic azo compound.The majority of this class of organic substances possesses good chromophoric properties.
The analysis of the influence of π-A'-block shows that there are no overall effect on the whole set of the designed molecules but separately for [1,2,5]oxadiazolo and [1,2,5]selenadiazolo derivatives the effects were found.For [1,2,5]oxadiazolo [3,4-c] derivatives the π-A'-block have no significant influence on the HOMO energy, but the compounds with thienyl bridge in π-A'-block (rs fragment) possess less value of LUMO energy than the compounds with phenyl bridge (qs fragment) on average by 0.14 eV that reduces the gap between LUMO and HOMO, excitation energy and correspondingly provides a bathochromic shift in UV-Vis spectra.Actually, the mean values of the gap between LUMO and HOMO for the compounds with thienyl bridge in π-A'-block (rs fragment) is smaller than that for the compounds with phenyl bridge (qs fragment) on average by 0.14 eV, the excitation energy is smaller on average by 0.10 eV and the wavelength is greater on average by 23 nm.The mean value for the [1,2,5]selenadiazolo [3,4-c] derivatives with phenyl bridge is 516 nm while for the compounds with thienyl bridge the mean value is 538 nm.
π-A'-block have no significant influence on the HOMO and LUMO energies [1,2,5]selenadiazolo[3,4-c] derivatives, but in a contrast to [1,2,5]oxadiazolo [3,4-c] derivatives the compounds with thienyl bridge in π-A'block (rs fragment) possess greater gap between LUMO and HOMO than the compounds with phenyl bridge (qs fragment) on average by 0.08 eV that provides an increase of excitation energy and correspondingly provides a hypsochromic shift in UV-Vis spectra.The excitation energy greater on average by 0.07 eV and the wavelength is smaller on average by 20 nm.The mean value for the [1,2,5]selenadiazolo[3,4-c] derivatives with phenyl bridge is 580 nm while for the compounds with thienyl bridge the mean value is 560 nm.
No significant dependency of the HOMO and LUMO energies, the excitation energy and the wavelength on the quantum derived characteristics of D-block and on the substitution type in the A-block (h, i, j or k, l, m in Scheme 2) is observed, so the calculation of more than 500 descriptors was performed using ChemoSophia online software (www.chemosophia.com) 28both for whole molecules (molecules 1 -126 in Table 1) and for Dblocks (building blocks a -g in Scheme 2) in radical state.Analysis of the descriptors showed that the HOMO energy has relationships with enthalpy of formation (ΔHf) of D-blocks calculated within ChemoSophia Elastic Model 28 but separately for [1,2,5]oxadiazolo derivatives and for [1,2,5]thiadiazolo/[1,2,5]selenadiazolo derivatives.The enthalpies of formation for D-blocks are represented in Table 5.The relationships are shown in Fig. 1 and can be described using polynomial equation (1)   where E is HOMO energy; a0, a1 and a2 are coefficients.For [1,2,5]oxadiazolo derivatives the cofficients in the equation ( 1) are a0 = -3.3709;a1 = 1.071•10 -3 and a2 = -8.270•10 - .The correlation coefficient R = 0.936; the standard deviation S = 0.036.
Thus the values of a0 and a2 coefficients for [1,2,5]oxadiazolo, [1,2,5]thiadiazolo and [1,2,5]selenadiazolo derivatives coincide.So, the only difference in equations is the terms a1 that means the slope of the curve varies by half but the curvatures for [1,2,5]thiadiazolo / [1,2,5]selenadiazolo derivatives and for [1,2,5]oxadiazolo derivatives and the beginning of the relationships are the same.Fig. 1 and equation (1) show that the reduction of ΔHf leads to increase of HOMO energy and in turn may provide a reduction of excitation energy and a bathochromic shift.Table 5 shows that the lower values are the characteristic feature of D-blocks (e, f, g) we proposed.These building blocks include alkylic and alicyclic components in their structure.Thus, the D-blocks (e, f, g) containing organic sensitizers should be more prospective than earlier proposed D -blocks (a, c, b, d).Actually, the e, f, g containing compounds possess greater HOMO energy on average 0.28 eV and LUMO energy on average 0.17 eV, smaller mean gap between LUMO and HOMO on average 0.11 eV, smaller mean excitation energy on average 0.05 eV and greater wavelength on average 11 nm than that for a, c, b, d containing compounds.
Additionally, correlations between HOMO energy and dipole moment of D-blocks calculated within ChemoSophia Elastic Model are observed.The greater polarity of the blocks provides the greater HOMO energy (the correlation coefficients 0.86, 0.86 and 0.83 for [1,2,5]oxadiazolo, [1,2,5]thiadiazolo and [1,2,5]selenadiazolo derivatives, correspondingly).The mean value of the dipole moment of e, f, g containing compounds (8.9 Debye) is greater than that for a, c, b, d containing compounds on average by 3.6 Debye.
An analysis of the oscillator strengths shows the following: [1,2,5]oxadiazolo [3,4-c]pyridine derivative 31 in Table 1 (dhrs) possesses the maximum value of the oscillator strength 1.233 among the designed molecules.Generally, the largest oscillator strengths are observed for the pyridine derivatives with completely planar D- block (c and d fragments).The mean value of the oscillator strengths for the pyridine derivatives with c or d fragments is 1.02 ± 0.12 while for other pyridine derivatives, the mean value of the oscillator strengths is 0.42 ± 0.15.Moreover, the diapasons of oscillator strengths for c and d containing pyridine derivatives and for all other do not intersect.The minimal value of oscillator strength among c and d containing pyridine derivatives is 0.7999 (molecule 10 dhqs in Table 1) while maximal value of oscillator strength among all other molecules is 0.7386 (molecule 64, akrs in Table 1).The planar structure of the D-block of c and d containing molecules provides in turn the greater planarity and the minimal deviation from π-plane of the A-block for the whole c and d containing molecules.So, the standard deviation of atoms from π-plane of the A-block for the whole c and d containing pyridine derivatives is 4.94 ± 0.14 Å while for the other molecules, the standard deviation is 6.73 ± 0.42 Å. Additionally the D-block d contains the minimal number of electrono-poor atoms (hydrogens) among all Dblocks that provides stronger electron donor property.Therefore, it is necessary to find more planar D-blocks containing minimal number of hydrogens for the design of novel prospective organic sensitizers with the maximal oscillator strength.

Correlation and regression analysis of quantum calculations results
It is interesting to study the effect of each component on the overall electron structure of D-A-π-A' organic sensitizers.For this aim, the sums of Mulliken atomic charges derived from the DFT B3LYP 6-311G(d,p) computations of the whole molecules (see above item) were calculated separately for each component, i.e.
where   ,   and  −′ are the charges of A-, D-and π-A'-blocks, correspondingly; where , and are the Mulliken atomic charges for the atoms belonging to A-, D-and π-A'-blocks, correspondingly.
It was found that all the D-blocks are actually electron donors.No one of the D-blocks has negative charge.The charges of these blocks for all molecules are in the range 0.041 -0.186 e.It is interesting that most of π-A'-blocks are also positively charged despite the fact that they include A'-block which is usually considered as acceptor.Only 19 of 126 compounds possess a weak negative charge in π-A'-block.The minimal value is only -0.022 e (molecule djqs, Table 1).The negative charges are observed only for some [1,2,5]thiadiazolo and [1,2,5]selenadiazolo derivatives.Generally, the charge of π-A'-block lies in the range -0.022 -0.087 e (mean value is 0.027).Therefore, the π-A'-block rather donor than acceptor, contrary to traditional views.All the Ablocks are negatively charged and their charges are in the range -0.251 --0.066 e.So, the A-blocks display distinct electron acceptor properties.
It should be noted that the heteroatom Y (Scheme 1) has an even greater impact on the electronic structure of the sensitizers that is not surprising, because Y is much closer to the substituents D and π-A' than heteroatom X.The charges of D-, A-and π-A'-blocks for pyridine (Y = CH) and pyridazine (Y = N) derivatives are represented in Table 4.The negative charge of A-blocks for pyridazine derivatives almost twice as much as that for pyridine derivatives.Consequently, the positive charge of D and π-A'-blocks of pyridazine derivatives significantly greater than that for pyridine derivatives.Table 6.Correlation coefficients matrix for Mulliken partial charges of A-, π-A'-, D-blocks (QA, Qπ-A', QD, correspondingly) and atoms of A-block (qN7, … , qN5) Qπ Qπ-A' QDA qN7 1.000 -0.083 -0.181 -0.735 0.994 -0.985 0.093 -0.056 0.110 -0.776 0.710 0.550 qC6 -0.083 1.000 -0.272 -0.065 -0.150 0.193 0.453 -0.455 0.451 0.013 -0.085 0.045 qC3 -0.181 -0.272 1.000 0.762 -0.156 0.111 -0.433 0.388 -0.432 0.203 -0.326 -0.039 qC4 -0.735 -0.065 0.762 1.000 -0.730 0.701 -0.300 0.249 -0.311 0.618 -0.611 -0.403 qC9 0.994 -0.150 -0.156 -0.730 1.000 -0.989 0.079 -0.043 0.095 -0.777 0.705 0.555 qY -0.985 0.193 0.111 0.701 -0.989 1.000 -0.000 -0.034 -0.018 0.770 -0.656 -0.581 qN2 0.093 0.453 -0.433 -0.300 0.079 -0.000 1.000 -0.998 0.999 -0.380 0.441 0.199 qX -0.056 -0.455 0.388 0.249 -0.043 -0.034 -0.998 1.000 -0.997 0.357 -0.407 -0.192 qN5 0.110 0.451 -0.432 -0.311 0.095 -0.018 0.999 -0.997 1.000 -0.395 0.452 0.212 Qπ -0.776 0.013 0.203 0.618 -0.777 0.770 -0.380 0.357 -0.395 1.000 -0.719 -0.854 Q π-A' 0.710 -0.085 -0.326 -0.611 0.705 -0.656 0.441 -0.407 0.452 -0.719 1.000 0.253 QDA 0.550 0.045 -0.039 -0.403 0.555 -0.581 0.199 -0.192 0.212 -0.854 0.253 1.000 It is interesting to elucidate the mutual influence of atoms within A-block and with π-A'-and D-blocks in the organic sensitizers.The correlation coefficients matrix for Mulliken partial charges of A-, π-A'-, D-blocks (QA, Q π-A', QD, correspondingly) and atoms of A-block (qN7, qC6, qC3, qC4, qC9, qY, qN2, qX, qN5) were calculated and presented in Table 6.Analyzing Table 6 it should be noted that heteroatom X has a significant influence only on the closest neighbors, i.e. atoms N2 and N5.The correlation coefficients (R) for the dependencies qX -qN2 and qX -qN5 are -0.998 and -0.997, correspondingly.Minus sign indicates that the increase in the charge of X leads to a reduction of N2 and N5 charges.The correlation coefficients of X charge dependency with the charges of other atoms and blocks are less than 0.5 that indicates a weak dependency of their charges on the charge of X heteroatom.Atom Y also has a significant effect on the closest neighbors, i.e.C9 and N7 atoms (correlation coefficients are -0.989 and -0.985, correspondingly) but its influence extends to the atom C4 (R = 0.701).Here we can see an alternation effect: increase in the charge of Y leads not only to a reduction of the closest neighbors C9 and N7 charges (negative R) but also to an increase of the next C4 charge (positive R).Moreover, the charge of whole A-block also correlates with q Y , q C9 and q N7 (R = 0.770, -0.777 and -0.776, correspondingly).Therefore, this fragment determines mostly the charge of A-block.Additionally, the charges of these atoms have a correlation with the π-A'-block charge (R = -0.656,0.705 and 0.710).So, this fragment is responsible for electron transfer between A-and π-A'-blocks.It draws an attention that the A-block charge has a significant dependency on the D-blocks charge.The dependency is shown in Fig. 2. The figure shows that the positive charge of D-blocks is linearly related with the negative charge of A-blocks that confirms the electron donor properties of the Dblocks.The correlation coefficient for this dependency is -0.854.The dependency of A-blocks charge on the charge of π-A'-blocks is weaker (R = -0.719).Minus sign in the both dependencies indicates that the A-block attracts electron density from both D-and π-A'-blocks, but more strongly from the first one.A multiple regression analysis of charges of X, Y atoms, D-and π-A'-blocks was performed for more detailed description of mutual influence of the organic sensitizers' atoms and fragments.So, the Mulliken partial charge of X heteroatom is dependent on the charges of N2, C6, N7 and N5 atoms (and vice versa their charges are dependent on the charge of X heteroatom) in accordance with the following equation.qX = -0.1073-1.23•qN2 -2.25•qC6 + 0.207•qN7 -1.17•qN5 (2) This equation and the following equations were obtained using "Backward Stepwise" techniques.Correlation coefficient R = 0.9989; Standard error of estimate S = 0.0067.The terms in this equation and the following equations are arranged in descending order of their statistical significance.The equation shows that the increase of X heteroatom partial charge yields in the decrease of N2, C6 and N5 charges and in weak increase of N7 charge.Each coefficient shows an influence of each atom on X heteroatom (and conversely, an influence of X heteroatom on the atoms).So, the increase in the N2 charge by 0.1 yields in decrease of qX by 1.23•0.1 = 0.123.So the equation shows a comparable influence of X heteroatom on neighboring N2 and N5 atoms decreasing their charge, stronger effect on C6 atom and a weak increase of N7 charge due to decrease of neighboring C6 electron density.The dependence of qy is much more extensive qY = -0.453-1.543•qN7 + 1.02•qC6 -0.141•QD + 0.0201•qN5 + 0.0200•qN2 -3.15•qC3 + 3.09•qC4 (3) R = 0.9956; S = 0.0070.
An increase of Y charge leads to decrease of the neighboring N7 charge and then to increase of the next C6 charge, decrease of the next C3 charge and increase of the next C4 charge.Thus, there is a pronounced charges alternation effect.C3 and C4 atoms are exposed to a particularly strong and almost the same influence of Y. N2 and N5 atoms are far from Y and exposed to a weak and almost the same influence of Y.There are no terms with C9 atom whose charge is apparently determined not only by Y but also by substituent R'.At the same time, some influence of Y to the D-block is observed yielding in an attraction of the electron density from the latter to Y.
An increase of the electron density of π-A'-block leads to an attraction of the electron density from neighboring C6 and to an increase of all the A-block's nitrogens electron density (a strong increase for N7 and the weak and approximately equal for N2 and N5).Moreover, π-A'-block provides some attraction of the electron density from D-block.
There is no a regularity of the influence of D-block on the atoms of A-block or on the π-A'-block of the pyridazine derivatives (Y = N, molecules 85 -126 in Table 1) but it has a significant influence on the atoms of Ablock and on the π-A'-block of pyridine derivatives (Y = CH, molecules 1 -84 in Table 1) describing by the following equation ) R = 0.9928; S = 0.0021.
The equation shows that D-block strictly donates the electron density to the atoms of A-block and to the π-A'-block since all the terms except the free term of the equation are negative, i.e. the decrease of D-block electron density yields in the increase of N7, X, Y, C6, N5, C4, C9, N2 electron density.Moreover, D-block donates the electron density to the π-A'-block that is in agreement with the above mentioned results.Only one atom C3 has no any regularities of the influence on the charge of D-block.
More important question allowing to obtain a key for the further design of prospective organic sensitizers: how the charge characteristics of atoms and fragments are related to the energy and spectral characteristics?The regression analysis reveals no significant relationship between the HOMO energy and charges of atoms and fragments.At the same time, the regression equation for the LUMO energy shows the good statistical characteristics LUMO = 1.27 -6.In accordance with the regression model an increase of the C9, C4, N2, X and N5 (red in Scheme 3) charges leads to decrease of the LUMO energy that can provide a red shift in UV-Vis spectra, while an increase of the Y and N7 (blue in Scheme 3) charges leads to increase of the LUMO energy.In accordance with Equation (3) the increase of C6, N2, N5 charge and decrease of N7 charge can be reached by increase of Y charge (i.e. using less electronegative X).The charge of π-A'-block has a contradictory impact on these atoms in accordance with other Equations ( 2), ( 4) and ( 5).In accordance with the regression model an increase of only C4 (red in Scheme 4) charge leads to decrease of excitation energy that provides the red shift in UV-Vis spectra, while an increase of the Y, N2 and N5 (blue in Scheme 4) charges leads to increase of excitation energy.It should be noted that a decrease of the N2 and N5 charge can be achieved by increase of X charge in accordance with Equation (2).Thus, the less electronegative X the less excitation energy.So, the usage of X = S or Se is preferable rather than O.In accordance with Equation (4), a The excitation energy can be described using the following regression model Ee = 4.33 + 6.54•qY -16.4•qC4 + 2.11•qN2 + 2.10•qN5 R = 0.951; S = 0.088.decrease of N2 and N5 charge can be achieved by a decrease of π-A'-block charge (i.e. using π-A'-block with the strengthened acceptor properties).Other Equations ( 3) and ( 5) show contradictory impact on these atoms.In accordance with the regression model the increase of only C4 (red in Scheme 5) charge leads to the batochromic shift in UV-Vis, while an increase of the Y, N5, N2 and X charges (blue in Scheme 5) leads to decrease of wavelength.All these atoms Y, C4, N5, N2 and X determining wavelength can be confirmed as a chromophoric fragment of these organic sensitizers.It should be noted that the decrease of the Y, N5, N2 and X charges can be achieved by an increase of D-block charge in accordance with Equation (5).Thus, the increase of the donor properties of D-blocks yields in the desirable changes in the charges.In accordance with Equation (4) the decrease of N2 and N5 charge can be achieved by decreasing of π-A'-block charge (i.e. using π-A'-block with the strengthened acceptor properties).Other Equations ( 2) and (3) show contradictory impact on these atoms since an increase of X or Y charge yields in decrease of N2 and N5 charges.At the same time this dependency is the best obtained for the oscillator strength.In accordance with the regression model, an increase of D-block charge leads to increase of the oscillator strength, while increase of the N7 and C6 charges leads to decrease of the oscillator strength.It should be noted that the decrease of the N7 and C6 charge along with an increase of D-block charge can be achieved by increase of the donor properties of D-block in accordance with Equation (5).Other equations show contradictory impact on these atoms and block.
This methodology performing a Correlation and Regression Analysis of Quantum Calculations results was named CRAQC techniques.

Conclusions
Thus, a molecular design and theoretical study of novel prospective organic sensitizers based on D-A-π-A' concept has been fulfilled.Like in previous researches, it has been shown that the A-block plays a key role for the wavelength shift in UV-Vis spectra, also we have shown that a presence of an element of higher periods of the Periodic System in the A-block provides noticeable bathochromic shift.This study determines that the thienyl bridge in the π-A'-block provides red shift with respect to the phenyl bridge for [1,2,5]oxadiazolo [3,4c]pyridine derivatives and vice versa the phenyl bridge in the π-block provides a red shift with respect to the thienyl bridge for [1,2,5]selenadiazolo [3,4-c]pyridine derivatives.No significant influence of the bridge in π-A'block is observed for [1,2,5]thiadiazolo [3,4-c]pyridine derivatives.The maximal wavelengths values have been observed for D-blocks with aliphatic and alicyclic fragments we proposed.It has been shown in the study that the reduction of the enthalpy of formation and increase of polarity of D-blocks yields in bathochromic shift in UV-Vis spectra.The quantitative dependencies of wavelengths on the enthalpies of formation and dipole moments are determined.It has been shown that the maximal values of the oscillator strength are observed for the planar D-block.Therefore, it has been shown in this study that the conditions for the design of compounds possessing the maximal wavelength and the maximal oscillator strength are different: the latter presupposes the planar structure of D-block while the first presupposes an existence of non-planar aliphatic and/or alicyclic fragments in D-block.Thus, the next steps for the molecular design of novel prospective organic sensitizers should include some combination of these concepts.Correlation and Regression Analysis of Quantum Calculations results (CRAQC techniques) has been invented.Mutual influences of blocks, fragments and atoms in the molecules and their influence on the energy and spectral properties of organic sensitizers are studied using the CRAQC.A method for determining of chromophoric fragments has been proposed.It has been shown that the wavelengths for the elucidated organic sensitizers is determined by Y, C4, N5, N2 and X atoms while the oscillator strengths is determined by N7, C6 atoms and D-block.It allowed to pick up an additional factordonor properties of D-block permitting to increase both wavelength and oscillator strength.

Figure 2 .
Figure 2. Dependency of A-block charge (QA) on the D-block charge (QD).

Scheme 3 .
Scheme 3. Influence of atoms on LUMO energy.

Scheme 4 .
Scheme 4. Influence of atoms on the excitation energy.

Scheme 5 .
Scheme 5. Influence of atoms on the UV-Vis spectra.

Table 1 .
. Quantum characteristics for designed molecules (HOMO is the energy of highest occupied molecular orbital, eV; LUMO is the energy of lowest unoccupied molecular orbital, eV; Δ is the gap between LUMO and HOMO, eV; Ee is the excitation energy, eV; OS is the oscillator strength; λ -is the wave length, nm; ΔHf is the enthalpy of formation of A-block, kcal/mole; µ is the dipole moment of A-block, D) 4011 -0.7393 2.6618 2.2935 0.2642 541 50.1 6.99 10 dhqs -3.6173 -0.7747 2.8427 2.6014 0.7999 477 68.6 6.39

Table 5 .
Enthalpies of formation for D-blocks